1D Nanomaterials for Sodium-Ion Batteries

review
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1D Nanomaterials: Design, Synthesis, and Applications
in Sodium–Ion Batteries
Ting Jin, Qingqing Han, Yijing Wang, and Lifang Jiao*
Dr. T. Jin, Q. Han, Prof. Y. Wang, Prof. L. Jiao
Key Laboratory of Advanced Energy Materials Chemistry
(Ministry of Education)
College of Chemistry
Nankai University
Tianjin 300071, China
E-mail: jiaolf@nankai.edu.cn
Prof. L. Jiao
Collaborative Innovation Center of Chemical Science
and Engineering (Tianjin)
Tianjin 300071, China
DOI: 10.1002/smll.201703086
1. Introduction
With the global energy crisis aggravation and environmental
deterioration, it is imperative to develop energy storage systems
(ESSs).[1,2] Among various ESSs, rechargeable batteries are
considered as the most successful technology that can sustain-
ably generate green energy from stored materials and convert
chemical energy into electrical energy.[3] Over the past years,
lithium–ion batteries (LIBs), are the most promising ESSs, have
captured the current worldwide rechargeable battery markets
due to the outstanding energy and power capability, especially
playing a dominant role in portable electronic devices.[4] How-
ever, lithium reserves are relatively low on the Earth’s crust,
and the distribution of lithium resources is mainly concen-
trated in South America, resulting in a high price of lithium,
which seriously limits the development and application of
Sodium–ion batteries (SIBs) have received extensive attention as ideal
candidates for large-scale energy storage systems (ESSs) owing to the rich
resources and low cost of sodium (Na). However, the larger size of Na+ and
the less negative redox potential of Na+/Na result in low energy densities,
short cycling life, and the sluggish kinetics of SIBs. Therefore, it is neces-
sary to develop appropriate Na storage electrode materials with the capa-
bility to host larger Na+
and fast ion diffusion kinetics. 1D materials such as
nanofibers, nanotubes, nanorods, and nanowires, are generally considered to
be high-capacity and stable electrode materials, due to their uniform struc-
ture, orientated electronic and ionic transport, and strong tolerance to stress
change. Here, the synthesis of 1D nanomaterials and their applications in
SIBs are reviewed. In addition, the prospects of 1D nanomaterials on energy
conversion and storage as well as the development and application orientation
of SIBs are presented.
Sodium–Ion Batteries
LIBs. Therefore, it is an urgent demand
for alternative energy storage devices with
low cost and remarkable performance. In
contrast to LIBs, sodium–ion batteries
(SIBs) are considered more promising for
medium and large-scale stationary energy
storage owing to the abundant resources
and low cost of sodium.[5]
However, the
larger Na+ than Li+ (1.02 Å vs 0.76 Å in
radius) and the higher standard electro-
chemical potential of Na+/Na compared
with Li+/Li (−2.71 and −3.04 V vs SHE,
respectively) lead to low power and energy
densities, hindering further development
of SIBs. Hence, it is greatly significant to
find appropriate electrode materials that
can host larger Na+ and possess fast ion
diffusion kinetics.
Nanoscale electrode materials attract
much attention due to their smaller size,
larger specific surface area and facile stress relaxation pro-
cesses. The small nanoparticles can shorten the length of Na+
diffusion. The large specific surface area not only increases
the electrode/electrolyte contact area but also improves charge
storage by electrical double-layer and surface redox processes.[6]
The benefit of stress relaxation is to relieve the volume varia-
tion of electrode materials during cycling processes. Among
various nanoscale materials, 1D nanomaterials, including
nanowires, nanofibers, nanobelts, nanorods, and nanotubes,
are recognized as a class of most promising materials in ESSs.
The unique structure of 1D nanomaterials can offer facile elec-
tronic and ionic transport and strong tolerance to stress change,
contributing to the high performance of ESSs. Although several
reviews devoted to 1D nanostructured materials have intro-
duced their applications in ESSs,[7–10] a review systematically
summarizing the fabrication and application of 1D nanomate-
rials in SIBs is still needed.
In this review, the synthetic routes for 1D nanomaterials,
mainly including electrospinning method, gas-phase route,
solution-phase route, and template-assisted method are empha-
sized. In addition, the most-recent advances and prospects of
1D nanostructured materials in SIBs are also covered. Repre-
sentative examples are listed in Tables 1 and 2. Moreover, the
effects of various morphologies and structural features on
electrochemical properties in SIBs are highlighted. Finally, the
bottlenecks and issues with fabrication of 1D nanomaterials
are discussed and a summary of the future development of 1D
nanomaterial applications in electrochemical energy storage
devices are presented.
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2. Advantages and Synthetic Routes of 1D
Nanostructured Materials
1D nanostructured electrode materials possess a variety of
advantages for obtaining high capacity, long-term cycling, and
superior rate performance in SIBs:
(i) The fast Na+ diffusion kinetic in 1D nanostructured
materials.
As far as we know, the kinetic diffusion of electrode materials
determines the power density of SIBs, which mainly include
two parts: diffusion in electrolyte and diffusion in electrode
materials. The latter is the key step. Generally, the diffusion
of Na+ in electrode materials is associated with the diffusion
length (L) and diffusion coefficient (D), which can be depicted
in Equation (1)
τ = /2
L D (1)
where τ is diffusion time in electrode materials, which is
proportional to L2 and in inverse proportion to D. The small
nanoparticles in 1D nanostructured materials can shorten the
diffusion length (L) of Na+, thus decreasing the Na+ diffusion
time in electrode materials, resulting in the enhanced specific
capacity and the improved power density of SIBs.
(ii) 1D nanostructured materials provide direct current path-
ways, which is beneficial to electrical transport compared
with other electrodes.
(iii) The large specific surface area of 1D nanostructured mate-
rials can enlarge the electrode/electrolyte contact area and
reduce the charge–discharge time.
(iv) 1D nanomaterials can accommodate the volume variation
of electrode materials in charge/discharge processes, hin-
dering the pulverization and aggregation of electrode mate-
rials and leading to long-term cycling performance.
2.1. Electrospun
2.1.1. Principles of Electrospun
The electrospinning technique is a versatile top-down method
for manufacturing 1D continuous fibers (from the nanometer
to micrometer scale) by electrostatic forces. The general elec-
trospinning equipment mainly contains three parts: (i) a spin-
neret; (ii) a high voltage power supplier; and (iii) a grounded
conductive collecting substrate (often a metal screen or rotating
mandrel). To date, the precursor solution is primarily a poly
mer-containing. There are mainly two types of polymers used
in electrospinning. One type is water-soluble polymers, such as
polyvinyl pyrrolidone (PVP), polyethylene oxide (PEO), and poly-
vinyl alcohol (PVA). Another type is non-water-soluble polymers,
such as polyacrylonitrile (PAN), polystyrene (PS), polyimide
(PI), polyvinylidene fluoride, polymethacrylate (PMMA), and
polyvinylchloride. In non-water-soluble polymers, the solvents
mainly include N,N-dimethylformamide (DMF), N-methyl-
2-pyrrolidone, and ethanol. In a typical process, when a voltage
Ting Jin received her B.Sc.
in materials chemistry from
Northwest University, China
(2015). Currently, she is a
Ph.D. student in the group
of Associate Prof. Lifang Jiao
at Nankai University, China.
Her research interests focus
on the design and fabrica-
tion of high-performance
electrode material for energy
storage and conversion,
mainly including sodium–ion and potassium-ion
batteries.
Qingqing Han received her
B.Sc. in chemistry from
Anyang Normal University,
China (2016). At present, she
is a graduate student under
the supervision of Associate
Prof. Lifang Jiao at Nankai
University, China. She cur-
rently works on the design and
preparation of advanced elec-
trode materials for recharge-
able sodium–ion batteries.
Lifang Jiao is an Associate
Professor at Nankai
University, China. She
received her Ph.D. from
Nankai University, China
(2005). She has coau-
thored over 190 relevant
peer-reviewed publications,
including 10 ESI highly cited
papers. Her current research
is focused on energy conver-
sion and storage (including
lithium, sodium, and magnesium secondary batteries, and
supercapacitors), hydrogen storage materials and electro-
catalytic hydrogen evolution.
(typically in the kV range) is applied between the spinneret and
the collector, the electrostatic forces acted on the droplet and
the surface tension of droplet make it become a conical shaped
droplet, which is called the Taylor cone.[11] Once the voltage
increases to a critical value, the repulsive electrostatic force over-
come the surface tension of the droplet and then the precusor
solution eject from the tip of Taylor cone and elongate with the
help of electrostatic force until it is deposited onto the collector.
Finally, the continuous and solid nanofibers are formed on the
ground collector with the evaporation of solvent. Furthermore,
the electrospun nanofibers are controllably synthesized and
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influenced by the following parameters: (i) The system parame-
ters (such as the molecular weight of polymers and the polymer
solution properties (conductivity, viscosity, dielectric constant,
and surface tension)). (ii) The operating parameters (such as
needle gauge, nozzle type, voltage, flow rate, distance between
the spinneret and collector). (iii) Ambient parameters (such
as temperature and humidity in the enviroment). Moreover, it
is worth mentioning that the subsequent process parameters
(e.g., heating temperature, heating rate, and heating time) are
greatly significant for obtaining inorganic materials with var-
ious nanostructures.
2.1.2. Preparation via Electrospun Method
Over the past years, the solid nanofibers fabricated by a facile
single-nozzle conventional electrospinning method are the
most common and have been widely applied in ESSs.[12–15] Cao
and co-workers successfully synthesized Sb–C nanofibers via a
single-nozzle electrospinning technique and subsequent heat
treatment.[13] Figure 1a schematically illustrates the synthetic
process of Sb–C nanofibers. The transmission electron micros-
copy (TEM) image in Figure 1b reveals that as-obtained Sb–C
nanofibers possess the structure of Sb nanoparticles homog-
enously embedded in carbon nanofibers. Such structure is
much stable and provides enough space for Na+ intercalation.
Recently, with the rapid development of nanotechnology and
energy technology, the single solid nanofibers are no longer
able to meet the demands of energy storage. Hence, in order to
achieve much superior performance of electrospun nanofibers
in ESSs, many researchers attempt to design and fabricate
electrospun nanofibers with various morphologies and struc-
tures (such as porous nanofibers,[16–20]
hollow nanofibers,[21–25]
core–shell nanofibers,[26–28]
tube-in-tube nanofibers,[29,30]
and
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Table 1. Electrochemical performance of 1D nanomaterials as anode materials for SIBs.
Anode materials Nanostructure Synthetic method Electrochemical performance Ref.
Carbon-based materials Carbon Hollow nanowires Template method 251 mA h g−1 at 50 mA g−1 [148]
N-doped C Nanofibers Electrospinning 377 mA h g−1 at 100 mA g−1 after 100 cycles [151]
Carbon Nanofibers Electrospinning 233 mA h g−1
at 50 mA g−1 [146]
B, N-dual doped C Nanofibers Template method 581 mA h g−1
at 100 mA g−1
after 120 cycles [150]
Graphene/C Nanowires Electrospinning 432.3 mA h g−1 at 100 mA g−1 [147]
Ti-based materials TiO2/C Nanofibers Electrospinning 164.9 mA h g−1 at 2 A g−1 [155]
TiO2@C Nanofibers Electrospinning 159.1 mA h g−1
at 800 mA g−1 [154]
rGO@TiO2 Nanofibers Electrospinning 124 mA h g−1
at 1.675 A g−1 [156]
Na2Ti7O15 Nanotubes Hydrothermal 130 mA h g−1
at 1 A g−1
Alloy-type materials Red P Nanorods Template method 971 mA h g−1
at 0.25 C [101]
P Nanofibers Template method 731 mA h g−1
at 0.1 A g−1
Sb Nanorod arrays Template method 557.7 mA h g−1
at 20 A g−1 [167]
Sb/C Nanofibers Electrospinning 350 mA h g−1
at 0.1 A g−1
Sb@C Nanotubes Template method 407 mA h g−1 at 0.1 A g−1 after 240 cycles [122]
Sb–C Nanofibers Electrospinning 631 mA h g−1 at 40 mA g−1 [13]
SnSb Nanofibers Electrospinning 198 mA h g−1 at 5 A g−1 after 140 cycles [168]
Sn/C Nanofibers Electrospinning 483 mA h g−1
at 2 A g−1
TiO2–Sn@C Pipe-wire nanofibers Electrospinning/ALD 413 mA h g−1 at 0.1 A g−1 after 400 cycles [166]
Transition metal oxides,
sulfides, and phosphides
Fe3O4 Nanotubes Hydrothermal 196 mA h g−1
at 2400 mA g−1 [174]
FeS Nanofibers Electrospinning 353 mA h g−1 at 5 A g−1 [178]
MnFe2O4@C Nanofibers Electrospinning 400 mA h g−1 at 5 A g−1 with 100 cycles (full cell) [170]
CuCo2O4@C Nanofibers Electrospinning 314 mA h g−1 at 1 A g−1 after 1000 cycles [18]
Co3O4@C Nanofibers Electrospinning 300 mA h g−1 up to 100 cycles at 50 mA g−1 [173]
WS2 Nanowires Solvothermal method 605.3 mA h g−1 at 100 mA g−1 [70]
WSx/WO3 Nanofibers Electrospinning 2th discharge capacity of 791 mA h g−1 [27]
MoS2 Nanofibers Electrospinning 840 mA h g−1 at the second cycle [184]
Bi2O3/C Nanofibers Electrospinning 430 mA h g−1 at 0.1 A g−1 after 200 cycles [175]
Sb2S3 Nanorods Hydrothermal 337 mA h g−1
at 2 A g−1 [71]
Sb2Se3/rGO Nanorods Solvothermal 682 mA h g−1 at 0.1 A g−1 after 50 cycles [72]
CoP Nanofibers Template method 615.29 mA h g−1 at 0.1 C at the 3rd cycle [186]

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multichannel nanofibers).[31–34] These particular structured
nanofibers have unique properties. For instance, porous and
hollow nanofibers can provide interconnected ion diffusion
pathways between electrolyte and the electrode materials,
thus improving the utilization rate of electrode materials. In
general, the pores in nanofibers are mainly generated from
the gas generation in heat treatment[16–18] and the templates
removal by chemical etching.[19,20] Guo and co-workers pre-
pared silicon nanoparticles embedded in porous carbon hybrid
via electrospinning method and subsequent template removal,
as illustrated in Figure 1c.[19] In order to increase the disper-
sity of Si in PAN/DMF solution, the surface of Si nanoparti-
cles was transformed into SiOx by calcining in air at 800 °C
for 1 h. The as-prepared Si@SiOx/PAN nanofibers were sta-
bilized in air firstly and then carbonized in argon atmosphere
to obtain Si@SiOx nanoparticles confined in carbon nanofibers
(Si@SiOx@CNF). Finally, the 10% HF solution was used to
etch SiOx in Si@SiOx@CNF, leading to the generation of pores
in Si@CNF. What’s more, Zhang and co-workers synthesized
macroporous active carbon fibers (MACFs) via electrospinning
and template strategy.[20] In the typical process, silica spheres
(SS) as the template were dispersed into PAN as the precursor
solution. The MACFs were obtained by electrospinning, fol-
lowed by carbonization process and SS template was removed
by hydrofluoric acid. The field emission scanning electron
microscopy (SEM) images presented in Figure 1d,e reveal that
the MACFs are smooth and continuous with a large amount
of uniform pores. Moreover, the MACFs are flexible and can
be used as binder-free and self-standing electrodes for further
application. Different from the traditional electrospinning
method, Mai and co-workers designed the gradient electro-
spinning to fabricate various types of mesoporous nanotubes
and pea-like nanotubes.[35]
In this way, different molecular
weight PVA (low, middle, high) and different inorganic mate-
rials were mixed as precursor solution. The low-, middle-, and
high-molecular-weight PVA were separated into three layers
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Table 2. Electrochemical performance of 1D nanomaterials as cathode materials for SIBs.
Cathode materials Nanostructure Synthetic method Electrochemical performance Ref.
Transition-metal oxides Graphene@VO2 Nanorod arrays CVD/hydrothermal 110 mA h g−1 at 18 A g−1 after 1500 cycles [199]
VO2 Nanowires Hydrothermal 160 mA h g−1
at 1 A g−1
H2V3O8 Nanowires Hydrothermal 168 mA h g−1
at 10 mA g−1
V2O5 Nanobelts Stirring method 61 mA h g−1
at 10 C [197]
Na0.44MnO2 Nanofibers Electrospinning 69.5 mA h g−1 at 10 C [195]
Na2/3(Fe1/2Mn1/2)O2 Nanofibers Electrospinning 53 mA h g−1 at 15 C [192]
Polyanionic compounds Na7V4(P2O7)4(PO4) Nanorods Sol–gel method 92.1 mA h g−1
at 0.05 C [208]
Na7V4(P2O7)4(PO4)/C Nanorods Hydrothermal 51.2 mA h g−1
at 80 mA g−1 [209]
Na6.24Fe4.88(P2O7)4@C@rGO Nanofibers Electrospinning 99 mA h g−1
at 40 mA g−1
Na3V2(PO4)3 Nanofibers Solvothermal 107 mA h g−1 at 0.2 C [204]
Na3V2(PO4)3 Nanorods Nanocasting 78 mA h g−1
at 5 C after 2000 cycles [102]
Na3V2(PO4)3/C Nanofibers Electrospinning 20 mA h g−1
at 20 C [12]
Na3V2(PO4)3 Nanofibers Template method 94 mA h g−1
at 100 C [203]
NaVPO4F/C Nanofibers Electrospinning 126.3 mA h g−1 at 1 C [16]
Figure 1. a) Schematic illustration of the preparation process for the
Sb–C nanofibers. b) TEM image of the Sb–C nanofibers. Reproduced with
permission.[13]
Copyright 2014, Royal Society of Chemistry. c) Schemetic
illustration of the synthetic process of Si@PCNF. Reproduced with
permission.[19]
Copyright 2013, Wiley-VCH. d,e) SEM images of MACF.
Reproduced with permission.[20]
Copyright 2016, Wiley-VCH.

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under the strong electrostatic tension force. Owing to the dif-
ferentials of the mass loss (M) and temperature (T) (dM/dT) of
the low-weight PVA is smallest, the inner low molecular weight
PVA was first decomposed, followed by the middle layer of
PVA, eventually all of the preliminarily decomposed PVA and
inorganic materials moved towards to the outer tubes. Finally,
inorganic mesoporous nanotubes only composed of tiny inor-
ganic nanoparticles were obtained after sintering in air. Other-
wise, all of the PVA carbonized at Ar atmosphere under high
temperature, forming composite mesoporous nanotubes con-
sisted of uniform inorganic nanoparticles and ultrathin carbon
nanotubes. The pea-like nanotubes were also obtained by con-
trollable heat treatments. The prepared precursor nanofibers
were directly preheated in air at 300 °C. At this temperature,
three types of PVA were all decomposed while the inorganic
precursor materials were still in the original position. Then
the samples were annealed at higher temperature under Ar
atmosphere, the outer preliminarily decomposed PVA carbon-
ized, nanoparticles formed and uniformly embedded in the
nanotubes. Eventually, pea-like nanotubes were achieved. The
synthetic procedures of the gradient electrospinning and con-
trolled pyrolysis method are clearly depicted in Figure 2a,b.
Various inorganic materials were electrospun into mesoporous
nanotubes and pea-like nanotubes according to this method,
including multielement oxides (Li3V2(PO4)3, Na3V2(PO4)3,
Na0.7Fe0.7Mn0.3O2, and LiNi1/3Co1/3Mn1/3O2), the binary-metal
oxides (LiMn2O4, LiCoO2, NiCo2O4, and LiV3O8) and single-
metal oxides (CuO, Co3O4, SnO2, and MnO2). In addition,
some pea-like nanotubes (Co, LiCoO2, Na0.7Fe0.7Mn0.3O2, and
Li3V2(PO4)3) have also been successfully obtained. The typical
structures are presented in Figure 2c–e.
Coaxial electrospinning as an attractive, simple and effec-
tive method for the fabrication of core–shell, hollow, and mul-
tichannel nanofibers has been widely researched, in which a
spinneret contains two coaxial capillaries with a polymer solu-
tion as the shell and a different viscous fluid or a nonviscous
fluid even or a solid powder as the core. Hwang et al.[26]
pre-
pared a core–shell structure of Si nanoparticles warpped with
carbon shell via a feasible electrospinning step using a dual
nozzle. Figure 3a exhibits the preparation process of Si/C com-
posites. The PMMA solutions containing Si nanoparticles and
the PAN solution were used as the core and shell precursor,
respectively. In this process two important points should
be noted. The first point is that PMMA was contained in the
core polymer solution in order to form some void space and
then accommodate the volume expansion of Si in cycling pro-
cesses, which is due to that PMMA can be evaporated at rela-
tively low temperature. The second point is that acetone was
added in the core solution in order to prevent the mixing of
the core and the shell solution because PAN is precipitated
in acetone. SEM cross-sectional image displayed in Figure 3b
reveals the indeed core–shell structure with Si nanoparticles
encapsulated in the core. Selected area electron diffraction
(SAED) pattern derived from the core section match well with
(111), (220), and (311) planes of Si and thus confirm the pres-
ence of Si in the core (Figure 3c). The hollow nanofibers can
also be synthesized by removing the core in the core–shell
structure obtained by coaxial electrospinning.[21,23] For instance,
TiO2 hollow nanofibers were fabricated by Han et al.[23] They
used the mineral oil as the inner core material, then the oil was
removed by calcination and finally the hollow structure formed.
Interestingly, some novel 1D nanostructures are also designed
and prepared by electrospinning. For example, lotus root-like
multichannel carbon (LRC) nanofibers were prepared by Lou
and co-workers via the method of emulsion single nozzle
cospinning electrospinning.[34] A microemulsion making
from PS is added into the PAN solution, which could be
stretched into nanofibers. The channel structures of LRC
nanofibers can be controlled by changing the weight ratio of
PAN and PS from 1: 0.1 to 1:1. The schematic diagrams and
TEM images (Figure 3d–g) show that both the channel diam-
eters and channel numbers in the nanofibers increase with
the increasing of the PS content. Yu and co-workers fabricated
Sn nanoparticles encapsulated in porous multichannel carbon
microtubes by single-nozzle electrospinning.[31] Figure 3h
presents the cross section of as-prepared nanofibers, which
exhibits the multichannel tubular structure with an average
channel diameter of ≈150 nm. The average thickness of the
channel walls is ≈100 nm with a great number of small holes
(diameter ≈100 nm). What’s more, multichannel microtubes
have been reported by using compound nozzle comprising two
or more metal capillaries, as illustrated in Figure 3i–m.[32]
2.2. Solution-Phase Route
Solution-phase routes, mainly including hydrothermal
method and solvothermal method, have been widely used in
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Figure 2. Schematic of gradient-electrospinning and controlled-pyrolysis
processes for preparation of a) mesoporous nanotubes and b) pea-
like nanotubes. TEM images of c) LiV3O8 and d) MnO2 mesoporous
nanotubes, scale bars, 100 nm. e) TEM image of Li3V2(PO4)3 pea-like
nanotubes, scale bar, 200 nm. The scale bar for the inset TEM image
are 100 nm. Reproduced with permission.[35] Copyright 2015, Nature
Publishing Group.

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the fabrication of 1D nanomaterials. Multifarious nanostruc-
tures have been obtained by the control of reaction conditions,
such as concentration, pH, pressure, temperature, duration
time, etc. The hydrothermal and solvothermal technique both
are the simple and universal synthesis approaches that uti-
lize solvents (aqueous and nonaqueous, respectively) at low
temperatures and high pressures for fabricating 1D nano-
materials. The chemical reactions take place in a stainless
steel autoclave and the processes are accompanied with three
stages: formation of the supersaturated solution, nucleation
and crystal growth. Generally, 1D nanostructured precur-
sors usually require an annealing treatment in which phase
transition, oxidation, reduction or pyrolysis are carried out to
obtain the target materials. A great deal of metal oxides with
1D nanostructure have been successfully synthesized, such
as CoO,[36,37] Co3O4,[38–43] SnO2,[44,45] VO2,[46] MoO3,[47] NiO,[48]
and TiO2.[49–51] For instance, our group have successfully fab-
ricated hierarchical CoO nanowire clusters on copper foil via
hydrothermal method.[37] As illustrated in Figure 4a, cobalt
carbonate hydroxide (Co(CO3)0.5(OH)·0.11H2O) precursor was
formed through the nucleation on the copper foil and “Ostwald
ripening” process. After heat treatment, the CoO nanowire
clusters were obtained in situ on the copper foil. SEM image
in Figure 4b implies that the CoO nanowire clusters covered
on the entire surface of the copper foil. From the TEM image
in Figure 4c, it can be seen that the CoO
nanoparticles with mesopores uniformly
distributed on the nanowires. What’s more,
the cobalt oxide nanowires grown on other
substrates (such as Ti,[36,40]
Si,[39]
Ni,[41,43]
carbon paper)[42]
have also been reported.
Such materials grown on conductive sub-
strates can be directly used in energy storage
devices, thus leading to the enhanced energy
density.
Multistep hydrothermal method has been
used to fabricate 1D hierarchical core–shell
nanomaterials, such as TiO2-B nanowire@α-
Fe2O3 nanothorn,[52]
NiCo2O4@MnO2,[53]
Co3O4/NiO, and ZnO/NiO.[54]
Xia et al.[52]
fabricated TiO2–B nanowire@α-Fe2O3 nan-
othorn core–shell arrays via a facile two-
step hydrothermal approach. As shown in
Figure 4d, the synthesis processes contain
two steps. In the first step, Na2Ti2O5·H2O
nanowire arrays grown on Ti foil were syn-
thesized by an alkaline hydrothermal reac-
tion with NaOH and then H2Ti2O5·H2O
formed by an ion exchange with HCl. Finally,
H2Ti2O5·H2O converted to TiO2-B nanowire
arrays by calcination. In the second step,
FeOOH nanorods were deposited on the
TiO2-B nanowire arrays by the hydrolysis of
Fe3+. Then FeOOH nanorods were calcinated
and then converted to hollow Fe2O3 nano-
thorns, resulting in the formation of hierar-
chical TiO2-B nanowire@Fe2O3 nanothorn
core-branch arrays on Ti foil. TEM image
presented in Figure 4e can clearly reveal the
core-branch structure of TiO2-B@α-Fe2O3. Figure 4f displays
the high resolution (HR)TEM image of the interface between
TiO2-B and α-Fe2O3. The lattice fringes of both TiO2-B and
α-Fe2O3 can be observed, further confirming the core-branch
architecture of TiO2-B@α-Fe2O3.
In addition, 1D nanostructure vanadium compounds
(including Na1.25V3O8 nanowires,[55] NaV3O8 nanowires,[56] VN
nanowires,[57] Na3V2(PO4)3/C nanowires,[58] Na3V2(PO4)3/C
nanorods,[59] and Li3V2(PO4)3/C mesoporous nanowires)[60] as
well as titanium compounds (such as Na2Ti6O13 nanorods,[61]
Na2Ti3O7 nanowires,[62] and Na2Ti3O7 nanowires)[63] have been
reported. Nanoparticles can shorten the length of ion transport
and improve the electrochemical reaction kinetics. However,
they are very easy to aggregate due to their high surface energy.
Hence, many researchers attempt to embed the nanoparticles
in 1D nanomaterials for improving the stability of active mate-
rials in ESSs.[58–60] Yu and co-workers embedded Na3V2(PO4)3
nanoparticles in carbon nanowires by hydrothermal method
following by heat treatment.[58] When used as cathode in SIBs,
Na3V2(PO4)3/C nanowires demonstrate long-life and high-rate
performance.
In recent years, the phosphides have also attracted
increasing attention in batteries due to their high theoretical
capacities. However, they experience huge volume changes
during charge/discharge processes.[64]
Electrode materials
Small 2018, 14, 1703086
Figure 3. a) The preparation process of core–shell structure Si/C composites. b) A cross-sec-
tional SEM view of a single Si/C. c) TEM image of a single Si@C. The inset is SAED pattern for
the region in the white box with the diffraction rings indexed. Reproduced with permission.[26]
Copyright 2012, American Chemical Society. d–g) Schematic diagrams and TEM images of
LRC nanofibers based on various PAN/PS weight ratio, scale bars, 200 nm. Reproduced with
permission.[34]
Copyright 2015, Nature Publishing Group. h) The cross section SEM image of
Sn nanoparticles encapsulated in porous multichannel carbon microtubes. Reproduced with
permission.[31]
Copyright 2009, American Chemical Society. i) Side-view SEM image of sample
after the organics have been removed. j–m) SEM images of multichannel tubes with variable
diameter and channel number from two to five. The inset in each figure shows the cross section
illustration of spinneret that was used to fabricate the tube. Reproduced with permission.[32]
Copyright 2007, American Chemical Society.

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NANO MICRO
are particularly easy to be damaged in the discharge/charge
process, leading to pulverization and eventually fast capacity
fading. To solve the abovementioned issues, some 1D nano-
structured phosphides have been fabricated.[65,66] Cu3P nanow-
ires have been synthesized through in situ growth and phos-
phidation directly on copper current collector.[65] The digital
images in Figure 5a exhibit obvious color changes at different
stages of the electrode fabrication processes, directly indicating
the generation of Cu(OH)2 and Cu3P and their full coverage
on Cu foil. As shown in Figure 5b, SEM images of Cu(OH)2
and Cu3P present that they both are nanowires with an average
length around 5 µm and their morphologies are very similar.
TEM image of Cu3P indicates that Cu3P nanowires are con-
sisted of a large number of small nanoparticles (Figure 5c).
Furthermore, the EDS mapping images of an individual
nanowire of Cu3P shown in Figure 5d reveal the uniform dis-
tribution of both Cu and P elements throughout the whole
nanowire.
The chalcogenide materials have been considered as greatly
promising host materials in batteries due to their various struc-
tural types and fascinating electrochemical activities.[67–69] How-
ever, they suffer from poor cyclic stability caused by the large
volume change during the conversion and alloying reactions.
To overcome this limitation, 1D nanostructured chalcogenide
materials have been prepared, such as WS2 nanowires,[70]
Sb2S3 nanorods,[71]
Sb2Se3 nanorods,[72]
Sb2Se3 nanowires.[73]
Free-standing membranes composed of ultralong Sb2Se3
nanowires have been fabricated via a facile hydrothermal
method by Mai and co-workers.[73] Benefited from the strain
accommodation ability and fast charge transport of 1D nano
wires, as-prepared free-standing Sb2Se3 nanowires exhibit
excellent cycling stability and rate performance for the lithium
and sodium storage.
2.3. Electrochemical Deposition
Electrochemical deposition is a versatile technique for the
fabrication of 1D nanostructured materials owing to its rela-
tively simple control of the process parameters. Some conduc-
tive substrates (such as Ni foam, stainless steel substrate, Ti
foil, etc.) are usually applied in the electrodeposition process
for the fabrication of binder-free electrodes. There are two
categories of electrochemical deposition: template-assisted
method[74–76] and template-free method.[77–81] Au nanoparti-
cles coating Ni nanowires have been prepared by Kim et al.
through the method of electrodeposition.[74] As illustrated
in Figure 6a, the researchers used anodic aluminium oxide
(AAO) membrane as the template and made Ni nanoparti-
cles deposite on AAO as seeds for the growth of Ni nanow-
ires. Subsequently, Ni nanowires can be obtained after Ni
plating solution filling into the holes and the use of voltage.
Finally, after Ni nanowires substrate was immersed in the
solution consisted of HAuCl4·3H2O and NH4Cl, and followed
by applying voltage, Ni nanowires coated with Au nanoparti-
cles were obtained. SEM image presented in Figure 6b and
Small 2018, 14, 1703086
Figure 4. a) Schematic illustration of the formation of hierarchical CoO nanowire clusters on copper foil. b) SEM image viewed from the top of CoO
nanowire clusters. c) TEM image of a single CoO nanowire. Reproduced with permission.[37]
Copyright 2015, Wiley-VCH. d) Schematic illustration of
the formation process of the TiO2–B nanowire@α-Fe2O3 nanothorn core–shell arrays. e) TEM and f) HRTEM images of the TiO2–B@α-Fe2O3 hybrid
nanowires. Reproduced with permission.[52]
Copyright 2014, Springer.

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TEM image displayed in Figure 6c clearly exhibit that the Au
nanoparticles coated on the surface of Ni nanowires. Because
the deposition of semiconductors (such as Si and Ge) is ther-
modynamically impossible, some researchers attempt to use
the electrochemical liquid–liquid–solid (ec-LLS) process[77]
and ionic liquid[78,79] for obtaining nanowires by template-free
electrodeposition.
Electrochemical deposition method is also used to pre-
pare 1D core–shell structural materials through combining
with other methods such as hydrothermal method. TiO2–
MoO3,[82] Co3O4/NiO,[83] Co3O4/Co(OH)2,[84] and NiCo2O4@
MnO2
[85]
core/shell nanowire arrays have been synthesized by
a facile hydrothermal method followed by a controllable elec-
trodeposition route. For instance, Co3O4/Co(OH)2 core/shell
nanowire arrays grown on nickel foam have been synthesized
by Fan and co-workers.[84] Figure 6d presents the formation
process of the porous hydroxide nanosheets formed by elec-
trodeposition on preformed nanowire arrays. It is noteworthy
that the hydroxide shell has a highly porous structure with
a great number of pores, and when converted to oxides by
thermal annealing, the porous structure is preserved. Co3O4/
Co(OH)2 with the typical core/shell structure can be clearly
observed from SEM image of the individual nanowire in
Figure 6e.
2.4. Template-Assisted Method
The template-assisted method is one of the
most widely routes to synthesize 1D nano-
materials. There are two types of templates,
depending on the diverse growth mecha-
nisms, used in the synthesis of 1D nanoma-
terials: confined template and the oriented
template.
2.4.1. Confined Template Method
To date, various confined templates,
including AAO membranes, mesoporous
templates (such as SBA-15, CMK-3) and so
on, have been used to fabricate 1D nano-
materials. Among several confined template
materials, AAO is the most universal and
successful template for the preparation of 1D
nanostructured arrays owing to its low-cost
accessibility, easy scalability, and uniformity
of the nanopores.[86] A great deal of 1D nano-
structured materials have been synthesized
with the assistance of AAO template.[87–94]
Recently, oxygen vacancies-containing amor-
phous SnO2 ordered arrays have been fab-
ricated through a template-assisted method
and atomic layer deposition (ALD) followed
by a subsequent annealing in N2 atmos-
phere.[91] Figure 7a shows the SEM image of
the as-prepared AAO template with hexago-
nally arranged pores with a size of ≈180 nm,
clearly demonstrating the long-range and
high ordering of the pore arrays. Ordered
Ni nanorod arrays as current collector were prepared using
the AAO template (Figure 7b). Then, a layer of amorphous
SnO2 was deposited on the surface of the Ni nanorod arrays
using ALD method. As depicted in Figure 7c, the rough and
enlarged nanorods confirm that amorphous SnO2 layers cov-
ered on the surface of Ni nanorods. Finally, oxygen vacancies-
containing amorphous SnO2 ordered arrays were obtained after
an annealing process in N2 atmosphere. Figure 7d presents
the SEM image of the heterostructured nanoarrays. It can be
observed that the nanorod arrays have no agglomeration or col-
lapse. Apart from AAO, SBA-15[95–100] and CMK-3,[101–103] as the
widespread mesoporous templates, are also extensively used
to prepare 1D nanostructured materials. Mesoporous peapod-
like Co3O4@carbon nanotube (Co3O4@CNT) arrays have been
constructed via a template-assisted approach.[95] The synthesis
process is illustrated in Figure 7e. CNT@SBA-15 was prepared
by furfuryl alcohol as a carbon precursor followed by calcina-
tion process. Next, the functional CNT@SBA-15 was filled with
Co(NO3)2 and then calcinated for obtaining Co3O4@CNT@
SBA-15. Finally, Co3O4@CNT was obtained with the SBA-15
leaching out. The obtained Co3O4@CNT composites have a
high surface area and large pore size. The Co3O4 nanoparticles
are carefully confined and uniformly embedded in the intra-
tubular mesopores of the CNT. The unique structure greatly
Small 2018, 14, 1703086
Figure 5. a) Digital images of Cu foil, Cu(OH)2 nanowires and Cu3P nanowires. b) SEM images
of Cu(OH)2 nanowires and Cu3P nanowires; inset in b) are their digital images. c) TEM images
of a single Cu3P nanowire. d) The EDS mapping images of an individual Cu3P nanowire. Repro-
duced with permission.[65]

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improves the accessibility of the Co3O4 nanoparticles as well
as facilitates fast ion diffusion transport, contributing to out-
standing electrochemical performance.
2.4.2. Oriented Template Method
CNTs and CNFs are the most widespread oriented templates
for the design and fabrication of 1D nanostructured mate-
rials. Usually, core–shell structure can be directly obtained
by growing other materials on the surface of template, while
after the template is removed in the following process the
hollow structure can be obtained. Various 1D nanostructured
materials have been synthesized by using CNTs or CNFs as
the templates, mainly including CNTs@SnO2@C coaxial
nanocables,[104] CNT@CuS,[105] CNT@Ni3S2,[106] ZnMn2O4
hollow nanotubes,[107] LiMn2O4 nanotubes,[108] hollow TiO2
nanotube,[109] Co3O4 nanotubes,[110] CNFs@MoS2,[111] NiO
hollow nanostructure,[112] and CNFs@MnO coaxial nano-
cables.[113] 1D hierarchical structure composed of Ni3S2
nanosheets grown on CNTs has been reported by Lou and co-
workers.[106] Due to the strong reaction between nickel ions
and silica, they first coated a uniform layer of mesoporous
silica on the surface of the functionalized CNTs to form
CNT@SiO2@NiSilicate core-double shell 1D nanostructure.
Subsequently, the NiSilicate nanosheets were converted into
Ni3S2 nanosheets by a chemical conversion route with Na2S.
The hierarchical CNT/Co3O4 microtubes,[114]
carbon-doped
Co3O4 hollow nanofibers[115]
and N-doped carbon@CoS coaxial
nanotubes[116]
have been fabricated by using electrospun PAN
nanofibers as the template. Yan et al.[115]
prepared carbon-
doped Co3O4 hollow nanofibers by using PAN nanofibers. In
the fabrication process, PAN nanofibers not only was used
as template but also taken as the carbon resource. First of
all, Co(OH)2 grown on the surface of PAN nanofibers was
obtained by solvothermal method. Then, PAN@Co(OH)2 pre-
cursor was calcinated in air, the hollow structure was formed
in this step and carbon was obtained by the decomposition
of PAN. Finally, carbon-doped Co3O4 hollow nanofibers
were successfully obtained. Similarly, N-doped carbon@CoS
coaxial nanotubes have also been constructed.[116] PAN was
converted to hydrosoluble polyacrylic acid (PAA) during a
simple hydrothermal process and then PAA was completely
removed to create a hollow tubular. As confirmed by SEM
and TEM images in Figure 8a,b, a hollow structure can be
clearly observed. HRTEM image illustrated in Figure 8c
confirms that the double-layered tube wall is consisted of
carbon and CoS layers. Further, two sets of lattice fringes with
distances of 0.292 and 0.34 nm can be respectively ascribed
to the (100) plane of the hexagonal CoS and the (002) plane
of carbon (Figure 8d). EDS mapping images in Figure 8e–i
further confirm the hollow structure of N-doped carbon@CoS
coaxial nanotubes.
In addition, some 1D nanostructured metal or metal oxides
can also be used as the templates in the fabrication of 1D
nanomaterials. ZnO nanorods are one of the most common
templates and have been used to fabricate plenty of 1D nano-
materials.[10,117–119]
Li4Ti5O12–C nanotube arrays,[10]
Pt–Ni–P
composite nanotube arrays,[119]
Ni@Pt core–shell nanotube
arrays,[118]
and NiO nanotube arrays[117]
are all synthesized
by taking ZnO nanorods as the template. Other 1D nano-
structured templates, such as MnOx nanowires,[120]
Co3O4
Small 2018, 14, 1703086
Figure 6. a) Schematic of the fabrication process of Au nanoparticles deposited on a Ni nanowire substrate. b) SEM and c) TEM images of a Au/Ni
electrode. Reproduced with permission.[74]
Copyright 2015, Wiley-VCH. d) The formation process of the porous hydroxide nanosheets formed by elec-
trodeposition on preformed nanowire arrays. e) SEM image of the individual Co3O4/Co(OH)2 with core/shell structure. Reproduced with permission.[84]

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nanowires,[121] Sb2S3 nanorods,[122] MoO2 nanowires,[123–125]
MoO3 nanorods,[126] and Cu nanowires[127] have also been
used. For instance, Lou and co-workers designed and syn-
thesized three-layered TiO2@carbon@MoS2 hierarchical
nanotubes via oriented template of MoO2 nanowires.[124] The
synthesis process is illustrated in Figure 8j, they first coated
a TiO2 layer on the surface of MnO2 for producing the core–
shell MnO2@TiO2 nanowires. Subsequently, a layer of poly-
dopamine (PDA) was deposited on the outside of MnO2@
TiO2 as carbon resource. Afterward, the resultant composite
nanowires were carbonized in N2 atmosphere and treated in
acid to remove the MnO2 templates for obtaining TiO2@N-
doped carbon (TiO2@NC) nanotubes. Finally, ultrathin MoS2
nanosheets were decorated on the surface of TiO2@NC nano-
tubes through a hydrothermal reaction and a subsequent
annealing process to produce the three-layered hierarchical
TiO2@NC@MoS2 tubular nanostructures. TEM image in
Figure 8k clearly illustrates the hierarchical tubular nano-
structure of TiO2@NC@MoS2.
2.5. Gas-Phase Route
Vapor deposition techniques, mainly containing chemical vapor
deposition (CVD) and ALD, offer precise control over the thick-
ness and uniformity of coating layers at the nanometer or even
angstrom level. Hence, gas-phase routes are usually used to
prepare 1D nanostructured materials, especially hierarchical
structured materials.
2.5.1. Chemical Vapor Deposition
CVD is an especially attractive method to produce 1D nano-
structured materials. Semiconductor (such as Si, Ge) nano
wires are the most common 1D nanomaterials prepared
by CVD.[128–130]
In the typical process, the foreign metal
nanoparticles can catalyze the decomposition of the semicon-
ductor-containing gas, as well as promote 1D growth. Au-cata-
lyzed Ge nanowires grown on Si (001) and Si (111) have been
fabricated by CVD method,[129] which grown in a limited tem-
perature range ≈320–380 °C. Yang and co-workers synthesized
Si nanowires by using SiCl4 as the precursor gas in a CVD
system.[128] The obtained nanowire arrays were grown vertically
aligned with respect to the substrate.
Apart from semiconductor nanowires, CNTs is another
widely prepared object through CVD route. Magrez et al.[131]
synthesized CNTs by CVD method based on the oxidative
dehydrogenation reaction of C2H2 with CO2. In addition,
metal sulfides (CoS, MnS),[132] SnO2@Si core–shell nanowire
arrays,[133] SiC@SiO2 core–shell nanowires,[134] CNTs-silicon
core–shell nanowire,[135] and coaxial Zn2GeO4@carbon nano
wires[136] have also been reported. Zn2GeO4@carbon nanow-
ires directly grown on a Cu foil (ZGO@C/Cu) have been
constructed by Chen et al.[136] The synthesis strategy is sche-
matically illustrated in Figure 9a. In the synthesis process,
the precursor consisted of ZnO, GeO2, and carbon powder
were heated up to 1000 °C under a flow gas mixture of N2
and O2. This reaction process can be expressed as follows
ZnO s + C s Zn v + CO/CO v2( ) ( ) ( ) ( )→ (2)
GeO s + C s Ge v + CO/CO v2 2( ) ( ) ( ) ( )→ (3)
Zn v + Ge v + CO/CO v Zn GeO @C s2 2 4( ) ( ) ( ) ( )→ (4)
where s represents to solid state and v represents to vapor state.
TEM and HRTEM images presented in Figure 9b–g exhibit the
coaxial structure of the ZGO@C nanowires with a Zn2GeO4
core and a uniformly coated carbon shell. And with the reaction
time increasing, both the diameter of Zn2GeO4 core and carbon
shell increased.
2.5.2. Atomic Layer Deposition
In particular, ALD is an attractive vapor-based self-termi-
nating thin film growth technique, which can deliver a con-
formal coverage of layered materials with well-controlled
thickness.[137]
In recent years, ALD has been a viable method
Small 2018, 14, 1703086
Figure 7. SEM images of a) AAO template, b) Ni nanorod arrays,
c) SnO2/Ni ordered arrays, and d) amorphous SnO2/Ni heterostructured
nanoarrays after annealing in N2 atmosphere. Reproduced with permis-
sion.[91] Copyright 2017, Elsevier. e) Schematic illustration of the forma-
tion process of the mesoporous peapod-like Co3O4@CNT. Reproduced
with permission.[95] Copyright 2015, Wiley-VCH.

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NANO MICRO
used to fabricate 1D nanomaterials due to its precise thick-
ness control and large-scale uniformity. It is noteworthy that
ALD method is usually applied to prepare 1D nanomaterials
by combining with other methods, especially template-assisted
method. Through the multistep ALD method, several mul-
tiwalled nested TiO2–Pt nanotubes in series have been suc-
cessfully fabricated with the assistance of porous AAO and
microporous Si templates (Figure 10a).[138]
What’s more, a great deal of 1D hierarchical structures have
been prepared by ALD, such as hierarchical TiO2@Fe2O3 hollow
nanostructure,[139] TiO2 nanotube@SnO2 nanoflake core-branch
arrays,[140] core–shell Ge@Graphene@TiO2 nanofibers,[141]
various helical oxide nanotubes (Al2O3, SiO2, TiO2, HfO2, and
ZnAl2O4),[142] SnO2-in-TiO2 wire-in-tube nanostructure.[143]
For instance, hierarchical TiO2@Fe2O3 hollow nanostruc-
tures grown on Ni foam have been constructed by Fan and
co-workers by employing ALD and liquid-phase self-assembly
(Figure 10b).[139] Fan and co-workers also designed and fabri-
cated hollow SnO2@TiO2 wire-in-tube nanostructure combined
vapor deposition with ALD.[143] The synthesis process is sche-
matically shown in Figure 10c. TEM image depicted in Figure
10d clearly confirms the wire-in-tube structure of SnO2-in-TiO2.
3. The Application of 1D Nanomaterials in
Sodium–Ion Batteries
SIBs as a promising candidate of LIBs attract increasing atten-
tion. However, the larger radius (1.02 Å vs 0.76 Å) and heavier
mass (23 g mol−1 vs 6.9 g mol−1) of Na+ compared to Li+ lead
to the terrible cycling and rate performance. 1D nanomaterials
possess substantial advantages when used as electrode mate-
rials. For example, 1D nanomaterials can shorten the electrons/
ions diffusion length, support greater contact surface area of
electrolyte and electrode and relieve the large volume change
in some degrees. Therefore, to develop 1D nanostructured elec-
trode materials is a greatly efficient way to improve the perfor-
mance of SIBs.
3.1. Anode
With the fast development of SIBs, electrode materials as an
important component in batteries have been studied profoundly
and widely. Anode materials mainly include intercalation-
type materials (such as carbon-based materials and titanium-
based materials), alloy-type materials (such as Sn, Sb, Si),
conversion reaction materials (e.g., transition metal oxides
and sulfides) and so on. As far as we know, the volume change
during the cycling process of anode materials is still a key
obstacle to prevent development of SIBs, especially for the
alloy-type materials. Among multifarious materials, 1D nano-
materials with unique structure have been paid tremendous
attention on account of accommodating the large volume vari-
ation to enhance the electrochemical performance. Hence, 1D
nanomaterials are very promising anode materials for high
capacity, superior-rate capability and long-term cycling proper-
ties SIBs.
Small 2018, 14, 1703086
Figure 8. a) FESEM, b,c) TEM, and d) HRTEM images, and e–i) EDX mapping of the N-doped carbon@CoS coaxial nanotubes after calcination. Repro-
duced with permission.[116] Copyright 2016, Wiley-VCH. j) Schematic illustration of the synthesis process of TiO2@NC@MoS2 tubular nanostructures.
k) TEM image of TiO2@NC@MoS2 nanotubes. Reproduced with permission.[124] Copyright 2017, Wiley-VCH.

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3.1.1. Carbon-Based Materials
SIBs are similar to LIBs in working mechanism, but the storage
behaviors of Na and Li in carbon-based anodes are quite dif-
ferent. The application of graphite to SIBs is limited to the
larger size of Na+. However, many other carbon-based materials
(e.g., graphene nanowires,[144] carbon nanotubes,[145] carbon
nanofibers)[146,147] with relatively low potential, high revers-
ible capacity, and good cycling stability are still the promising
anode materials for SIBs. Particularly, 1D carbon nanomate-
rials (nanotubes, nanofibers, nanorods, nanobelts, nanowires)
possessing short Na+ transfer pathways and strong tolerance
to stress change have gained great attentions. Carbon-based
1D nanostructured materials have been widely studied. Cao
et al.[148] fabricated hollow carbon nanowires (HCNWs) by pyro-
lyzing hollow polyaniline nanowires directly. The as-prepared
HCNWs are about 150 nm in diameter and the surface is rough
with many nanohemispheres. The HCNWs electrodes show
good cyclic stability, as shown in Figure 11a, a high capacity of
206.3 mA h g−1
can be still retained even after 400 cycles and the
capacity retention ratio reach up to 82.2%. Moreover, HCNWs
exhibit excellent rate performance with initial reversible capaci-
ties of 252, 238, 216 mA h g−1
at 0.2, 0.5, 1 C (1 C = 250 mA g−1
)
respectively, and a high reversible capacity of 149 mA h g−1
also
can be obtained even at 500 mA g−1
(Figure 11b).
In addition, doping heteroatoms in carbon nanofibers is an
effective way to promote the electrochemical performance. For
example, it is reported that doping nitrogen atom contributes to
improving the electronic conduction and offers some defects as
open-paths and active sites for Na+ insertion. Yu and co-workers
successfully fabricated N-doped porous carbon nanofibers by
pyrolysis of Ppy.[149] Due to the larger interlayer distance and
easier charge transfer properties of fibre-like morphology,
N-doped porous carbon nanofibers deliver a reversible capacity
of 296 mA h g−1 at 0.05 A g−1 when used as anode for SIBs. Yu
and co-workers also designed boron (B), nitrogen (N) codoping
3D interconnected carbon nanofibers (denoted as BN-CNFs) to
enhance sodium storage performance.[150] B, N codoping pro-
vides synergistic effects of increased active sites and enlarged
carbon layer spacing for Na+ insertion and improved elec-
tronic conductivity. Additionally, the continuous pores in 1D
nanostructure can offer interconnected ion diffusion pathways
between the electrolyte and active materials. Consequently,
the porous BN-CNFs show high reversible capacity and supe-
rior cycling performance (a high capacity of 277 mA h g−1
at
10 A g−1
even after 1000 cycles).
Apart from the performance of electrode material itself,
energy density is also very significant for practical applica-
tions. Some inactive components (such as metal substrate,
binder, etc.) in conventional electrodes could pull down the
Small 2018, 14, 1703086
Figure 9. a) A schematic illustration of the CVD synthesis of ZGO@C/Cu. b–d) TEM and e–g) HRTEM images of ZGO@C nanowires synthesized with
different reaction times. Reproduced with permission.[136]
Copyright 2015, Royal Society of Chemistry.

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NANO MICRO
energy density. Therefore, to fabricate free-standing and
binder-free electrodes is a greatly efficient way to improve
the energy density. Particularly, Lou and co-workers designed
and fabricated free-standing N-doped carbon nanofiber films
(N-CNFs) as the anode for SIBs.[151] N-CNFs derived from
PAA precursor through two treatments of imidation and
carbonization. After carbonization, the PI nanofibers films
converted into N-CNFs with shrinkage of ≈20% (Figure 11c).
Figure 11d shows that the N-CNFs possess the morphology
with long and uniform straight nanofibers. In addition, it can
be clearly seen that a single nanofiber possesses a rough sur-
face with a large number of pores (Figure 11e), which make
easy contact of electrolyte and Na ions. These nanofibers
interconnect into conductive network to promote the moving
of Na+ and electrons. Thus, N-CNFs, as the anode for SIBs,
exhibit a reversible capacity of 377 mA h g−1 at a current
density of 0.1 A g−1 after 100 cycles and show great cycling
stability (Figure 11f). Figure 11g displays excellent rate capa-
bility (a high capacity of 154 mA h g−1 even at the 15 A g−1)
of N-CNFs and the superior stability of the electrode (the
reversible capacity recovered to 305 mA h g−1 when the cur-
rent density returned to 50 mA g−1).
3.1.2. Ti-Based Anode Materials
Titanium-based materials are getting considerable attention on
account of reasonable operation voltage, no safety concerns,
nontoxicity, structural stability, and abundant titanium resource.
Therefore, an increasing number of new promising titanium-
based compounds are applied to SIBs. Various Ti-based mate-
rials, mainly including TiO2 (anatase, rutile, brookite, and
TiO2-B in nature) and sodium-titanate compounds (Na2Ti7O15,
Na2Ti3O7, Na4Ti5O12, Na2Ti6O13), have been reported as anode
materials for SIBs. Among the four polymorphs of TiO2, the
anatase TiO2 is deemed to the most appropriate and promising
anode material for SIBs.[152] Over the past years, 0D mate-
rials (nanoparticles and spheres), 1D materials (nanotubes,
nanorods, and nanowires),[153] 2D materials (nanosheets),
and suchlike different nanostructures of TiO2 materials have
been extensively studied. Among them, 1D nanostructured
TiO2 with short diffusion distance for Na+ and fast transport
of electrons are beneficial to achieve higher reversible capacity
and rate performance. Ge et al.[154] prepared TiO2@carbon
nanofibers (TiO2@CNFs) via electrospinning method and used
it as anode for SIBs. TiO2@CNFs deliver a reversible capacity of
237.3 mA h g−1 at 30 mA g−1, which may be attributed to the
well retained 1D structure and anatase crystal phase. Xiong
et al.[155] embedded the anatase TiO2 nanocrystals with a diam-
eter of ≈12 nm into the electrospun carbon naofibers. The long
straight 1D fibers with strong structural stability contribute to
a high capacity of ≈302.4 mA h g−1
and good rate performance
(a capacity of 164.9 mA h g−1
at a large current density of
2000 mA g−1
).
Except for carbon-coating,[156]
doping heteroatoms (such
as N, B, S, F) is also an effective measure to enhance the
Small 2018, 14, 1703086
Figure 10. a) SEM image of multiwalled nested TiO2–Pt nanotubes. Reproduced with permission.[138]
Copyright 2011, Springer. b) TEM image of a hier-
archical hollow TiO2@Fe2O3 core–shell nanostructure. Reproduced with permission.[139]
Copyright 2013, Wiley-VCH. c) Schematics of the fabrication
process of the SnO2@TiO2 wire-in-tube nanostructure. d) TEM image of SnO2@TiO2 wire-in-tube nanostructures. Reproduced with permission.[143]

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electrochemical performance of TiO2. Remarkably, self-sup-
ported nanotube arrays of S-doped TiO2 (S-TiO2) grown on Ti
foil substrate were fabricated and used as binder-free anode
for SIBs.[157] As schematically illustrated in Figure 12a, S-TiO2
nanotube arrays were prepared by an anodization route fol-
lowed by sulfidation. Gray TiO2 tubes transformed to black
S-TiO2 nanotubes in the sulfidation process. SEM image dis-
played in Figure 12b indicates S-TiO2 nanotube with an external
diameter of 80 nm and the tube thickness of about 12 nm. The
ordered nanotube structure contributes to the fast transfer of
Na+. In addition, S doping can drastically accelerates the move-
ment of electrons within the nanotubes. Consequently, S-TiO2
nanotube arrays exhibit prominent cycling stability (a capacity
of 136 mA h g−1
is retained at 3350 mA g−1
after 4400 cycles)
(Figure 12c). Pan and co-workers synthesized Sn-doped TiO2
nanotubes by a facile sol–gel method and a subsequent hydro-
thermal route.[158]
Moderate Sn doping is beneficial to electrical
conductivity, and consequently enhances the electrochemical
performance.
In contrast to anatase-TiO2, TiO2(B) is not very desirable in
view of its impuissant ion diffusion and low electronic con-
ductivity. According to previous report,[159] oxygen vacancies
(OVs) can act as an electronic charge carrier to enhance the
electronic conductivity, thus leading to fast ion/electronic trans-
port and enlarged Na+ diffusion coefficient in SIBs. Hence, Ji
and co-workers[160] constructed OVs evoked blue TiO2(B) nano-
belts (B-TiO2(B)) as the anode for SIBs. The carbon nanobelts
coating can prevent OVs from the intervention of electrolyte
decomposition and SEI (solid electrolyte interphase, which is
formed by the reaction between the electrolyte and the surface
of the electrode material) formation. OVs can expand inter-
layer spacing and reduce the energy barrier of sodiation, which
is beneficial to preserve the intact crystal structure and accel-
erate storage kinetics. As shown in Figure 12d,e, the B-TiO2(B)
Small 2018, 14, 1703086
Figure 11. a) Cycle performance of the HCNW electrode at a current density of 50 mA g−1. b) Rate performance of the HCNW electrode. Reproduced
with permission.[148] Copyright 2012, American Chemical Society. c) Photographs of PI film and N-CNF film. d) SEM image of N-CNF (the inset is the
corresponding high-magnification image). e) SEM image of a single nanofiber in N-CNF. f) Cycling performance and g) rate performance of N-CNF.

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exhibits excellent cycling stability and robust rate performance
(the capacities of 204.6, 182.5, 162.9, 150.9, 134.7, 114.9, and
106.8 mA h g−1 can be obtained at 0.25, 0.5, 1, 2.5, 5, 10, and
12.5 C, respectively), which are much better than the electrode
of TiO2(B) without the OVs and carbon species (termed as
W-TiO2(B).
Except for TiO2, Wei and co-workers[161] reported Na2Ti7O15
nanotubes with porous structure and 3D network grown on the
Ti substrate through an in situ hydrothermal synthetic route.
Taking advantages of the large interface and channels of 1D
nanotubes and porous Ti substrate, Na2Ti7O15 nanotubes exhibit
high specific capacity and stable cycling performance. Nanoparti-
cles (especially within 10 nm) are helpful to mitigate the absolute
strain and effectively retard pulverization of electrode materials.
Meanwhile, dispersing nanoparticles in carbon conductive matrix
can buffer volume change and prevent particles aggregation.
Therefore, Xie et al.[162] prepared ultrasmall MgTi2O5 nanoparti-
cles confined in carbon nanorods. MgTi2O5–C nanocomposites
were synthesized by an in situ carbonization process. Due to
the enhanced conductivity of MgTi2O5, fast transport of Na+
and
effective utilization of hard carbon, MgTi2O5–C nanocompos-
ites demonstrate great enhancement in reversible capacity, rate,
cycling capability, and coulombic efficiency.
3.1.3. Alloy-Type Anode Materials
Compared with carbonaceous materials and titanium-based
materials, alloy-based materials acquire growing attention
because of their high theoretical capacities, low working poten-
tials and simple preparation methods. The study of alloy-based
anode materials mainly focus on IVA and VA in periodic table
of chemical elements,[163] such as P, As, Sb, Sn or their alloys,
which can alloy with Na to construct alloy compounds such
as Na3.75Sn (847 mA h g−1), Na3Sb (660 mA h g−1), and Na3P
(2560 mA h g−1). Specifically noting, the theoretical specific
capacity of Ge in SIBs (369 mA h g−1) is lower than in LIBs,
and the electrochemical performance of Si applied in SIBs
has not been verified experimentally. Though alloying reaction
materials can obtain much higher capacities than hard carbon
and titanium oxides, they also suffer from large volume expan-
sion, which leads to a consecutive pulverization of the electrode
materials and a rapid capacity decay. To solve the problem, a lot
of novel 1D nanostructured materials are applied to restrain
volume variation of the alloy-type anodes during Na+
inser-
tion and extraction processes. Especially, an effective approach
is to blend the active nanomaterials with inert materials or
less volume variation materials. The most studied binary or
Small 2018, 14, 1703086
Figure 12. a) Illustration of the fabrication process of S–TiO2 nanotube arrays. b) SEM image of S–TiO2 nanotube arrays. c) Cycling performance of
S–TiO2 nanotube arrays at a rate of 10 C. Reproduced with permission.[157] Copyright 2016, Wiley-VCH. d) Cycling performance and e) rate performances
of W–TiO2(B) and B–TiO2(B). Reproduced with permission.[160] Copyright 2017, Wiley-VCH.

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multielement compounds are composite with 1D carbon
nanomaterials. Remarkably, Ruan et al.[164]
reported the red
phosphorus/N-doped carbon nanofibers composite as the anode
for high-performance SIBs. The carbon nanofibers possessing a
large volume of mesopores and high surface area could relieve
the volume changes of the P particles during discharge/charge
processes. In addition, Red P confined in CMK-3 (P@CMK-3)
has been reported by Yu and co-workers. When used in SIBs,
P@CMK-3 demonstrates high specific capacity and great cycling
performance.[101]
As a versatile top-down method, electrospin-
ning technique is widely used to prepare 1D carbon nanofibers.
Our group reported Sn nanodots confined in porous N-doped
carbon nanofibers (Sn NDs@PNC) as anode for SIBs.[165]
As-prepared Sn NDs@PNC membrane has good integrity
and suppleness, which can be used as free-standing anode
in SIBs. As shown in Figure 13a,b. Many extremely small Sn
nanoparticles (≈1–2 nm) are confined in 1D carbon nanofibers
homogeneously. Such unique structure greatly shortens the dif-
fusion length of Na+, improves the conductivity and efficiently
relieves the volume changes of Sn in cycling processes. Con-
sequently, with the increasing of current density from 200 to
10000 mA g−1, the capacity decrease slightly and the reversible
capacity reach as high as 633 mA h g−1 at 200 mA g−1. More
surprisingly, it can still exhibit a high capacity of 450 mA h g−1
even at the excessively large current density of 10 000 mA g−1,
which show excellent cycling performance (Figure 13c). Simi-
larly, Wang and co-workers prepared pipe-wire TiO2-Sn@carbon
nanofibers (TiO2-Sn@CNFs) as the anode for SIBs by com-
bining an electrospining method and ALD technique.[166] Dis-
persing Sn nanoparticles in the 1D nanofibers can buffer the
stress change and the TiO2 pipe can also relieve Sn nanopar-
ticles aggregation and restrain volume fluctuation. Hence, the
pipe-wire TiO2-Sn@CNFs electrodes demonstrate excellent high
reversible capacity and good cycling stability.
Metal Sb has also received a great deal of attention as
anode material for SIBs due to its high theoretical capacity
(660 mA h g−1). Wu et al.[13] prepared the Sb–C nanofibers
as an anode for SIBs by simple electrospining technique. Sb
nanoparticles with the diameter of ≈15–20 nm are uniformly
embedded in the carbon nanofibers. The Sb–C electrodes
show excellent rate capability with a capacity of 337 mA h g−1
even at a large current density of 3000 mA g−1. Additionally,
Sb nanorod arrays with uniform large interval spacing were
prepared by nanoimprinted AAO template technique coupled
with an electrodeposition process.[167] The fabrication pro-
cesses are schematically illustrated in Figure 13d. 1D highly
ordered Sb nanorod arrays as anode for SIBs with many
advantages such as good vertical alignment and large interval
spacing, leading to fast electrons/ions transport and the
accommodation of volume variation. Therefore the integrated
electrodes display high capacity, cycle stability and rate capa-
bility. As is known to us, carbon nanotube as one of widely
used 1D carbon-based materials possesses many advantages.
For example, the tube with void space can accommodate the
large volume expansion and the carbon shell can prevent the
aggregation of nanoparticles. Liu et al.[122]
synthesized Sb@C
coaxial nanotubes by means of carbon-coating and thermal
reduction. Firstly, PDA was coated on Sb2S3 by hydrothermal
method to fabricate Sb2S3@PDA core–shell nanorods, after
calcination, Sb2S3 was reduced into Sb by carbonized PDA
layer. The obtained 1D tube structure can provide electronic/
ionic transport pathway and strong tolerance to volume change
of Sb. Ji et al.[168]
prepared SnSb nanoparticles dispersed in
porous electrospun carbon nanofibers as SIBs anode. They
mainly studied the influence of FEC in electrolyte on the for-
mation of SEI films and further on the electrochemical prop-
erties. Eventually, they got that the thin, uniform, flexible SEI
films and the novel 1D porous nanofibrous structures jointly
contribute to the high specific capacity of 345–350 mA h g−1
at 0.2 C and excellent rate capability of over 110 mA h g−1
at 20 C.
3.1.4. Transition Metal Oxides, Sulfides, and Phosphides
Transition metal oxides (TMOs), transition metal sulfides
(TMSs), and transition metal phosphides (TMPs) have been
supposed as promising anode materials for SIBs because of
their high theoretical specific capacities. However, the con-
tinuous pulverization of electrode materials caused by huge
volume change during Na insertion and extraction process is
still the tough issue at present. In term of the reaction mecha-
nism during Na+ insertion/extraction processes, a wide range
of these compounds can be classified into the conversion reac-
tion or the combination of conversion reaction and alloying
reaction. The concept of conversion reaction was first put up by
Alca´ntara et al.[169] who studied spinel NiCo2O4 as SIBs anode.
Later Jiao and co-workers[18] prepared CuCo2O4@C nanofibers
as the anode for SIBs. 1D CuCo2O4@C nanofibers effectively
accelerate the electronic/ionic transport and display out-
standing performance. Liu et al.[170] synthesized MnFe2O4@C
(MFO@C) nanofibers through the electrospinning technique
as SIBs anode. Figure 14a reveals that the smooth and contin-
uous MFO@C nanofibers with an average diameter of 180 nm
interlink into a 3D network and the MFO@C electrode still
retains its original morphology and structure after long-term
cycling. The unique structure with large surface area and ultra-
small MnFe2O4 nanodots of MFO@C exhibit excellent cycling
stability, high reversible capacity and good rate performance in
half cell of SIBs. Furthermore, when evaluated MFO@C in full
cell by assembling an Al-plastic film soft package battery with
Na3V2(PO4)2F3/C, the full battery delivers a discharge capacity
of 406 mA h g−1 at 500 mA g−1 and an average output voltage
of ≈2.3 V (Figure 14b). And the full cell affords a reversible
capacity of 392 mA h g−1 (a high capacity retention ≈96.5%)
and coulombic efficiency of 99% after 100 cycles (Figure 14c),
which further demonstrate MFO@C nanofibers have potential
for application.
Except for binary-metal oxides, there are plentiful single-
metal oxides with 1D nanostructure such as CuO,[171] SnO2,[172]
Co3O4,[173] Fe3O4,[174] Bi2O3,[175] etc. Polymer binder is a vital
ingredient of electrode materials, but it exacerbates the cycling
stability and irreversible capacity losses. To overcome the
problem, Yuan et al.[171]
reported the binder-free porous CuO
nanorod arrays (CNA) by in situ engraving Cu foil method
(Figure 14d). Cu substrate not only plays supporting role but
also integrates CuO nanorod arrays. In addition, the unique
nanorod arrays with sufficient pores and open space can realize
Small 2018, 14, 1703086

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fast electron transport and facile diffusion of electrolyte into the
internal electrodes (Figure 14e), which is beneficial to achieve a
specific capacity of 290.6 mA h g−1 and a high coulombic effi-
ciency of ≈100% even at a high current density of 200 mA g−1
after 450 cycles.
Similar to TMOs, TMSs (such as MoS2,[176,177] WS2,[70]
FeS,[178] and SnSbSx)[179] have also been widely researched and
applied. For instance, layered MoS2 is one of the most impor-
tant members of TMSs. According to the different layer-stacking
sequences, MoS2 exists in three forms: the one-layer-stacked
trigonal 1T-MoS2, the two-layer-stacked hexagonal polymorph
2H-MoS2, and the three-layer-stacked rhombohedral 3R-MoS2.
Among them, 2H-MoS2 is a room-temperature stable product.
Sodium storage mechanism of 2H-MoS2 contains two reactions:
intercalation-type reaction at higher potential windows and con-
version-type reaction at lower potential windows, as depicted in
Equation (5) and (6)
x x x2H MoS Na e 1T Na MoS2 2− + + ↔ −+ −
(5)
x xx1T Na MoS 4 Na 4 e Mo 2NaS2 2( ) ( )− + − + − ↔ ++ −
(6)
It was found that the structure of MoS2 could be partially
restored if x value in the 1T-NaxMoS2 less than 1.5. Nev-
ertheless, with further Na+
intercalation, 1T-NaxMoS2 was
Small 2018, 14, 1703086
Figure 13. a) TEM and b) HRTEM images of Sn NDs@PNC nanofibers. c) Rate capability and cycling performance of Sn NDs@PNC, lower Sn
content (L–Sn@PNC), and higher Sn content (H–Sn@PNC) electrodes., Inset: SEM, TEM, and HRTEM images of Sn NDs@PNC after 300 cycles.
Copyright 2015, Wiley-VCH. d) Schematic illustration of the fabrication processes of Sb nanorod arrays by using the
nanoimprinted AAO templating technique with the assistance of an electrodeposition process. Reproduced with permission.[167]
Copyright 2015, Royal
Society of Chemistry.

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decomposed to metal Mo nanoparticles and Na2S and its
structure could not be recovered anymore.[180] When used as
anode in SIBs, MoS2 suffers severe cycling problem and large
voltage polarization due to the shuttle effects of polysulfides
and irreversible reaction. To solve these problems, one of
the most effective ways is to fabricate 1D nanomaterials of
MoS2.[181–183] Yu and co-workers synthesized 1D MoS2-carbon
composites through electrospinning method.[181] As illustrated
in Figure 14f, TEM image of MoS2–carbon composites exhibits
uniform nanofibers with diameters of about 50 nm. Moreover,
extremely thin and small MoS2 layers are randomly embedded
in the thin amorphous carbon fibers (Figure 14g–i). For
sodium storage, this unique 1D nanocomposites demonstrate
outstanding performance: a high capacity of 854 mA h g−1 is
obtained at 0.1 A g−1, and the capacity of 253 mA h g−1 can
be delivered at a ultrahigh current density of 10 A g−1 after
100 cycles. In addition, our group prepared WS2 nanowires
via solvothermal method followed by a heat treatment.[70] As-
prepared WS2 nanowires possess expanded interlayer spacing
of 0.83 nm through the intercalation of NH4
+
, fast Na+ diffu-
sion kinetics, open channels and abundant active sites for
rapid Na+ intercalation/deintercalation. Thus, WS2 nanowires
display a remarkable capacity of 415 mA h g−1 at 200 mA g−1
after 500 cycles. Ryu et al.[27] prepared core–shell heteroge-
neous WSx/WO3 with thorn-bush nanofiber (NF) architectures
for SIBs. The unique hierarchical structure affords plenty of
active sites for Na+
intercalation and efficiently restrains the
dissolution of sulfur, leading to the extraordinary performance.
They also prepared vine-like MoS2 nanofibers coated with TiO2
as the anode for SIBs.[184]
Notably, TiO2 plays a crucial role in
restraining sulfur dissolution and 1D vine-structure offers a
huge surface area, good strain accommodation, fast Na+ and
electron diffusion. Additionally, Sb2S3 has been identified as
a promising material for SIBs due to its high reversible theo-
retical capacity of 946 mA h g−1. Kim and co-workers not only
utilized hydrothermal method to synthesize carbon-coated van
der Waals stacked Sb2S3 nanorods but also emphatically ana-
lyzed the reaction mechanism.[71] They found that robust struc-
ture of carbon-coated Sb2S3 nanorods can effectively relieve
volume expansion due to following two reasons: (i) the metal
chalcogenide electrodes usually display a smaller volume
expansion than the homologous metal electrodes because
of the additional conversion reaction of metal chalcogenide
and Na+ ions; (ii) the amorphous carbon coating can relieve
volume expansion. S and Se both belong to the sixth family
of the periodic table, therefore they have many similarities. Ou
et al.[72] fabricated reduced graphene oxide (rGO)-overcoated
Sb2Se3 nanorods by a facile solvothermal method as the anode
for high-performance SIBs. The carbonaceous matrix rGO
not only can relieve large volume as a buffer agent but also
can enhance the electronic conductivity. There are only a few
reports about 1D TMP used in SIBs. Yan and co-workers fab-
ricated 1D nanostructured CoP and FeP4 supported by carbon
using amphiphilic self-assembling fibrous elastin proteins as
template.[185,186]
The CoP and FeP4 nanoparticles (5–10 nm in
diameter) are decorated in the fibers with diameters around
50 nm and lengths about 2 µm. When used as anode in SIBs,
CoP exhibits a high capacity (615.29 mA h g−1
at 0.1 C at the
third cycle) as well as superior rate performance (a capacity of
300 mA h g−1
can be achieved at 5.0 C after 1000 cycles).
Small 2018, 14, 1703086
Figure 14. a) SEM image of MFO@C nanofibers. b) Charge/discharge curves, and c) cycling performance of an Al-plastic film soft package Na–ion full
battery with MFO@C–Na3V2(PO4)2F3/C (inset in b) tested in the voltage range of 1.0–3.5 V at 500 mA g−1
. Reproduced with permission.[170]
Copyright
2016, American Chemical Society. d) SEM image of CuO nanorod arrays. e) Schematic diagram showing the strategy for a binder-free CNA electrode.
Copyright 2014, Wiley-VCH. f) TEM, g) HRTEM image of MoS2/carbon composite. h,i) Corresponding HRTEM images
from the marked region in (f) and (g), respectively. Reproduced with permission.[181]

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3.1.5. Organic Materials
Organic anodes have received considerable attention owing
to their advantages of abundant resources, sustainability, and
possible multielectron reactions. However, high solubility and
sluggish kinetics also hinder their development. Hu and co-
workers first reported disodium terephthalate (Na2C8H4O4) as
an anode for SIBs.[187]
The Na2C8H4O4 electrode exhibits low
Na insertion voltage at 0.29 V, and delieves a reversible capacity
of 250 mA h g−1
corresponding to two electron transfer. Wang
and co-workers prepared 1D 2,5-dihydroxy-1,4-benzoquinone
disodium salt (DHBQDS) nanorods by in situ formed on a
Cu current collector.[188]
In addition, a thin layer of Al2O3 with
thickness of 1–2 nm was coated on the DHBQDS nanorod elec-
trodes via ALD method to relieve the dissolution of DHBQDS
in the electrolyte during cycling processes. Furthermore, owing
to the fast ionic and electronic conductivity of DHBQDS-carbon
nanocomposite, Al2O3 coated DHBQDS nanorod electrodes
exhibit increased coulombic efficiency, high capacity and long-
term cycling performance.
3.2. Cathodes
Cathode materials mainly contain transition-metal oxides, poly-
anionic compounds (such as phosphates, fluorophosphates,
pyrophosphates, and others), prussian-blue analog, and organic
compounds. As an important component of SIBs, highly revers-
ible cathode materials are greatly necessary to be designed to
improve the electrochemical performance of SIBs.[189,190]
3.2.1. Transition-Metal Oxides
Transition-metal oxides are usually categorized into tunnel-
structured oxides and layered oxides. They have been exten-
sively researched and used as cathode materials in SIBs for
their high capacities, simple structures, and feasible syn-
thesis methods. Layered metal oxides mainly include O3-type
(ABCABC stacking), P2-type (ABBA stacking), and P3-type
(ABBCCA stacking) on the basis of the stacking sequence of
oxygen layers. Among them, the most researches concentrate
on the P2-type and O3-type, especially the P2-type materials due
to their high specific capacities and superior stability. Owing
to abundant Mn resource on Earth and the high capacities of
manganese oxides, manganese compounds (e.g., NaMnO2,
Na0.67Co0.5Mn0.5O2, Na0.60MnO2, and Na0.44MnO2) attract great
interest in cathode materials of rechargeable SIBs. Zhong
et al.[191] prepared P2-type Na0.7MnO2.05 nanotube/carbon nano-
tube (NMO/CNT) core/branch composites via the hydrothermal
method followed by a CVD route. Such core–shell hierarchical
netlike structure is favorable to faster transfer of ions. There-
fore, when applied to SIBs, a high capacity, rate capability and
long cycling ability can be obtained by NMO/CNT. In addition,
P2-type Na2/3(Fe1/2Mn1/2)O2 hierarchical nanofibers was syn-
thesized via electrospinning technique by Kalluri et al.[192]
Such
unique hierarchical nanofibers as the cathode for SIBs display
an initial discharge capacity of ≈195 mA h g−1
and improved
cycling performance with a capacity retention of 86.4% after
80 cycles. Among tunnel-structure materials, Na0.44MnO2
[193,194]
is particularly attractive on account of its wide tunnel structure
which can greatly facilitate the insertion/extraction of Na+
. Fu
et al.[195]
fabricated two types of Na0.44MnO2 hierarchical struc-
tures (nanofibers and nanorods) by optimized electrospinning
and controlled subsequent annealing procedure. Both of them
demonstrate outstanding electrochemical performance due to
diverse reasons. The superior rate performance of Na0.44MnO2
nanofibers is attributed to its 1D ultralong and continuous
fibrous network structure. While the excellent cyclic perfor-
mance of the Na0.44MnO2 nanorods can be ascribed to its large
S-shaped tunnel structure with a single crystalline structure. In
addition, Cao et al.[196]
reported single crystalline Na4Mn9O18
nanowires as SIBs cathode via a polymer-pyrolysis method. Due
to the high crystallinity and a homogeneous nanowire structure,
Na4Mn9O18 nanowires supply a mechanically stable structure
and a short diffusion path for Na+
intercalation and extraction.
Consequently, a high reversible capacity of 128 mA h g−1
at
0.1 C and exceptional cycling performance (77% capacity reten-
tion after 1000 cycles at 0.5 C) are delivered.
Vanadium oxides are attractive electrode materials owing to
their low cost, natural abundance and the wide range of vana-
dium valence states (V0–V5). However, the poor electrochemical
kinetics and low electronic conductivity lead to the low-rate and
terrible cycling stability. In order to solve this limitation, various
1D nanostructured vanadium oxides have been constructed.
Rui et al.[197] synthesized V2O5 nanobelts by a cost-effective
and facile process under ambient condition. The commercial
V2O5 powder as precursor was dissolved in the NaCl solution,
after 72 h vigorous stirring, and eventually was transformed
into V2O5 nanobelts (Figure 15a). In order to study the growth
mechanism of V2O5 nanobelts, the intermediates with various
morphologies are collected during different time periods, as
shown in Figure 15b. After generating free vanadium species
(such as [V10O28]6− and VO2+), recrystallization and continuous
deposition of VO2
+
onto the crystalline seeds, belt-like nano-
structured V2O5 formed. When applied in LIBs and SIBs, V2O5
nanobelts exhibit an outstanding electrochemical performance
(for SIBs, it exhibits a reversible capacity of ≈116 mA h g−1 at
20 C after 500 cycles). Apart from V2O5, VO2 is another prom-
ising cathode material for SIBs. However, it also suffers from
fast capacity fade and poor rate performance. Wang et al.[198]
have fabricated VO2 nanowire, nanobelt and nanosheet arrays
by a simple hydrothermal method to improve the performance
of VO2. In addition, designing tailored nanoarchitecture and
surface engineering is an effective way to enhance the perfor-
mance of batteries. Fan and co-workers[199] used VO2 arrays
growing on graphene network by bottom-up growth and
coated with graphene quantum dots (VO2@GQD) as binder-
free cathode for SIBs. Benefitted from many advantages (e.g.,
fast ion diffusion, the improved stability taken from graphene)
provided by the tailored unique structure, VO2@GQD displays
high reversible capacity and long cycling stability. The hierar-
chical zigzag-shaped Na1.25V3O8 (NVO) nanowires were pre-
pared by Mai and co-workers using a facile topotactic interca-
lation method.[55]
The morphologies of products with different
amounts of CTAB are depicted in Figure 15c–f. NVO-C1 (0.05 g
CTAB), NVO-C2 (0.1 g CTAB), NVO-C3 (0.2 g CTAB), and
NVO without CTAB have been studied. Figure 15c shows no
Small 2018, 14, 1703086

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© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimSmall 2018, 14, 1703086
hierarchical structure NVO without CTAB addition, NVO-C1
appears kinked nanowire structure while some stackings can
be observed (Figure 15d). With increasing CTAB to 0.1 g, TEM
image of NVO-C2 shown in Figure 15e clearly presents that
the obvious and uniform kinked nanowires are composed of
many repeated small interconnected nanorods. For NVO-C3
(Figure 15f), the nanowire structure becomes ambiguous and
the nanorods are stacked with each other disorderly. As a result,
the specific discharge capacities of NVO, NVO-C1, NVO-C2,
and NVO-C3 are 83.5 mA h g−1
, 154.3 mA h g−1
, 171.9 mA h g−1
,
and 126.6 mA h g−1, respectively. Obviously, the NVO-C2 with
the most uniform structure shows the best electrochemical per-
formance. Compared with the nontopotactically synthesized
nanowire structure, the hierarchical zigzag nanowires structure
owns short Na+
diffusion pathways, enlarged electrode/elec-
trolyte contact area, strong strain accommodation and intact
morphology, which are helpful to alleviate the structural deg-
radation and self-aggregation during Na+
ions intercalation/
deintercalation process, as shown in Figure 15g. Therefore,
the novel as-prepared hierarchical zigzag-shaped Na1.25V3O8
Figure 15. a) Schematic diagram of synthesis and assembly of V2O5 nanobelts as cathode materials for SIBs and LIBs. b) Schematic illustration of
the formation process of the V2O5 nanobelts. Reproduced with permission.[197] Copyright 2016, Elsevier. c–f) Morphologies of products synthesized
with different amounts of CTAB. TEM images of NVO c), NVO–C1 d), NVO–C2 e), and NVO–C3 f), and the insets in (c–f) show the morphologies in
different scales (scale bar for c inset: 200 nm; for d–f insets: 1 µm). g) Schematic illustration of the electrochemical process for the non-topotactically
and topotactically synthesized nanowire structure of Na1.25V3O8. Reproduced with permission.[55] Copyright 2015, Royal Society of Chemistry.

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nanowires can provide excellent electrochemical performance
when used as cathode in SIBs.
3.2.2. Polyanionic Compounds
Polyanionic compounds can be classified into phosphates,
fluorophosphates, pyrophosphates, sulfates and so on.[200]
Polyanionic compounds are getting increasing attention
owing to their structural stability, safety and suitable operating
potentials. However, the poor electronic conductivity results
in relatively low electrochemical utilization, slow rates, and
the restriction in practical applications. An effective solution
is to combine active electrode materials with conductive 1D
carbon matrix. NASICON (sodium super ion conductor) struc-
tured Na3V2(PO4)3 (NVP) with 3D open framework and high
theoretical energy density has been identified as one of the
most attractive phosphates for SIBs, nevertheless, poor elec-
tronic conductivity is still the short slab of NVP electrodes. To
settle the matter, some researchers prepared Na3V2(PO4)3/C
nanofibers via electrospinning method.[12,201,202] Yu and co-
workers fabricated Na3V2(PO4)3/C nanofibers via electrospin-
ning method.[12] In the Na3V2(PO4)3/C nanafibers, Na3V2(PO4)3
nanoparticles with the diameter of 20–30 nm uniformly dis-
persed in 1D carbon nanofibers. The unique 1D morphology,
efficient electrochemical coupling and 3D conductive net-
work lead to superior rate capability. Jiang et al.[102] confined
carbon-coated Na3V2(PO4)3 nanoparticles into ordered carbon
matrix CMK-3 (NVP@C@CMK-3) as the high performance
cathode for SIBs. NVP nanoparticles coated by double carbon
are beneficial to the fast transport of Na+ ions and electrons,
which plays a significant role in enabling high-power perfor-
mance (a reversible capacity of 103 mA h g−1 after 1000 cycles
at 1 C and 78 mA h g−1 at 5 C after 2000 cycles). Ren et al.[203]
utilized a facile self-sacrificed route prepared 3D Na3V2(PO4)3
nanofiber network composed of 1D nanofibers. Both the half
and full SIBs with Na3V2(PO4)3 nanofiber network as the
cathode demonstrate excellent electrochemical performance,
which can be ascribed to the fast ion diffusion and improved
structural integrity. In addition, 3D networks consisted of
Na3V2(PO4)3 nanofibers possess short ion diffusion pathways,
the large contact surface area between electrodes and electro-
lytes, structural stability to avoid agglomeration and higher
packing density compared with particle materials. Hence,
Na3V2(PO4)3 nanofibers as highly reversible cathode is also
applied in the development of all-solid-state SIBs.[204]
The good ionicity of fluorides resulted from their higher
electronegativity can increase the operating voltage of the elec-
trodes. Therefore, fluorophosphates materials as the cathode
get great attentions. Our group prepared novel 1D NaVPO4F/C
nanofibers as self-standing cathode material for SIBs via an
electrospinning method.[16] The formation process is illus-
trated schematically in Figure 16a. As shown in Figure 16b,
the obtained NaVPO4F/C show the smooth and continuous
nanofibers with a uniform diameter of about 150 nm, which
interlink into a 3D network. This 3D network can support more
pathways for the ionic and electronic transport and restrain the
aggregation of NaVPO4F nanoparticles in the cycling process. As
illustrated in Figure 16c, TEM image and particle size analysis
of NaVPO4F confirm that NaVPO4F nanoparticles (≈6 nm)
uniformly disperse in the carbon nanofibers. When applied in
SIBs, NaVPO4F/C nanofibers exhibit a high capacity, a superior
rate capability, and long-term cycling ability (Figure 16d). The
outstanding electrochemical performance can be attributed to
the following main merits: (i) the small NaVPO4F nanoparticles
(≈6 nm) shorten the length of Na–ion transport, which facili-
tates Na+
ions diffusion kinetics; (ii) 1D nanofibers significantly
improve the ionic and electronic transport; (iii) the self-standing
electrode can improve the energy density.
Sulfates with strong electronegativity and higher redox
potentials attract a great deal of attentions. Among sulfate-
based materials, the alluaudite-type Na2Fe2(SO4)3 have gained
extensive attention due to its higher operating potential with
3.8 V (vs Na+
/Na) and high theoretical energy density.[205]
Deng
and co-workers explored the off-stoichiometric member of
Na2 + 2xFe2 − x(SO4)3.[206]
They constructed Na2 + 2xFe2 − x(SO4)3@
porous carbon nanofibers (PCNF) hybrid film by combining
electrospinning with electrospraying method. The synthetic
process is shown in Figure 16e–h. First, the electrospun
nanofibers were carbonized to imporous carbon nanofibers,
and then porous carbon nanofibers were fabricated by removing
SiO2 clusters in HF solution. Next, they synthesized the hydrate
sulfate@PCNF by electrospraying and vacuum impregnation.
Finally, after calcination, they successfully synthesized the all-
uaudite-type Na2 + 2xFe2 − x(SO4)3@PCNF hybrid with excellent
flexibility, stable architecture, and high-efficiency electron/ion
transport pathways. Benefited from the porous and conductive
1D nanofibers network, the free-standing hybrid film obtained
superior electrochemical performance when used as cathode
in SIBs.
Pyrophosphate compounds are also considered as the promi
sing materials due to their unique 3D (P2O7)4− framework,
vast ionic transportation pathways, and rich structural varia-
tion. In order to improve the relatively low electronic conduc-
tivity of Na6.24Fe4.88(P2O7)4, Niu et al.[207] developed a novel
structure of graphene-wrapped Na6.24Fe4.88(P2O7)4 nanofibers
(NFPO@C@rGO) as the SIBs cathode by electrospinning
method. The materials display higher reversible capacity,
long-term cycle life, and superior rate performance com-
pared with the pure NFPO and NFPO@C composite. Deng
et al.[208] designed and synthesized the 1D nanostructured
Na7V4(P2O7)4(PO4) as high-potential and superior-performance
cathode material for SIBs. The mixed polyanion compounds
were synthesized by sol–gel process, calcination process, and
purification process. Na7V4(P2O7)4(PO4) has two high potential
of 3.8713 V (V3+/V3.5+) and 3.8879 V (V3.5+/V4+), respectively.
Later, the group designed Na7V4(P2O7)4(PO4)/C nanorods using
more facile and low-cost hydrothermal-assisted strategy.[209]
These 1D nanostructured materials efficiently improved the
electrode ionic/electronic conductivity and enhanced struc-
tural stability, thus leading to the improved sodium storage
performance.
3.2.3. Organic Materials
Organic materials are considered to be a class of attrac-
tive cathodes due to their abundant resources, low cost,

1703086 (22 of 26)
small
NANO MICRO
environmental friendliness, recyclability, and high theore
tical capacity.[210,211]
However, organic compounds suffer
from low electrical conductivity and are prone to dissolve into
organic electrolyte solutions, resulting in inferior electro-
chemical performance. A nanorod structured carbonyl-based
organic salt Na2C6O6, sodium rhodizonate (SR) dibasic, was
prepared for high-performance SIBs.[212]
Benefited from the
enhanced reaction kinetics and high electrochemical activity
of SR nanorods, a high reversible capacity (≈190 mA h g−1
at 0.1 C after 100 cycles) and an outstanding rate perfor-
mance (50% of the capacity can be delievered at 10 C) can
be obtained.
Figure 16. a) Schematic illustration of the formation process of NaVPO4F/C nanofibers. b) SEM images of NaVPO4F/C nanofibers. c) TEM images
of NaVPO4F/C nanofiber (inset in (c): particle size analysis of NaVPO4F/C). d) Cycling performance of NaVPO4F/C nanofibers. Reproduced with
permission.[16] Copyright 2017, Wiley-VCH. e–h) Schematic of the synthetic approach for Na2 + 2xM2x(SO4)3@PCNF hybrid film.[206] Copyright 2016,
Royal Society of Chemistry.

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small
NANO MICRO
4. Summary and Outlook
In this Review, the recent progress in the design and fabrica-
tion of 1D nanomaterials is summarized, and their applications
in SIBs are highlighted. The structures, synthesis methods and
the electrochemical performance for SIBs of the typical 1D
nanomaterials have been listed in Tables 1 and 2. As described
in this review, a great number of 1D nanostructured electrodes
have been designed and synthesized for SIBs and exhibit out-
standing electrochemical performance. The excellent perfor-
mance can be ascribed to the features of 1D nanomaterials:
short ion diffusion pathways, good mechanical strength and
high surface areas. Such characteristics efficiently improve Na+
diffusion kinetics, alleviate the volume expansion during charge
and discharge (especially in anode materials), enhance the sta-
bility of electrode structure and increase the utilization rate of
active materials, thus effectively improving the cycling and rate
performance of SIBs. Despite the considerable achievements
made so far, there are still several bottlenecks and substantial
development room for the fabrication and application of 1D
nanomaterials:
(i) All the methods for the preparation of 1D nanomaterials still
remain obstacles. Electrospinning technology still cannot
prepare uniform nanofibers with diameters below 50 nm.
There are usually toxic and corrosive organic solvents in
the preparation of the precursor solutions. Hydrothermal
method is not well controlled in the synthesis process and
the reproducibility is terrible. CVD and ALD routes are very
limited. For instance, only several types of 1D nanomaterials
(e.g., semiconductor carbide, nitride, etc.) can be prepared
by CVD. The ALD method is limited to its high cost, and
it can only produce 1D nanomaterials in the presence of
the template or cover several thin layers on the surface of
original 1D nanomaterials. These limitations and problems
need to be solved in the near future.
(ii) With the growing demands for portable and wearable devic-
es, electrospun flexible energy storage devices or 1D nano
materials fabricated on the flexible conductive substrates
may be the promising and interesting filed.
(iii) To date, large-scale production for 1D nanomaterials is still
not fully achieved. The main reason may be the terrible
controllability and poor repeatability in the synthesis pro-
cess from my personal point of view. Moreover, the high
production cost and strict conditions in certain synthesis
routes are also the key problems on the way of large-scale
production.
The design and preparation of 1D nanomaterials have been
comprehensively discussed in this review. However, more spe-
cific fabrication and application are needed to be attempted and
optimized. For large-scale and low-cost production, the required
synthesis procedures, which are relatively complex, must be
simplified and engineered. We firmly believe that the future of
1D nanomaterials have a bright development prospects in the
field of energy storage and conversion. In addition, to develop
SIBs with high energy density, high safty and long-term cycling
capacity is a major challenge for the development and applica-
tion of SIBs. Furthermore, the optimization of electrolytes may
provide an opportunity for improving the electrochemical per-
formance of SIBs.
Acknowledgements
This work was financially supported by the National Natural Science
Foundation of China (51622102, 51231003, 51571124), MOST
(2016YFB0901502), and the 111 Project (B12015).
Conflict of Interest
The authors declare no conflict of interest.
Keywords
1D, nanofibers, nanotubes, nanowires, sodium–ion batteries
Received: September 7, 2017
Revised: October 3, 2017
Published online: December 11, 2017
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