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2100856 (1 of 11) © 2021 The Authors. Advanced Energy Materials published by Wiley-VCH GmbH
Perspective
Perspective on Carbon Anode Materials for K+
Storage:
Balancing the Intercalation-Controlled and Surface-Driven
Behavior
Jiaying Yang, Yixuan Zhai, Xiuhai Zhang, En Zhang, Hongqiang Wang, Xingrui Liu,
Fei Xu,* and Stefan Kaskel*
DOI: 10.1002/aenm.202100856
lithium-ion batteries (LIBs) face challenges
to satisfy the ever-increasing demand for
large-scale and low-cost electrical energy
storage in areas like electric vehicles and
smart grids.[1,2] Alternative battery tech-
nologies relying on more earth-abundant
elements such as Na (23 000 ppm) and
K (17 000 ppm) have become a rapidly
developing area.[3] In this context, potas-
sium-ion batteries (PIBs) employing K+
as charge carriers have recently drawn
increasing attention.[3–7] Apart from the
abundance and low cost of K, PIBs are also
characterized by several features. First, the
redox potential of K+/K is −2.93 V versus
standard hydrogen electrode (SHE), much
closer to that of Li+/Li (−3.04 V vs SHE)
as compared to Na+/Na (−2.71 V vs. SHE),
indicative of higher output voltage and
thus better energy density for full cells.[8,9]
Second, K+ in electrolyte demonstrates
higher ionic conductivity owing to its weaker Lewis acidity with
smaller solvated K+ and lower desolvation energy, which are
vital to achieve high rate capability.[10] Third, graphite is able to
accommodate reversible K+ intercalation/deintercalation via the
formation of fully stage-one compound KC8 with a capacity of
279 mAh g−1, akin to Li+ intercalation process;[11] Whereas the
Na+ intercalation is not feasible in carbonate electrolyte.[2,12]
Consequently, PIBs are potentially adaptable to the well-estab-
lished graphite anode industry of LIBs. Additionally, it is fea-
sible to employ Al foil as current collector instead of Cu used
in LIBs, because K does not alloy with Al at low potentials, thus
allowing reduced production cost.[13]
Inspired by these striking
advantages, rapid progress has been achieved in the design
and development of cathode and anode materials. High-perfor-
mance anode concepts are crucial for advancing the field, and
various anode materials have been developed including carbon
materials, metals/alloys, metal oxides, metal sulfides/selenides,
and their corresponding composites. Several comprehensive
overviews of various anode materials have been summarized
recently.[9,13–17]
Among these anode materials, carbon materials
have been identified as potential leading anode materials due to
their good conductivity, physicochemical stability, low cost, and
environmental sustainability. [2,18–20]
The past several years witnessed an explosive growth in pub-
lications regarding the design of various graphitic or amorphous
Potassium-ion batteries (PIBs) have emerged as a compelling complement to
existing lithium-ion batteries for large-scale energy storage applications, due to
the resource-abundance of potassium, the low standard redox potential and high
conductivity of K+
-based electrolytes. Rapid progress has been made in identi-
fying suitable carbon anode materials to address the sluggish kinetics and huge
volume variation problems caused by large-size K+
. However, most research into
carbon materials has focused on structural design and performance optimization
of one or several parameters, rather than considering the holistic performance
especially for realistic applications. This perspective examines recent efforts to
enhance the carbon anode performance in terms of initial Coulombic efficiency,
capacity, rate capability, and cycle life. The balancing of the intercalation and
surface-driven capacitive mechanisms while designing carbon structures is
emphasized, after which the compatibility with electrolyte and the cell assembly
technologies should be considered under practical conditions. It is anticipated
that this work will engender further intensive research that can be better aligned
toward practical implementation of carbon materials for K+
storage.
J. Y. Yang, Y. X. Zhai, Dr. X. H. Zhang, Prof. H. Q. Wang,
Dr. X. R. Liu, Prof. F. Xu
State Key Laboratory of Solidification Processing
Center for Nano Energy Materials
School of Materials Science and Engineering
Northwestern Polytechnical University and Shaanxi Joint Laboratory
of Graphene (NPU)
Xi’an 710072, P. R. China
E-mail: feixu@nwpu.edu.cn
Dr. E. Zhang, Prof. S. Kaskel
Inorganic Chemistry I
Technische Universität Dresden
01062 Dresden, Germany
E-mail: stefan.kaskel@tu-dresden.de
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/aenm.202100856.
1. Introduction
Restricted by the scarcity (20 ppm) and heterogeneous distri-
bution of Li resources in Earth’s crust, the current prevalent
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carbons (e.g., hard/soft carbons), in which K+ can be accommo-
dated in graphene layers, pores/nanovoids, or defect sites.[21–26]
The graphitic carbons generally suffer from insufficient rate
capability and poor cyclic stability, due to the inferior intercalation
kinetics and large volume variation (58%).[11,15] While many recent
publications report on much improved capacity, rate capability,
and cyclability using amorphous porous carbons with expanded
interlayer and tremendous defects/nanovoids for pseudoca-
pacitive contribution;[2,18,27] However, the low initial Coulombic
efficiency (ICE), high-potential sloping curves and low material
density should be cautiously considered for practical applica-
tions. In this perspective, the proposed K+ storage mechanisms
of various carbons are briefly discussed first, along with their
respective strengths and weaknesses in determining the multi
ple
performances. We then outline that further research must
refocus on balance of intercalation versus surface-driven charge
storage mechanisms in designing carbon structures, aiming at
comprehensively optimizing of performance. Meanwhile, the
compatibility between the carbon and electrolyte and the cell
assembly technologies under realistic conditions should be con-
sidered. Our systematic analysis and estimation are approximate
and at least enable us to rule out materials that would be unac-
ceptable in performances from a practical viewpoint, such as low
ICE, high-potential sloping curve, and low tap density.
2. K+
Storage Mechanism in Carbon Anode
The operation mechanism of PIBs is similar to that of LIBs, in
which K+
travels between the anode and cathode in a rocking-
chair fashion during the charge/discharge process (Figure 1a).
Unlike the major intercalation/deintercalation mechanism
in the cathode, various K+
storage processes are involved in
the anode. Depending on the microstructure and discharge
behavior of carbons, several charge storage mechanisms have
been proposed including intercalation, adsorption, pore filling,
etc.[18]
Generally, two main mechanisms have been identified:
diffusion-controlled intercalation process and surface-driven
capacitive process (Figure 1b).[2,28]
In addition, K+
plating is
also proposed to form metallic state in the nanopore/voids at
the low potential, similar to Na+
plating.[29,30]
The intercala-
tion process is based on K+
intercalation/deintercalation into/
from interlayers of graphitic carbon layers, and thus the storage
capacity relies on ionic state, interlayer structure, and so on.
The surface-driven capacitive K+ storage mainly happens at the
surface or near surface region and does not pose damage to the
electrode. Thus, the storage capacity is related to specific sur-
face area, intrinsic carbon defect/edges, and heteroatom-doped
functional sites within carbon materials.
2.1. Graphitic Carbon with Intercalation Involved K+ Storage
Due to the remarkable layer stacking with certain interlayer
spacing, K+ storage in graphite materials mainly comes from
its intercalation into graphitic layers.[11,31] Such intercalation
process adopts a similar staging mechanism as Li-intercalation
of graphite layer, in which the K+ completely intercalates into
graphite layers until occupying neighboring graphite layers.[12]
More specifically, this stage was divided into pseudocapacitive
intercalation behavior (C to KC24) and diffusion-limited interca-
lation (KC24 to KC8).[32] The intercalation process occurs at lower
working potential (e.g.,<0.5 V vs K+/K), as confirmed by Ji et al.
in 2015, delivering a high reversible capacity of 273 mAh g−1,
approaching the theoretical capacity of a stage-one KC8 forma-
tion (279 mAh g−1). Meanwhile, sequential formation of KC36,
KC24, and KC8 were identified during the intercalation/deinter-
calation process, as revealed by X-ray diffraction (XRD) patterns
(Figure 2).[11]
Afterward, the intercalation mechanism was
thoroughly studied with operando characterizations and theo-
retical simulations, revealing the reversible staging transition:
C-KC60-KC48-KC36-KC24/KC16-KC8.[33]
Despite the intercalation mechanism is under development,
graphite anode is still considered as a promising candidate in
consideration of the low-potential plateau curve for obtaining
higher full cell voltage and energy density. However, diffusion
of K+
into graphite layers is sluggish and leads to as high as
58% volume expansion with the graphitic structure collapse,
thus causing low-rate performance and serious capacity fading
during repeated cycling.[27]
Designing an appropriate structure
of graphite materials is promising to tolerate large volumetric
deformation. For example, a new carbon-polynanocrystalline
graphite with hollow shape and porous walls was prepared
by chemical vapor deposition, which exhibited disorder at
Figure 1. a) Schematic illustration of a “rocking-chair” fashion in PIBs. Reproduced with permission.[17] Copyright 2017, ACS. b) Two main proposed
K+ storage mechanisms in carbon materials.
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nanometric scales but strict order at atomic scales.[34] This novel
structure buffers volumetric expansion during intercalation pro-
cess, showing 50% capacity retention over 240 cycles. In addi-
tion, expanding the layer spacing of graphite materials is also an
effective strategy for promoting rapid K+ intercalation/deinterca-
lation. An et al. presented an expanded graphite with wider inter-
layer spacing as anode materials for PIBs,[27] which enhanced K+
diffusion coefficient and delivered a capacity of 263 mAh g−1
at
the rate of 10 mA g−1
and managed to improve the cyclability to
500 cycles. Despite these efforts, there is still enough room for
further enhancement of both cyclic stability and rate capability.
2.2. Amorphous Carbon with Surface-Driven Capacitive K+
storage
Amorphous carbons include hard and soft carbons, which
are different from graphite with long-range order in crystal-
lographic structure. Hard carbon consists of highly disordered
carbon layers and is nongraphitizable, while soft carbon is
partially disordered with short-range order and less defects,
and can be graphitized at higher temperatures.[2,12]
Therefore,
the microstructure (such as defects, edges, pores, functional
groups, and graphitization degree) directly affects K+
storage
performances.[30,35,36]
In addition to the partial intercalation
of K+, surface-driven capacitive K+ storage is predominated in
amorphous carbons. This capacitive process is crucial to facilitate
rate capability and cycle performance,[37–39] because the pseudo-
capacitive behavior mainly takes place at surface or near surface
region with fast kinetics and little damage to the bulk electrode.
In this context, potassiation/depotassiation curves appear as the
sloping characteristics (Figure 3a,b). The notable capacitive con-
tribution can be estimated by cyclic voltammetry (CV) profiles
with different scan rates. As can be seen from Figure 3c, the
surface-driven capacitive contribution is 71.4% at 0.2 mV s−1
. [40]
With increasing the scan rate, the surface-driven contribution
gradually increases and a value as high as 86.5% can be achieved
at 1.2 mV s–1
(Figure 3d).[40]
Relatively massive investigations
have been conducted to improve the capacitive contribution for
pursing high-rate and ultralong stability. The strategies include:
1) enhancing specific surface area by introducing nanopores or
designing various nanostructures,[41–52]
2) producing tremen-
dous intrinsic carbon layer defects,[36,53]
and 3) incorporating
doped heteroatom functional groups.[39,40,54–97]
Some representa-
tive electrochemical results in terms of capacity, cycle stability,
and rate performances are summarized in Figure 3e.
As an example, the construction of carbon materials with
developed pore structure is helpful to reduce K+
diffusion dis-
tance and increase the K-storage capacity.[49,51,52]
For instance,
Figure 2. a) The initial discharge/charge profile of a graphite electrode at C/10 (1C = 279 mA g−1). b) The XRD patterns of electrodes corresponding to
the marked selected states of charge in panel a), and c) structure diagrams of different K-graphite intercalation compounds, side view (top row) and
top view (bottom row). Reproduced with permission.[11] Copyright 2015, American Chemical Society.
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Li et al. synthesized a series of carbon materials with different
pore structures and found that the carbon anode synthesized at
900 °C showed the best performance due to its appropriate sur-
face area (2205 m2
g−1
), high mesoporous volume (1.07 cm3
g−1
)
and large number of active sites for K+
adsorption. A capacity of
284.8 mAh g−1
was obtained after 200 cycles at 0.1 A g−1
.[49]
More-
over, defects can be created in carbon materials by adjusting
the carbonization temperature. Low temperature induces more
abundant defects, which is beneficial to the effective adsorp-
tion of K+
for capacitive charge storage.[35,36,53]
For example,
polyaniline-co-polypyrrole derived carbon at 500 °C shows
a high capacity of 305 mAh g−1
after 660 cycles at 0.2 A g−1
,
while the carbon obtained at higher temperature of 900 °C dis-
plays the lower capacity of 62 mAh g−1
after 420 cycles at the
same current.[36]
For some carbons, too much decreasing tem-
perature will cause incomplete carbonization and deceased con-
ductivity.[36,93,94]
When heteroatoms are doped into the carbon
structure, the materials tend to produce more active sites,
thus boosting the adsorption of K+
.[39,82,83,92]
A nitrogen-doped
carbon microsphere using chitin as raw material exhibits a
high cycle stability with 154 mAh g−1 after 4000 cycles at 72 C
(1C = 280 mA g−1
).[60]
Furthermore, it is found that N doping,
especially edge N species can effectively promote K+
adsorption
with fast reaction kinetics.[36,53,84,86]
Based on this, the material
with 9.34 at% of edge-N shows impressive K-storage stability
of 252 mAh g−1
at 1 A g−1
after 6000 cycles.[53]
Although the
introduction of heteroatoms can improve the performances,
their optimum content and configuration remains elusive.
For example, some carbons have a nitrogen content as high as
22.7 at%, but their performances are moderate.[91]
Therefore,
clarifying the optimized heteroatom content along with their
suitable chemical configuration is worthy of further inves-
tigation. Besides, incorporation of multiple-element doping
is found to work synergistically, such as N/B,[72]
N/P,[37,73,85]
S/N,[39,40,71,77,80,81,83,88,89]
N/O,[30,74,76,78,79,90]
S/O,[82]
and O/F.[28]
Despite of the enhancement in rate and cycling perfor-
mances, these capacitive-dominated carbons generally face
the following challenges. First, the ICE is low, normally in the
Figure 3. Galvanostatic discharge/charge curves of N/O dual-doped hierarchical porous hard carbon a) for the first and following cycles at 50 mA g−1
,
and b) at different current densities. Reproduced with permission.[30]
Copyright 2018, Wiley-VCH. c) The CV profile showing the capacitive contribution
ratio at 0.2 mV s−1
. d) Histogram of capacitive contribution ratios at different scan rates of N/S dual-doped graphene nanosheets. Reproduced with
permission.[40]
Copyright 2020, ACS. A summary of e) capacity, cycle stability, and rate performances and f) ICE of carbon anodes with surface-driven
capacitive K+
storage.
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range of 10–50% (Figure 3f). The large irreversible capacity
loss in the first cycle (Figure 3a) is mainly originated from
the high specific surface area with the irreversible electrolyte
decomposition, instability of formed solid electrolyte interface
(SEI) and the irreversible adsorption of ions at defect sites, etc.
Recently, the ICE can be raised to 60% by regulating the het-
eroatom-doped carbon structure.[36,82] For example, Alshareef
et al. designed a defect-rich, edge-nitrogen doped carbons with
a high ICE of 65%.[36] In fact, there is no need to pursue too
high specific surface area. The material with a specific surface
area as high as 2372.5 m2 g−1 has an ICE of 52.3%.[52] Second,
the sloping discharge/charge profile from such surface-driven
behavior would decrease the average voltage and energy density
of the full cell when paired with a cathode. Cathode materials
are classified into four categories in PIBs, including Prussian
blue and its analogs, layered metal oxides, polyanion com-
pounds, and organic cathode materials.[98–100] The redox poten-
tial depends on the types of cathode. For instance, KVP2O7
deliver higher potential up to 4.2 V.[101] When using highly
porous carbons as cathode materials, potassium ion hybrid
capacitors can be developed,[102–105] which exhibit lower energy
density but higher power density than PIBs. Finally, unsatis-
factory volumetric capacity is another problem of amorphous
carbon material, which is due to the lower packing density with
massive void space (such as hollow carbon of 0.25 g cm−3 and
activated carbon frequently below 0.5 g cm−3),[106,107] as com-
pared to that of graphite (e.g., 2.3 g cm−3).
3. Design of Carbon Materials with Intercalation/
Capacitive Hybrid Storage Mechanisms
Rational balancing of surface-driven and intercalation mecha-
nism is desirable to incorporate the merits of high ICE, low-
potential plateau, remarkable cyclability, and rate capability
(Figure 4). Currently, there are several strategies to construct
carbon structure with balanced intercalation/capacitive pro-
cesses to enhance these performances. The key issue is to
rationally control the crystalline graphitic structure and dis-
ordered microcrystallite nanodomains within the carbons.
Various approaches have been adopted including 1) adjusting
carbonization temperature,[108,109] 2) compositing different pre-
cursors with distinct properties,[110–112]
and 3) constructing pore
and micro/nanostructures in graphitic materials.[23,113–116]
The pyrolysis temperature plays a key role in determining
the carbon microstructure. When using polyacrylonitrile as the
precursor, it was found that the resulting carbon materials tran-
sitioned from disordered structure, partially ordered structure,
finally to ordered structure when increasing the carbonization
temperature from 650, 1250, to 2800 °C (Figure 5a).[108]
The vari-
ation of structure leads to distinct K+
storage mechanism, there-
fore reflecting different discharging/charging behaviors. Com-
pared with disordered carbon (treated at 600 °C) and ordered
graphitic carbon (treated at 2800 °C), partially ordered carbon
at the mediate carbonization temperature of 1250 °C exhibited
intercalation-induced faradaic reaction and surface capacitive
behavior (Figure 5b), thus delivering an optimized capacity of
180 mAh g−1
at 1 A g−1
. This strategy is also applicable to soft
carbon materials. Liu et al. obtained pitch-derived soft carbon
with different structures through a simple thermal control
and found that the structure of materials changes from nearly
amorphous to long-range ordered structures with the increase
of temperature.[109] And the materials with partially disordered
species managed to achieve a sloping/plateau profile, sug-
gesting a high-energy-density anode material.[109] Apart from
carbonization temperature, direct mixing of distinct precur-
sors, which are responsible for the formation of hard, soft or
graphitic carbons, is another promising strategy to balance
the sloping/plateau charge storage behavior. For example, by
mixing graphite with pitch precursor in the desired mass ratio
of 3:1, graphite-soft carbon composites with optimized per-
formances were synthesized.[111] In this respect, the material
not only has the intercalation function of graphite structure,
but also could inhibit the unstable SEI formation and provide
structural protection for graphite layers with the existence of
soft carbon structure. Thus, a high ICE of 67.3%, reversible
capacities of 280.2 mAh g−1 and a plateau-dominated profile
were obtained with the graphite-soft carbon composite.[111]
Meanwhile, compositing graphite with the hard carbon is also
effective. By coating amorphous N-doped carbon nanosheets
on multilayer graphite (random orientations of the flake gra-
phene), the resulting composite exhibits a flat discharge pla-
teau from graphite and enhanced cyclic stability (215.7 mAh g−1
after 1000 cycles at 0.2 A g−1) and ICE (from uncoated graphite
of 43.43% to 61.83%). This is due to the improved K diffusion
coefficient and volume buffer ability.[112]
Moreover, by using metal specie-containing precursor and
special carbonization, graphitic structure can be designed with
few layers graphene microspheres. Lu et al. reported a sulfur-
assisted method (Figure 6a) that changed benzene rings of tet-
raphenyltin into 3D few layer graphene microspheres (FLGMs,
Figure 6b).[113] A dominated low-potential plateau curve was
observed, and an ICE of 94% was achieved assisted by prepo-
tassiation (Figure 6c). Unlike planar graphene and graphite,
FLGMs was not layer-by-layer stacked compactly and thus was
more suitable for facilitating intercalation kinetics and fast
transport for K+, and exhibited an excellent cycling stability with
capacity around 230 mAh g−1 after 1000 cycles at 200 mA g−1
(Figure 6d).[113] Similarly, constructing nanostructure/pores/
defect into graphitic carbons is beneficial for introducing certain
surface-controlled capacitive charge storage to solve the slug-
gish kinetics and poor cyclability involved in K+
intercalation.
As an example, Qian et al. utilized a hydrothermal treatment to
produce a graphitic carbon with nanospring structure.[114]
The
participation of water for the formation of nanospring struc-
ture is important to achieve excellent K-storage performances
(Figure 7a,b). This not only induces a great amount of edge-plane
active sites but also generates the mesoporous structure, which
together make the material remain intact after potassiation com-
pared with the graphite (Figure 7a). Such combination allows
the obtained nanospring structure to integrate the intercalation-
controlled and surface-driven K+
storage behavior, producing a
sloping/plateau discharging characteristics (Figure 7c). Mean-
while, the cyclability was striking with an ultralong life span of
99.9 mAh g−1
after 10 000 cycles at 2000 mA g−1
(Figure 7d). Dou
et al. used potassium hydroxide as etching agent to adjust the
graphite structure through high temperature annealing. Com-
pared to the untreated graphite, the material has a larger layer
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spacing and forms a lot of carbon nanosheets, leading to rapid
transport of K+ and high reversible capacity of 100 mAh g−1
after 100 cycles at 0.2 A g−1.[115] Likewise, graphitic carbon with
engineered defects can help to facilitate fast kinetics and obtain
refined discharging–charging behavior.[23] It is worth noting
that with excessive pore-making, cyclability and rate capability
could be improved, but at the expense of adsorption-dominated
sloping profile and packing density.[64] Evidently, designing spe-
cial micro/nanostructures into graphitic carbons is essential for
ensuring the low-potential plateau, high rate and superior cycla-
bility, and even higher ICE.
4. Carbon Structure with Electrolyte Adaption
For carbon anode materials, electrolyte adaption is also impor-
tant for enhancing the electrode stability and thus K storage
performance. According to the carbon structures with different
K storage mechanisms, several criteria should be considered
for the electrolyte selection, including the solute, solvent, and
concentration. For those intercalation-controlled graphitic car-
bons, large volume variation usually occurs during the intercala-
tion/deintercalation process.[11,15] Thus, to mitigate the cracking
of SEI under stress and impede further passivation and exces-
sive side reactions, the selected electrolyte should help to build
mechanically robust SEI layers or to reduce the volume varia-
tion.[117,118] The first is the selection of K salt in electrolyte. It is
reported that potassium bis(fluorosulfonyl)imide (KFSI)-based
electrolyte can lead to more inorganic-rich SEI originating from
the decomposition of FSI− anions, which is more robust than
KPF6-derived organics/inorganics blended SEI.[119] The second
is the choice of solvent, and it is demonstrated that the use of
ether-based solvent can enable the co-intercalation of K+ along
with solvent molecules into graphitic layers, different from
ester-based electrolyte.[118,120,121] Such co-intercalation leads to
low volume variation and high K+ diffusion coefficient, and
therefore is responsible for high-rate and long cyclability perfor-
mances. For example, Chou et al. studied the electrochemical
performance of graphite in diethylene glycol dimethyl ether
(DEGDME), and found that diffusion kinetics of K+-DEGDME
Figure 5. a) Correlation between microstructure and annealing temperature of carbon materials, b) dQ/dV curves of CNF films at the oxidation. The
inset shows the illustrations of main K+
storage mechanism in different regions. Reproduced with permission.[108]
Copyright 2019, Elsevier.
Figure 4. Schematic illustration of balancing the intercalation-controlled and surface-driven capacitive K+ storage with their respective strengths and
weaknesses.
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complex (about 10−9 cm2 s−1) is faster than K+ (about 10−11 cm2 s−1).
Therefore, the battery exhibited higher rate performance
(77.8 mAh g−1 at 10 Ag−1) and cycling stability (capacity reten-
tion of 88.5% after 100 cycles). Besides, a superior ICE of
62.9% was achieved.[121] The third consideration is electrolyte
concentration. Increasing the salt concentration can facilitate
the decomposition of anion in salt, thus forming more inor-
ganic-containing SEI with robust and dense structure.[122,123]
Fan et al. revealed that an inorganic-rich SEI film was formed in
the graphite electrode with an electrolyte of KFSI/ethyl methyl
carbonate (molar ratio of 1: 2.5), and thus excellent cycling
stability over 17 months was achieved.[122] Likewise, design of
the localized high-concentration electrolyte is also beneficial to
the formation of more inorganic-rich SEI layer.[123]
For surface-driven amorphous carbons, the capacitive K+
storage induces little volume variation during cycling. Con-
sequently, there is relatively less requirement on the electro-
lyte selection. The benchmark electrolyte is KPF6 in carbonate
solvent,[30,36,39] benefiting from the well-established knowl-
edge of lithium-ion battery studies. Considering the existence
of disordered graphitic microcrystallite in hard/soft carbons,
partial intercalation/deintercalation behavior also presents in
surface-driven amorphous carbons, and thus the electrolyte
adaption follows similar rules. For example, Lu et al. reported
Figure 6. a) Schematic illustration for the preparation of few layered graphene microspheres composite (FLGMs). b) TEM image of FLGM. c) The first
discharge/charge curves of FLGMs electrode. d) Cycling performance of FLGM, commercial graphene, and graphite at a current density of 200 mA g−1
.
Reproduced with permission.[113]
Copyright 2021, RSC.
Figure 7. a) Schematic illustration of potassiation process of G (graphite, left) and OGCS (high-oriented mesoporous graphitic carbon nanospring,
right) electrodes. b) HRTEM images of OGCS. c) The first discharge/charge curves of G, GC (graphitic carbon), and OGCS at 50 mAg−1. d) Cycling
performance at a high current rate of 2000 mAg−1. Reproduced with permission.[114] Copyright 2019, Wiley-VCH.
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that O-doped carbon anodes delivered long cycle performance
(capacity retention of 91.5% after 3000 cycles at 0.5 A g−1)
in high concentration electrolyte of 3 m KFSI in 1,2-dimeth-
oxyethane.[124] Despite the striking advantages of KFSI and
ether-based electrolytes, there also exist some adverse effects.
For example, FSI− anion-based electrolytes could corrode Al
foil during anodic polarization,[125] high salt concentration with
denser inorganic-rich SEI probably suppress the cointercalation
in ether-based solvent,[123,126] and ether-based solvent is not tol-
erant to high-voltage cathode. Therefore, judicious electrolyte
selection is necessary in consideration of not only performances
but also carbon structures with different K storage mechanisms.
5. Areal Capacity and Cell Components
For the areal capacity, it is related to the carbon mass loadings
in the electrode. Considering the areal capacity of 4 mAh cm−2
(the value of commercially available LIBs) and the anode
capacity is 300 mAh g−1 for carbon in PIBs, a practical carbon
anode mass loading of 13.4 mg cm−2 is required. However, we
found that amorphous carbons with surface-driven capacitive
storage generally show low mass loading less than 1.5 mg cm−2,
corresponding to low areal capacity less than 0.4 mAh cm−2,
which is far from practical requirements. This might be due
to the low tap density with massive void space, which is also
unfavorable for volumetric capacity. Recently, Lu et al. elevated
mass loading of carbons with hollow architecture to produce
high areal capacity.[83] The areal capacity of 0.47 mAh cm−2 is
obtained when the mass loading is 5.65 mg cm−2 at the cur-
rent density of 0.1 A g−1. While for intercalation-dominated
graphitic carbons, the majority of literatures also adopt a low
mass loading for better electrolyte infiltration. However, there
are a few studies strived to increase the mass loading with
areal capacity close to or even higher than commercialization
requirement. For example, Fan et al. compared cycling perfor-
mance of graphite electrode with different mass loadings.[122]
As expected, with increasing mass loading to 28.56 mg cm−2,
the battery delivered an areal capacity of 7.36 mAh cm−2. As for
lab-level coin-cell assembling, excess of electrolyte dosage and
large volume glass fibers separators are often used for evalu-
ation, which are also unfavorable for large-scale applications.
There are several reports on the investigation of full cells paired
with the cathode, and the average discharge potential is in the
range of 1.1–3.5 V.[99]
6. Conclusion
PIBs have recently attracted much attention due to resource-
abundance of potassium, the low standard redox potential and
high conductivity of K+
-based electrolyte. Particularly, consid-
ering the cost-effectiveness and environmental-friendliness,
carbon materials stand out as the most promising candidates
for PIBs anode. Carbon materials are available in various
forms with different compositions and microstructures, which
have a crucial influence on the performance of potassium
storage. As for graphite materials, their K-storage originates
from K+
intercalation in the graphite layer. However, diffusion
of K+ into graphite layers is sluggish and leads to high volume
expansion coupled with collapse of the graphitic structure,
thus causing insufficient rate capability and poor cyclic sta-
bility. While amorphous porous carbons with expanded
interlayer and tremendous defects/nanovoids can improved
capacity, rate capability, and cyclability via surface-driven capac-
itive K storage. However, the low ICE, high-potential sloping
curves and low material density are problematic for a practical
application. Therefore, well balancing of both mechanisms is
necessary, and further investigation should consider interca-
lation-dominated graphitic carbons with suitable micro/nano-
structures and high tap density, which can deliver high ICE,
low-potential plateau, enhanced cyclability and high energy
density. Carbon model materials with well-defined pore size,
doping, or microstructure may play an important role to
achieve a better understanding of K-storage mechanisms. The
compatibility of electrolyte with different carbon structures
should be also considered. Other critical parameters and full
cell considerations are also of importance. Besides, the devel-
opment of novel analytical techniques has a great future for
better understanding of SEI formation, K+ storage mechanism,
specific electroadsorption sites, K+ mobility and anode degra-
dation mechanisms. With the guidelines for rational design
of these critical parameters, the K+-based energy storage tech-
nology can be further enlightened and fostered into practical
applications within decades.
Acknowledgements
This work was supported by the National Natural Science Foundation
of China (Nos. 51972270 and 21603175), Natural Science Foundation of
Shaanxi Province (No. 2020JZ-07), the Fundamental Research Funds for the
Central Universities (3102019JC005) and the Research Fund of the State Key
Laboratory of Solidification Processing (NPU), China (No. 2021-TS-03), and
F.X. acknowledges support by the Alexander von Humboldt Foundation.
Open access funding enabled and organized by Projekt DEAL.
Conflict of Interest
The authors declare no conflict of interest.
Keywords
carbon anode materials, initial Coulombic efficiency, intercalation,
low-potential plateau curve, potassium-ion batteries, surface-driven
capacitive process
Received: March 13, 2021
Revised: May 18, 2021
Published online: June 12, 2021
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Jiaying Yang received her bachelor degree from Northeast Forestry University in 2019. She
is currently a master degree candidate in the School of Materials Science and Engineering,
Northwestern Polytechnical University, China, under the supervision of Prof. Fei Xu. Her research
interest focuses on the design and synthesis of hollow carbon spheres and their applications in
energy storage.
Fei Xu received his bachelor (2009) and Ph.D. (2015) degrees from Sun Yat-sen University.
Currently, he is an associate professor in the School of Materials Science and Engineering,
Northwestern Polytechnical University. He worked at the Institute for Molecular Science,
Japan (2012–2014), and then the Dresden University of Technology, Germany (2018–2020), as
an Alexander von Humboldt Fellow. His research interests include the design, synthesis, and
functional exploration of molecularly designed porous polymer/carbon architectures and their
composites for challenging energy and environmental issues.
Stefan Kaskel studied chemistry and received his Ph.D. in Tübingen (1997). After a postdoc in
the group of J. D. Corbett, he was a group leader at the Max Planck Institute for Coal Research.
Since June 2004, he is full professor of Inorganic Chemistry at TU Dresden, and since 2008 also
the head of the business unit Chemical Surface Technology at the Fraunhofer Institute (IWS). His
research interests are focused on porous and nanostructured materials for applications in energy
storage, catalysis, batteries, and separation technologies.
Adv. Energy Mater. 2021, 11, 2100856

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