The document discusses using density functional theory calculations to investigate nickel phosphorus trisulfide (NiPS3) monolayers as an anode material for lithium-ion batteries. The results show that lithium atoms can strongly adsorb on both nickel and sulfur sites in the NiPS3 structure, allowing for multiple lithium adsorption. This provides a theoretical capacity of 608 mAh/g, one of the highest among two-dimensional transition metal compounds. The low lithium diffusion barrier of 0.279 eV also enables fast charging and discharging. Overall, the study suggests that creating multiple active sites in electrode materials could significantly boost lithium storage capacity.

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Applied Surface Science
journal homepage: www.elsevier.com/locate/apsusc
Full length article
Boosting the high-capacity with multi-active centers: A first-principles
investigation of NiPS3 monolayer as an anode material
Zhen Maa,b
, Fangfang Wanga
, Min Doua
, Qingnian Yaoa
, Fang Wua,⁎
, Erjun Kanb,⁎
a
College of Information Science and Technology, Nanjing Forestry University, Nanjing, Jiangsu 210037, PR China
b
Department of Applied Physics, Nanjing University of Science and Technology, Nanjing, Jiangsu 210094, PR China
A R T I C L E I N F O
Keywords:
Density functional theory
NiPS3 monolayer
Lithium ion battery
A B S T R A C T
The specific capacity of lithium-ion batteries (LIBs) is one of the key challenge, which determines the perfor-
mance in practical devices. Here, we explored that multi-active centers of electrode materials will significantly
boost the Li storage capacity. Based on the comprehensive first-principles calculations of NiPS3 monolayer, our
results show that Li atom can be strongly adsorbed on Ni and S atoms, which provides the possibility of multi-
adsorption of Li atoms. Significantly, the lowest energy barrier of Li diffusing on NiPS3 monolayer is only
0.279 eV, benefiting the ultrafast charging/discharging process. Moreover, the estimated open circuit voltage is
about 0.55 eV, implying NiPS3 monolayer as a suitable anode material for LIBs. Importantly, the calculated
theoretical specific capacity of NiPS3 monolayer is about 608 (mA h)/g, which is almost the highest value among
two-dimensional transition-metal compounds. Thus, our results clearly show that creating multi-active sites in
transition-metal compounds has great promise for developing anode materials of high-performance LIBs.
1. Introduction
Lithium-ion batteries (LIBs) become more and more important in
todays' electric technology, because they can storage and release sus-
tainable electric power [1–6]. Although LIBs have many advantages,
such as no memory effect and long life, the low storage capacity and
slow charging and discharging significantly degrade their performance
and limit their applications [7]. Therefore, finding substitutable anode
materials with high capacity and high-rate charging/discharging pro-
cess has become the most concerned issue in the field of LIBs [7–10].
Considering the large and flat surfaces, two-dimensional (2D) ma-
terials are of special interest as electrode materials for LIBs, which can
enable fast ion diffusion and offer easy ion insertion channels [7,11].
Graphene, which is the firstly explored 2D material, has been in-
vestigated as the electrode materials [12–14]. Although the flat plane
leads to the fast ion diffusion, the storage capacity is not high due to the
inert chemical surface. Similar situation is also found in other non-
metal 2D materials [15,16]. Recent research shows that germanium
doped graphene can significantly improve the specific capacity, which
may provide one possible way for developing high-performance LIBs
[17].
On the other hand, transition metal (TM) atoms are naturally active
centers for Li adsorption because of their multi-valence states. For ex-
ample, Tang et al. have predicted that 2D Ti3C2 material shows low ion
diffusion barrier and the specific capacity is about 320 mAh/g [18].
Similarly, such phenomena are also found in other 2D TM compounds
[19–22]. However, because of the large atomic mass of TM atoms and
the moderate adsorptive number of Li atoms, improving the specific
capacity of 2D TM compounds seems quite difficult.
Recently, a new kind of 2D materials MnPSe3 exfoliated from its
bulk counterpart was first theoretically proposed by Li et al. [23]. It
indicates that the MnPSe3 nanosheet is an antiferromagnetic semi-
conductor, whereas carriers doping induces its transition from anti-
ferromagnetic semiconductor to ferromagnetic half-metal. In the next
following year, a series of MPS3 (M = Fe, Mn, Ni, Cd, Zn) and MPSe3
(M = Fe, Mn) were successfully synthesized in experiment by Du et al.
[24]. In NiPS3 bulk structure, the metal cations are located in the center
with a [P2S6]4−
framework forming layers that are weakly bonded
together through van der Waals interactions and stacked in an ABC
sequence [25–27]. Recently, previous reports have shown that 2D
NiPS3 can be obtained by exfoliating its bulk with the cleavage energy
of ~0.4 eV. It demonstrated that exfoliated 2D NiPS3 nanosheet can
provide higher strain relaxation, larger surface area and shorter charge
carrier diffusion length for electrochemical applications in experiments
[28–30]. Also, 2D NiPS3 has been demonstrated with big potential in
energy field and spintronics [25,26]. As an anode for LIBs, NiPS3 na-
nosheets show high-performance electrochemical active properties in
terms of high reversible capacity and rate capabilities [31,32].
https://doi.org/10.1016/j.apsusc.2019.143534
Received 7 June 2019; Received in revised form 17 July 2019; Accepted 30 July 2019
⁎
Corresponding authors.
E-mail addresses: fangwu@mail.ustc.edu.cn (F. Wu), ekan@njust.edu.cn (E. Kan).
Applied Surface Science 495 (2019) 143534
Available online 30 July 2019
0169-4332/ © 2019 Elsevier B.V. All rights reserved.
T

2. Motivated by the experimental results, here, we investigated the
possibility of multi-active centers for LIBs. Previously, the lithium can
be stably adsorbed on Ni, S or P atoms in NiO [33], MoS2 [34] and black
phosphorus [35]. It means that Li atoms may be simultaneously ad-
sorbed on the Ni and S atoms on the NiPS3 monolayer with suitable
electronic states. Through comprehensive first-principles calculations,
we demonstrated that Li atoms are strongly adsorbed on Ni and S
centers because of the partial covalence character of NieS bonds.
Consequently, the specific capacity of NiPS3 is almost the highest one
for all realized 2D TM compounds. Therefore, our results provide an
important way to boost the specific capacity, benefiting the develop-
ment of high-performance LIBs.
2. Computational methods
Our first-principles calculations were based on density functional
theory (DFT), as implemented in Vienna ab initio simulation package
(VASP) [36]. The electron exchange-correlation interaction was de-
scribed by the generalized gradient approximation (GGA) [37] in the
scheme of Perdew–Burke–Ernzerhof (PBE). To appropriately account
for the strongly correlated electrons, the GGA + U method was used to
deal with the partially filled d orbitals of the Ni atoms. We simulated Li
adsorption and diffusion on NiPS3 monolayer with a 2 × 2 × 1 super-
cell that one Li atom was used, and the interactions between two Li
atoms can be ignored. At the same time, a vacuum space of 20 Å was set
between NiPS3 layers to keep away from mirror interactions. The
convergence criteria of was set to be 1 × 10−5
eV in energy and
0.01 eV/Å in force, respectively. In all calculations, the cut-off energy
was placed to be 500 eV for the plane-wave basis. The Brillouin zone
was performed using a Monkhorst–Pack grid of 3 × 3 × 1 in the com-
putations of structure relaxation and self-consistent field (SCF). We
investigated the diffusion barrier for Li in the NiPS3 monolayer using
the climbing image nudged elastic band (CI-NEB) method [38,39],
which was known as an effectively tool for finding the energy of tran-
sition state (TS) [40,41]. The purpose of the NEB method is to find
saddle points and minimum energy paths (MEPs) between a given in-
itial state (IS) and final state (FS).
3. Results and discussions
3.1. Structural and electronic properties of monolayer NiPS3
Firstly, we focused on the structural and electronic properties of
NiPS3 monolayer. Similar with that of phosphorene, NiPS3 is a puckered
structure instead of a planar structure, as shown in Fig. 1(a). In order to
determine the ground state of NiPS3, SCF calculations of the anti-
ferromagnetic (AFM) and ferromagnetic (FM) states were both carried
out. It indicated that the NiPS3 monolayer tends to be antiferromagnetic
(AFM) coupling, which is more stable than the ferromagnetic (FM) state
as that in MnPS3 monolayer. Energy band shows that the 2D NiPS3
monolayer is an indirect semiconductor with a band gap of 1.63 eV,
consistent with the previous report [25]. To get further information
about the energy band, we plotted the partial charge densities of the
conduction band minimum (CBM) and valence band maximum (VBM),
as shown in Fig. 1b. Interestingly, both the CBM and VBM are domi-
nated by the covalence components of NieS bonds. It is well known
that fully occupied orbitals will become inert for Li adsorption. Thus,
the partial covalence characters of NieS bonds implies that both Ni
atoms and S atoms may be the possible active centers for Li adsorption.
3.2. Li adsorption and diffusion on monolayer NiPS3
In order to avoid interaction between lithium atoms, we used a
2 × 2 × 1 NiPS3 supercell. According to the symmetry of the crystal
structure, there are two possible positions which the lithium atom can
be doped in the 2D NiPS3, namely, the top site directly above one Ni
atom (H1) or S atom (H2), as shown in Fig. 2(b). The preference sites
for Li adsorption is judged by the adsorption energy, which is defined as
= − −
E E E nE n
( )/
b NiPS Li NiPS Li
n
3 3 (1)
where Eb is the binding energy of Li on NiPS3 monolayer, ENiPS3 is the
energy of NiPS3 monolayer supercell without Li atom, ELi is the total
energy of a single Li atom in a bulk BCC structure, and ENiPS3Lin
is the
total energy of NiPS3 monolayer with adsorbed n Li atoms. For ad-
sorption on H1 site, Li atom is located above Ni atom, and the corre-
sponding adsorption energy is −1.06 eV. For H2 site, Li atom is ad-
sorbed on the top of S atoms, and the calculated adsorption energy is
−0.88 eV, slightly smaller than that on Ni atoms.
Now, we discuss the intrinsic mechanism response for the different
adsorptive behavior. Our previous calculations on the pristine NiPS3
monolayer have explored that both the CBM and the VBM are con-
tributed by the covalence character of NieS bonds. The partial density
of states (PDOS) of Li adsorption indicates that charge has been trans-
ferred from Li atom into NieS covalence bonds. As shown in both
Fig. 3a and b, Li atom denotes the electrons to the Ni and S atoms in all
two adsorptive configurations with quite similar structures. Therefore,
both Ni atoms and S atoms can be the active sites of Li adsorption for
LIBs. The strength of Li and NiPS3 monolayer bonding determines the
adsorptive stability. Consequently, when Li is adsorbed on Ni or S
atoms, the structures become more stable.
On the other hand, rate of charging and discharging in LIBs is re-
lated to the mobility of Li ions on the NiPS3 monolayer. In order to
study how fast the Li atom diffuses on monolayer NiPS3, we calculated
the diffusion energy barrier along possible diffusion paths, which was
defined by energy difference between the relative energies on the sites
along the diffusion pathway and that of the corresponding initial state.
We selected different diffusion pathways: E1 → E2, E1 → E4, as shown
in Fig. 4(a). The corresponding energy barriers of diffusion path as a
function of the distance are summarized in Fig. 4(b). The minimum
diffusion barrier of the Li atoms diffusion on the NiPS3 monolayer is
about 0.279 eV, which obviously benefits the real application as an
anode material.
3.3. Average open circuit voltage and Li storage capacity of monolayer
NiPS3
The open circuit voltage (OCV) data is a main index to characterize
the performance of lithium batteries. To obtain the OCV, we consider
the reaction
+ →
NiPS xLi Li NiPS
x
3 3 (2)
where x is the number of adatoms inserted in the unit cell of NiPS3. In
theory, the OCV can be computed from the energy difference if the
volume and entropy effects are not considered. In other words, the
binding energy can be interpreted as OCV used in the battery field:
≈ −
∆
= −
− +
E
x
E xE E
x
OCV
( )
f Li NiPS Li NiPS
x 3 3
(3)
where ELixNiPS3
is the total energy of the composite system with x metal
atoms adsorbed in the unit cell of NiPS3, ELi is the total energy of a
single Li atom in a bulk BCC structure, and ENiPS3
is the total energy of
an isolated NiPS3.
Based on this method, a variety of properties related to Li-Batteries
can be predicted by filtering a series of LixNiPS3 ratios (x values). In
order to evaluate the average OCV and theoretical Li storage capacity of
the NiPS3 monolayer, it is necessary to estimate the Li adsorption ca-
pacity with the possible maximum containment. We considered a series
of structures basing on the stoichiometric ratio of LixNiPS3 (1 ≤ x ≤ 5),
as shown in Fig. 5. Considering that Li atoms can form strong adsorp-
tion with Ni and S atoms, the double-side adsorption of Li to NiPS3 has
the highest ratio (xmax) about 5, as shown in Fig. 5(c), and the calcu-
lated open circuit voltage is 0.55 eV. A weak binding is observed for 11
Z. Ma, et al. Applied Surface Science 495 (2019) 143534
2

3. Li atoms adsorbing on NiPS3, which indicates that the structure of
LixNiPS3 becomes less stable due to the strong electrostatic repulsive
interactions between adjacent Li atoms. Higher concentrations of Li
atoms inserting into a single layer of NiPS3 will cause bond breaking,
which destroys the monolayer NiPS3 structure and the lithium atom
insertion/extraction process is irreversible. The theoretical specific ca-
pacity of NiPS3 LIBs depending on the above structures is about 608
(mA h)/g, which is almost the highest one among 2D transition metal
compounds [18,22,42–46] and obviously higher than that of the cur-
rent commercial electrode material TiO2 (< 200 (mA h)/g) [47]. We
summarize the estimated open circuit voltage and theoretical specific
capacity of some available 2D materials in Table 1.
4. Discussion and conclusion
Taking 2D NiPS3 monolayer as an example, we have demonstrated
the importance of multi-active centers for developing high-performance
LIBs. To be noted, similar with the structure of NiPS3 monolayer, there
are many other TMPS3 monolayers, which are the potential candidate
anode materials. Furthermore, since multi-active centers can sig-
nificantly improve the specific capacity, synthesizing 2D TM com-
pounds with more covalence character will benefit the specific capacity.
In conclusion, with first principles calculations, we systematically
studied the Li adsorption and diffusion on NiPS3 monolayer, and then
explored the potentials of NiPS3 monolayer as Li ion battery anodes. We
found that Li atoms can strongly adsorbed on Ni and S atoms because of
the covalence character of NieS bonds. Because of the multi-active
centers, the maximum of Li adsorption can reach the stoichiometric
Fig. 1. (a) Top and side view of 2D NiPS3 monolayer. (b) Calculated band structures and partial charge density of conduction band minimum (CBM) and valence
band maximum (VBM) for the 2D NiPS3 monolayer. The blue, pink and yellow balls denote nickel, phosphorus and sulfur atoms, respectively.
Fig. 2. (a) Top and side views of single lithium atom
adsorbed on NiPS3 monolayer. Taking Li adsorption
above Ni atom as an example. (b) The H1 and H2
indicate two representative sites for Li atom ad-
sorption. The top site directly above one Ni atom
(H1), and S atom (H2). The blue, pink, yellow and
red balls denote nickel, phosphorus, sulfur and li-
thium atoms, respectively.
Z. Ma, et al. Applied Surface Science 495 (2019) 143534
3

4. Fig. 3. Calculated total density of states (TDOS) and partial density of states (PDOS) plot of the Li-absorbed NiPS3 monolayer of (a) H1 site, and (b) H2 site, the black
dotted lines denote the locations of the Fermi level.
Fig. 4. (a) Considered diffusion pathways for Li atom on NiPS3 monolayer. (b) (c) Diffusion barriers of Li atom on NiPS3 monolayer from E1 to E2(E4). E1, E2, E3, E4
indicate the initial and final position of different diffusion pathways. The blue, pink and yellow balls denote nickel, phosphorus and sulfur atoms, respectively.
Fig. 5. Top and side view of the structure that (a) six Li atoms, (b) eight Li atoms and (c) ten Li atoms are bilaterally adsorbed above the Ni and S atoms of the NiPS3
monolayer double-side. The blue, purple, yellow and red balls denote nickel, phosphorus, sulfur and lithium atoms, respectively.
Z. Ma, et al. Applied Surface Science 495 (2019) 143534
4

5. ratio of Li5NiPS3, and the specific capacity of NiPS3 monolayer is esti-
mated as 608 (mA h)/g, almost the highest one among 2D TM com-
pounds. The calculated Li diffusion barrier can be as low as 0.279 eV,
implying the possibility of fast rate-performance. Furthermore, the
calculated average intercalation potential is only 0.55 eV, which is
quite suitable for anode materials. Considering the various compounds
of TMPS3, our results not only explored a new mechanism in developing
high-performance LIBs, but also offered many potential candidate
anode materials.
Author contributions
The manuscript was written through contributions of all authors. All
authors have given approval to the final version of the manuscript.
Acknowledgment
The work was supported by the NSFC (11474165), the Outstanding
Youth Fund of Nanjing Forestry University (NLJQ2015-03), and the
Postgraduate Research & Practice Innovation Program of Jiangsu
Province. We also acknowledge the support from the Shanghai
Supercomputer Centre.
Declaration of competing interest
There are no conflicts to declare.
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Table 1
The estimated open circuit voltage (V) and the theoretical specific capacity
(mA h/g) of some investigated promising anode materials for LIBs.
Species Open circuit voltage Theoretical specific capacity Ref.
NiPS3 0.55 608 This work
Ti3C2 0.62 320 18
Mo2C 0.14 526 42
VS2 0.93 466 43
Zr2B2 0.63 526 45
orth-Mo2B2 0.41 444 44
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