Applied Surface Science 563 (2021) 150264

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Applied Surface Science

journal homepage: www.elsevier.com/locate/apsusc

Full Length Article

First-principle study of electronic properties and quantum capacitance of

lithium adsorption on pristine and vacancy-defected O-functionalized
Ti2C MXene
Xiao-Hong Li a, b, *, Shan-Shan Li a, Xing-Hao Cui a, Rui-Zhou Zhang a, Hong-Ling Cui a
a
College of Physics and Engineering, Henan University of Science and Technology, Luoyang 471023, China
b
Provincial and Ministerial Co-construction of Collaborative Innovation Center for Non-ferrous Metal New Materials and Advanced Processing Technology, Luoyang
471023, China

A R T I C L E I N F O A B S T R A C T

Keywords: The electronic structure, surface charge storage and quantum capacitance of lithium adsorption on pristine and
MXene vacancy-defected Ti2CO2 MXene are theoretically investigated by density functional theory. Negative adsorption
Density functional theory energies (Eads) of pristine Ti2CO2 (PT) and C-vacancy Ti2CO2 (VT) monolayers adsorbed by Li atom indicate that
Vacancy-defect
the adsorption processes of Li atom on systems are exothermic and favorable to adsorption. The most stable
Electronic properties
Quantum capacitance
configurations are confirmed for the Li-adsorbed PT and VT monolayers. Pristine Ti2CO2 (PT) monolayer is a
semiconductor with the bandgap of 0.2035 eV, while C-vacancy Ti2CO2 (VT) monolayer has the metallic nature.
The adsorption of Li atom on PT monolayer makes the system undergo the semiconductor-metal transition, while
the adsorption of Li atom on VT monolayer doesn’t change the metal nature. The magnetism is observed for the
system with Li atom directly adsorbed on top of hollow site (VT-LH), and the total magnetic moments is 0.18 μB.
Charge density differences of the adsorption of Li atom on PT and VT monolayers are further explored. The
introduction of C-vacancy improves the quantum capacitances of the Li-adsorbed systems at 0 V, and Li-adsorbed
PT and VT monolayers have high quantum capacitance because of the large density of states near Fermi level,
and are suitable for cathode material.

1. Introduction promising anode material for LIBs. Researches have confirmed that two-
dimensional (2D) materials, such as silicone, fullerene, MoS2, and MnO2,
Lithium-ion batteries (LIBs) have attracted most attention and are potential anode materials for LIBs [12–15].
become the main power sources for portable electronic vehicles, 2D materials have attracted great interest because of their large
implantable medical systems because of their compact size and high surface area, excellent physical and chemical properties [16]. MXene, a
energy density [1,2]. However, the safety and cost issues of LIBs hinder new family of 2D materials, was synthesized by etching the A layer from
their development [3], so the main challenge for LIBs is to search or the three-dimensional (3D) Mn+1AXn phases [17] by hydrofluoric acid
develop new materials with high-rate capability and safety, better (HF) solutions [18]. In etching process, O, F, and OH groups are often
cycling performance and stability, and higher capacity [4–7]. Graphite terminated on the surface of MXene, so the formula of functionalized
is the first anode material for commercial LIBs, but its low specific ca­ MXene is Mn+1XnTx, where T is the surface termination.
pacity limits its performance in LIBs. Recent experiments indicated that MXenes are functionalized by the surface groups such as F, OH and O
lowering the dimensionality of materials can improve their performance groups, which depends on etching agents and subsequent treatment.
of energy storage. For example, graphene has been investigated to Research indicates that O and OH terminated MXenes are more stable
replace graphite as anode in LIBs [8,9]. Yu [10] investigated the gra­ [19], and OH groups is easily be converted into O terminations [20].
phene with nitrogen-doped defects. He thought that the performance of MXenes with O terminating group are suitable for high performance,
N-doped graphene is improved as anode materials for LIBs. Yu [11] also while MXenes with OH and F terminating groups are not. There are
reported that graphenes and their allotropes such as graphenylene are other effective process to turn F or OH groups to O groups on MXenes:

* Corresponding author at: College of Physics and Engineering, Henan University of Science and Technology, Luoyang 471023, China.
E-mail address: lorna639@126.com (X.-H. Li).

https://doi.org/10.1016/j.apsusc.2021.150264
Received 26 April 2021; Received in revised form 24 May 2021; Accepted 29 May 2021
Available online 6 June 2021
0169-4332/© 2021 Elsevier B.V. All rights reserved.
X.-H. Li et al. Applied Surface Science 563 (2021) 150264

(1) annealing treatment under vacuum or inert atmosphere [20,21], (2) Table 1
N-Butyllithium treatment [22]. Therefore, we chose O-functionali­ The bond length d (Å), the nearest distance (dLi-Ti2CO2), adsorption energy (Eads,
zed MXene to investigate its properties. Gao et al. [23] investigated the eV) and charge transfer (CT) for PT monolayer adsorbed by Li atom.
adsorption behavior of mercury on Ti2CO2. Yu [24] investigated the System dTi-O dTi-C Eads dLi-Ti2CO2 CT(e)
adsorption behavior of sodium on Ti3C2Tx (Tx = O, F, OH). Xin et al. [25] PT 1.974 2.178 – – –
reported the quantum capacitance and work function of Nbn+1Cn PT-LT 2.052 2.076 − 3.278 2.082 0.900
MXenes. PT-LC 2.025 2.163 − 3.542 1.998 0.887
MXene has been regarded as a novel electrochemical double-layer PT-LO 2.056 2.151 − 2.935 1.734 0.916
capacitor (EDLC) because of its superior electrochemical properties
[26]. EDLC is one kind of supercapacitor with high capacitance, long-life
monolayers, respectively. ELi is the total energy of bulk Li.
span, and better electrical conductivity. However, many practical ap­
The charge redistribution between Li atom and substrate are visu­
plications are limited because of poor volumetric surface area and
alized by charge differential density as follows:
insufficient capacitance [27]. So it is necessary to search better electrode
materials with high capacitance. For EDLCs, the energy-storage perfor­ Δρ = ρLi/substrate − ρsubstrate − ρLi (3)
mance can be evaluated by the total capacitance (CT) of materials with
The differential quantum capacitance is defined as:
1/CT = 1/CQ + 1/CD [28,29]. Here, CQ reflects the quantum capaci­
tance, which depends on the intrinsic electronic structure [30], while CD dσ
Cdiff = = e2 DOS( − Ve) (4)
reflects the electro-chemical double-layer capacitance, which depends dϕG
on the electrode–electrolyte interfacial structure [31]. Recent investi­
where dσ and dϕG are the differentials of local charge density and
gation [32] indicates that CT is mainly determined by CQ. And CQ can be
potential. The excess charge density can be expressed as [46]
improved by vacancy effect [33,34] and co-doping [35].
∫ +∞
Vacancy defect is inevitably introduced in MXene during the process
of etching and can affect the properties and functionalities of 2D ma­ ΔQ = D(E)[f (E) − f (E − eϕG )]dE (5)
− ∞
terials [36,37]. Sang et al. [24] thought that vacancy can influence the
metallic nature of Ti3C2Tx and the surface morphology. Wang et al. [38] where D(E), ƒ(E) and E are DOS, Fermi-Dirac distribution function
reported the related properties of vacancy-defected Ti2CO2. Hu et al. and the energy relative to the Fermi level, respectively. e is elementary
[39] thought that C-vacancy is much easier to from in MXene when electric charge. For two-dimensional materials, Cdiff can be [32]:
compared with other 2D materials. Our previous studies also reported ∫ +∞
the adsorption of NH3 gas on pristine and vacancy-defected Ti2CO2 Cdiff = e2 D(E)FT (E − eϕG )dE (6)
− ∞
monolayer [40,41].
As electrode materials for LIBs, MXene has high Li storage capacity Where FT(E) is the thermal broadening function and is obtained by
and is comparable to graphite [42]. Little reports on the performance of [47]
lithium storage are available. With this motivation, the first-principles (
E
)
calculation is used to study the electronic properties, surface storage FT (E) = (4κB T)− 1 sech2 (7)
2κB T
charge and quantum capacitances of pristine Ti2CO2 (PT) and C-vacancy
Ti2CO2 (VT) monolayers absorbed by Li atom. The effect of vacancy and Here, κB is the Boltzmann’s constant, T is equal to 300 K. The storage
adsorption on electronic structure and quantum capacitance is further charge on electrode surface is obtained by
investigated. Our investigation will help to develop or design better ∫ ϕG
performance of MXene-based electrode materials. Q= Cdiff dϕ (8)
0

2. Computational details The integrated quantum capacitance (Cint) can be calculated by

integrating Cdiff over the charge/discharge cycle [48].
All calculations are performed by density functional theory (DFT) Q 1 V
∫
(9)
′ ′
using the projected augmented wave method implemented in the Vienna Cint (V) = =
V Ve 0
Cdiff (V )dV
Ab-initio Simulation Package (VASP) [43]. The generalized gradient
approximation (GGA) with the parametrization scheme of Perdew-
3. Results and discussion
Burke-Ernzerhof (PBE) is selected for the exchange–correlation poten­
tial [44]. The plane wave basis with cut-off energy of 650 eV is used. A
3.1. Structural and electronic properties of PT monolayer absorbed by Li
vacuum layer of 25 Å was set in the direction normal to MXene layers.
atom
The convergences in energy and force are 1.0 × 10-7 eV and 0.01 eV Å− 1,
respectively. The spin-polarization is considered. A 7 × 7 × 1 k-grid and
Here, we considered all the possible initial adsorption configura­
11 × 11 × 1 k-grid are adopted for the geometry optimization and the
tions: Li atom directly adsorbed on top of Ti (PT-LT), C (PT-LC), O (PT-
static self-consistent-field calculation, respectively. 3 × 3 × 1PT and VT
LO) atoms, bridge sites between neighboring Ti and C (PT-LTC), Ti and
(one carbon removal from PT monolayer) supercells were used to
O (PT-LTO), C and O (PT-LCO) atoms. Figure S1 in Supporting Infor­
investigate the adsorption of Li atom. The van der Waals interaction is
mation presents the initial structures of PT-LT, PT-LC, PT-LO, PT-LTC,
considered using a dispersion correlation term with the DFT-D3 method
PT-LTO, and PT-LCO monolayers. For PT-LTO monolayer, the Li atom is
[45].
on the top site of Ti atom after optimization, so PT-LTO monolayer has
The adsorption energy (Eads) can be used to assess the stability of Li
changed to PT-LT monolayer after optimization. Similarly, for PT-LTC
adsorbed on PT and VT monolayers. The adsorption energy (Eads) of a Li
and PT-LCO monolayers, Li atom is on the top of C atom after optimi­
atom on PT or VT monolayer is calculated by the following equations:
zation, which means that PT-LTC and PT-LCO monolayers have changed
EP− ads (Li) = ELi− PT − EPT − ELi (1) to PT-LC monolayer. So Table 1 only presents the lattice constant, Eads,
and the charge transfer of PT-LT, PT-LO, and PT-LC monolayers. In order
EV− ads (Li) = ELi− VT − EVT − ELi (2) to have a comparison, the related data of PT monolayer are also included
in Table 1. Fig. 1 presents the optimized structures of PT-LT, PT-LC, and
where EP-ads and EV-ads are the total energies of Li adsorbed on PT and
PT-LO monolayers in 2 × 2 × 1 supercell in order to make the structures
VT monolayers, respectively. EPT and EVT are the energies of PT and VT

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X.-H. Li et al. Applied Surface Science 563 (2021) 150264

Fig. 1. The optimized structures of PT (a), PT-LT (b), PT-LO (c), and PT-LC (d) monolayers. The red, light blue, brown, and light green balls denote the oxygen,
titanium, carbon, and lithium atoms, respectively.

(a) PT (b) PT-LT

1 1

Energy (eV)
Energy (eV)

0 0
0.2035 eV

-1 -1

(c) PT-LC (d) PT-LO

1 1
Energy (eV)
Energy (eV)

0 0

-1 -1

Fig. 2. The band structures of PT (a), PT-LT (b), PT-LC (c), and PT-LO (d) systems.

more clearly. PT-LT, PT-LC and PT-LO monolayers, respectively, which are in agree­
From Table 1, the Eads of PT-LT, PT-LC, and PT-LO monolayers are ment with the Li-O bond length of Stournara’s results [50]. Compared
− 3.278, − 3.542, and − 2.935 eV, respectively, which are larger than with PT monolayer, for PT-LT monolayer, the adsorption of Li atom
that (-1.34 eV) of graphene absorbed Li atom [49]. This indicates that makes Ti move downwards, resulting in the increase of Ti-O bond and
the interactions between Li atom and PT monolayer for the three sys­ the decrease of Ti-C bond. For PT-LO monolayer, the O atom absorbed
tems are stronger than that between Li atom and graphene. Negative Eads by Li atom moves upwards slightly, resulting in the increase of Ti-O
of the three systems indicates that the adsorption process of Li on PT bond. For PT-LC monolayer, we can notice that the adsorption of Li
monolayer is exothermic and favorable to adsorption. From the analysis atom has smaller effect on the PT monolayer when compared with PT-LT
of bonding of adsorbed atoms, Li atom has the interaction with Ti and and PT-LO monolayers. The calculated Bader charge transfer of Li atom
neighboring three O atoms for PT-LT monolayer, while Li atom only has is 0.900, 0.887, and 0.916 e for PT-LT, PT-LC, and PT-LO monolayers,
the interaction with O atom for PT-LO monolayer. For PT-LC system, Li respectively. Positive Q values indicated that Li atom acts as donors, and
atom has the interaction with neighboring three Ti and three O atoms. PT monolayer acts as acceptors. The larger Eads and Q indicate the strong
So the minimum Eads (-3.542 eV) of PT-LC monolayer indicates that PT- interaction between Li atom and PT monolayer.
LC monolayer is the most stable adsorption configuration and has Fig. 2 plots the band structures of PT, PT-LT, PT-LC, and PT-LO
stronger interaction. monolayers. PT monolayer is an indirect semiconductor, and the band
The nearest bond distances between Li atom and PT monolayers are gap is 0.2035 eV from M to Γ, which is close to the results of Xie et al.
all Li-O bonds, and the bond lengths are 2.082, 1.998, and 1.734 Å for [51] and Zha et al. [52]. The adsorption of Li atom on PT monolayer

3
X.-H. Li et al. Applied Surface Science 563 (2021) 150264

(a) PT (b) PT-LT

40
Ti Ti
40 O O
C C

DOS (arb. unit)

Li
DOS (arb. unit)

20
20

0 0
-10 -5 0 -10 -5 0
Energy (eV) Energy (eV)

(c) PT-LC (d) PT-LO

60
40 Ti Ti
O O
C C
DOS (arb. unit)

Li
Li DOS (arb. unit) 40

20
20

0 0
-10 -5 0 -10 -5 0
Energy (eV) Energy (eV)
Fig. 3. The PDOS of PT (a), PT-LT(b), PT-LC(c), and PT-LO(d) monolayers.

Fig. 4. The optimized structure of VT (a), VT-LH (b), VT-LC1(c) and VT-LC (d) monolayers. The red, light blue, brown, and light green balls denote the oxygen,
titanium, carbon, and lithium atoms, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of
this article.)

makes the Fermi level shift up into the conduction band and results in 3.2. Structural and electronic properties of VT monolayer absorbed by Li
the metallization of PT-LC, PT-LT, and PT-LO monolayers. No magne­ atom
tism is found for the four monolayers.
The partial density of states (PDOS) of PT, PT-LT, PT-LC, and PT-LO For VT monolayer, all the possible initial adsorption sites are Li atom
monolayers are presented in Fig. 3. It is noted that there is little directly adsorbed on top of Ti (VT-LT), C (VT-LC), O (VT-LO) atoms and
contribution of Li atom at Fermi level. The strong interaction between Li hollow (VT-LH), bridge sites between neighboring Ti and C (VT-LTC), Ti
atom and PT monolayer induced the blue shift of Fermi energy, which and O (VT-LTO) atoms. Figure S2 in Supporting Information presents the
results in the metallic nature of PT-LC, PT-LT, and PT-LO monolayers. initial structures of VT-LT, VT-LC, VT-LO, VT-LTC, VT-LTO, and VT-LH
The symmetry of spin-up and spin-down DOS in Fig. 3 further confirms monolayers. For VT-LTO and VT-LTC monolayers, the Li atom moves to
the non-magnetism of the four monolayers. the top of the nearest C atom after optimization, so VT-LTO and VT-LTC
monolayers change to VT-LC monolayer. For VT-LT monolayer, the Li
atom is on the top of hollow sites, so VT-LT monolayer changes to VT-LH

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X.-H. Li et al. Applied Surface Science 563 (2021) 150264

Table 2 and VT-LC1 monolayers, when compared with that of VT monolayer. All
The bond length (Å), the nearest distance (dLi-Ti2CO2), adsorption energy (Eads, adsorptions of Li on VT monolayers are all exothermic because of the
eV) and charge transfer (CT) for Li atom absorbed on VT monolayer. negative Eads. VT-LC1 monolayer is the most favorable adsorption
System Ti-O Ti-C Eads dLi-Ti2CO2 CT(e) configuration and has the stronger interaction because of the minimum
VT 1.946 2.181 – – –
Eads (-3.669 eV) among the three monolayers. The Eads of VT-LH, VT-LC,
VT-LH 1.996 2.131 − 3.498 1.965 0.883 and VT-LC1 monolayers are all larger than that (-2.66 eV) of mono-
VT-LC 1.999 2.121 − 3.435 1.947 0.885 vacancy graphene absorbed Li atom. This indicates that the in­
VT-LC1 2.033 2.127 − 3.669 1.985 0.885 teractions between Li and VT monolayers are stronger than that between
Li and graphene [53].
We notice that the charge transfers of Li atom on VT monolayer are
monolayer. For VT-LO monolayer, the O atom moves to the top of C
all about 0.88 e for the three monolayers, which indicates the similar
atom far away from C-vacancy and changes to VT-LC1 monolayer (the
interactions between Li atom and VT monolayer. Li atom is donor with
optimized structure of VT-LO monolayer).
positive charge transfer, while VT monolayer is acceptor. For Li-
In order to make the structure more clearly, Fig. 4 presents the
adsorbed VT monolayers, the introduction of C-vacancy makes the
optimized structures of VT, VT-LH, VT-LC, and VT-LC1 monolayers in 2
transitional-metal atom Ti coupled with the localized defect states,
× 2 × 1 supercell.
which results in large adsorption energies. So the Li-adsorbed VT
Table 2 lists the related parameters of VT, VT-LH, VT-LC, and VT-LC1
monolayers are generally more stable with the smaller Eads, when
monolayers. VT monolayer is obtained by the removal of a carbon atom
compared with the Li-adsorbed PT monolayer.
from PT monolayer. Compared with PT monolayer, the introduction of C
The band structures of VT, VT-LH, VT-LC, and VT-LC1 monolayers
vacancy results in the charge redistribution, the decrease of Ti-O bond
are listed in Figure S3 of Supporting Information. Fig. 5 plots the density
length and the increase of Ti-C bond length. From Table 2, we note that
of states of VT, VT-LH, VT-LC, and VT-LC1 monolayers in order to
the adsorption of Li atom on VT monolayer makes the increase of Ti-O
explore the interaction between the states of Li and VT monolayer.
bond length and the decrease of Ti-C bond length for VT-LH, VT-LC

VT VT-LH
30
40 Ti-up
Ti-down
O-up 15
20 O-down
C-up
DOS (arb. unit)
DOS (arb. unit)

C-down 0
Ti-up
0
-15 Ti-down
O-up
O-down
-20 -30 C-up
C-down
Li-up
-45
-40 Li-down

-10 -5 0 -10 -5 0
Energy (eV) Energy (eV)

(a) (b)
VT-LC VT-LC1
Ti-up Ti-up
30 Ti-down Ti-down
O-up O-up
O-down 30 O-down
C-up
DOS (arb. unit)

15 C-up
C-down
DOS (arb. unit)

Li-up C-down
Li-down Li-up
0 Li-down
0

-15

-30
-30

-10 -5 0 -10 -5 0
Energy (eV) Energy (eV)

(c) (d)
Fig. 5. The density of states of VT (a), VT-LH (b), VT-LC (c), and VT-LC1 (d) monolayers.

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X.-H. Li et al. Applied Surface Science 563 (2021) 150264

Fig. 6. The charge density differences of PT-LT (a), PT-LO (b), PT-LC (c), VT-LH (d), VT-LC (e), and VT-LC1(f) monolayers. The isosurface value was set as 0.02e/Å3.
Yellow (blue) represents the accumulation (depleting) of electrons. The red, light blue, brown, and light green balls denote the oxygen, titanium, carbon, and lithium
atoms, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Combine with Figure S1 and Fig. 5, the introduction of C vacancy in­ on PT and VT monolayers, charge depletion is mainly from Li atom, little
duces the transformation of PT monolayer from semiconductor to from the upper Ti atoms, while charge accumulation appears around O
conductor. Compared with VT monolayer, the adsorption of Li atom atoms adjacent to the adsorption atom. The closer to the Li atom, the
doesn’t change the metal nature of VT monolayer. All Li-adsorbed VT more charge the O atoms gain. No visible charge transfer is observed
monolayers have the metal characteristics. From Fig. 5(a), no magne­ around the lower O and Ti atoms. The Li atom and the base PT (or VT)
tism is observed for VT monolayer, while the adsorptions of Li atom on monolayer are both stronger polarized because of the charge redistri­
VT monolayer don’t change the magnetism of VT-LC and VT-LC1 bution, which is consistent with the larger adsorption energies.
monolayers. The magnetism is observed for VT-LH monolayer due to
the asymmetry of spin-up and spin-down DOS near the Fermi level, with
the total magnetic moments of 0.18 μB. From Fig. 5(b), the magnetism is 3.3. Quantum capacitance of PT and VT monolayers absorbed by Li atom
mainly from the Ti and O atoms.
In order to further explore which states have the contribution to the Fig. 7 presents the Cdiff, Cint and Q under the potential from − 0.6 V to
magnetism, the partial density of state of VT-LH monolayer is illustrated + 0.6 V. From Fig. 7(a), the adsorption sites of Li atom have little effect
in Figure S4 of Supporting information. The magnetism mainly origi­ on the Cdiff of PT-LO, PT-LT, and PT-LC monolayers. The Cdiff exhibits
nated from Ti-d and O-p states. The spin-up and spin-down states of Li the U-shape feature in the negative potential range. For PT-LO mono­
and C atoms are slightly asymmetrical because of the interaction be­ layer, Cdiff exhibits the maximum of 3372.3 μF/cm2 at 0 V and the
tween Li and VT monolayer. The spin density distribution of VT-LH minimum of 58.8 μF/cm2 at − 0.24 V. For PT-LT monolayer, the
monolayer is presented in Figure S4(b) in order to further understand capacitance is enhanced with the maximum of 3511.3 μF/cm2 at 0 V and
the magnetism. It is noted that the spin density is mainly located around the minimum of 38.3 μF/cm2 at − 0.21 V. Compared with PT-LO and PT-
Ti and O atoms, which is in agreement with the analysis of the density of LT monolayers, PT-LC monolayer has the largest capacitance with the
states (Fig. 5). maximum of 3520.2 μF/cm2 at 0 V and the minimum of 32.9 μF/cm2 at
Fig. 6 presents the charge density differences of the adsorption of Li − 0.21 V. In the positive range, PT-LC monolayer has the largest Cdiff of
atom on PT and VT monolayers in order to further understand the 10,904 μF/cm2 at 0.51 V. The largest Cdiff of PT-LO and PT-LT mono­
adsorption sensitivity. A significant variation of charge density at the layers are 10,812 μF/cm2 at 0.51 V and 10,993 μF/cm2 at 0.48 V.
interface is observed because of the adsorption of Li atom. In Fig. 6, the Compared to Li-adsorbed PT monolayers, the Cdiff of Li-adsorbed VT
regions of charge depletion and accumulation are displayed in blue and monolayers exhibits the lower capacitance in the negative potential
yellow colors, respectively. For the systems of the adsorption of Li atom range, while significant improvement is observed near zero-voltage re­
gion of quantum capacitance with 6594, 5222, 3520, 4431 μF/cm2 at 0

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X.-H. Li et al. Applied Surface Science 563 (2021) 150264

From Fig. 7(b), we can note that the storage charge Q of Li-adsorbed
PT-LO
PT-LT PT and VT monolayers exhibits an asymmetric behavior, and is mainly
9000 PT-LC 6000 stored at positive range. This indicates that Li-adsorbed PT and VT
monolayers are suitable for cathode, which is accordance with the
Cdiff( F/cm2)

analysis of Cdiff. The adsorption sites of Li atom have little effect on the
6000 4000
surface storage charge for PT monolayer. The Q values of Li-adsorbed PT
monolayers increase with the increasing ϕG. Compared with PT mono­
VT
3000 2000 VT-LH layer, the introduction of C-vacancy decreases the Q, and the adsorption
VT-LC
VT-LC1
sites of Li atom have great effect on the Q. With the increasing potential,
the Q value increases gradually, and reaches the maximum of 3740,
3097, 3411, and 3486 μC/cm2 at 0.6 V for VT, VT-LH, VT-LC, and VT-
0 0
-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6
LC1 monolayers, respectively.
Potential (V) Potential (V)
Integrated quantum capacitance Cint is also an important parameter,
(a)
which the total storage capacity relies on. From Fig. 7(c), we can see that
4000 Cint of PT and VT monolayers in positive ϕG is generally much larger
VT than that in negative ϕG. This indicates that PT and VT monolayers are
PT-LO
PT-LT
3000 VT-LH more potential for cathode materials, consisting with the analysis of Cdiff
3000
PT-LC VT-LC and Q as discussed above. Cint at zero-voltage region are 3372, 3511, and
2000
VT-LC1
3520 μF/cm2 for PT-LO, PT-LT and PT-LC monolayers, respectively.
Then Cint undergoes a significant increase, and reaches the maximum at
Q( C/cm2)

2000
0.6 V. Compared with PT monolayer, the introduction of C-vacancy
1000 greatly increases Cint at near zero-voltage region, with 6594, 5222,
1000 5224, and 4430 μF/cm2 for VT, VT-LH, VT-LC, and VT-LC1 monolayers
0 at 0 V, respectively. In addition, the adsorption of Li atom doesn’t
improve Cint of VT monolayer.
0
-1000
4. Conclusion
-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6
Potential (V) Potential (V) The electronic properties and quantum capacitances of Li-adsorbed
(b) PT and VT monolayers are investigated systematically with density
functional theory. PT monolayer is an indirect semiconductor, VT
monolayer has the metallic nature because of the existence of C-va­
6000 cancy, while the Li-adsorbed PT and VT monolayers all have the metallic
6000
nature. The introduction of C-vacancy makes Ti atoms coupled with the
localized defect states and results in large adsorption energies, so the Li-
adsorbed VT monolayers are generally more stable than the Li-adsorbed
Cint ( C/cm2)

4000 PT monolayers. VT-LH monolayer has the magnetism with the total
4000
magnetic moment of 0.18 μB. Li-adsorbed PT and VT monolayers have
VT high quantum capacitances and are suitable for cathode materials. The
PT-LO VT-LH introduction of C vacancy enhances the quantum capacitances of the Li-
VT-LC
2000 PT-LT adsorbed VT monolayers at the zero-voltage region. Our results can help
PT-LC VT-LC1
2000 to understand the effect of adsorption and vacancy on the performance
of electrode materials.

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6
CRediT authorship contribution statement
Potential (V) Potential (V)
(c)
Xiao-Hong Li: Conceptualization, Supervision, Writing - original
Fig. 7. (a) The differential quantum capacitance; (b) Surface storage charge; (c) draft. Shan-Shan Li: Data curation, Visualization. Xing-Hao Cui:
The integrated quantum capacitance of PT and VT monolayer adsorbed by Investigation, Software. Rui-Zhou Zhang: Writing - review & editing.
Li atom. Hong-Ling Cui: Validation, Writing - review & editing.

V for VT, VT-LH, VT-LC, and VT-LC1 monolayers, respectively. Srivas­ Declaration of Competing Interest
tava et al. [39] thought that the vacancy defect can improve the quan­
tum capacitance of graphene. Compared with the Li-adsorbed PT The authors declare that they have no conflict of interest.
monolayer, the adsorption of Li atom on VT monolayers enhances Cdiff at
zero-voltage region. The significant improvement of Cdiff can be Appendix A. Supplementary material
explained by the increased DOS near EF when compared with PT
monolayer. In the positive range, VT monolayer gradually increases and Supplementary data to this article can be found online at https://doi.
has the maximum Cdiff of 7592 μF/cm2 at 0.36 V. The adsorption of Li org/10.1016/j.apsusc.2021.150264.
atom has great effect on the Cdiff. VT-LC1, VT-LC, and VT-LH monolayers
have the Cdiff of 6866 μF/cm2 at 0.54 V, 6592 μF/cm2 at 0.54 V, and References
6039 μF/cm2 at 0.51 V, respectively. Su et al. [54] reported the quantum
capacitance of 9.9 μF/cm2 for pristine graphene, which is much smaller [1] K.T. Nam, D.W. Kim, P.J. Yoo, C.Y. Chiang, N. Meethong, P.T. Hammond, Y.
than those of PT and VT monolayers. From Fig. 7(a), Cdiff in the positive M. Chiang, A.M. Belcher, Virus-enabled synthesis and assembly of nanowires for
lithium ion battery electrodes, Science 312 (2006) 885–888.
scope is much larger than that in the negative scope, indicating that Li [2] J.M. Tarascon, M. Armand, Issues and challenges facing re-chargeable lithium
adsorbed PT and VT monolayers are promising cathode materials. batteries, Nature 414 (2001) 359–367.

7
X.-H. Li et al. Applied Surface Science 563 (2021) 150264

[3] J.B. Goodenough, Y. Kim, Challenges for Rechargeable Li Batteries, Chem. Mater. [30] A. Das, S. Pisana, B. Chakraborty, S. Piscanec, S.K. Saha, U.V. Waghmare, K.
22 (2009) 587–603. S. Novoselov, H.R. Krishnamurthy, A.K. Geim, A.C. Ferrari, A.K. Sood, Monitoring
[4] H.Y. Kang, Y.C. Liu, K.Z. Cao, Y. Zhao, L.F. Jiao, Y.J. Wang, H.T. Yuan, Update on dopants by Raman scattering in an electrochemically top-gated graphene
anode materials for Na-ion batteries, J. Mater. Chem. A. 3 (2015) 17899–17913. transistor, Nat. Nanotechnol. 3 (2008) 210–215.
[5] Z. Jian, W. Luo, X. Ji, Carbon electrodes for K-ion batteries, J. Am. Chem. Soc. 137 [31] J. Huang, B.G. Sumpter, V.A. Meunier, Universal model for nanoporous carbon
(2015) 11566–11569. supercapacitors applicable to diverse pore regimes, carbon materials, and
[6] M.C. Lin, M. Gong, B.G. Lu, Y.P. Wu, D.Y. Wang, M.Y. Guan, A. Michael, C.X. Chen, electrolytes, Chem. Eur. J. 14 (2008) 6614–6626.
J. Yang, B.J. Hwang, H. Dai, An ultrafast rechargeable aluminum ion battery, [32] E. Paek, A.J. Pak, G.S. Hwang, A computational study of the interfacial structure
Nature 520 (2015) 324–328. and capacitance of graphene in [BMIM][PF6] ionic liquid, J. Electrochem. Soc. 160
[7] B. Anasori, Y. Xie, M. Beidaghi, J. Lu, B. C. Hosler, L. Hultman, R. C. Kent Paul, Y. (2013) A1–A10.
Gogotsi, W. Barsoum, Two-dimensional, ordered, double transition metals carbides [33] T. Sruthi, T. Kartick, Route to achieving enhanced quantum capacitance in
(MXenes), ACS Nano. 9(2015) 9507-9516. functionalized graphene based supercapacitor electrodes, J. Phys, Condens. Matter
[8] X. Zhao, C.M. Hayner, M.C. Kung, H.H. Kung, Flexible holey graphene paper 31(2019)475502.
electrodes with enhanced rate capability for energy storage applications, ACS Nano [34] A. Srivastava, B. SanthiBhushan, Trade-off between quantum capacitance and
5 (2011) 8739–8749. thermodynamic stability of defected graphene: an implication for supercapacitor
[9] L.S. Zhang, L.Y. Jiang, H.J. Yan, W.D. Wang, W. Wang, W.G. Song, Y.G. Guo, L. electrodes, Appl. Nanosci. 8 (2018) 637–644.
J. Wan, Mono dispersed SnO2 nanoparticles on both sides of single layer graphene [35] L. Xu, L. Chen, L. Li, X. Li, Effects of the N/S codoping configuration and ternary
sheets as anode materials in Li-ion batteries, J. Mater. Chem. 20 (2010) doping on the quantum capacitance of graphene, J. Mater. Sci. 54 (2019)
5462–5467. 8995–9003.
[10] Y.-X. Yu, Can all nitrogen-doped defects improve the performance of graphene [36] R. Bourrellier, S. Meuret, A. Tararan, O. Stéphan, M. Kociak, L.H.G. Tizei,
anode materials for lithium-ion batteries? Phys. Chem. Chem. Phys. 15 (2013) A. Zobelli, Bright UV single photon emission at point defects in h-BN, Nano Lett. 16
16819–16827. (2016) 4317–4321.
[11] Y.X. Yu, Graphenylene: a promising anode material for lithium-ion batteries with [37] D. Wong, J. Velasco, L. Ju, J. Lee, S. Kahn, H.-Z. Tsai, C. Germany, T. Taniguchi,
high mobility and storage, J. Mater. Chem. A 1 (2013) 13559–13566. K. Watanabe, A. Zettl, F. Wang, M.F. Crommie, Characterization and manipulation
[12] S.M. Seyed-Talebi, I. Kazeminezhad, J. Beheshtian, Theoretical prediction of of individual defects in insulating hexagonal boron nitride using scanning
silicene as a new candidate for the anode of lithium-ion batteries, Phys. Chem. tunneling microscopy, Nat. Nanotechnol. 10 (2015) 949–953.
Chem. Phys. 17 (2015) 29689–29696. [38] C. Wang, H. Han, Y. Guo, Stabilities and electronic properties of vacancy-doped
[13] E. Buiel, J. Dahn, Li-insertion in hard carbon anode materials for Li-ion batteries, Ti2CO2, Comput. Mater. Sci. 159 (2019) 127.
Electrochim. Acta 45 (1999) 121–130. [39] T. Hu, J. Yang, X. Wang, Carbon vacancies in Ti2CT2 MXenes: defects or a new
[14] C. Feng, J. Ma, H. Li, R. Zeng, Z. Guo, H. Liu, Synthesis of molybdenum disulfide opportunity? Phys. Chem. Chem. Phys. 19 (2017) 31773–31780.
(MoS2) for lithium ion battery applications, Mater. Res. Bull. 44 (2009) [40] X.H. Li, S.S. Li, Y.L. Yong, R.Z. Zhang, Adsorption of NH3 onto vacancy-defected
1811–1815. Ti2CO2 monolayer by first-principles calculations, Appl. Surf. Sci. 504 (2020),
[15] J.-H. Li, J. Wu, Y.-X. Yu, Toward large-capacity and high-stability lithium storages 144325.
via constructing quinone− 2D-MnO2–pillared structures, J. Phys. Chem. C 125 [41] X.H. Li, H.L. Cui, R.Z. Zhang, S.S. Li, First-principles study of biaxial strain effect on
(2021) 3725–3732. NH3 adsorbed Ti2CO2 monolayer, Vacuum 179 (2020), 109574.
[16] F. Xia, H. Wang, D. Xiao, M. Dubey, A. Ramasubramaniam, Two-dimensional [42] D. Er, J. Li, M. Naguib, Y. Gogotsi, V.B. Shenoy, Ti3C2 MXene as a high capacity
material nanophotonics, Nat. Photonics 8 (2014) 899–907. electrode material for metal (Li, Na, K, Ca) ion batteries, ACS Appl. Mater.
[17] Y. Xie, M. Naguib, V.N. Mochalin, M.W. Barsoum, Y. Gogotsi, X. Yu, K.-W. Nam, X.- Interfaces 6 (2014) 11173–11179.
Q. Yang, A.I. Kolesnikov, P.R.C. Kent, Role of surface structure on Li-Ion energy [43] G. Kresse, J. Furthmuller, Efficient iterative schemes for ab initio total-energy
storage capacity of two dimensional transition-metal carbides, J. Am. Chem. Soc. calculations using a plane-wave basis set, Phys. Rev. B 54 (1996) 11169.
136 (2014) 6385–6394. [44] J.P. Perdew, K. Burke, M. Ernzerhof, Generalized gradient approximation made
[18] M. Naguib, M. Kurtoglu, V. Presser, J. Lu, J. Niu, M. Heon, L. Hultman, Y. Gogotsi, simple, Phys. Rev. Lett. 77 (1996) 3865–3868.
M.W. Barsoum, Two-dimensional nanocrystals produced by exfoliation of Ti3AlC2, [45] S. Grimme, J. Antony, S. Ehrlich, H. Krieg, A consistent and accurate ab initio
Adv. Mater. 23 (2011) 4248–4253. parametrization of density functional dispersion correction (DFT-D) for the 94
[19] X. Zhang, J. Lei, D. Wu, X. Zhao, Y. Jing, Z. Zhou, A Ti-anchored Ti2CO2 monolayer elements H-Pu, J. Chem. Phys. 132 (2010), 154104.
(MXene) as a single-atom catalyst for CO oxidation, J. Mater. Chem. A 4 (2016) [46] D. John, L. Castro, D. Pulfrey, Quantum capacitance in nanoscale device modeling,
4871–4876. J. Appl. Phys. 96 (2004) 5180–5184.
[20] Y. Xie, M. Naguib, V.N. Mochalin, M.W. Barsoum, Y. Gogotsi, X. Yu, K.W. Nam, X. [47] P. Hirunsit, M. Liangruksa, P.Khanchaitit, Electronic structures and quantum
Q. Yang, A.I. Kolesnikov, P.R.C. Kent, J. Am. Chem. Soc. 136 (2014) 6385–6394. capacitance of monolayer and multilayer graphenes influenced by Al, B, N and P
[21] J. Li, X. Yuan, C. Lin, Y. Yang, L. Xu, X. Du, J. Xie, J. Lin, J. Sun, Achieving high doping, and monovacancy: Theoretical study, Carbon 108(2016)7–20.
pseudocapacitance of 2D titanium carbide (MXene) by cation intercalation and [48] B.C. Wood, T. Ogitsu, M. Otani, J. Biener, First-principles-inspired design strategies
surface modification, Adv. Energy Mater. 7 (2017) 1602725. for graphene-based supercapacitor electrodes, J. Phys. Chem. C 118 (2013) 4–15.
[22] X. Chen, Y. Zhu, M. Zhang, J. Sui, W. Peng, Y. Li, G. Zhang, F. Zhang, X. Fan, N- [49] N. Dimakis, I. Salas, L. Gonzalez, O. Vadodaria, K. Ruiz, M.I. Bhatti, Li and Na
butyllithium-treated Ti3C2Tx MXene with excellent pseudocapacitor performance, adsorption on graphene and graphene oxide examined by density functional
ACS Nano 13 (2019) 9449–9456. theory, quantum theory of atoms in molecules, and electron localization function,
[23] X. Gao, Y. Zhou, Y. Tan, Z. Cheng, B. Yang, Y. Ma, Z. Shen, J. Jia, Exploring Molecules 24 (2019) 754.
adsorption behavior and oxidation mechanism of mercury on monolayer Ti2CO2 [50] M.E. Stournara, V.B. Shenoy, Enhanced Li capacity at high lithiation potentials in
(MXenes) from first principles, Appl. Surf. Sci. 464 (2019) 53–60. graphene oxide, J. Power Sources 196 (2011) 5697–5703.
[24] Y.-X. Yu, Prediction of mobility, enhanced storage capacity, and volume change [51] Y. Xie, P.R.C. Kent, Hybrid density functional study of structural and electronic
during sodiation on interlayer-expanded functionalized Ti3C2 MXene anode properties of functionalized Tin+1Xn (X=C, N) monolayers, Phys. Rev. B 87 (2013),
materials for sodium-ion batteries, J. Phys. Chem. C 120 (2016) 5288–5296. 235441.
[25] Y. Xin, Y.-X. Yu, Possibility of bare and functionalized niobium carbide MXenes for [52] X.H. Zha, K. Luo, Q. Li, Q. Huang, J. He, X. Wen, S. Du, Role of the surface effect on
electrode materials of supercapacitors and field emitters, Mater. Des. 130 (2017) the structural, electronic and mechanical properties of the carbide MXenes,
512–520. Europhys. Lett. 11 (2015) 26007.
[26] Y. Wen, T.E. Rufford, X. Chen, N. Li, M. Lyu, L. Dai, L. Wang, Nitrogen-doped [53] I. A. Pašti, A. Jovanovic,
́ A. S. Dobrota, S. V. Mentus, B. J. Johansson, N. V.
Ti3C2Tx MXene electrodes for high-performance supercapacitors, Nano Energy 38 Skorodumova, Atomic adsorption on graphene with a single vacancy: systematic
(2017) 368–376. DFT study through the periodic table of elements, Phys. Chem. Chem. Phys. 20
[27] C.G. Liu, Z.N. Yu, D. Neff, A. Zhamu, B.Z. Jang, Graphene-based supercapacitor (2018) 858–865.
with an ultrahigh energy density, Nano Lett. 10 (2010) 4863–4868. [54] F.Y. Su, L. Huo, Q.Q. Kong, L.J. Xie, C.M. Chen, Theoretical study on the quantum
[28] J. Xia, F. Chen, J. Li, Tao, N. Measurement of the quantum capacitance of capacitance origin of graphene cathodes in lithium ion capacitors, Catalysts 8
graphene, Nat. Nanotechnol. 4(2009)505-509. (2018) 444.
[29] M.D. Stoller, C.W. Magnuson, Y. Zhu, S. Murali, J.W. Suk, R. Piner, R.S. Ruoff,
Interfacial capacitance of single layer graphene, Energy Environ. Sci. 4 (2011)
4685–4689.