A density functional theory study of high-performance pre-lithiated MS2 (M = Mo, W, V) Monolayers as the Anode Material of Lithium Ion Batteries.

Recent experimental study shows that the pre-lithiated MoS2 monolayer exhibits an enhanced electrochemical performance, coulombic efficiency of which is 26% higher than the pristine MoS2 based anode. The underlying mechanism of such significant enhancement, however, has not yet been addressed. By means of density functional theory (DFT) calculations, we systematically investigated the adsorption and diffusion behavior of lithium (Li) atoms on the MS2 (M = Mo, W, V) monolayers. On the pre-lithiated MS2 monolayers, the adsorption energy of extra Li ions are not significantly changed, implying the feasibility of multilayer adsorption. Of importance, the Li diffusion barriers on pre-lithiated MS2 are negligibly small because of the charge accumulation between the diffusing Li ions and the pre-lithiating Li layer. Correspondingly, we report that the pre-lithiation should be a general treatment which can be employed on many transition-metal di-chalcogenides to improve their storage capacities and charge-discharge performance in Li ion batteries. In addition, we propose that the pre-lithiated VS2 may serve as an outstanding anode material in LIBs.


Results and Discussion
In two-dimensional transition-metal di-chalcogenides, the atomic layer of metal elements are sandwiched between two S layers. As shown in Fig. 1(a), the Mo−S bond length of the 2H-MoS 2 is 2.41 Å, and the Mo−S − Mo bond angle is 80.68°, agreeing well the previous theoretical results 37 . Two binding sites are considered for analyzing the adsorption of Li ions on the MoS 2 . The top site (T site) is directly above one Mo atom, while the hollow site (H site) is above the center of a hexagon, as shown in Fig. 1(b). We have also examined the other possible adsorption sites (e.g. above the S atom), however, the adsorbed Li ion is observed to move to the neighboring T site after structural relaxation. The binding energy of metal atoms on the MS 2 is defined as: The E nLi-MS2 is the total energy of the coupled structure, in which n Li ions adsorbing on the MS 2 . E Li is the energy of an isolated Li atom in a vacuum. E MS2 is the energy of an isolated MS 2 monolayer. And n is the number of adsorbed Li atoms. According to such definition, a more negative binding energy indicates a more favorable exothermic interaction between MS 2 and Li atoms. As shown in Fig. 1, the adsorption of a Li ion at the T site (−1.94 eV) is more stable than that on the H site (−1.78 eV), with a Li-S distance being 2.37 Å, consisting well with previous theoretical studies 28,37 . In addition to Li ions, the adsorption of other metal elements which possess potential barrier applications have also been calculated. The binding energies of different adsorbing atoms and their corresponding cohesive energies are shown in Fig. 1(c). It can be seen that the binding of Li, Na and K atoms www.nature.com/scientificreports www.nature.com/scientificreports/ on MoS 2 are stronger than the metallic bonds in their bulk structures. This suggests that the MoS 2 may also be employed as anode materials for Na and K ion batteries.
Subsequently, the Li storage capacities of MS 2 monolayer (M = Mo, W, V) were investigated. A series of Li/ MS 2 configurations with different stoichiometry of LixMS 2 (x = 0.125, 0.222, 0.500, 1.000, and 2.000) were constructed by adding one Li ion on each side of the (4 × 4), (3 × 3), (2 × 2), (2 × 1) and (1 × 1) supercells, respectively. As shown in Fig. 2, the binding energies of Li ions decreases with increasing Li coverages. It is worthy to note that the Li binding energies on VS 2 are much larger than on other MS 2 . When x = 2, full Li coverages are achieved on both sides of MS 2 . It is seen that the averaged binding energies of Li ions on fully covered VS 2 , MoS 2 and WS 2 are −2.58 eV, −1.35 eV and −1.56 eV, respectively. This indicates strong attractive interactions between Li ions and MS 2 monolayers at the full coverage. The Li 2 MS 2 represents the highest Li storage capacity on bare MS 2 . At this coverage, the theoretical capacity can be calculated with the following equation:  21 . In order to obtain an in-depth understanding, the adsorption and diffusion of extra Li atoms on the pre-lithiated MS 2 are investigated. Firstly, as shown in Fig. 3, two possible pre-lithiated configurations have been considered, the layered Li atoms prefer to adsorb above the T M site of the MS 2 monolayer with the binding energies being −1.81 eV (MoS 2 ), −1.82 eV (WS 2 ), and −2.86 eV (VS 2 ), respectively. The corresponding Li-S distances are 2.45 Å (MoS 2 ), 2.51 Å (WS 2 ) and 2.32 Å (VS 2 ). Figure 4 shows the configurations and corresponding binding energies of extra Li atoms adsorbing the pre-lithiated VS 2 (Li 2 VS 2 ) monolayer with various coverages. As seen, the binding energies of the Li ions on  www.nature.com/scientificreports www.nature.com/scientificreports/ Li 2 VS 2 monolayer decreases gradually with the elevation of the related storage ratio (x). On the pre-lithiated VS 2 , the Li ions used for pre-lithiation are assumed anchored on the VS 2 and thus the Li storage capacity is defined as: Li MS 2 2 maximum theoretical capacity of the Li atoms on the pre-lithiated MS 2 (M = Mo, W, V) monolayers were 308.14, 204.70 and 415.67 mA•h • g −1 respectively. Thus, from the point of the binding energy and the theoretical capacity, Li 2 VS 2 is relatively more suitable for LIBs anode materials for the higher binding energy and theoretical storage capacity.
The performance of an electrode material is closely related the mobility of the adsorbed Li ions 38 . In general, a lower diffusion barrier means a higher diffusion rate 39,40 . Thus it is necessary to study the diffusion behavior Li ions when the Li 2 MS 2 monolayers are used as the substrates. The migrations of the Li atom among the T site and the H site are studied using the CI-NEB method. The red circles and black arrows in Fig. 5 represent the diffusion pathway of the Li atom from the most stable adsorption site (T site or H site) to the next equivalent  www.nature.com/scientificreports www.nature.com/scientificreports/ stable adsorption site. As seen in Fig. 5(a-c)   www.nature.com/scientificreports www.nature.com/scientificreports/ the temperature 43 . As can be seen, the diffusion constant increases exponentially with the decreasing diffusion barrier at a constant temperature. Please note that on the three Li 2 MS 2 substrates, the pre-lihtiated VS 2 monolayer is the most optimized anode material in terms of high Li binding energy and low diffusion barrier and the high Li adsorption capacity. Figure 6 summarizes the LDOS of the initial states (IS) and transition states (TS) of Li diffusion on Li 2 MS 2 monolayers.
One of important factors for estimating the performance of LIB anode materials is the electric conductivity. Many pristine MS 2 are semiconductors with large band gaps, implying poor electric conductivity [44][45][46] . As seen, all Li 2 MS 2 monolayers are conducting materials. More detailed analysis of the LDOS plots shows that when a Li ion adsorbs on the Li 2 MS 2 monolayer, the electronic states of Li ions are more hybridized, indicating that the interactions between the adsorbed Li and pre-lithiating Li layer are chiefly metallic bonding. This is consisted with the previous theoretical investigates 47 .
The differential charge densities were calculated in order to identify the bonding characteristics between the diffusing Li ion and the Li 2 MS 2 substrates. As clearly shown in Fig. 7, the electrons are accumulated between the diffusing Li ion and the Li 2 MS 2 . In addition, the areas of such charge accumulations expand on three neighing Li ions in the Li 2 MS 2 , indicating that the accumulated electrons are delocalized. This agree well with the PDOS analysis that the interactions are metallic bonding. As a result, the migration of the diffusing Li ion does not need to break the Li-Li 2 MS 2 bonds. Correspondingly, the Li diffusion barrier on Li 2 MS 2 should be very small, which is in good consistence with our CI-NEB calculations.

conclusions
In conclusion, the adsorption of Li ions on the surface of the pristine/pre-lithiated MS 2 monolayer (Li 2 MS 2 , M = Mo, W, V) are systematically investigated. Our calculations showed that the optimal adsorption sites of Li ions on the pristine MS 2 is the on-top site of the metal atoms. A pre-lithiating Li layer is formed when all the on-top sites are occupied by a Li ion. The pre-lithiation of MS 2 (M = W and V) will enhance the adsorption and diffusion of Li ions. Although the Li binding energy on the clean MS 2 and the pre-lithiation are not significantly different, the Li diffusion barriers on the pre-lithiated MS 2 are much less than those on the clean MS 2 , implying a fast charge-discharge property. In particular, we report that the pre-lithiated VS 2 is a very promising anode materials in the Li ion barriers, due to strong Li binding interactions and negligibly small Li diffusion barriers on the Li 2 VS 2 . Thus, this work not only interprets the in-depth working principles of the reported pre-lithiation for MoS 2 , but also propose that the pre-lithiated VS 2 may serve as one of the best anode materials in LIBs.