Properties of S-Functionalized Nitrogen-Based MXene (Ti2NS2) as a Hosting Material for Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries have received extensive attention due to their high theoretical specific capacity and theoretical energy density. However, their commercialization is hindered by the shuttle effect caused by the dissolution of lithium polysulfide. To solve this problem, a method is proposed to improve the performance of Li-S batteries using Ti2N(Ti2NS2) with S-functional groups as the sulfur cathode host material. The calculation results show that due to the mutual attraction between Li and S atoms, Ti2NS2 has the moderate adsorption energies for Li2Sx species, which is more advantageous than Ti2NO2 and can effectively inhibit the shuttle effect. Therefore, Ti2NS2 is a potential cathode host material, which is helpful to improve the performance of Li-S batteries. This work provides a reference for the design of high-performance sulfur cathode materials.


Introduction
Presently, the continuous development of electric vehicles and electronic devices puts more requirements on rechargeable batteries [1]. At present, the technology of lithium batteries is relatively mature, but the low theoretical capacity of them cannot meet the needs of future development [2]. Therefore, new rechargeable battery technologies need to be developed. In the next generation of rechargeable batteries, Li-S batteries have received widespread attention because of their high theoretical specific capacity and high energy density. The charge and discharge of Li-S batteries are based on a chemical reaction: S 8 + 8Li 2 ↔ 8Li 2 S. During discharging process, the lithium anode is oxidized to form lithium ions and electrons. The lithium ions and electrons travel to the cathode via a membrane and an external circuit, respectively. Sulfur is reduced at the cathode and reacts with lithium ions and electrons to first form soluble intermediates Li 2 S 8 , Li 2 S 6 , Li 2 S 4 , and then form insoluble Li 2 S 2 and Li 2 S (Figure 1). The charging process is reversed [3,4]. The theoretical specific capacity of Li-S batteries can reach 1675 mAh·g −1 , and the theoretical energy density can reach 2600 Wh·kg −1 [5][6][7]. In addition, as a cathode material, the sulfur has the advantages of large storage capacity, low cost, environmental friendliness, and non-toxicity [8][9][10]. However, in the process of charging and discharging, the long-chain soluble polysulfides (Li 2 S 8 , Li 2 S 6 , Li 2 S 4 ) produced at the cathode of Li-S batteries are easily dissolved in the electrolyte. These soluble lithium polysulfides (LiPSs) can shuttle with the electrolyte to the anode ("shuttle effect"), causing the loss of cathode active materials. As a result, the coulombic efficiency of the Li-S batteries is reduced, and the cycle stability is To solve the above-mentioned problems, a lot of efforts have been made. Phys confinement is one of the effective methods [15]. Various structures, such as the open rous structure [16] and the lithium permeable shell [17], have been shown to inhibit shuttle effect. In 2009, Nazar et al. [18] used CMK-3, a mesoporous carbon, as a conduc skeleton loaded with elemental sulfur, greatly improving the performance of the catho In 2018, Ma [19] et al. used the hollow carbon sphere structure as the main body of sulfur cathode, which effectively improved the stability of the lithium-sulfur batteries Another effective method is chemical binding, which uses host materials with h conductivity and appropriate affinity to capture LiPSs [20]. Therefore, a variety of m rials have been introduced into sulfur cathodes as host materials, such as graphene [ 23], two-dimensional transition metal sulfides and oxides [24][25][26], phosphorene [27etc., which have been proved to be possible as cathode host materials.
Recently, MXenes, a new type of two-dimensional materials, have received extens attention due to their high specific surface area, good electrical conductivity and sta structure [30][31][32]. It is considered to have great potential to become excellent sulfur ca ode host materials. In 2015, Xiao Liang et al. [33] introduced Ti2C into the cathode of L batteries and produced the 70 wt.% S/Ti2C composite materials, which proved that th was a strong interaction between LiPSs and Ti atoms on the surface of Ti2C. This allow the specific capacity of the sulfur cathodes to reach 1200 mAh·g −1 , therefore improving cycle performance of the sulfur cathodes. The sulfur cathodes still had a capacity retent rate of 80% after 400 cycles of charging and discharging at a rate of 0.5 C. In 2018, Ch Du et al. [34] used Ti2O hollow nanospheres to wrap sulfur, which was then embedded the Ti2C interlayer to produce the S@Ti2O/Ti2C composite materials as the cathodes of S batteries. When the S@Ti2O/Ti2C composite cathode was charged and discharged rate of 0.2 C, its initial capacitance reached 1408.6 mAh·g −1 . Under the conditions of and 5 C high-rate charging and discharging after 200 cycles, it could maintain the spec capacities of 464.0 mAh·g −1 and 227.3 mAh·g −1 , respectively.
As MXenes are etched with HF acid, functional groups are inevitably left on the s face of MXenes [35]. Common natural functional groups are -O, -F, -OH [36]. The p ence of functional groups affects the anchoring effects of MXenes on LiPSs. In 20 Dashuai Wang et al. [37] studied the anchoring effects of Ti3C2 surface functional gro on LiPSs through first-principles calculations. The results showed that the anchoring fects of O-functionalized Ti3C2 (Ti3C2O2) on LiPSs were better than those of F-function ized Ti3C2 (Ti3C2F2). Recently, some studies have shown that it is possible to introd non-natural functional groups, such as S-functional groups, through experimental mea To solve the above-mentioned problems, a lot of efforts have been made. Physical confinement is one of the effective methods [15]. Various structures, such as the open porous structure [16] and the lithium permeable shell [17], have been shown to inhibit the shuttle effect. In 2009, Nazar et al. [18] used CMK-3, a mesoporous carbon, as a conductive skeleton loaded with elemental sulfur, greatly improving the performance of the cathode. In 2018, Ma [19] et al. used the hollow carbon sphere structure as the main body of the sulfur cathode, which effectively improved the stability of the lithium-sulfur batteries.
Recently, MXenes, a new type of two-dimensional materials, have received extensive attention due to their high specific surface area, good electrical conductivity and stable structure [30][31][32]. It is considered to have great potential to become excellent sulfur cathode host materials. In 2015, Xiao Liang et al. [33] introduced Ti 2 C into the cathode of Li-S batteries and produced the 70 wt.% S/Ti 2 C composite materials, which proved that there was a strong interaction between LiPSs and Ti atoms on the surface of Ti 2 C. This allowed the specific capacity of the sulfur cathodes to reach 1200 mAh·g −1 , therefore improving the cycle performance of the sulfur cathodes. The sulfur cathodes still had a capacity retention rate of 80% after 400 cycles of charging and discharging at a rate of 0.5 C. In 2018, Chang Du et al. [34] used Ti 2 O hollow nanospheres to wrap sulfur, which was then embedded in the Ti 2 C interlayer to produce the S@Ti 2 O/Ti 2 C composite materials as the cathodes of Li-S batteries. When the S@Ti 2 O/Ti 2 C composite cathode was charged and discharged at a rate of 0.2 C, its initial capacitance reached 1408.6 mAh·g −1 . Under the conditions of 2 C and 5 C high-rate charging and discharging after 200 cycles, it could maintain the specific capacities of 464.0 mAh·g −1 and 227.3 mAh·g −1 , respectively.
As MXenes are etched with HF acid, functional groups are inevitably left on the surface of MXenes [35]. Common natural functional groups are -O, -F, -OH [36]. The presence of functional groups affects the anchoring effects of MXenes on LiPSs. In 2019, Dashuai Wang et al. [37] studied the anchoring effects of Ti 3 C 2 surface functional groups on LiPSs through first-principles calculations. The results showed that the anchoring effects of O-functionalized Ti 3 C 2 (Ti 3 C 2 O 2 ) on LiPSs were better than those of F-functionalized Ti 3 C 2 (Ti 3 C 2 F 2 ). Recently, some studies have shown that it is possible to introduce non-natural functional groups, such as S-functional groups, through experimental means. Unlike natural functional groups, there are few studies on non-natural functional groups. In this work, through first-principles calculations, the adsorption capacity, electronic properties and catalytic capacity of S-functionalized Ti 2 N (Ti 2 NS 2 ) for LiPSs are studied. The research results show that Ti 2 NS 2 has a moderate adsorption capacity for LiPSs, which is stronger than O-functionalized Ti 2 N (Ti 2 NO 2 ). In addition, Ti 2 NS 2 has good electrical conductivity, and it still has good electrical conductivity after adsorption of Li 2 S x species. Therefore, Ti 2 NS 2 has the potential to become host materials for the cathodes of Li-S batteries.

Method and Computational Details
In this work, all first-principles calculations are based on the CASTEP package. The exchange-correlation functional is described by the Perdew-Burke-Ernzerhof (PBE) functional within generalized gradient approximation (GGA) [38]. The Grimme of DFT-D2 is used to describe the van der Waals (vdW) interaction between the substrate and LiPSs [39,40]. The models of MXenes are constructed using 3 × 3 super cells. The size of the vacuum layer is set to 20 Å along the Z-axis to avoid layer-to-layer interaction. To ensure the accuracy of the calculation, 520 eV is selected as the cut-off energy of the plane wave base. The 6 × 6 × 1 k-point grid is used for structural optimization, and the 9 × 9 × 1 k-point grid is used for the calculation of the density of states. Meanwhile, the maximum values of the energy standard, force standard position and displacement standard for structural convergence are 2 × 10 −5 eV/atom −1 , 0.05 eV/Å −1 and 0.002 Å, respectively. The electron transfer is calculated using the Hirshfeld population analysis method.
The adsorption energy (E ads ) between Li 2 S x species and MXenes is defined by the following formula: where E species+MXene represents the energy of the entire system after MXenes adsorb Li 2 S x species, while E MXene and E species represent the energy of isolated MXenes and Li 2 S x species, respectively. By definition, the more negative the value, the stronger ability of MXenes to adsorb Li 2 S x species.

Structure and Adsorption Performance
First, the structures of Li 2 S x species are studied ( Figure 2). S 8 presents a folded ring structure, and the shortest S-S bond length is 1.96 Å. The shortest S-Li bond lengths of soluble Li 2 S 8 , Li 2 S 6 , and Li 2 S 4 are 2.39 Å, 2.41 Å, and 2.37 Å, respectively, and the shortest S-S bond lengths are 2.05 Å, 2.04 Å, and 2.08 Å, respectively. For insoluble Li 2 S 2 and Li 2 S, the shortest S-Li bond lengths are 2.24 Å and 2.11 Å, respectively. For insoluble Li 2 S 2 and Li 2 S, the shortest S-Li bond lengths are 2.24 Å and 2.11 Å. All molecules present a 3D structure, not a chain structure, which is consistent with previous work [41].
Secondly, we establish the model of Ti 2 N ( Figure S1). The lattice constant is a = b = 3.01 Å, and the Ti-N bond length is 2.07 Å. Based on Ti 2 N, the model of Ti 2 NS 2 is established ( Figure 3). The fully relaxed Ti 2 NS 2 presents the hexagonal structure. The lattice constant is a = b = 3.17 Å. The triangular carbon layer in the middle is sandwiched by two triangular titanium layers, while the outermost layer of S atoms is located directly above the lower layer of titanium. Compared with the original Ti 2 N, the Ti-N bond length of Ti 2 NS 2 changes from 2.07 Å to 2.18 Å. The bond length of the Ti-S bond is 2.39 Å. This is in line with the results of previous research [42], indicating the correctness of the Ti 2 NS 2 model. Secondly, we establish the model of Ti2N ( Figure S1). The lattice constant is a = b = 3.01 Å, and the Ti-N bond length is 2.07 Å. Based on Ti2N, the model of Ti2NS2 is established ( Figure 3). The fully relaxed Ti2NS2 presents the hexagonal structure. The lattice constant is a = b = 3.17 Å. The triangular carbon layer in the middle is sandwiched by two triangular titanium layers, while the outermost layer of S atoms is located directly above the lower layer of titanium. Compared with the original Ti2N, the Ti-N bond length of Ti2NS2 changes from 2.07 Å to 2.18 Å. The bond length of the Ti-S bond is 2.39 Å. This is in line with the results of previous research [42], indicating the correctness of the Ti2NS2 model.  Figure 4 shows the density of states of Ti2NS2. The dotted line represents the Fermi energy levels. It can be clearly seen from the figure that the Fermi level appears in the electronic state, which indicates that the Ti2N with S-functional group presents the metallicity. The metallicity is mainly provided by the d-orbital of titanium. At the same time, the p-orbital of the sulfur atom also contributes to the metallicity of Ti2NS2. The electrical conductivity of the host materials facilitates the charge-discharge reaction in Li-S batteries, since it can provide the electrons needed for the reaction.  Secondly, we establish the model of Ti2N ( Figure S1). The lattice constant is a = b = 3.01 Å, and the Ti-N bond length is 2.07 Å. Based on Ti2N, the model of Ti2NS2 is established ( Figure 3). The fully relaxed Ti2NS2 presents the hexagonal structure. The lattice constant is a = b = 3.17 Å. The triangular carbon layer in the middle is sandwiched by two triangular titanium layers, while the outermost layer of S atoms is located directly above the lower layer of titanium. Compared with the original Ti2N, the Ti-N bond length of Ti2NS2 changes from 2.07 Å to 2.18 Å. The bond length of the Ti-S bond is 2.39 Å. This is in line with the results of previous research [42], indicating the correctness of the Ti2NS2 model.  Figure 4 shows the density of states of Ti2NS2. The dotted line represents the Fermi energy levels. It can be clearly seen from the figure that the Fermi level appears in the electronic state, which indicates that the Ti2N with S-functional group presents the metallicity. The metallicity is mainly provided by the d-orbital of titanium. At the same time, the p-orbital of the sulfur atom also contributes to the metallicity of Ti2NS2. The electrical conductivity of the host materials facilitates the charge-discharge reaction in Li-S batteries, since it can provide the electrons needed for the reaction.  Figure 4 shows the density of states of Ti 2 NS 2 . The dotted line represents the Fermi energy levels. It can be clearly seen from the figure that the Fermi level appears in the electronic state, which indicates that the Ti 2 N with S-functional group presents the metallicity. The metallicity is mainly provided by the d-orbital of titanium. At the same time, the p-orbital of the sulfur atom also contributes to the metallicity of Ti 2 NS 2 . The electrical conductivity of the host materials facilitates the charge-discharge reaction in Li-S batteries, since it can provide the electrons needed for the reaction.
After understanding the structure of Li 2 S x species and Ti 2 NS 2 , the interaction between Li 2 S x species and Ti 2 NS 2 is studied. To find the stable structures, different positions of the Li 2 S x species on Ti 2 NS 2 are tried. For Li 2 S, the possible adsorption orientations include S-Top, Li-Side and S-Down ( Figure S2). Among the three adsorption orientations, the S-Down becomes the S-Top after optimization, and the adsorption energies of the S-Top and Li-Side are −3.42 eV and −1.56 eV, respectively. Therefore, the S-Top is the most favorable adsorption orientation. The adsorption of Li 2 S 2 , Li 2 S 4 , Li 2 S 6 , Li 2 S 8 and S 8 on Ti 2 NS 2 is considered in a similar manner. The final optimized structures are shown in Figure 5. Table 1 shows the adsorption energies (E ads ), shortest distances between Li 2 S x species and Ti 2 NS 2 (d), and transfer charge (Q) when Ti 2 NS 2 adsorbs Li 2 S x species. The ring structure of the S 8 molecule remains intact, parallel to the surface of Ti 2 NS 2 , and the adsorption energy is −0.57 eV. The shortest distance between the S atom of S 8 and the S atom on the surface of Ti 2 NS 2 is 3.52 Å. For insoluble Li 2 S and Li 2 S 2 , their Li atoms tend to combine with the S atoms of Ti 2 NS 2 . Li atoms of Li 2 S and Li 2 S 2 are surrounded by three S atoms on the surface of Ti 2 NS 2 , and the distances from the nearest S atoms are 2.38 Å and 2.43 Å, respectively, and the adsorption energies are −3.42 eV and −2.36 eV, respectively. As for the soluble Li 2 S 4 , Li 2 S 6 , Li 2 S 8 , their adsorption energies are −1.31 eV, −0.90eV, −0.95 eV, respectively. Similar to the insoluble Li 2 S and Li 2 S 2 , Li atoms tend to combine with the S atoms of the Ti 2 NS 2 , and the shortest distances between them are 2.47 Å, 2.54 Å and 2.51 Å, respectively. Generally speaking, the adsorption energies of Ti 2 NS 2 for Li 2 S x are between −0.57 eV~−3.42 eV, showing an increasing trend with the deepening of lithiation.  After understanding the structure of Li2Sx species and Ti2NS2, the interaction between Li2Sx species and Ti2NS2 is studied. To find the stable structures, different positions of the Li2Sx species on Ti2NS2 are tried. For Li2S, the possible adsorption orientations include S-Top, Li-Side and S-Down ( Figure S2). Among the three adsorption orientations, the S-Down becomes the S-Top after optimization, and the adsorption energies of the S-Top and Li-Side are −3.42 eV and −1.56 eV, respectively. Therefore, the S-Top is the most favorable adsorption orientation. The adsorption of Li2S2, Li2S4, Li2S6, Li2S8 and S8 on Ti2NS2 is considered in a similar manner. The final optimized structures are shown in Figure 5. Table  1 shows the adsorption energies (Eads), shortest distances between Li2Sx species and Ti2NS2 (d), and transfer charge (Q) when Ti2NS2 adsorbs Li2Sx species. The ring structure of the S8 molecule remains intact, parallel to the surface of Ti2NS2, and the adsorption energy is −0.57 eV. The shortest distance between the S atom of S8 and the S atom on the surface of    Since the shuttle effect is caused by the dissolution of soluble polysulfides (Li2S4,  Since the shuttle effect is caused by the dissolution of soluble polysulfides (Li 2 S 4 , Li 2 S 6 , and Li 2 S 8 ) into the electrolyte, we calculate the adsorption energies of electrolyte solvent molecules (DOL and DME) for Li 2 S 4 , Li 2 S 6 , and Li 2 S 8 ( Figure S3). The results show that the adsorption energies of electrolyte solvent molecules are between −0.76~−0.84 eV, which are fewer than those of Ti 2 NS 2 (−0.90~−1.31 eV). Furthermore, the adsorption energies of Ti 2 NS 2 are in the range of −0.8~−2.0 eV [43], and the adsorption energy intensity is moderate. Therefore, Ti 2 NS 2 can effectively inhibit the shuttle effect. In addition, to form a comparison, the model of Ti 2 NO 2 is constructed ( Figure S4). The structure of Ti 2 NO 2 is similar to that of Ti 2 NS 2 , presenting a hexagon structure. The lattice constant of Ti 2 NO 2 is a = b = 3.07 Å, and the length of the Ti-O bond is 1.85 Å, which is shorter than that of Ti 2 NS 2 , mainly because the size of the oxygen atom is smaller than that of the sulfur atom. After that, the adsorption energies of Ti 2 NO 2 for Li 2 S x species are calculated ( Figure S5, Figure 6). The results show that the adsorption energies of Ti 2 NO 2 for Li 2 S x species are −2.07 eV, −2.21 eV, −1.29 eV, −0.66 eV, −0.90 eV, −0.43 eV, respectively, which are fewer than those of Ti 2 NS 2 . Therefore, Ti 2 NO 2 is less effective than Ti 2 NS 2 in inhibiting the shuttle effect. The S-functional groups have an advantage over the O-functional groups. To further explore the adsorption mechanism of Ti2NS2, the charge transfer and charge density difference between Li2Sx species and Ti2NS2 are calculated.
It can be seen from Table 1 that the transferred electrons between S8 and Ti2NS2 are 0.13 e, which indicates that the force between S8 and the substrate is weak, and the adsorption energy depends on van der Waals force. Similar to S8, the transferred electrons of Li2S8 and Li2S6 are 0.15 e and 0.13 e, respectively, so the adsorption energies also mainly depend on van der Waals force. Later, with the deepening of lithium, the transferred electrons become more. The transferred electrons of Li2S4, Li2S2 and Li2S are 0.22 e, 0.34 e and 0.38 e, respectively. Meanwhile, the adsorption energies become higher, indicating that the transferred electrons affect the adsorption energies. Figure 7 shows the charge density difference between Li2Sx species and Ti2NS2. The blue regions indicate the accumulation of charge, and the red regions indicate the depletion of charge. The blue regions are mainly concentrated near the Li atoms of Li2Sx and the S atoms of Ti2NS2 surface, which indicates that the transferred electrons between Li2Sx species and Ti2NS2 surface are mainly provided by Li atoms of Li2Sx species. For long-chain sulfides (Li2S8, Li2S6, Li2S4), the blue regions are significantly smaller than those of short- To further explore the adsorption mechanism of Ti 2 NS 2 , the charge transfer and charge density difference between Li 2 S x species and Ti 2 NS 2 are calculated.
It can be seen from Table 1 that the transferred electrons between S 8 and Ti 2 NS 2 are 0.13 e, which indicates that the force between S 8 and the substrate is weak, and the adsorption energy depends on van der Waals force. Similar to S 8 , the transferred electrons of Li 2 S 8 and Li 2 S 6 are 0.15 e and 0.13 e, respectively, so the adsorption energies also mainly depend on van der Waals force. Later, with the deepening of lithium, the transferred electrons become more. The transferred electrons of Li 2 S 4 , Li 2 S 2 and Li 2 S are 0.22 e, 0.34 e and 0.38 e, respectively. Meanwhile, the adsorption energies become higher, indicating that the transferred electrons affect the adsorption energies. Figure 7 shows the charge density difference between Li 2 S x species and Ti 2 NS 2 . The blue regions indicate the accumulation of charge, and the red regions indicate the depletion of charge. The blue regions are mainly concentrated near the Li atoms of Li 2 S x and the S atoms of Ti 2 NS 2 surface, which indicates that the transferred electrons between Li 2 S x species and Ti 2 NS 2 surface are mainly provided by Li atoms of Li 2 S x species. For longchain sulfides (Li 2 S 8 , Li 2 S 6 , Li 2 S 4 ), the blue regions are significantly smaller than those of short-chain sulfides (Li 2 S 2 , Li 2 S), indicating that the transferred electrons of long-chain sulfides are fewer than those of short-chain sulfides, so Ti 2 NS 2 has a stronger adsorption capacity for short-chain sulfides. sorption energy depends on van der Waals force. Similar to S8, the transferred electrons of Li2S8 and Li2S6 are 0.15 e and 0.13 e, respectively, so the adsorption energies also mainly depend on van der Waals force. Later, with the deepening of lithium, the transferred electrons become more. The transferred electrons of Li2S4, Li2S2 and Li2S are 0.22 e, 0.34 e and 0.38 e, respectively. Meanwhile, the adsorption energies become higher, indicating that the transferred electrons affect the adsorption energies. Figure 7 shows the charge density difference between Li2Sx species and Ti2NS2. The blue regions indicate the accumulation of charge, and the red regions indicate the depletion of charge. The blue regions are mainly concentrated near the Li atoms of Li2Sx and the S atoms of Ti2NS2 surface, which indicates that the transferred electrons between Li2Sx species and Ti2NS2 surface are mainly provided by Li atoms of Li2Sx species. For long-chain sulfides (Li2S8, Li2S6, Li2S4), the blue regions are significantly smaller than those of shortchain sulfides (Li2S2, Li2S), indicating that the transferred electrons of long-chain sulfides are fewer than those of short-chain sulfides, so Ti2NS2 has a stronger adsorption capacity for short-chain sulfides.  In addition, to better explore the influence of van der Waals forces on adsorption, we take Li 2 S 2, Li 2 S 4 and Li 2 S 6 as examples to calculate the ratio of vdW interaction (R), as shown in Figure 8. The R is defined as follows: where E vdW ads and E novdW ads represent the adsorption energies with and without the vdW interaction, respectively. It is clear that the ratio of van der Waals forces decreases and the ratio of chemical interactions increases as the degree of lithium increases. For long-chain sulfides, van der Waals force is the main source of adsorption energy.

Electronic Properties
It is well known that good conductivity is very important for batteries. However, sulfur, the cathode material of Li-S batteries, is very poor in conductivity. An excellent cathode host material should not only have good conductivity itself, but also have good conductivity after absorbing Li 2 S x species. Therefore, the density of states of the whole systems after Ti 2 NS 2 adsorbed Li 2 S x species is calculated. Figure 9a shows the density of states of the whole system after Ti 2 NS 2 adsorbed S 8 , and the dotted line in Figure 9 represents the Fermi energy level. Similar to the density of states of Ti 2 NS 2 , S 8 @Ti 2 Ns 2 composites still possess metallic properties due to Ti atoms. The electronic properties of S 8 are changed by Ti 2 NS 2 . In addition, Figure 9b-f show the density of states of the systems which are formed after the adsorption of long-chain sulfides Li 2 S 8 , Li 2 S 6 , Li 2 S 4 and short-chain sulfides Li 2 S 2 and Li 2 S by Ti 2 NS 2 . Affected by Ti 2 NS 2 , the composite materials formed by Li 2 S x species and Ti 2 NS 2 still have an electronic state at the Fermi level. All systems exhibit metallic properties, including S 8 , Li 2 S, and Li 2 S 2 , which are originally poor conductivities. This indicates that the sulfur cathodes can maintain high conductivity during the entire lithiation and delithiation process. This is very beneficial for improving the cycle performance and rate performance of Li-S batteries. In addition, to better explore the influence of van der Waals forces on adsorption, we take Li2S2, Li2S4 and Li2S6 as examples to calculate the ratio of vdW interaction (R), as shown in Figure 8. The R is defined as follows: where vdW ads E and novdW ads E represent the adsorption energies with and without the vdW interaction, respectively. It is clear that the ratio of van der Waals forces decreases and the ratio of chemical interactions increases as the degree of lithium increases. For long-chain sulfides, van der Waals force is the main source of adsorption energy.

Electronic Properties
It is well known that good conductivity is very important for batteries. However, sulfur, the cathode material of Li-S batteries, is very poor in conductivity. An excellent cathode host material should not only have good conductivity itself, but also have good conductivity after absorbing Li2Sx species. Therefore, the density of states of the whole systems after Ti2NS2 adsorbed Li2Sx species is calculated. Figure 9a shows the density of states of the whole system after Ti2NS2 adsorbed S8, and the dotted line in Figure 9 represents the Fermi energy level. Similar to the density of states of Ti2NS2, S8@Ti2Ns2 composites still possess metallic properties due to Ti atoms. The electronic properties of S8 are changed by Ti2NS2. In addition, Figure 9b-f show the density of states of the systems which are formed after the adsorption of long-chain sulfides Li2S8, Li2S6, Li2S4 and shortchain sulfides Li2S2 and Li2S by Ti2NS2. Affected by Ti2NS2, the composite materials formed by Li2Sx species and Ti2NS2 still have an electronic state at the Fermi level. All systems exhibit metallic properties, including S8, Li2S, and Li2S2, which are originally poor conductivities. This indicates that the sulfur cathodes can maintain high conductivity during the entire lithiation and delithiation process. This is very beneficial for improving the cycle performance and rate performance of Li-S batteries.

Conclusions
In this work, the performance of S-functionalized Ti2N (Ti2NS2) as the host materials for the cathodes of Li-S batteries is studied through first-principles calculations. The results show that the adsorption energies of Ti2NS2 are moderate, stronger than those of