A Novel Two-Dimensional ZnSiP2 Monolayer as an Anode Material for K-Ion Batteries and NO2 Gas Sensing

Using the crystal-structure search technique and first-principles calculation, we report a new two-dimensional semiconductor, ZnSiP2, which was found to be stable by phonon, molecular-dynamic, and elastic-moduli simulations. ZnSiP2 has an indirect band gap of 1.79 eV and exhibits an anisotropic character mechanically. Here, we investigated the ZnSiP2 monolayer as an anode material for K-ion batteries and gas sensing for the adsorption of CO, CO2, SO2, NO, NO2, and NH3 gas molecules. Our calculations show that the ZnSiP2 monolayer possesses a theoretical capacity of 517 mAh/g for K ions and an ultralow diffusion barrier of 0.12 eV. Importantly, the ZnSiP2 monolayer exhibits metallic behavior after the adsorption of the K-atom layer, which provides better conductivity in a period of the battery cycle. In addition, the results show that the ZnSiP2 monolayer is highly sensitive and selective to NO2 gas molecules.

As a new family of 2D materials, phosphorus carbides (PCs) with α phase and β phase are semiconductors that exhibit highly anisotropic electronic characters with high carrier mobilities. More importantly, α-PC and β-PC, as promising anode materials for Li-, Na-, and K-ion batteries, having high capacities and fast diffusion channels for Li, Na, and K ions [10,11]. It has also been predicted that α-PC, as a promising gas sensor, exhibits superior selectivity and sensitivity for NO 2 [12]. Buckled-graphene-like PC 6 , as a semiconductor, has been predicted to have ultrahigh carrier mobility and, as an anode for Li-ion batteries, a high capacity of 717 mAh/g and an open-circuit voltage of 0.21 V [13]. Furthermore, typical 2D metal-phosphide δ-InP 3 exhibits high electron mobility and has been shown to be usable as a N-based gas sensor with high selectivity and sensitivity and good reversibility [16]. In addition, metal oxides, such as two-dimensional WO 3 and Pd-loaded ZnO monolayers, are important semiconductors applied in gas sensors, with high sensitivities [19,20].
Apart from the excellent performances of binary semiconductors, ternary 2D semiconductor materials have also attracted special interest. Using the epitaxial growth technique,

Structure and Stability
By using the global-structure search method, we found a new ZnSiP 2 monolayer with the space group Pmc21 (no. 26) containing two formula units. The structure crystalized in an orthorhombic structure, and the optimized lattice parameters were a = 3.7251 and b = 6.1398 Å. As shown in Figure 1a, a remarkable feature is that the ZnSiP 2 monolayer is stacked as a bilayer hexagonal lattice, and the two layers are bonded by Si and P, with a distance of 2.286 Å. Each layer was arranged alternately in two kinds of hexagonal rings. One ring was composed of one Si, two Zn, and three P atoms; the other ring was composed of one Zn, two Si, and three P atoms, which gave rise to two types of bonds: Si-P and Zn-P. To understand the chemical-bonding nature, the charge density difference was calculated and shown in Figure 1b, which is defined as the difference between the total electron density of the ZnSiP 2 monolayer and the charge density of isolated Zn, Si, and P atoms at their specified positions. It can clearly be seen that there is a strong non-polar covalent bond between Si and P [32]. Regarding the Zn-P bonds, the polar covalent bonds between Zn and P atoms were due to the transfer charges shifted toward P atoms.
The cohesive energy is a key factor in experimental synthesis, which is calculated by E coh = (2E Si + 2E Zn + 4E P − E ZnSiP 2 )/8, where E Si , E Zn , E P , and E ZnSiP 2 represent the energies of one Si, Zn, P, and perfect ZnSiP 2 , respectively. The calculated cohesive energy of the ZnSiP 2 monolayer was 4.36 eV/atom, which is comparable to those of phophorene (3.30 eV/atom) [33], germanene (3.26 eV/atom), silicene (3.98 eV/atom) [34], and SiP (4.16 eV/atom) [35]. We further calculated the formation energy of the ZnSiP 2 monolayer related to the SiP 2 monolayer and Zn metal to investigate its stability, which was calculated by E f = E ZnSiP 2 − µ SiP 2 − mµ Zn(bulk) , where µ SiP 2 , µ Zn(bulk) , and E ZnSiP 2 are the energies of two-dimensional SiP 2 [36], one Zn atom in bulk Zn metal, and the perfect ZnSiP 2 monolayer, respectively, and m is the number of Zn atoms. The calculated formation energy is −0.465 eV, the negative value further indicating that the ZnSiP 2 monolayer may be synthesized. The phonon spectrum was used to check the dynamic stability of the ZnSiP 2 monolayer. The calculated phonon dispersion curves for the ZnSiP 2 monolayer are shown in Figure 2a; all frequencies in the Brillouin region were positive, which means that the ZnSiP 2 monolayer is dynamically stable. Furthermore, thermal stability was checked by AIMD simulation running for 10 ps at 400 K ( Figure 2b); the structure remained almost intact at the end of the simulation, revealing that the ZnSiP 2 monolayer has good thermal stability. According to the above analysis, the predicted 2D ZnSiP 2 is promising for experimental synthesis.
Additionally, we further calculated the four independent elastic constants of the  11 22 12 C C C > [37]. Moreover, the diagrams for the in-plane Young's modulus and Poisson ratio with polar angle [38] could be obtained and are depicted in Figure 3, showing that the ZnSiP2 monolayer is anisotropic. The maximum Young's modulus (105 N/m) was higher than that reported for phosphorene (92 N/m) [39] and comparable to those of MoS2 (129 N/m) [40] and V2Te2O monolayers (115.3 N/m) [41]. The anisotropy characteristic of mechanical properties also has an important effect on electronic properties.   Additionally, we further calculated the four independent elastic constants of the  11 22 12 C C C > [37]. Moreover, the diagrams for the in-plane Young's modulus and Poisson ratio with polar angle [38] could be obtained and are depicted in Figure 3, showing that the ZnSiP2 monolayer is anisotropic. The maximum Young's modulus (105 N/m) was higher than that reported for phosphorene (92 N/m) [39] and comparable to those of MoS2 (129 N/m) [40] and V2Te2O monolayers (115.3 N/m) [41]. The anisotropy characteristic of mechanical properties also has an important effect on electronic properties.   Additionally, we further calculated the four independent elastic constants of the ZnSiP 2 monolayer, which were C 11 = 106.2 Nm −1 , C 22 = 86.0 Nm −1 , C 12 = 8.6 Nm −1 , and C 44 = 26.1 Nm −1 . According to the obtained elastic constants, the ZnSiP 2 monolayer satisfied the mechanical stability standard: C 11 > 0; C 44 > 0; C 11 C 22 > C 2 12 [37]. Moreover, the diagrams for the in-plane Young's modulus and Poisson ratio with polar angle [38] could be obtained and are depicted in Figure 3, showing that the ZnSiP 2 monolayer is anisotropic. The maximum Young's modulus (105 N/m) was higher than that reported for phosphorene (92 N/m) [39] and comparable to those of MoS 2 (129 N/m) [40] and V 2 Te 2 O monolayers (115.3 N/m) [41]. The anisotropy characteristic of mechanical properties also has an important effect on electronic properties.

Electronic and Adsorption Properties
The calculated band structure and density of states for the ZnSiP2 monolayer are shown in Figure 4a,b. The valence band maximum (VBM) is at point Γ, and the conduction band minimum (CBM) is at point Y. Therefore, as an indirect semiconductor, the band-gap values derived from the PBE and HSE calculations were 1.04 and 1.79 eV, respectively. The band dispersion near the VBM and CBM shows an anisotropic character, which results in the anisotropy of the effective masses. According to the formula , the obtained electron effective masses near the CBM were 1.364 m0 and 0.333 m0 along the x-and y-directions, while the hole effective masses near the VBM were 1.019 m0 and 0.433 m0 along the x-and y-directions, respectively. The density of states in Figure 4b shows that the VBM and CBM are both mainly contributed to by P 2p and Zn 4d orbitals. To further study the performance of the ZnSiP2 monolayer as an electrode material, we investigated the adsorption properties of one K atom on its surface using a 3 × 2 × 1 supercell as the substrate. According to the structural symmetry, ten possible K-atom adsorption sites (S1-S10) with adsorption energies based on Equation (1) were considered and calculated, as shown in Figure 5. After geometric-structure optimization, we

Electronic and Adsorption Properties
The calculated band structure and density of states for the ZnSiP 2 monolayer are shown in Figure 4a,b. The valence band maximum (VBM) is at point Γ, and the conduction band minimum (CBM) is at point Y. Therefore, as an indirect semiconductor, the band-gap values derived from the PBE and HSE calculations were 1.04 and 1.79 eV, respectively. The band dispersion near the VBM and CBM shows an anisotropic character, which results in the anisotropy of the effective masses. According to the formula m * = 2 ∂E 2 /∂k 2 , the obtained electron effective masses near the CBM were 1.364 m 0 and 0.333 m 0 along the x-and ydirections, while the hole effective masses near the VBM were 1.019 m 0 and 0.433 m 0 along the x-and y-directions, respectively. The density of states in Figure 4b shows that the VBM and CBM are both mainly contributed to by P 2p and Zn 4d orbitals.

Electronic and Adsorption Properties
The calculated band structure and density of states for the ZnSiP2 monolayer are shown in Figure 4a,b. The valence band maximum (VBM) is at point Γ, and the conduction band minimum (CBM) is at point Y. Therefore, as an indirect semiconductor, the band-gap values derived from the PBE and HSE calculations were 1.04 and 1.79 eV, respectively. The band dispersion near the VBM and CBM shows an anisotropic character, which results in the anisotropy of the effective masses. According to the formula , the obtained electron effective masses near the CBM were 1.364 m0 and 0.333 m0 along the x-and y-directions, while the hole effective masses near the VBM were 1.019 m0 and 0.433 m0 along the x-and y-directions, respectively. The density of states in Figure 4b shows that the VBM and CBM are both mainly contributed to by P 2p and Zn 4d orbitals. To further study the performance of the ZnSiP2 monolayer as an electrode material, we investigated the adsorption properties of one K atom on its surface using a 3 × 2 × 1 supercell as the substrate. According to the structural symmetry, ten possible K-atom adsorption sites (S1-S10) with adsorption energies based on Equation (1) were considered and calculated, as shown in Figure 5. After geometric-structure optimization, we To further study the performance of the ZnSiP 2 monolayer as an electrode material, we investigated the adsorption properties of one K atom on its surface using a 3 × 2 × 1 supercell as the substrate. According to the structural symmetry, ten possible K-atom adsorption sites (S1-S10) with adsorption energies based on Equation (1) were considered and calculated, as shown in Figure 5. After geometric-structure optimization, we found some equivalent sites due to the transfer of K atoms from one site to another site. As can be clearly seen in Figure 5b, the equivalent sites were S 1 = S 2 = S 3 = S 9 = S 10 and S 5 = S 7 = S 8 , so only four sites S 2 , S 4 , S 5 , and S 6 were left, with adsorption energies of −0.68, −0.57, −0.55, and −0.35 eV, respectively. Thus, the adsorption energy of the K atom at S 2 site was the lowest, which means that the adsorbed K atoms prefer to stay at the bridge position of Si-P to reduce the Coulomb repulsion between K and Zn. The nearest K-P, K-Zn and K-Si distances are 3.30 Å, 3.92 Å and 3.53 Å, respectively.
found some equivalent sites due to the transfer of K atoms from one site to another site. As can be clearly seen in Figure 5b, the equivalent sites were S1 = S2 = S3 = S9 = S10 and S5 = S7 = S8, so only four sites S2, S4, S5, and S6 were left, with adsorption energies of −0.68, −0.57, −0.55, and −0.35 eV, respectively. Thus, the adsorption energy of the K atom at S2 site was the lowest, which means that the adsorbed K atoms prefer to stay at the bridge position of Si-P to reduce the Coulomb repulsion between K and Zn. The nearest K-P, K-Zn and K-Si distances are 3.30 Å, 3.92 Å and 3.53 Å, respectively. To assess the adsorption behavior of the K atoms, we calculated the charge-density differences shown in Figure 6a, which is defined by: n Z Si P surface.
The diffusion barrier of K ions is a key parameter in estimating the performance of a battery. Next, the diffusion of one K ion on the ZnSiP2 surface was investigated. The possible diffusion path (inset of Figure 6b) between the lowest-energy adsorption sites and the calculated results is shown in Figure 6b. The diffusion barrier of the path was 0.12 eV, which is comparable to the result for ReN2 (0.127 eV) [42]. Compared with other anode materials, ZnSiP2 has a low K-ion diffusion barrier that is smaller than those of BP (0.155 eV) [43], PC6 (0.26 eV) [44], and SnC (0.17 eV) [45]. However, this value is larger than those of GeS (0.05 eV) [46], Ti3C2 (0.103 eV) [47], and C6BN (0.087 eV) [48]. The low diffusion barrier can result in ultrafast charging-discharging cycles in K-ion batteries. To assess the adsorption behavior of the K atoms, we calculated the charge-density differences shown in Figure 6a, which is defined by: where ρ(Zn 12 Si 12 P 24 ), ρ(KZn 12 Si 12 P 24 ), and ρ(K) are the charge densities of the Zn 12 Si 12 P 24 monolayer with adsorbed K atoms, the substrate Zn 12 Si 12 P 24 , and an isolated K atom, respectively. Obvious charge transfer could be observed, and the K atoms had a net charge of 0.84 |e| based on the Bader charge analysis, which implies charge transfer from the K atoms to the adjacent P and Si atoms in the Zn 12 Si 12 P 24 surface.

Capacity and Open-Circuit Voltage
After studying the adsorption and diffusion behavior of one K atom on the supercell of the ZnSiP2 monolayer, we then explored the behavior of K adsorption concentration. Five K concentrations (KxZn2Si2P4, x = 1-4, 6) were considered, and the average adsorption energies acquired according to Equation (2) were −0.30, −0.46, −0.16, −0.12, and −0.03 eV, respectively. It is to be noted that the K concentration reached x = 6, still showing negative adsorption energy, which means that K atoms can be adsorbed on the ZnSiP2 monolayer. The three stable adsorption configurations (K2Zn2Si2P4, K4Zn2Si2P4, and K6Zn2Si2P4) are shown in the inset of Figure 7. The first and the second K atom layers are located at S2 and S5 sites, with both sides of the ZnSiP2 monolayer. As for the third The diffusion barrier of K ions is a key parameter in estimating the performance of a battery. Next, the diffusion of one K ion on the ZnSiP 2 surface was investigated. The possible diffusion path (inset of Figure 6b) between the lowest-energy adsorption sites and the calculated results is shown in Figure 6b. The diffusion barrier of the path was 0.12 eV, which is comparable to the result for ReN 2 (0.127 eV) [42]. Compared with other anode materials, ZnSiP 2 has a low K-ion diffusion barrier that is smaller than those of BP (0.155 eV) [43], PC 6 (0.26 eV) [44], and SnC (0.17 eV) [45]. However, this value is larger Molecules 2022, 27, 6726 6 of 13 than those of GeS (0.05 eV) [46], Ti 3 C 2 (0.103 eV) [47], and C 6 BN (0.087 eV) [48]. The low diffusion barrier can result in ultrafast charging-discharging cycles in K-ion batteries.

Capacity and Open-Circuit Voltage
After studying the adsorption and diffusion behavior of one K atom on the supercell of the ZnSiP 2 monolayer, we then explored the behavior of K adsorption concentration. Five K concentrations (K x Zn 2 Si 2 P 4 , x = 1-4, 6) were considered, and the average adsorption energies acquired according to Equation (2) were −0.30, −0.46, −0.16, −0.12, and −0.03 eV, respectively. It is to be noted that the K concentration reached x = 6, still showing negative adsorption energy, which means that K atoms can be adsorbed on the ZnSiP 2 monolayer. The three stable adsorption configurations (K 2 Zn 2 Si 2 P 4 , K 4 Zn 2 Si 2 P 4 , and K 6 Zn 2 Si 2 P 4 ) are shown in the inset of Figure 7. The first and the second K atom layers are located at S 2 and S 5 sites, with both sides of the ZnSiP 2 monolayer. As for the third K-atom layer, the K atom prefers to stay at the S 2 site. The stoichiometry K 6 Zn 2 Si 2 P 4 can provide the maximal storage capacity 517 mAh/g, according to Equation (4), which is higher than other reported values for 2D materials, such as GeS (256 mAh/g) [46], ReN 2 (250 mAh/g) [42], Ti 3 C 2 (191 mAh/g) [47], MoS 2 /Ti 2 CS 2 (141 mAh/g) [49], and MoN 2 (432 mAh/g) [50], but lower than the capacities for BC 3 (858 mAh/g) [51], BC 6 P (1410 mAh/g) [25], C 6 BN (533 mAh/g) [48], BP (570 mAh/g) [43], and V 2 S 2 O (883.6Ah/g) [41]. Based on Equation (3), OCVs were obtained and are shown in Figure 7, and the calculated values for different concentrations, KZn 2 Si 2 P 4 , K 2 Zn 2 Si 2 P 4 , K 3 Zn 2 Si 2 P 4 , K 4 Zn 2 Si 2 P 4 , and K 6 Zn 2 Si 2 P 4 , were 0.30, 0.46, 0.16, 0.12, and 0.03 V, respectively. The Bader analysis showed that every K atom transfers 0.58 e to ZnSiP 2 when two K atoms are absorbed on the surface of 2D ZnSiP 2 , while every K atom transfers 0.51 e to ZnSiP 2 when only one K atom is absorbed on the surface of 2D ZnSiP 2 , implying that two K atoms are more easily absorbed on the surface of 2D ZnSiP 2 than one K atom. So, the OCV increases as x increases from 1 to 2, as shown in Figure 7. However, the overall voltage decreases as the capacity increases.  Importantly, the density states of the three stable adsorption configurations (KZn 2 Si 2 P 4 , K 2 Zn 2 Si 2 P 4 , and K 3 Zn 2 Si 2 P 4 ) were calculated using the PBE functional, and ZnSiP 2 , after the adsorption of K atoms, showed metallic behavior, as shown in Figure 8, which is beneficial for the ZnSiP 2 monolayer as an electrode material.

Gas-Sensing Properties
To further study the gas-sensing ability of the ZnSiP2 monolayer, we systematically studied the adsorption behavior of gas molecules (CO, CO2, SO2, NO, NO2, and NH3) on its surface by first-principles simulations. The most stable configurations of the gas molecules adsorbed on the ZnSiP2 monolayer are shown in Figure 9, and the corresponding adsorption energies (Ead), adsorption distances (d0), band gaps after molecule adsorption (Eg), and charge transfers (Q) are listed in Table 1. A positive charge for Q means charge transfer from the monolayer to the gas molecules. The equilibrium distance of 1.53 Å between NO2 and the monolayer revealed that NO2 forms a stable chemical bond. Moreover, the NO2 molecules showed high adsorption energies, indicating that ZnSiP2 is more sensitive to NO2 molecules than the other five molecules. As shown in Table 1, the Bader charge analysis indicated that there were 0.24, 0.12, 0.67, and 0.13 electron trans- Figure 8. The total densities of states of KZn 2 Si 2 P 4 , K 2 Zn 2 Si 2 P 4 , and K 3 Zn 2 Si 2 P 4 .

Gas-Sensing Properties
To further study the gas-sensing ability of the ZnSiP 2 monolayer, we systematically studied the adsorption behavior of gas molecules (CO, CO 2 , SO 2 , NO, NO 2 , and NH 3 ) on its surface by first-principles simulations. The most stable configurations of the gas molecules adsorbed on the ZnSiP 2 monolayer are shown in Figure 9, and the corresponding adsorption energies (E ad ), adsorption distances (d 0 ), band gaps after molecule adsorption (E g ), and charge transfers (Q) are listed in Table 1. A positive charge for Q means charge transfer from the monolayer to the gas molecules. The equilibrium distance of 1.53 Å between NO 2 and the monolayer revealed that NO 2 forms a stable chemical bond. Moreover, the NO 2 molecules showed high adsorption energies, indicating that ZnSiP 2 is more sensitive to NO 2 molecules than the other five molecules. As shown in Table 1, the Bader charge analysis indicated that there were 0.24, 0.12, 0.67, and 0.13 electron transfers between the molecules and the substrates for SO 2 , NO, NO 2 , and NH 3 , which further implies that NO 2 molecules have strong chemical interactions with the ZnSiP 2 monolayer. The electronic band structures and densities of states for gas-ZnSiP 2 are shown in Figures 10 and 11, respectively. All the systems, except for NO and NO 2 , that adsorbed the ZnSiP 2 monolayer became direct band-gap semiconductors, and both VBM and CBM were at the Gamma point. It can be clearly seen from Figures 10 and 11 that the NO and NO 2 adsorbed on the ZnSiP 2 monolayer introduced a high density of states at the Fermi surface, which made the ZnSiP 2 exhibit a metallic character and changed the electronic properties of the ZnSiP 2 monolayer easily. The adsorption of CO, CO 2 , and NH 3 had no significant effect on the band structure, and the band gaps did not change much. For SO 2 adsorption (see Figure 10c), the shallow donor energy levels were introduced into the energy band, resulting in the narrowing of the band gap. Combining all the above results, we can conclude that the ZnSiP 2 monolayer is promising as a sensor of NO 2 gas molecules with high selectivity and sensitivity.

Computational Methods
To find the lowest energy structure of 2D ZnSiP2, a swarm-intelligence-based PSO method, implemented in CALYPSO code [52,53], combined with first-principles calculations, was employed, which has been used to successfully predict many 2D systems, such as Cu2Si, PC6, SnP3, and B2N3 [19,34,54,55]. The structures of 2D ZnSiP2 were searched Figure 11. The total densities of states (TDOSs) derived from the PBE functional of the molecules adsorbed on the ZnSiP 2 monolayer.

Computational Methods
To find the lowest energy structure of 2D ZnSiP 2 , a swarm-intelligence-based PSO method, implemented in CALYPSO code [52,53], combined with first-principles calculations, was employed, which has been used to successfully predict many 2D systems, such as Cu 2 Si, PC 6 , SnP 3 , and B 2 N 3 [19,34,54,55]. The structures of 2D ZnSiP 2 were searched with the simulation cells containing 1-4 formula units. The population size and the number of generations were both set to 30, which have been tested to give convergent results. In the first generation, a population of the structures was generated randomly. In the following generation, 60% of the population was generated from the lowest energy structures in the previous generation and all of the structures were fully relaxed, including the atomic positions and the lattice parameters.
The first-principles calculations based on density functional theory were performed using the projector-augmented wave (PAW) method, as implemented in VASP software [56][57][58]. The exchange correlation potential was described using Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation [59] and corrected by the van der Waals (vdW) interaction in the calculation of the adsorption properties of ZnSiP 2 . The plane-wave energy cut-off and Monkhorst-Pack K-point mesh density were set to 500 eV and 2π × 0.03 Å −1 , respectively. All geometries were optimized and relaxed until a total energy change smaller than 10 −6 eV and a force tolerance acting on each atom less than 0.001 eVÅ −1 was achieved. In order to make the band-gap calculation more accurate for semiconductors, the HSE06 functional was employed [60]. A vacuum thickness of 25 Å was used to avoid the interlayer interactions. The nudged elastic band (NEB) method was used to obtain the K-ion diffusion energy barrier. To assess the dynamic stability, phonon spectra were calculated using the PHONOPY code [61]. In addition, ab initio molecular dynamics (AIMD) were explored with the NVT ensemble to examine the thermal stability.
In order to study the interactions between metals (gas molecules) and substrates, adsorption energies and adsorption distances were systematically calculated, according to the following equation: where E total , E ZnSiP 2 , and E metal(gas) represent the total energy of the metal (gas molecules) adsorbed on the ZnSiP 2 monolayer, the perfect ZnSiP 2 monolayer, and the metal in the bulk metal or gas molecules, respectively, and n is the number of adsorbed metal atoms. The adsorption stability of the K-ion layer on the ZnSiP 2 monolayer is estimated by average adsorption energy, which is calculated using the following formula: where E ntotal and E (n−1)total refer to the total energies of the ZnSiP 2 monolayer with n and (n−1) layers and m is the number of K atoms in every layer. For a given concentration x of K x Zn 2 Si 2 P 4 , the open-circuit voltage (OCV) can be obtained with the following equation: where E(x 2 ) and E(x 1 ) are the total energies of K x Zn 2 Si 2 P 4 at two adjacent K-ion concentrations x 2 and x 1 , e is the element charge, and E K is the energy of one K atom in the bulk K metal. The theoretical capacity can be evaluated from: where c is the number of adsorbed K atoms per ZnSiP 2 unit, F is the Faraday constant (26,801 mAhmol −1 ), and M is the molar weight of ZnSiP 2 in gmol −1 .

Conclusions
In summary, we predicted the ZnSiP 2 monolayer as a new 2D semiconductor material which can be used as an anode material for K-ion batteries and NO 2 gas sensors by the global-optimization algorithm combined with first-principles calculation. Phonon simulation, molecular dynamics, and elastic-constant calculations confirmed its stability. The calculated electronic structure and mechanical properties indicate that ZnSiP 2 has an indirect band gap of 1.79 eV and exhibits anisotropic mechanical characteristics. Furthermore, we investigated 2D ZnSiP 2 as an anode for KIBs. The ZnSiP 2 monolayer has a theoretical capacity of 517 mAh/g for K-ions and a low diffusion barrier of 0.12 eV. In addition, we also investigated the gas-sensing properties of the ZnSiP 2 monolayer with six gas molecules (CO, CO 2 , SO 2 , NO, NO 2 , and NH 3 ). The results show that the ZnSiP 2 monolayer is a promising gas sensor for NO 2 with high sensitivity and selectivity.
Author Contributions: Conceptualization, C.P. and D.Z.; formal analysis, C.P., Z.W. and X.T.; writing-original draft preparation, C.P.; writing-review and editing, D.Z. and J.C.; funding acquisition, D.Z., C.P. and J.C. All authors have read and agreed to the published version of the manuscript.

Conflicts of Interest:
The authors declare no conflict of interest.
Sample Availability: Samples of the compounds are available on request from the authors.