First-principles study on interfacial reaction of SiCO/LTaO for all-solid-state lithium-ion batteries

Silicon oxycarbide is an excellent anode material for all solid batteries, characterized by excellent stability and some oxidation resistance. The structure of the SiCO/LTaO system has been investigated using first principles. We have investigated the electron density, DOS, and energy band structure at the SiCO/LiTaO interface. The calculations note that this particular configuration is expected to be applied to such as lithium-ion batteries and supercapacitors. This work will provide some theoretical support for the difficult problem of lowering impedance in the preparation of all-solid-state lithium-ion batteries.


Introduction
Silicon oxycarbide is a silicon-based polymer-derived ceramic characterized by excellent stability and some oxidation resistance [1][2][3].The presence of silicon as a neutral mixed bond tetrahedron and amorphous carbon nanographene in the SiCO structure is considered to be the reason for the higher lithium storage.However, the prepared all-solid-state battery still suffers from high interfacial impedance as well as lithium dendrites during cycling, and the interface of the electrode-electrolyte is a key element, which is related to space charge.LTaO is considered to be a very promising candidate due to its high chemical stability, good mechanical properties, and small volume change during lithium embedding/detachment. Several research groups have also investigated LTaO as an ASSB material.However, the electrochemical mechanisms on the atomic scale have not been fully investigated so far.In the present work, we have studied the DOS, electron density as well and the energy band structure of the SiCO/LTaO hybrid system by first principles.The computational results show that LTaO material is expected to be a high-quality buffer layer material for solid lithium batteries with high impedance.Provides theoretical support for the commercialization of all solid batteries.

Computational method
All calculations in this paper are performed using the first principles (DFT) method [4] and are implemented using the CASTEP module [5].PBE-GGA (generalized gradient approximation) is chosen as the gradient correction.The energy cutoff of 500 eV and a 3×3×2 k-point lattice.A vacuum layer with a thickness of 15 Ȧ is constructed.
The system is constructed from supercells SiCO and LiTaO 3 , and to eliminate the problems posed by lattice mismatches, 2×2 and 3×3 supercell operations are performed on SiCO and LiTaO 3 , respectively.Geometric optimization is used to relax the SICO/LTaO interface until the force per atom is less than 10 -2 eV/Å, and the energy convergence is less than 10 -5 eV.

Discussion
The geometry of the SiCO/LTaO heterogeneous interfacial system has been completely relaxed as shown in Figure . 1.    matching.In order to study the diffusion path of lithium atoms at the SICO/LTO interface, the activation energies of lithium atoms migrating to the SICO/LTO layer interface or within the electrode layer were calculated by the elastic band-climbing method, and two migration paths were selected in this paper.For convenience of illustration, we name the path of lithium atoms diffusing from the electrolyte LTO body to the SICO body as Path2, and the path of lithium atoms diffusing from the electrolyte body to the SICO/LTO interface as Path1, which is calculated as shown in Fig. 5.The calculated energy barrier from path 1 is 0.48 eV,its value is small, which indicates that it is easier for lithium atoms to migrate to the interface of SICO and LTO, while the calculated migration barrier value from path 2 is 1.25 eV,its value is more than twice as much as that of the migration to path 1, which indicates that it is harder for lithium atoms to diffuse into the body of SiCO.Only a small amount of lithium atoms diffuse into the body, which is due to the fact that lithium atoms are adsorbed by the oxygen atoms on the surface of SICO when they migrate into the body of SiCO.As a result, a large number of lithium atoms migrate to the SICO/LTO interface, consistent with the above findings.

Conclusions
We investigated the interfacial structure of SICO/LTaO by Density Functional theory.We have studied the electron density, DOS, and energy band structure at the SiCO/LiTaO interface.The calculations note that this particular configuration is expected to be applied to such as lithium-ion batteries and supercapacitors.This work will provide some theoretical support for the difficult problem of lowering impedance in the preparation of all-solid-state lithium-ion batteries.

Figure 1 .
Figure 1.Heterogeneous interfaces for the SiCO/LTO systems: (a) up view of SiCO/LTaO; (b) side view of SiCO/LTaO; (c) A single supercell for the SiCO/LTaO system.As can be seen in Figure.2(a), after 40 iterations of geometry optimization, the structure remains stable and its equilibrium energy is -127250.08eV.The maximum ion force, maximum ion displacement, and total energy convergence set for this study are 0.1E-04/atom, 0.27000E-01, and 0.400E-02Ȧ, respectively.The calculated results are shown in Figure.2(b) as 1.578336E -6 eV/atom, 2.9501534E - 2 eV/Ȧ, and 6.02645E -4 Ȧ.All the values are lower than our preset values, which indicates that our calculated results are in line with the computational accuracy.

Figure 2 .
Figure 2. (a) Geometric optimization, (b) energy optimization convergence convergences.The energy band structure of the SiCO/LTaO system is shown in Figure.3, and the band gaps of the two materials constituting the system are 0.291 eV and 0.064 eV, respectively, which are optimized from the original semiconducting properties to the metallic properties, proving that the charge insulating layer at the SiCO/LTao interface has disappeared.

Figure 3 .
Figure 3.The band gap of (a) SiCO (b) LTaO (c) SiCO/LTaO (d) DOS of SiCO/LTaO, SiCO, LTaO.The electron density maps before and after adsorption at the two interfaces are shown in Figure.4,which noted that the middle charge layer has been conducted after the interface matching, which indicates that the SiCO/LTaO composite can improve the conductivity of the electrode.

Figure 4 .
Figure 4. (a) Charge density map before interface matching (b) charge density map after interfacematching.In order to study the diffusion path of lithium atoms at the SICO/LTO interface, the activation energies of lithium atoms migrating to the SICO/LTO layer interface or within the electrode layer were calculated by the elastic band-climbing method, and two migration paths were selected in this paper.For convenience of illustration, we name the path of lithium atoms diffusing from the electrolyte LTO body to the SICO body as Path2, and the path of lithium atoms diffusing from the electrolyte body to the SICO/LTO interface as Path1, which is calculated as shown in Fig.5.The calculated energy barrier from path 1 is 0.48 eV,its value is small, which indicates that it is easier for lithium atoms to migrate to the interface of SICO and LTO, while the calculated migration barrier value from path 2 is 1.25 eV,its value is more than twice as much as that of the migration to path 1, which indicates that it is harder for lithium atoms to diffuse into the body of SiCO.Only a small amount of lithium atoms diffuse into the body,

Figure 5 .
Figure 5. Two path lines of SICO/LTO and corresponding lithium ion jump energy barriers