Study on the Stability of Sulfide Solid-State Electrolyte Based upon Electronic Structure Calculated from First Principles Calculation
Authors: Kim Hyungjun , Hwang Taesoon , Cho Maenghyo , Cho Kyeongjae 
Published: February 2020

With the increasing demand for electric vehicles (EVs) and portable electronic devices, interest in high energy density lithium-ion batteries (LIBs) is continuously growing. However, to enhance the overall properties of LIBs, alternatives are needed from the material aspect due to the intrinsic limitations of current commercialized LIBs such as instability of liquid electrolyte. As an alternative to mitigate thermal instability of LIBs, all-solid-state lithium batteries (ASSLBs) employing a solid-state electrolyte is receiving attention, which would mitigate flammability and internal short circuit. However, studies on the intrinsic properties of these solid-state electrolyte have been limited comparing to a growing interest on ASSLBs. Therefore, in this study, the stability of Li10GeP2S12 (LGPS), one of the sulfide solid-state electrolytes, is investigated based on first principles calculation.

Designing a unit crystal structure of the LGPS to be used for first principles calculations is difficult because of the need to independently consider the Ge/P arrangements and lithium ions along c-axis arrangements in LGPS. Various researches have been suggested a complex LGPS crystal structure for first principles calculation, but all of the proposed models are slightly different. Recently, Oh et al. used the model considering Ge/P arrangements, lithium ions equi-distance arrangements, and the symmetry of the crystal system. As this model having reliability by considering all the possibilities with high throughput DFT calculations, all the calculations in this study will be conducted based on the crystal structure of LGPS in Fig 1. (a).

We investigated the diffusion coefficients of lithium ions in LGPS at various temperature using ab initio molecular dynamics (AIMD) simulations. At the beginning of the simulation, the desirable temperature of the system was set by velocity scaling method. After the temperature of the system matched with assigned temperature, the AIMD simulation was executed until the diffusivity converged in the NVT ensemble with a Nosé-Hoover thermostat. With the calculated diffusivity at several high temperature organized on a table in Fig 1. (b), the diffusivity of LGPS at room temperature was extrapolated and well match with experimental results.

After verifying the reliability of the unit crystal structure of LGPS, the electronic structure of LGPS was calculated to elucidate the electrochemical stability of LGPS along various temperature. As shown on a table in Fig 1. (c), the nature of lithium inside LGPS at room or high temperature is ionic state, almost +1 charged state, inside the solid-state electrolyte. Moreover, the other elements, Ge, P, and S, have the constant charge states at different temperature, showing the nature of solid-state electrolyte. Furthermore, in order to qualitatively compare the electrochemical reactivity with regard to temperature, the density of states (DOS) was plotted in Fig 1. (d). The DOS of the elements at different temperature shows the properties of insulator of LGPS, and the reduced band gap at high temperature. From the results of the electronic structure, the Bader charge and the DOS, indicate that LGPS states in constant charge, but increasing electrochemical reactivity by narrowing the band gap as temperature increase.

In this study, we elucidate the stability of LGPS by temperature. The electronic structure calculation based on first principles calculation confirms that the change of electric charge of each atom is small according to temperature, but the possibility of electrochemical reaction become high. As a result, LGPS can act as a solid-state electrolyte even at high temperature, but the dangerousness of internal short circuit could be increased if operating temperature of LGPS become extremely high considering band gap variation along temperature.

Figure 1