Understanding of Correlation between Li Diffusion and Local Environments in Solid Electrolyte Based on Kinetic Energy Barriers As Local Structure Variations By First Principles Calculation
Authors: Hwang Taesoon , Kim Hyungjun , Cho Maenghyo , Cho Kyeongjae 
Published: February 2020

Recently, demands on energy storage systems of high capacity and efficiency are continuously increased as the growth in commercialization of new renewable energy generation, such as wind and solar power, electric vehicle and portable electronic devices. Li-ion battery(LIB) has been introduced in diverse electronic products as one of the promising energy storage devices because of good energy density and long cycle life to answer these demands. For these reasons, study on LIB actively progressed to improve the performance of LIB. However, there are critical obstacles to make advance in energy density, such as increase in voltage and capacity, of conventionally used LIB. The commercialized LIB system use liquid electrolytes as for Li-ion migration between two electrodes. This liquid electrolyte deteriorate the stability of LIB, bring about overheating and explosion as temperature changes originated mainly from increase in operating voltage. In addition, leakage of organic based liquid electrolyte from the inside of LIB could have noxious effect on health. Therefore, alternatives on liquid electrolyte have been highly required to resolve these limitations.
As increase in efforts to develop new electrolyte types, Inorganic solid electrolytes have received attentions to substitute it for liquid electrolytes. Inorganic solid electrolytes is one of the promising alternatives of liquid electrolyte because of their excellent thermal stability and good Li ion conductivity. Among inorganic electrolytes, Li10GeP2S12 (LGPS) indicates the great Li ion conductivity to be comparable with Li ion conductivity of liquid conductivity. Despite of this superior Li ion conductivity, the understanding of mechanism for Li ion diffusion in LGPS has not been clearly understood. This study explains correlation between Li ion diffusion and local environments of LGPS to understand mechanism of Li ion migration by first principles calculation.
LGPS has enough Li ion concentrations and two types of migration paths. Li ion could migrate in frameworks of Ge-S and P-S(fig.1a). These frameworks secure c-axis and ab-plane paths for Li ion diffusions. There are also two types of methods for first principles calculation of Nudged Elastic Band(NEB) method of Li ion diffusion. One is defect migration (single ion migration) and the other is multi ion concentration migration(fig.1). Firstly, defect migration was analyzed by NEB. We setup three local environments for understanding the correlation in c-axis direction migration. The three situations are only one Li ion without any other Li ions in LGPS(fig.1b), one Li ion in the migration path around Li ions existent in the other non-migration paths(fig.1c) and only one vacancy with existent of Li ions in the migration path(fig.1b).
Although the first and second cases of Li ion migration indicates similar tendency and energy barrier for Li ion migration, the third case shows that energy barrier in same migration path is more higher than the energy barrier of the other cases. This means that the factor influencing the Li ion diffusion is not the local Li ions in non-migration path but the near Li ions in the migration path. In addition to single ion diffusion, multi ion concentration migration is examined and this type has no vacancy and assumes that the all Li ions in the path diffuse all together. The energy barrier is lower than the energy barrier in all of single ion migration cases(fig.1f). This indicates that simultaneous migration of all Li ions in migration path is energetically beneficial.
The ab-plane migration has two migration paths and the energy barriers are relatively higher than the energy barrier of c-axis migration path(fig.1e). This means that Li ion migration is mainly progressed in c-axis rather than ab-plane regardless of local environments in LGPS. Therefore, this understanding of mechanism for Li ion diffusion in LGPS would be effective guide line for developing the superior inorganic solid electrolyte based on LGPS.

Figure 1