Electrochemical Impedance Spectroscopic Studies of All Solid State battery with Li10GeP2S12 as Electrolyte

Abstract

All-solid-state batteries will be the main direction of lithium-ion batteries in the future. Current research mainly focuses on improving the conductivity of solid-state electrolytes, but there are few studies on the electronic and ionic transport in all solid state batteries. In this paper, we synthesized Li10GeP2S12 through high temperature solid phase method. The ionic conductivity of Li10GeP2S12 at room temperature is 2.02×10-3 S/cm and it's activation energy calculated from Arrhenius plots is 29.8 kJ/mol. The all solid-state battery of LiNbO3@LiNi1/3Co1/3Mn1/3O2/Li10GeP2S12/Li was successfully fabricated and characterized by galvanostatic charge/discharge (DC), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The first discharge capacity of the all-solid-state battery is 121.2 mAh/g, the coulombic efficiency stabilize at 99.8% after 40 weeks and the capacity retention rate is 93.7% after 100 weeks. After analyzing the electrochemical impedance spectroscopy, the typical impedance spectra of the battery is composed of high frequency semicircle (HFS), middle frequency semicircle (MFS) and low frequency line (LFL). And HFS belongs to the impedance of electrolyte (Rel), MFS belongs to charge transfer impedance (Rct) and LFL belongs to diffusion process of lithium ion in active material. The continuous increase of Rel between 3.8 V and 4.3 V is due to the decomposition of LGPS to GeS2, S and P2S5 at high potential, which results in the decrease of grain conductivity. On the other hand, the voltage range of 3.8~4.3 V is near the charging and discharging plateau at which concentration polarization is large. The stress in the crystal may lead to the breakup of some grains which resulting in the generation of more grain boundaries and the increase of grain boundary impedance. According to the fitting results of Rct, we find that Rct decreases with the increase of potential until 4.3 V at which Rct reaches the minimum value in the first process of charging and it is a reversible process while discharging.