Development of Li10GeP2S12-Type Solid Electrolytes with Electrochemical Stability to Li Metal Anode
Authors: Hori Satoshi , Sun Xueying , Suzuki Kota , Hirayama Masaaki , Kanno Ryoji 
Published: February 2020

Solid-state Li-ion battery promises potentially high-power characteristics, although increase in energy density remains as an issue to be solved.1,2 Therefore solid electrolytes that possess electrochemical stability to Li metal electrode as well as high ionic conductivity are urgently demanded. This study synthesized solid electrolytes using Li10GeP2S12(LGPS),3 a supersonic conductor showing Li ion conductivity over 10−2 S cm−1, as a structural template; we performed material search for LGPS-derivatives in the Li–M–P–S–O–X (M = Ge, Si, Sn; X = F, Cl, Br, I) system in order to elucidate effects of doped- anion/cation towards the stability to Li metal electrode.
We discovered various LGPS-type electrolytes through optimization of synthesis condition for each chemical composition. For the Li–M–P–S–X system, heating at 400–550°C is required, for Li–P–S–O–X at 200–280°C, and for Li–Si–P–S–O at 950–1000°C, respectively. The synthesized solid electrolytes were characterized by structure analysis based on diffraction data and AC impedance method. Electrochemical stability for electrolytes was evaluated by charge–discharge cycling tests of solid-state cell, in which the synthesized electrolyte, Li metal, and the mixture of LiNbO3-coated-LiCoO2 and LGPS was used as a separator, anode, and cathode, respectively. The retention rate of discharge capacity was used as an indicator for evaluating stability towards Li metal.
The effects of oxygen doping were clarified in the Li–M–P–S–O and Li–P–S–O systems. The cell using Li9.42Si1.02P2.1S9.96O2.04 or Li3.2PS3.7O0.3 respectively showed retention rate of more than 98% whereas Li–M–P–S cells showed less than 20% after 5th cycle.4 However, these oxygen-doped electrolytes suffer from lower conductivity on the order of 10−4 S cm−1. These results indicate oxygen doping improves electrochemical stability but at the cost of decrease in conductivity. On the other hand, the Li–P–S–X system showed 99% capacity retention after 5th cycle as well as a high conductivity of 4×10−3 S cm, suggesting doping halogen elements is a promising technique for achieving a better balance between stability and conductivity for LGPS-type electrolytes.
References

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Hori, S.; Suzuki, K.; Hirayama, M.; Kato, Y.; Kanno, R., Lithium Superionic Conductor Li9.42Si1.02P2.1S9.96O2.04 with Li10GeP2S12-Type Structure in the Li2S–P2S5–SiO2 Pseudoternary System: Synthesis, Electrochemical Properties, and Structure–Composition Relationships. Frontiers in Energy Research 2016, 4 (38).