Improvement of Electrochemical Performance of Bulk-Type All-Solid-State Lithium Secondary Batteries By Using Sulfide Solid Electrolyte Thin Films

All-solid-state batteries using inorganic solid electrolytes are widely studied as next generation batteries. Two types (bulk or thin-film) of batteries have been investigated. Especially, bulk-type batteries, which use composite electrodes of active material and electrolyte powders, are anticipated for power sources with high energy density. Solid electrolyte thin films are useful for bulk-type batteries with an ideal electrode-electrolyte interface. Furthermore, the solid electrolyte contents in composite electrodes can be reduced significantly. We previously fabricated amorphous Li₂S-P₂S₅ and Li₂S-GeS₂ thin films with high lithium-ion concentrations by pulsed laser deposition (PLD) [1, 2], and this technique was used for coating solid electrolyte thin films on active material particles. Consequently, the energy density of all-solid-state batteries normalized by the total weight of the composite electrode increased [2, 3]. In order to further improve the battery performance, solid electrolyte thin films with higher lithium-ion conductivity are indispensable. We prepared the 33Li₄GeS₄·67Li₃PS₄ (mol%) thin film with an ionic conductivity of 1.1×10^-4 S cm^-1, and the conductivity was increased to 1.8×10^-3 S cm^-1 by a heat treatment at 200 ^oC. The all-solid-state cell using LiCoO₂ particles coated with the 33Li₄GeS₄·67Li₃PS₄ (SE) thin film was charged and discharged with a larger capacity of 67 mAh g^-1 than that using non SE-coated LiCoO₂ particles. Moreover, the capacity of all-solid-state cell increased to 80 mAh g^-1after enhancing the ionic conductivity of SE-coating layer after the heat treatment. The coating of highly-ion conductive solid electrolyte thin films on active material particles is considered to be effective in forming an ideal interface between electrode and electrolyte, resulting in the increase of energy density in bulk-type all-solid-state batteries.

Acknowledgement: This study is financially supported by Toyota Motor Corporation.

References

[1] A. Sakuda et al., J. Am. Ceram. Soc., 93(2010) 765-768.

[2] Y. Ito et al., Solid State Ionics, 236(2013) 1-4.

[3] A. Sakuda et al., J. Power Sources, 196 (2011) 6735-6741.