As next generation batteries, all-solid-state batteries using inorganic solid electrolytes are widely investigated. 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 the formation of an ideal electrode-electrolyte interface in bulk-type batteries. Furthermore, the solid electrolyte contents in composite electrodes can be reduced significantly. We previously reported the preparation of amorphous Li₂S-P₂S₅ and Li₂S-GeS₂ thin films by pulsed laser deposition (PLD), and this technique was applied for solid electrolyte coatings on LiCoO₂ particles. In order to further improve the battery performance, solid electrolyte thin films with higher lithium-ion conductivity are demanded. We fabricated the Li₂S-GeS₂-P₂S₅ 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. In this study, bulk-type all-solid-state batteries using LiCoO₂ particles coated with Li₂S-GeS₂-P₂S₅thin films were constructed. For the fabricated batteries, we investigated charge-discharge performances, electrode-electrolyte interfacial resistances, and microstructural changes in the composite electrode. Moreover, Raman mapping was conducted for the composite electrode to investigate the local SOC distributions.

The surface of LiCoO₂ particles was uniformly covered with Li₂S-GeS₂-P₂S₅ (SE) thin films. The thickness of SE-coating layer was estimated to be about 180 nm, corresponding to the amount of 3 wt% SE in the composite electrode. In SEM images of positive electrode using LiCoO₂ particles with SE-coatings, SE-coating layer was in close contact with LiCoO₂ particles. However, a lot of voids were observed in the electrode layer. On the other hand, the number of voids in the positive electrode decreased considerably in SEM images of positive electrode using SE-coated LiCoO₂ particles with heat treatment at 200 ^oC. The all-solid-state cell using SE-coated LiCoO₂ particles was charged and discharged with a larger capacity than that using non SE-coated LiCoO₂ particles. Moreover, the all-solid-state battery using SE-coated LiCoO₂ with heat treatment showed a larger capacity and better cycle performance. At the impedance plots after charging process, the interfacial resistance between positive electrode and SE-coating layer decreased to 5.8 Ω cm² after heat treatment. The cell using SE-coated LiCoO₂ particles with heat treatment was discharged under high current density of more than 3.9 mA cm^-2 at room temperature. Raman mapping image for composite electrode using LiCoO₂particles with SE-coating and subsequent heat treatment indicated that electrochemical reactions did proceed more uniformly.

Sulfide electrolyte coatings on active material are considered to be effective in forming an ideal electrode-electrolyte interface, resulting in the increase of energy density in bulk-type all-solid-state batteries. Furthermore, the effect of heat treatment for SE-coated LiCoO₂particles is considered to be the increase of ionic conductivity in SE-coating layer, and the decrease of voids in the composite electrode. Especially, the decrease of voids is effective in the reduction of the loss of lithium-ion conduction paths. These effects of SE-coating and heat treatment would lead to a larger discharge capacity, better cycle performance, and better rate performance in the fabricated all-solid-state cells.