Electrochemical and structural evaluation for bulk-type all-solid-state batteries using Li4GeS4-Li3PS4 electrolyte coating on LiCoO2 particles

Bulk-type all-solid-state batteries, which use composite electrodes with a powder mixture of active materials and solid electrolytes, are anticipated for large-scale power sources. However, conventional powder mixing protocols are insufficient to maintain ion-conductive pathways within composite electrodes. Herein, sulfide electrolyte coatings have attracted attention as a promising means to overcome this difficulty. We assessed the effects of sulfide electrolyte coatings for active materials on the electrochemical properties and structural changes in all-solid-state cells. A favorable electrode-electrolyte interface was formed by coating significantly small amounts (ca. 3 wt%) of Li4GeS4-Li3PS4 solid electrolyte (SE) onto LiCoO2 particles via vapor phase process. The all-solid-state cell (In/Li2S-P2S5/SE-coated LiCoO2) was charged and discharged with a larger capacity than that using non-SE-coated LiCoO2 particles, indicating that the SE-coating is effective in forming a favorable ion-conductive pathway to LiCoO2 particles. Improvement of the cell performance after heat treatment was considered to derive not only from the enhancement of ionic conductivity in the SE-coating layer, but also from the reduction of voids in the composite electrode. Less ionic resistance and denser environment are beneficial for the Li-ion supply to the deepest part in the composite electrode, which results in more homogeneous electrochemical reaction in all-solid-state cells.