Trigger of the Highly Resistive Layer Formation at the Cathode-Electrolyte Interface in All-Solid-State Lithium...

Interfacial materials design is critical in the development of all-solid-state lithium batteries. We must develop an electrode-electrolyte interface with low resistance and effectively utilize the energy stored in the battery system. Here, we investigated the highly resistive layer formation process at the interface of a layered cathode: LiCoO₂, and a garnet-type solid-state electrolyte: Li_6.4La₃Zr_1.4Ta_0.6O₁₂, during the cosintering process using in situ/ex situ high-temperature X-ray diffraction. The onset temperature of the reaction between a lithium-deficient Li_xCoO₂ and Li_6.4La₃Zr_1.4Ta_0.6O₁₂ is 60 °C, while a stoichiometric LiCoO₂ does not show any reaction up to 900 °C. The chemical potential gap of lithium first triggers the lithium migration from the garnet phase to the Li_xCoO₂ below 200 °C. The lithium-extracted garnet gradually decomposes around 200 °C and mostly disappears at 500 °C. Since the interdiffusion of the transition metal is not observed below 500 °C, the early-stage reaction product is the decomposed lithium-deficient garnet phase. Electrochemical impedance spectroscopy results showed that the highly resistive layer is formed even below 200 °C. The present work offers that the origin of the highly resistive layer formation is triggered by lithium migration at the solid-solid interface and decomposition of the lithium-deficient garnet phase. We must prevent spontaneous lithium migration at the cathode-electrolyte interface to avoid a highly resistive layer formation. Our results show that the lithium chemical potential gap should be the critical parameter for designing an ideal solid-solid interface for all-solid-state battery applications.