Electro-Chemo-Mechanical Evolution at the Garnet Solid Electrolyte-Cathode Interface

Solid-state batteries promise higher energy density and improved safety compared with lithium-ion batteries. However, electro-chemomechanical instabilities at the solid electrolyte interface with the cathode and the anode hinder their large scale implementation. Here, we focus on resolving electro-chemo-mechanical instability mechanisms and their onset conditions between a state-of-the-art cathode, LiNi_0.6Mn_0.2Co_0.2O₂ (NMC622), and the garnet Li₇La₃Zr₂O₁₂ (LLZO) solid electrolyte. We used thin-film NMC622 on LLZO pellets to place the interfacial region within the detection depth of the X-ray characterization techniques. The experimental probes of the near-interface region included in operando X-ray absorption spectroscopy and ex situ focused ion beam scanning electron microscopy. Electrochemical degradation was not observable during cycling at room temperature with 4.3 V versus Li/Li⁺ charge voltage cutoff, or with stepwise potentiostatic hold up to 4.1 V versus Li/Li⁺. In contrast, secondary phases including reduced transition metal species (Ni²⁺, Co²⁺) were found after cycling up to 4.3 V versus Li/Li⁺ at 80 °C and during potentiostatic hold at 4.3 V versus Li/Li⁺ (Ni²⁺). Intergranular cracks between NMC622 grains and delamination at the NMC622|LLZO interface occurred readily after the first charge. These interface reaction products and mechanical failure lowered the capacity and cell efficiency due to partial loss of the NMC622 phase, partial loss of contact at the interface, and a higher polarization resistance. Electrochemical instability between delithiated NMC622 and LLZO could be mitigated by using a low charge voltage cutoff or cycling at lower temperature. Ways to engineer the mechanical properties to avoid crack deflection and delamination at the interface are also discussed for enhancing mechanical stability.