Distinguishing the Effects of the Space-Charge Layer and Interfacial Side Reactions on Li₁₀GeP₂S₁₂-Based All-Solid-State Batteries with Stoichiometric-Controlled LiCoO₂

All-solid-state lithium batteries (ASSLBs) with high volumetric energy density and enhanced safety are considered one of the most promising next-generation batteries. Elucidating the capacity-fading mechanism caused by the space-charge layer (SCL) and the interfacial side reaction (ISR) is crucial for the future development of high-energy-density ASSLBs with a longer cycle life. Here, a systematic study to probe the electrochemical performance of Li₁₀GeP₂S₁₂-based ASSLBs with stoichiometric-controlled Li_xCoO₂was performed with the aid of density functional theory (DFT) calculations, X-ray photoelectron spectroscopy (XPS), focused ion beam-field emission scanning electron microscopy (FIB-SEM), and solid-state nuclear magnetic resonance (NMR) spectroscopy. We discovered that the overstoichiometric Li_1.042CoO₂shows a high capacity at first cycle with the smallest overpotential, but the capacity gradually decreases, which is ascribed to the weak SCL effect and strong interfacial side reactions. On the contrary, the lithium-deficient Li_0.945CoO₂achieves the best cycling stability with a very low capacity associated with the strongest SCL effect and weak interfacial side reactions. The SCL effect is indeed coupled with ISR, which eventually leads to capacity fading in long-term operation. We believe that the new insights gained from this work will accelerate the future development of LiCoO₂/LGPS-based ASSLBs with both a mitigated SCL effect and a longer cycle life.