Tailoring grain boundaries of solid-state electrolytes for building better all-solid-state lithium batteries

Grain boundaries (GBs), which are inherent and unavoidable microstructural features in polycrystalline materials, play a decisive role in determining the overall performance of solid-state electrolytes (SSEs). Although SSEs hold great promise for enabling safer and higher-energy-density all-solid-state lithium batteries (ASSLBs), their functional properties are largely dominated by GBs. These GBs severely impede ionic transport, promote lithium dendrite propagation, and accelerate interfacial degradation, thereby compromising the performance and lifespan of the cell. However, despite the significant research on GB regulation, a comprehensive review of their formation mechanisms and improvement strategies specific to ASSLBs is still lacking. In this review, we highlight the formation mechanism of GBs and the effect of their fundamental physical and electrochemical characteristics on the safety and electrochemical performance of SSEs, integrating recent insights from experimental and computational analyses. The effective strategies for suppressing and mitigating GB-related limitations are summarized to enhance the performance of SSEs, along with proposed future research directions in GB engineering. By consolidating current knowledge, this review aims to connect fundamental understanding to practical battery design and provide actionable guidance for addressing the GB challenges in ASSLBs.