Elemental Segregation at LLZO Grain Boundaries: Eliminating Its Detrimental Role in Conductivity and Lithium Nucleation

Garnet Li ₇ La ₃ Zr ₂ O ₁₂ (LLZO) electrolyte is considered a key enabler of solid-state batteries (SSBs) with Li metal anodes, but the formation of grain boundaries (GBs) impairs its performance. To date, the mechanistic understanding of GB structures and its impact on cell performance remains elusive. Here, we show that element segregation at LLZO GBs critically governs Li transport and nucleation behavior by forming an intricate GB structure comprising GB core and edges. During conventional sintering, Al, Ta, and La segregate at GBs, locally depleting Li and creating space-charge layers that lower total ionic conductivity. Simultaneously, this segregation leads to higher electronic conductivity along GBs, which promotes microscale Li nucleation at GB edges and increases the risk of dendrite formation. The underlying mechanism of segregation is contributed to the interplay between thermodynamic driving forces and diffusion kinetics along the Li-ion migration pathways during sintering. Building on this mechanistic understanding, we develop a universal strategy to achieve segregation-free GBs through a rapid yet precisely controlled sintering protocol that utilizes the onset of solid-state softening, identified by an inflection in the densification curve, and is applicable to various ceramic materials. This approach yields transparent, polycrystalline LLZO with negligible GB impedance and markedly enhanced dendrite tolerance. By elucidating the structural origins and electrochemical consequences of GB segregation, this work provides a blueprint for the rational development of next-generation electrolytes with superior performance in SSBs.