Atomic-Scale Engineering of Grain Boundary Chemistry and Potential for Li-Garnet Solid-State Electrolytes

Abstract

With increasing demands to replace flammable liquid electrolytes, solid-state electrolytes have sparked interests both in academic and industrial sectors. In particular, Li-garnet LLZO (Li7La3Zr2O12) exhibits outstanding electrical, chemical, and mechanical properties. Despite the promise, LLZO suffers from the formation of lithium (Li) dendrites at high current operations, thereby limiting the safety and performance of garnet-based solid-state batteries. Li metal nucleation and growth reportedly initiate and grow along the locality of grain boundaries (GBs) in such polycrystalline electrolytes, due either to low ionic or high electronic conductivity of the boundaries. However, limited understanding yet exists on the origins of the local GB conductivity mismatch. The mechanistic understanding should be an important prerequisite to develop engineering strategies of the failing GB interfaces. In this work, we first apply grain boundary core-space charge layer model to understand the basis of Li metal nucleation at GBs. Grain boundary core of LLZO is modeled to have excess negative charge potential, leading to the subsequent Li ionic carrier depletion and electronic carrier accumulation in the adjacent space charge layers. The carrier concentration variations then drive the local electronic conductivity surge and detrimental Li metal nucleation at the GBs. Both electrochemical and microscopic characterization techniques are employed to investigate the atomic-level composition and space charge potential at the GBs. This understanding offers us new strategies for controlling Li nucleation. First, sintering atmosphere is selectively chosen to reduce the potential barrier by suppressing intrinsic defect accumulation at the GB core. Second, an aliovalent dopant, with net-positive charge, is introduced to segregate to GB and mitigate the negative core potential. Both approaches are shown effective in lowering the electronic conductivity and to impede Li metal penetration. We successfully achieve critical current densities (CCD) of ~ 1 mA cm-2,a two-fold increase over prior reports. Our theoretical understanding and the engineering approaches will aid in interpreting the failure origin of the garnet electrolytes, and provide a blueprint on how to engineer GBs for improved safety and performance of all-solid-state Li metal batteries.