Transport Kinetics: A New Perspective on Failure of Garnet Solid Electrolytes

Solid-state rechargeable lithium-metal batteries with garnet-type (Li₇La₃Zr₂O₁₂) solid electrolytes (SEs) represent promising candidates of the next-generation high-energy batteries yet their practical use are hindered by a short cycle life usually due to dendrite nucleation and penetration through the garnet. In the previous works, the dendrite nucleation is ascribed to poor wettability of Li metal at the alkaline-residue-covered garnet surface, and high electronic conductivity of garnet that invites Li⁺-electron recombination at grain boundary. In this work, it is showed by constructing a mathematical model on a residue-free garnet particles, that grain size of the garnet has profound influence on Li⁺ transport kinetics, and therefore, the dendrite nucleation. Smaller garnet grains tend to show faster Li⁺ transport in the bulk yet they also involve higher Li⁺ flux diffusing across grain boundaries and Li-garnet interface, which are considered kinetically more sluggish. As a result, more Li-ions tend to accumulate at the grain boundary and the interface, which accounts for unstable local environment and a sharply reduced electron migration barrier, and together they invite dendrite nucleation. Based on the findings, a new asymmetric garnet SE is proposed that features high ionic conductivity and dendrite suppression ability.