Electro-Chemo-Mechanical Phenomena Governing Dendrite Formation in Solid-State Batteries

Solid electrolyte-based Li-ion batteries can enable energy storage devices with high energy and power densities due to their compatibility with high voltage cathodes (&gt;5 V vs Li/Li⁺) and Li metal anode. Among numerous solid electrolytes, doped Lithium Lanthanum Zirconium Oxide (LLZO) has a high room temperature ionic conductivity (~10^-3 mS cm^-1) and electrochemical stability. LLZO when cycled with Li metal in a symmetrical cell configuration (Li-LLZO-Li), continuous stripping and plating results in the formation of Li metal filaments (dendrites) which nucleate on the cathode, propagate through the solid electrolyte, and short-circuit the cell. The exact phenomena governing the dendrite formation are unclear.<br/><br/>In this work, we have optimized the synthesis and sintering conditions of doped LLZO (both Al and Ga doped) and achieved low interfacial resistance between Li metal and solid electrolyte (&lt;20 Ω cm²). By cycling under constant pressure and temperature, and careful unidirectional plating, we have estimated the true critical current density (CCD) at which a dendrite nucleates and short-circuits the cell. We have also developed an electro-chemo-mechanical model for the nucleation and propagation of dendrites and have corroborated the model with experiments. We show that the process of Lithium ion (Li⁺) transfer from solid electrolyte to Li metal (Li⁰) in a prefilled crack can break open a hard ceramic solid electrolyte and thus create a pathway for Li⁰ to plate inside the crack. We also hypothesize the conditions that dictate whether a dendrite grows in a filled mode, i.e., when a dendrite and crack grow together or in a dry mode, i.e., where the crack grows much faster than the dendrite.