Mechanisms of Metal Penetration in Solid Electrolytes

Abstract

An important unresolved topic in materials science is the mechanism by which metals infiltrate solid electrolytes during electrodeposition. A deep understanding of this phenomenon in Li+-conducting solid electrolytes could determine whether these materials can enable fast-charging (> 3 mA cm⁻²) solid state batteries that are safer and more energy-dense than the state of the art. At present, it is thought that intensified stresses are generated at the largest surface flaws on the electrolyte during electrodeposition, and at the critical current density these stresses drive brittle fracture within the bulk to create paths for metal advancement.

This thesis demonstrates that metal penetration depends on two additional factors. The first is whether electric field focusing is present between the stripping and plating electrodes. We show that amplified electric fields, which correlate with increased local current densities, cause Li filled cracks to initiate and grow to penetration, overriding the presence of larger surface defects elsewhere. The second factor is the yield stress of the electrodeposited metal. We show that in Li⁺-, Na⁺-, and K⁺-conducting solid state systems, the critical current density scales inversely with the mechanical deformation resistance of the electrodeposited metal.

We then present two novel electrode architectures in which a liquid phase enables higher critical current densities via interfacial stress relief and current homogenization. First, biphasic (liquid-solid) Na-K alloys are shown to exhibit K⁺ critical current densities over 15 mA cm⁻², in contrast to 2.5 mA cm⁻² for pure K metal. Second, an interfacial film of Na-K liquid between Li metal and Li₆.₇₅La₃Zr₁.₇₅Ta₀.₂₅O₁₂ solid electrolyte doubles the critical current density compared to cells without the Na-K interlayer. These design approaches hold promise for overcoming mechanical stability issues that have heretofore limited the performance of solid state batteries.