Experimental Insights into the Limiting Mechanism of Vacancy Transport in Sodium Metal Anodes for Solid State Batteries

TL;DR

This study investigates the vacancy transport limiting mechanism in sodium metal anodes for solid-state batteries, revealing interfacial thermodynamics as the rate-limiting factor and proposing strategies for improved stability and performance.

Contribution

It identifies interfacial thermodynamics, rather than bulk diffusion, as the main bottleneck in vacancy transport in sodium anodes, and demonstrates how interlayers can mitigate this issue.

Findings

Critical current density exhibits exponential increase above a threshold.

Activation energy for vacancy buildup exceeds bulk diffusion energy.

Interfacial thermodynamics, not bulk diffusion, limits vacancy transport.

Abstract

Ceramic solid-state batteries with sodium (Na) metal electrodes promise enhanced safety and energy density compared to contemporary secondary batteries. However, the critical delamination of the Na metal electrode during discharge - when vacancies accumulate at the Na/ceramic interface - remains to be understood and avoided. The study investigates the underlying mechanism by applying a linear current ramp between two Na metal electrodes separated by a ceramic solid electrolyte to provoke vacancy buildup. Above a critical current density $j_\mathrm{crit}$ the anode voltage no longer increases linearly but in an exponential fashion. Arrhenius analysis of $j_\mathrm{crit}(T)$ for the three solid electrolytes $\mathrm{Na_{1.9}Al_{10.67}Li_{0.33}O_{17}}$, $\mathrm{Na_{3.4}Zr_2Si_{2.4}P_{0.6}O_{12}}$, and $\mathrm{Na_5SmSi_4O_{12}}$ yields an activation energy $E_\mathrm{A}$ of $0.13$ to $0.15\,\mathrm{eV}$. This exceeds the activation energy of $0.053\,\mathrm{eV}$ for the diffusive vacancy migration in bulk Na significantly. Further, $E_\mathrm{A}$ is insensitive to anode microstructure variation. Both observations rule out bulk diffusion as the transport bottleneck. A thin tin-sodium alloy interlayer lowers $E_\mathrm{A}$ to $(0.10\pm0.01)\,\mathrm{eV}$, implicating interfacial thermodynamics as rate-limiting. Sodiophilic, Na-conducting interlayers and low-tension interfaces emerge as key pathways to stable, high-rate Na-SSBs at practical stack pressures.