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

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 $j_{\mathrm{c}\mathrm{r}\mathrm{i}\mathrm{t}}$ the anode voltage no longer increases linearly but in an exponential fashion. Arrhenius analysis of $j_{\mathrm{c}\mathrm{r}\mathrm{i}\mathrm{t}}(T)$ for the three solid electrolytes $\mathrm{N}{\mathrm{a}}_{1.9}\mathrm{A}{\mathrm{l}}_{10.67}\mathrm{L}{\mathrm{i}}_{0.33}{\mathrm{O}}_{17}$, $\mathrm{N}{\mathrm{a}}_{3.4}\mathrm{Z}{\mathrm{r}}_2\mathrm{S}{\mathrm{i}}_{2.4}{\mathrm{P}}_{0.6}{\mathrm{O}}_{12}$, and $\mathrm{N}{\mathrm{a}}_5\mathrm{S}\mathrm{m}\mathrm{S}{\mathrm{i}}_4{\mathrm{O}}_{12}$ yields an activation energy $E_{\mathrm{A}}$ of $0.13$ to $0.15\mathrm{e}\mathrm{V}$. This exceeds the activation energy of $0.053\mathrm{e}\mathrm{V}$ 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 $(0.10\pm 0.01)\mathrm{e}\mathrm{V}$, 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.