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

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.

Figures (9)

• Figure 1: Schematic depiction of the simplified vacancy-transport mechanism. A sodium vacancy is generated at the interface by oxidation (red), it is thermally excited into the electrode bulk (blue), and diffusively transported within the electrode bulk (green).
• Figure 2: a) Schematic depiction of the three-electrode setup to measure void accumulation. b) Representative measurement of critical void accumulation at $I_\mathrm{crit}$ and $23\,\degree\mathrm{C}$.
• Figure 3: Determination of the critical current density $j_\mathrm{crit}$ of void formation at temperatures between $-30$ and $90\,\mathrm{\degree C}$. The points where the voltage deviates from linearity with current by $5\,\%$ are indicated. Per cell modification and temperature, the highest $j_\mathrm{crit}$ cell out of three sample cells is selected. The different cell modifications are schematically depicted, starting with a) $\mathrm{Na_{1.9}Al_{10.67}Li_{0.33}O_{17}}$, b) $\mathrm{Na_{3.4}Zr_2Si_{2.4}P_{0.6}O_{12}}$ and c) $\mathrm{Na_5SmSi_4O_{12}}$ SEs, followed by series d) with $\mathrm{Na_{3.4}Zr_2Si_{2.4}P_{0.6}O_{12}}$ SEs and a modified Na anode with reduced grain size and series e) with $\mathrm{Na_{3.4}Zr_2Si_{2.4}P_{0.6}O_{12}}$ and a Sn interlayer between SE and Na anode.
• Figure 4: Determination of activation energies $E_\mathrm{A}$ from Arrhenius fits of the critical current densities determined in Figure \ref{['fig:Measurement Data']}, plotted against inverse temperature.
• Figure 5: Optical microscopy of etched Na microstructure in two magnifications, a) unaltered microstructure, b) microstructure with reduced grain sizes after folding Ag nanoparticles ($2\,\mathrm{volume\,\%}$) $20$ times into Na.