All‐solid‐state batteries with an alkali metal anode have the potential to achieve high energy density. However, the onset of dendrite formation limits the maximum plating current density across the solid electrolyte and prevents fast charging. It is shown that the maximum plating current density is related to the interfacial resistance between the solid electrolyte and the metal anode. Due to their high ionic conductivity, low electronic conductivity, and stability against sodium metal, Na‐β″‐alumina ceramics are excellent candidates as electrolytes for room‐temperature all‐solid‐state batteries. Here, it is demonstrated that a heat treatment of Na‐β″‐alumina ceramics in argon atmosphere enables an interfacial resistance <10 Ω cm2 and current densities up to 12 mA cm−2 at room temperature. The current density obtained for Na‐β″‐alumina is ten times higher than that measured on a garnet‐type Li7La3Zr2O12 electrolyte under equivalent conditions. X‐ray photoelectron spectroscopy shows that eliminating hydroxyl groups and carbon contaminations at the interface between Na‐β″‐alumina and sodium metal is key to reach such values. By comparing the temperature‐dependent stripping/plating behavior of Na‐β″‐alumina and Li7La3Zr2O12, the role of the alkali metal in governing interface kinetics is discussed. This study provides new insights into dendrite formation and paves the way for fast‐charging all‐solid‐state batteries.
Na-β″-alumina
ceramics are archetypical ion conductors with excellent sodium-ion
conductivity. Their processing is, however, challenging and results
in large variations in reported conductivity measurements. We systematically
reexamine the impact of sintering conditions on microstructure and
sodium-ion conductivity of Na-β″-alumina ceramics. Depending
on sintering temperature and sintering time, we measure conductivities
between 0.04 and 0.37 S/cm at 300 °C on ceramics prepared from
identical starting powders. During sintering, formation of a liquid
phase is observed above 1500 °C, which promotes densification
but leads to abnormal grain growth for extended sintering times. While
such conditions result in the highest conductivities measured for
our sample series (0.37 S/cm at 300 °C), the corresponding microstructures
are mechanically fragile. For mechanically robust, densely sintered
samples, we identify the average grain size as the dominating factor
controlling ion conductivity. For average grain sizes between 1 and
6 μm, we obtain conductivities between 0.17 and 0.27 S/cm at
300 °C. The influence of porosity in undersintered, highly porous
samples is well accounted for by Archie’s law and results in
low ion conductivities down to 0.04 S/cm at 68% density. Our insights
into microstructural factors controlling ionic conductivity such as
grain size and density are instrumental for the successful integration
of Na-β″-alumina ceramic electrolytes into next-generation
batteries.
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