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.

Water‐in‐salt electrolytes have enabled the development of novel high‐voltage aqueous lithium‐ion batteries. This study explores the reasons why analogous sodium electrolytes have struggled to reach the same level of electrochemical stability. Solution structure and electrochemical stability are compared for 11 sodium salts, selected among the major classes of salts proposed for highly concentrated electrolytes. The water environment established for each anion is related to its position in the Hofmeister series and a surprisingly strong correlation between the chaotropicity of the anion and the resulting electrochemical stability of the electrolyte is found. The search for suitable sodium salts is complicated by the fact that higher salt concentrations are needed than for their lithium equivalents. Reaching such a high concentration of >25 mol kg−1 with one or a combination of multiple sodium salts that have the desired properties remains a major challenge. Hence, alternative approaches such as multisolvent systems should be explored. The water solubility of NaTFSI can be increased from 8 to 30 mol kg−1 in the presence of ionic liquids. Such a ternary electrolyte enables stable cycling of a 2 V class sodium‐ion battery based on the NaTi2(PO4)3/Na2Mn[Fe(CN)6] electrode couple for 300 cycles at 1C with a Coulombic efficiency of >99.5%.

Using galvanostatic techniques, an oxidative stability up to 4.6 V versus Li/Li+ and beyond has been reported for the prototypical polymer electrolyte consisting of 1 m lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in poly(ethylene oxide) (PEO). However, no long‐term cycling of a battery with this high cut‐off voltage has been demonstrated. Electrochemical and spectroscopic/spectrometric methods are employed to critically reinvestigate the electrochemical oxidation mechanisms of PEO electrolytes. It is found that the onset of PEO oxidation occurs at much lower voltage of around 3.2 V versus Li/Li+, at which the terminal OH group is deprotonated. At 3.6 V, the chain of the PEO is oxidized. Both processes result in the formation of the strong acid HTFSI, which in turn chemically attacks the PEO to form methanol and 2‐methoxyethanol. A stable cycling of a solid‐state lithium‐metal battery with a high‐energy LiNi0.8Mn0.1Co0.1O2 (NMC811) posititve electrode to an upper cut‐off voltage of 3.6 V versus Li/Li+ is demonstrated, however, resulting in enhanced capacity fading when increasing the upper cut‐off voltage to 3.8 V versus Li/Li+ or higher. Thus, operating PEO electrolytes beyond 3.6 V versus Li/Li+ requires protective layers at the positive electrode‐electrolyte interface to prevent PEO oxidation.

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