Room-temperature operation of high-voltage all-solid-state batteries requires solid electrolytes that combine high cation conductivity (≥1 mS cm− 1), a wide electrochemical stability window, and intimate electrode/solid electrolyte interfaces. However, none of the solid electrolytes has fulfilled these material properties so far. Hydroborates, a yet underexplored class of solid electrolytes,1 are highly conductive at room temperature and compatible with lithium and sodium metal anodes; however, the discharge cell voltage was limited up to 3 V in previous reports,2, 3 mainly due to the electrochemical oxidative stability of the solid electrolytes.4 Here we show that a hydroborate solid electrolyte, consisting of two kinds of hydroborate anions with different oxidative stability limits, can be effectively stabilized in contact with a 4 V-class cathode.5 At high voltages, the less stable anions tend to form a passivating interphase layer upon electrochemical oxidation at the cathode/solid electrolyte interface, while the more stable anions remain intact in the interphase layer, maintaining cation conduction. The self-forming interphase enables the first stable cycling of a 4 V-class hydroborate-based all-solid-state battery employing a sodium metal anode and a cobalt-free, high-voltage cathode. The cells exhibit a discharge capacity of 100 mAh g−1 at C/5 and an excellent (>85%) capacity and energy retention after 500 cycles at room temperature. Applying external pressure enables a discharge capacity of >110 mAh g−1 at C/10 with a high areal capacity close to 1.0 mAh cm−2. This work records the highest discharge cell voltage and specific energy (>440 Wh kg− 1) among all reported all-solid-state sodium batteries, demonstrating the attractive material properties and potential of hydroborates which surpass well-investigated oxides and sulfides as solid electrolytes for high-voltage all-solid-state batteries. References L. Duchêne, A. Remhof, H. Hagemann, and C. Battaglia, Energy Storage Mater., 25. 782–794 (2020).L. Duchêne, D. H. Kim, Y. B. Song, S. Jun, R. Moury, A. Remhof, H. Hagemann, Y. S. Jung, and C. Battaglia, Energy Storage Mater., 26, 543–549 (2020).S. Kim, H. Oguchi, N. Toyama, T. Sato, S. Takagi, T. Otomo, D. Arunkumar, N. Kuwata, J. Kawamura, and S. Orimo, Nat. Commun. 10, 1081 (2019).R. Asakura, L. Duchêne, R.-S. Kühnel, A. Remhof, H. Hagemann, and C. Battaglia, ACS Appl. Energy Mater., 2, 6924−6930 (2019).R. Asakura, D. Reber, L. Duchêne, A. Remhof, H. Hagemann, and C. Battaglia, submitted