Thermal and Electrochemical Interface Compatibility Between Hydroborate Solid Electrolytes and High-Voltage Cathodes for All-Solid-State Batteries

Stable long-term cycling of high-voltage all-solid-state batteries requires interfacial stability of solid electrolytes with cathodes and anodes simultaneously. Therefore, materials for solid electrolytes need to combine high cation conductivity (≥1 mS cm⁻¹), compatibility with lithium or sodium metal anodes and high-voltage cathodes, and intimate electrode/electrolyte interfaces. Hydroborates are a very promising material class,^1–7 in which some of the electrolytes are softer than sulfides and more electrochemically stable than oxides. By introducing a self-passivating functionality into this yet underexplored class of solid electrolytes, we recently demonstrated a 4 V all-solid-state sodium battery using a sodium metal anode at room temperature with an excellent long-term capacity and energy retention (~80 %) for &gt;800 cycles.⁸ This study records the highest average discharge cell voltage and cathode-based specific energy among all reported all-solid-state sodium batteries so far. However, the thermal stability of the solid electrolytes and their compatibility with battery electrodes at high temperatures needs to be further addressed to ensure stable cycling and high operational safety of all-solid-state batteries.<br/>Here we study the interfacial compatibility of a hydroborate solid electrolyte with 3 V-class cathode active materials: NaCrO₂, NaMnO₂, and NaFeO₂.⁹ Among these layered sodium transition metal oxide cathodes, NaCrO₂ shows the highest thermal compatibility in contact with the hydroborate solid electrolyte up to 525 °C in the discharged state. Furthermore, the electrolyte remains intact upon the internal thermal decomposition of the charged, i.e. desodiated, cathode (Na_0.5CrO₂) above 250 °C, demonstrating the potential for highly safe hydroborate-based all-solid-state batteries with a wide operating temperature range. The experimentally determined onset temperatures of thermal decomposition of the hydroborate solid electrolyte in contact with 3 V-class cathodes surpass those of sulfide and selenide solid electrolytes, exceeding previous thermodynamic calculations. Our results also highlight the need to identify relevant decomposition pathways of hydroborates to enable more valid theoretical predictions. We will further discuss strategies to overcome the electrochemical stability limit of hydroborate solid electrolytes to increase the operating cell voltage of all-solid-state batteries.<br/>References:<br/>L. Duchêne, A. Remhof, H. Hagemann, and C. Battaglia, <i>Energy Storage Mater.</i> <b>2020</b>, <i>25</i>, 782.<br/>M. Brighi, F. Murgia, R. Černý, <i>Cell Rep. Phys. Sci.</i> <b>2020</b>, <i>1</i>, 100217.<br/>L. Duchêne, R.-S. Kuehnel, E. Stilp, E. Cuervo Reyes, A. Remhof, H. Hagemann, C. Battaglia, <i>Energy Environ. Sci.</i> <b>2017</b>, <i>10</i>, 2609.<br/>L. Duchêne, D. H. Kim, Y. B. Song, S. Jun, R. Moury, A. Remhof, H. Hagemann, Y. S. Jung, and C. Battaglia, <i>Energy Storage Mater.</i> <b>2020</b>, <i>26</i>, 543.<br/>F. Murgia, M. Brighi, R. Černý, <i>Electrochem. Commun.</i> <b>2019</b>, <i>106</i>, 106534.<br/>S. Kim, H. Oguchi, N. Toyama, T. Sato, S. Takagi, T. Otomo, D. Arunkumar, N. Kuwata, J. Kawamura, and S. Orimo, <i>Nat. Commun.</i> <b>2019</b>, <i>10</i>, 1081.<br/>R. Asakura, L. Duchêne, R.-S. Kuehnel, A. Remhof, H. Hagemann, and C. Battaglia, <i>ACS Appl. Energy Mater.</i> <b>2019</b>, <i>2</i>, 6924.<br/>R. Asakura, D. Reber, L. Duchêne, S. Payandeh, A. Remhof, H. Hagemann, and C. Battaglia, <i>Energy Environ. Sci.</i> <b>2020</b>, <i>13</i>, 5048.<br/>R. Asakura, L. Duchêne, S. Payandeh, D. Rentsch, H. Hagemann, C. Battaglia, and A. Remhof, <i>ACS Appl. Mater. Interfaces</i> <b>2021</b>, accepted.