KB₃H₈·NH₃B₃H₇ Complex as a Potential Solid-State Electrolyte with Excellent Stability against K Metal

All-solid-state potassium batteries are promising candidates in the fields of large-scale energy storage owing to their intrinsic safety, stability, and cost-effectiveness. However, a suitable solid-state electrolyte with high ionic conductivity and favorable interfacial stability is a major challenge for the design and development of these batteries. Herein, we report the synthesis of new KB₃H₈·nNH₃B₃H₇ (n = 0.5 and 1) complexes to develop suitable solid-state K-ion conductors for batteries. Both the complexes undergo a reversible phase transition below the thermal decomposition temperature. The optimal KB₃H₈·NH₃B₃H₇ delivers a solid-state K-ion conductivity of 1.3 × 10^-4 S cm^-1 at 55 °C with an activation energy of 0.44 eV after a transition from a monoclinic to an orthorhombic phase, which is the highest value of K borohydrides reported to date and places KB₃H₈·NH₃B₃H₇ among the leading solid-state K-ion conductors. Moreover, KB₃H₈·NH₃B₃H₇ reveals a K-ion transference number of nearly 0.93, an electrochemical stability window of 1.2 to 3.5 V vs K⁺/K, a good capability of K dendrite suppression, and a remarkable stability against the K metal anode due to the formation of the stable interface. These performances make KB₃H₈·NH₃B₃H₇ a promising electrolyte for all-solid-state potassium batteries.