Room Temperature Operation of All‐Solid‐State Potassium‐Ion Battery Enabled by Unconventional Ionic Solid Electrolyte K6[Rh4Zn4(l‐cys)12O]·nH2O

Abstract The unconventional ionic solid, K 6 [Rh 4 Zn 4 (L‐cys) 12 O]· n H 2 O, exhibited excellent formability as well as high ionic conductivity. These features enabled a straightforward assembly of all‐solid‐state potassium‐ion battery: A cold‐pressed solid sample of K 6 [Rh 4 Zn 4 (L‐cys) 12 O]· n H 2 O, which was sandwiched by solid samples of K 2 Ni[Fe(CN) 6 ] and chloranil as cathode, and anode, respectively, displayed an initial discharge capacity of 21.6 mAh g −1 ‐electrodes at 25 °C at a current density of 0.5 mA cm −2 (∼0.34C). Energy dispersive X‐ray spectroscopy analysis confirmed that K + ions swiftly conduct through K 6 [Rh 4 Zn 4 (L‐cys) 12 O]· n H 2 O between the cathode and anode during charge–discharge cycles.