K metal disk electrodes, prepared by putting a chunk of K metal in a 1,2-dimethoxyethane (DME) solution containing potassium bis-(fluorosulfonyl)amide (KFSA) and 1,3,2-dioxathiolane 2,2-dioxide (DTD), exhibited suppressed polarization and interfacial resistance in continuous metal stripping−deposition reactions of a symmetric half-cell comprising a cross-linked polymer electrolyte. This simple and effective pretreatment process enhances passivation layer formation, enabling fabrication of the first 3 V-class all-solid-state K-ion battery operated at room temperature.

In this study, all-solid-state sodium batteries with dry polymer electrolytes are demonstrated based on Na half-cell optimization for the stable operation of a layered oxide positive electrode, P2-Na2/3[Ni1/3Mn2/3]O2, and a hard carbon negative electrode at 60 °C. In all-solid-state batteries, the dry-polymer electrolyte does not penetrate the pores of the composite electrode; thus, dispersibility of the active material, conductive carbon, and binder polymer inside the composite electrode is essential to obtain a stable charge–discharge reaction. Using Ketjen black as a conductive carbon, a uniformly dispersed composite electrode was successfully fabricated for an acceptable charge–discharge reaction. Instead of a conventional polyvinylidene fluoride binder, a polymer electrolyte must be used as a binder polymer to provide an effective ionic conduction path inside the composite electrode. All-solid-state Na // P2-Na2/3[Ni1/3Mn2/3]O2 batteries exhibited excellent cycle performance even at 60 °C, while deterioration was more pronounced in the liquid electrolyte at the same temperature. The capacity retention of the P2-Na2/3[Ni1/3Mn2/3]O2 // hard carbon cell was further improved by coating P2-Na2/3[Ni1/3Mn2/3]O2 with Al-oxide.