Comparative Study of Layered Manganese-based Oxides Doped with Iron, Titanium, and Aluminum: Positive Electrode Performance in K-Ion Battery Using an Ionic Liquid Electrolyte

K-ion batteries (KIBs) are regarded as a viable option for large-scale electrical energy storage devices on account of their abundant potassium resources. Although layered P3-type manganese oxides and their derivatives have been studied as positive electrodes for KIBs, most of them use organic solvent-based electrolytes. We recently reported the improved performance of P3-type KxMnO2 positive electrode for KIBs with a highly safe ionic liquid electrolyte. In the present study, iron, titanium, and aluminum were doped into potassium manganese oxide positive electrode, and their charge–discharge properties were evaluated in the above ionic liquid electrolyte. The doped materials exhibited enhanced rate capabilities and cycling performances owing to their reduced surface resistance. The Al-doped material demonstrated the best cycling performance and highest discharge capacity, reaching 64.5 mAh g−1 with 92.8 % capacity retention at 100 mA g−1 over 100 cycles, owing to its low surface resistance. In addition, the structural stabilities of the Fe-, Ti-, and Al-doped materials during the charge–discharge process were confirmed through reversible crystal structural evolution observed via ex-situ X-ray diffraction. Finally, a stable cycling performance was also demonstrated in a K-ion full cell composed of an Al-doped potassium manganese-oxide positive electrode and a graphite negative electrode.