Mn-Based Transition Metal Oxide Positive Electrode for K-Ion Battery Using an FSA-based Ionic Liquid Electrolyte

Abstract

Layered Mn-based transition metal oxides have gained interest as positive electrode materials for K-ion batteries due to their high capacity, excellent structural stability, and abundant resources. However, their practical utility is significantly hindered by insufficient electrochemical performances during operations. This study reports the successful synthesis of P3-K₀.₄₆MnO₂ via the solid-state method and investigates its charge–discharge behavior as a positive electrode working in an FSA-based (FSA= bis(fluorosulfonyl)amide) ionic liquid electrolyte at 298 K. The K₀.₄₆MnO₂ electrode demonstrates superior performance compared to previously reported KₓMnO₂ counterparts, delivering a reversible discharge capacity of about 100 mAh g⁻¹ at a current density of 20 mA g⁻¹ and a capacity retention of 68.3% over 400 cycles at 100 mA g⁻¹. Ex situ X-ray diffraction analyses confirm the occurrence of reversible structural changes during the charge–discharge process. Further, we explore potassium storage mechanisms through ex situ synchrotron soft X-ray absorption spectroscopy. Spectra obtained in Mn L-edge region suggest that Mn is reversibly oxidized and reduced during K⁺ deintercalation and intercalation processes. Remarkably, discharging the electrode below 2.3 V induces reversible formation of Mn²⁺ from Mn³⁺/⁴⁺ on the electrode surface. The study demonstrates superior electrochemical performance of K₀.₄₆MnO₂ positive electrode for K-ion battery using ionic liquid electrolyte.