Revisiting the K-Mn-O System for Green Cathode Materials in Potassium-ion Batteries: From K2MnO4 to K3MnO4

Abstract

This study reports the synthesis, structural characterization, and electrochemical evaluation of α-K3MnO4 and K2MnO4 as potential cathode materials for potassium-ion batteries (KIBs). A new solid-state route was developed to prepare K2MnO4 from an equimolar mixture of KMnO4 and K3MnO4. Both phases contain isolated MnO4 tetrahedra, but differ in potassium diffusion behaviour, as shown by bond valence energy landscape calculations. Electrochemical cycling shows a reversible Mn6+/Mn5+ redox couple at an average potential of 2.4 V vs. K+/K, delivering reversible capacities up to 110 mAh/g. Operando X-ray diffraction and absorption spectroscopy confirm the structural transitions and provide insights into the electronic evolution on Mn, including evidence of irreversible decomposition at high voltage. These results highlight the potential of the K–Mn–O system as a platform for developing high-voltage, Mn-based cathode materials for KIBs.