Introducing strong metal–oxygen bonds to suppress the Jahn-Teller effect and enhance the structural stability of Ni/Co-free Mn-based layered oxide cathodes for potassium-ion batteries

Abstract

Mn-based layered oxides (KMO) have emerged as one of the promising low-cost cathodes for potassium-ion batteries (PIBs). However, due to the multiple-phase transitions and the distortion in the MnO₆ structure induced by the Jahn-Teller (JT) effect associated with Mn-ion, the cathode exhibits poor structural stability. Herein, we propose a strategy to enhance structural stability by introducing robust metal–oxygen (M–O) bonds, which can realize the pinning effect to constrain the distortion in the transition metal (TM) layer. Concurrently, all the elements employed have exceptionally high crustal abundance. As a proof of concept, the designed K_0.5Mn_0.9Mg_0.025Ti_0.025Al_0.05O₂ cathode exhibited a discharge capacity of approximately 100 mA h g⁻¹ at 20 mA g⁻¹ with 79% capacity retention over 50 cycles, and 73% capacity retention over 200 cycles at 200 mA g⁻¹, showcased much better battery performance than the designed cathode with less robust M–O bonds. The properties of the formed M–O bonds were investigated using theoretical calculations. The enhanced dynamics, mitigated JT effect, and improved structural stability were elucidated through the in-situ X-ray diffractometer (XRD), in-situ electrochemical impedance spectroscopy (EIS) (and distribution of relaxation times (DRT) method), and ex-situ X-ray absorption fine structure (XAFS) tests. This study holds substantial reference value for the future design of cost-effective Mn-based layered cathodes for PIBs.