Unraveling the electrochemical K-ion intercalation kinetics of sol-gel synthesized Co-substituted K0.7MnO2 electrodes for K-ion batteries

Developing potassium-ion batteries could complement the existing lithium-ion battery technology in this digital era. The KxMnO2-based layered transition metal oxides can deliver high specific capacity due to the Mn 3 +/4 + redox couple. However, structural changes during the reversible insertion of K-ions affect the material's integrity, resulting in capacity fading. Herein, the substitution of cobalt for manganese was studied to understand the effects of altered structural characteristics on the electrochemical performance of K 0.7 Mn 1-y Co y O 2 (y = 0.0, 0.2, 0.3, and 0.5) cathodes in K-ion batteries. X-ray diffraction (XRD) and neutron powder diffraction (NPD) analysis revealed the formation of a solid solution into a P3-type structure by suppressing the secondary phases accommodating more K-ions (0.7 K+) in the interlayers. The NPD analysis further corroborated the better crystallinity with single-phase formation without impurities. Overlapping cyclic voltammetry (CV) curves and improved coulombic efficiency in galvanostatic charge-discharge (GCD) analyses revealed the inductive effect between Mn and Co ions. Among the prepared compositions, K 0.7 Mn 0.7 Co 0.3 O 2 demonstrated relatively better capacity retention of 89 % at 200 mA/g with a dominant contribution of Mn 3 +/4 + redox couple. The insertion kinetics of K-ions were further analyzed using the galvanostatic intermittent titration technique (GITT), and electrochemical impedance spectroscopy (EIS) techniques, which demonstrated better diffusion coefficients with low reaction resistance and decreased interfacial resistance. In-situ EIS revealed a significant decrease in the charge transfer resistance during potassiation. Redox-active Co 3 +/4 + could effectively mitigate severe structural transformations, ease the insertion of K+ ions, and promote K 0.7 Mn 0.7 Co 0.3 O 2 as a suitable cathode for K-ion batteries.