Improved K-Ion Diffusion Kinetics of Cobalt-Substituted P3-type K0.67MnO2 Electrodes for K-Ion Batteries

The scarcity of lithium in the earth's crust has directed researchers to focus on developing affordable and scalable battery technologies like sodium-ion (Na-ion) and potassium-ion (K-ion) batteries as alternatives to lithium-ion (Li-ion) batteries to fulfill future needs. However, capacity fading during cycling is challenging in K-ion battery cathodes, especially in layered metal oxides. Therefore, understanding the structural transitions for the smooth deintercalation and intercalation of large K-ions in the interlayers is fascinating. Therefore, the current study deals with preparing K0.67Mn1–yCoyO2 (y = 0, 0.05, and 0.10) by conventional ball milling and its K-ion intercalation properties. X-ray diffraction (XRD) and Rietveld refinement analysis inferred that the elongation of the (003) plane along the c-axis resulted in an increased unit cell volume. The microscopic images revealed that the particles obtained are submicrometer-sized, with an average size of 500 nm. The pristine K0.67MnO2 demonstrates five distinguished redox peaks corresponding to the Mn3+/4+ couples in the potential window from 1.5 to 3.9 V versus K/K+, revealed through a cyclic voltammogram (CV). The Co-ion substitution in the K0.67MnO2 structure induces a significant change in peak potentials below 3.25 V versus K/K+ with a reduced polarization potential (ΔE). The obtained reversible capacity of K0.67Mn0.95Co0.05O2 is 81 mAh/g, which elucidates the extraction of 0.36 K+-ions. Ex-situ XRD analysis inferred a minimum volume change in the K0.67Mn0.95Co0.05O2 structure, which paved the way for better electrode kinetics. This further concurs with the diffusion coefficients in the range 10–9–10–8 cm2/s from GITT analysis. Among the prepared electrodes, K0.67Mn0.95Co0.05O2 has better high-voltage characteristics and minimum reaction resistance calculated from the overpotential during each pulse and showed appreciable capacity retention even after 500 cycles at a current density of 200 mA/g.