K0.54[Co0.5Mn0.5]O2: New Cathode with High Power Capability for Potassium-Ion Batteries

Abstract

Results and Discussion We propose P3-K0.54[Co0.5Mn0.5]O2, which is rationally designed as a promising cathode material for high-performance potassium-ion batteries (KIBs). Its composition adopts the use of the valence state of Mn above 3.5+ to minimize the disruptive effect of Jahn–Teller distortion in the MnO6 octahedra during the electrochemical reaction. Unlike other types of layered materials that suffer from the sluggish diffusion of large potassium ions accompanying multi-step voltage profiles, P3-K0.54[Co0.5Mn0.5]O2 delivers a high specific discharge capacity of 120.4 mAh (g-oxide)-1 with smooth charge and discharge curves. First-principles calculations predict an activation barrier energy of ~260 meV for a large K+ diffusion, which is comparable to those observed in conventional layered cathode materials for lithium-ion batteries. As a result, even at 500 mA g-1, P3-K0.54[Co0.5Mn0.5]O2 is able to deliver a high discharge capacity of 78 mAh g-1, which is a retention of 65% versus the capacity obtained at 20 mA g-1. Combination studies using operando X-ray diffraction, the ex-situ X-ray absorption near-edge structure, and first-principles calculations elucidate the nature of the excellent potassium storage mechanism of K0.54[Co0.5Mn0.5]O2. This work provides a new insight for the development of efficient cathode materials for KIBs. Experimental The P3-K0.54[Co0.5Mn0.5]O2 powder was synthesized using a typical combustion method. An aqueous solution was stoichiometrically prepared by dissolving KNO3, Co(NO3)6H2O, Mn(NO3)26H2O, and citric acid as a chelating agent in distilled water. The starting solution was evaporated overnight on a hot plate at 110 °C under constant stirring. Then, the dried powders were further heated to 200 °C for the auto-combustion of the citric acid. The as-received powders were calcined at 500 °C for 3 h to decompose the nitrates, and then re-calcined at 800 °C for 5 h under an air atmosphere at a heating rate of 2 °C min-1. This was followed by slow cooling at a cooling rate of 1 °C min-1. The X-ray diffraction (XRD) with Cu kα radiation were conducted and structural crystallinity analyzed by Rietveld refinement. The particle morphologies and surface of the powders were observed using SEM and TEM. Galvanostatic charge/discharge test was performed using a R2032-type coin cell. The K-storage mechanisms in the charge and discharge processes were examined by means of ex-situ XRD, in operando XRD, and ex-situ X-ray absorption spectroscopy (XAS). Also, the structural variation of P3-K x [Co0.5Mn0.5]O2 was predicted as a function of the K+ content in the structure based on the first-principles calculation. Figure 1