K+ modulated K+/vacancy disordered layered oxide for high-rate and high-capacity potassium-ion batteries

Abstract

With high theoretical capacity and applicable operating voltage, layered transition metal oxides are potential cathodes for potassium-ion batteries (PIBs). However, a K⁺/vacancy ordered structure in these oxides limits the K⁺ transport kinetics and storage sites so that the PIBs still have poor rate performance and low achievable capacity. Here, to effectively resolve the problem, a K⁺/vacancy disordered P3-type structure is designed and synthesized by simply modulating the K⁺ contents in Mn/Ni-based layered oxides. The effect of the K⁺ contents in a series of K_xMn_0.7Ni_0.3O₂ (x = 0.4–0.7) oxides has been systematically studied and it is found that while the K⁺/vacancy ordered superstructure is stable at low K⁺ content (x < 0.6), a complete K⁺/vacancy disordered structure forms at high K⁺ content (x > 0.6), evidenced by selected area electron diffraction and voltage plateaus in the charge/discharge curves. The K⁺/vacancy disordered K_0.7Mn_0.7Ni_0.3O₂ exhibits much better rate performance and higher discharge capacity, compared to the K⁺/vacancy ordered K_0.4Mn_0.7Ni_0.3O₂. Molecular dynamic simulations confirm that the K⁺/vacancy disordered structure possesses interconnected continuous channels for K⁺ diffusion and more active storage sites. This discovery sheds light on rational design of K⁺/vacancy disordered layered oxide cathodes for next-generation high-performance PIBs.