Bisphosphate shell layer structure-decorated K0.45Rb0.05Mn0.85Mg0.15O2 cathode for boosting potassium/sodium storage

Abstract

Novel K_0.45Rb_0.05Mn_0.85Mg_0.15O₂ (KRMMO) cathode encapsulated by bisphosphate K₃PO₄/Mn₃PO₄ shell layer is delicately designed for boosting potassium/sodium storage. Benefiting from the bisphosphate layer, the volume expansion of KRMMO is effectively inhibited, K₃PO₄/MnPO₄ double-coated K_0.45Rb_0.05Mn_0.85Mg_0.15O₂ (D-KRMMO) has a high electronic conductivity and fast ionic diffusivity, which can stimulate potassium/sodium storage. Meanwhile, bisphosphate K₃PO₄/Mn₃PO₄ shell directly isolates the cathode from the electrolyte to alleviate side reactions occurring between the electrolyte and the material, thus the dissolution of Mn in KRMMO host can be inhibited during the cycling process. In potassium ion batteries (PIBs), the discharge specific capacity of D-KRMMO is 105.9 mAh g⁻¹ at a current density of 20 mA g⁻¹ compared with pure KRMMO (94 mAh g⁻¹ at 20 mA g⁻¹). A highly reversible discharge specific capacity of 112.8 mAh g⁻¹ at 100 mA g⁻¹ is shown in the sodium-ion batteries (SIBs). And acceptable C-rate performances of 112.8 mAh g⁻¹ and 44.8 mAh g⁻¹ are exhibited at 100 mA g⁻¹ and 2 A g⁻¹, respectively. EIS and GITT measurements have shown that D-KRMMO cathode has faster ion mobility and electron mobility, as well as higher pseudocapacitance contribution.