Double-layer phosphates coated Mn-based oxide cathodes for highly stable potassium-ion batteries

Manganese rich oxides with layered structure are now being developed as cathodes for potassium ion batteries (PIBs). Nevertheless, poor cycling stability and sluggish electrochemical kinetics have hindered their practical applications. Here, we construct a thin K3PO4/MnPO4 layer, which has high conductivity of both electron and potassium-ion, on the surface of K0.5Mn0.8Co0.2O2 (KMCO) and achieve improved cycling stability and capacity rate when the composite is used as cathodes in PIBs. Comparing to the pure KMCO, the K3PO4/MnPO4-coated K0.5Mn0.8Co0.2O2 (P-KMCO) has higher electron and K-ion conductivity, less surficial oxygen loss and layered-to-spinel-to-rock salt tri-phase transition, and less internal lattice expansion and contraction in cycling, as studied by aberration-corrected scanning transmission electron microscopy and in-situ X-ray diffraction. The synthesized P-KMCO exhibits a highly reversible specific capacity (101.3 mAh g−1 at 0.1 A g−1), excellent rate performance and stellar capacity retention of 80% after 500 cycles at 1 A g−1 compared to pure KMCO (78.6 mAh g−1 at 0.1 A g−1 with 67.3% capacity retention). The P-KMCO electrode shows also a supreme electrochemical performance when assembled in pouch cells. First-principal calculations imply that the surface modification can effectively prevent TM ions dissolution and oxygen release while cycling. Like those developed in lithium-ion batteries, this work demonstrates that the appropriate surface engineering is also an effective approach to development stable cathodes of PIBs.