A P3-Type K1/2Mn5/6Mg1/12Ni1/12O2 Cathode Material for Potassium-Ion Batteries with High Structural Reversibility Secured by the Mg–Ni Pinning Effect

Mn-based layered oxides are very attractive as cathodes for potassium-ion batteries (PIBs) due to their low-cost and environmentally friendly precursors. Their transfer to practical application, however, is inhibited by some issues including consecutive phase transitions, sluggish K + deintercalation/intercalation, and serious capacity loss. Herein, Mg–Ni co-substituted K 1/2 Mn 5/6 Mg 1/12 Ni 1/12 O 2 is designed as a promising cathode material for PIBs, with suppressed phase transitions that occurred in K 1/2 MnO 2 and improved K + storage performance. Part of Mg 2+ and Ni 2+ occupies the K + layer, playing the role of a “nailed pillar”, which restrains metal oxide layer gliding during the K + (de)intercalation. The “Mg–Ni pinning effect” not only suppresses the phase transitions but also reduces the cell volume variation, leading to the improved cycle performance. Moreover, K 1/2 Mn 5/6 Mg 1/12 Ni 1/12 O 2 has low activation barrier energy for K + diffusion and high electron conductivity as demonstrated by first-principles calculations, resulting in better rate capability. In addition, K 1/2 Mn 5/6 Mg 1/12 Ni 1/12 O 2 also delivers a higher reversible capacity owing to the participation of the Ni element in electrochemical reactions and the pseudocapacitive contribution. This study provides a basic understanding of structural evolution in layered Mn-based oxides and broadens the strategic design of cathode materials for PIBs.