Mesoporous LiMn1–xMxPO4 (M = Ni, Co) Precatalysts by Alcohol-Mediated Self-Assembly: Hydroxide Reconstruction and Suppression of Manganese Degradation for Stable Alkaline Oxygen Evolution

Abstract

Manganese is an earth-abundant, low-cost transition metal with considerable potential for sustainable electrocatalysis; however, its poor stability under oxidative alkaline conditions remains a major challenge. Herein, we report the fabrication of Ni(II)- and Co(II)-substituted olivine lithium manganese phosphate electrodes, LiMn1–xMxPO4 (M = Ni, Co; x = 0.05, 0.10, and 0.25), through an alcohol-mediated synthesis approach and investigate the effects of solvent and secondary metal incorporation on their structural evolution and alkaline oxygen evolution reaction (OER) performance. The precursor solutions exhibit markedly different precipitation behaviors depending on the solvent. Rapid precipitation of crystalline MnPO4·H2O occurs in ethanol and 1-butanol, whereas precipitation is slower and less extensive in methanol. Increasing Ni(II) concentration suppresses precipitation more effectively than Co(II), improving the stabilization of Mn(II) in solution. Upon calcination, ethanol- and 1-butanol-derived precursors yield predominantly mesoporous olivine LiMn1–xMxPO4 with minor Mn2P2O7 impurities, whereas methanol-derived solutions and supernatants produce phase-pure olivine LiMn1–xMxPO4. Electrochemical studies in 1 M KOH reveal rapid reconstruction of all phosphate electrodes into mixed-metal hydroxides, identifying these hydroxides as the active OER species. Secondary metal incorporation substantially enhances electrode stability by suppressing manganese degradation and promoting stable mixed (Mn,M)(OH)2 active phases.