Liquid-Assisted Redox Synthesis Enables High-Yield Metastable Tetragonal Spinel Oxides

Controlling phase selectivity for especially metastable multimetal oxides remains a central challenge, even though they are attractive for catalysis and energy conversion applications. The conventional synthesis method, like aqueous redox-precipitation, typically uses dilute reactant solutions, in which thermodynamic equilibrium favors stable products, limiting the selective synthesis of metastable phases. Here, we demonstrate a water-assisted reaction that plays a dual and decisive role in liquid-assisted redox synthesis (LRS) to obtain a metastable tetragonal cobalt-manganese spinel (CMO), rather than the readily formed cubic spinel. Limited initial hydration (0.5-1.5 mL) creates a highly concentrated microenvironment that enhances local acidity and shifts redox toward Mn³⁺ formation rather than Mn⁴⁺. Dehydration at 160 °C enables the incorporation of unstable Mn³⁺ into the solid lattice, resulting in high selectivity and phase purity toward the metastable tetragonal spinel. A low liquid-to-solid ratio (η = 0.27-0.81 μL mg^-1) coupled with elevated dehydration temperatures (up to 200 °C) enabled the selective formation of metastable tetragonal spinel. Increasing the initial water content (≥3.0 mL) without complete water dehydration further restores bulk-solution-like behavior, promotes Mn³⁺/Mn⁴⁺ equilibration, and yields the thermodynamic cubic spinel. The optimized condition delivers 88% yield (∼27× higher than conventional aqueous ARP, 3.2%), and the tetragonal phase exhibits enhanced alkaline (oxygen evolution reaction) OER activity relative to the cubic analog.