Development of Electrochemical Anion Doping Technique for Expansion of Functional Material Exploration

Abstract Instead of conventional cation doping strategy, anion doping is a promising new strategy for advances of energy conversion and storage technologies such as batteries, catalysts, electrolysis, and fuel cells. To synthesize mixed‐anion compounds, novel synthesis techniques such as topochemical reaction, high‐pressure reaction, solvothermal reaction have been developed. Despite these excellent synthesis techniques, synthesizable mixed‐anion compounds are still limited. For further expansion of the material exploration of mixed‐anion compounds, herein, an electrochemical anion doping technique is developed, which can flexibly control a species of anion, the doping rate and the degree of anion doping. The concept of the new synthesis technique is verified by F doping to the perovskite oxide La 0.5 Sr 0.5 CoO 3− δ . Quantitative control of F in the perovskite host material is succeeded by using an electrochemical reactor composed of La 0.5 Sr 0.5 CoO 3− δ ‐BaF 2 |BaF 2 |PbF 2 ‐Pb, and phase‐pure F‐doped La 0.5 Sr 0.5 CoO 3− δ powder is obtained. Moreover, nano‐size crystalline domains with amorphous phase are formed on the particle surface under the high‐rate F doping, suggesting that tuning the anion doping rate enables the control of the formation of metastable phase. As demonstrated, the electrochemical anion doping technique opens up new possibilities for advances of energy materials by utilizing function of anionic species.