Layered Cu1–zMn1+zO2 Crednerite: Mapping the Phase Stabilization Region via Precise Compositional Control for Optimum Supercapacitor Performance

CuMnO2 is a prototype ABO2-type crednerite compound featured by transition metal ions of variable valence states essential for creating novel properties and optimum performance. However, the phase stabilization region of CuMnO2 has not yet been well established, restricting one's ability in comprehending this unique structure for functional applications. Here, layered Cu1-zMn1+zO2 crednerite was systematically synthesized and characterized by accurately regulating the reaction parameters of hydrothermal conditions, which led to a first demonstration of the phase diagram for CuMnO2 crednerite. The pure phase layered structure was uncovered to be stabilized under hydrothermal conditions as the temperature varies between 85 and 175 °C and the molar ratio of Cu to (Cu + Mn) varies between 0.45 and 0.55. For Cu1-zMn1+zO2, there appeared non-stoichiometric occupation of transition metal ions. Strikingly, different from many other layered oxides, the samples at a molar ratio of Cu:(Cu + Mn) = 0.55 showed a special structure, in which excess Cu2+ occupied the position of the Mn3+ site to form a Cu2+ (3d9)/Mn4+ (3d3) ionic pair and traces of corresponding cationic ordered phases. Such a configuration (3d9/3d3 ionic pair) gives rise to an optimum super-capacitor performance, as represented by a highest mass specific capacitance of 428.4 F/g at a current density of 1 A/g. The strategy reported in this work for mapping the phase diagram of layered CuMnO2 crednerite is fundamentally important, which may offer guidance to explore the potentials of other ABO2-type compounds for functional applications.