Layered Cu 1- z Mn 1+ z O 2 Crednerite: Mapping the Phase Stabilization Region via Precise Compositional Control for Optimum Supercapacitor Performance.

CuMnO 2 is a prototype ABO 2 -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 CuMnO 2 has not yet been well established, restricting one's ability in comprehending this unique structure for functional applications. Here, layered Cu 1- z Mn 1+ z O 2 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 CuMnO 2 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 Cu 1- z Mn 1+ z O 2 , 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 Cu 2+ occupied the position of the Mn 3+ site to form a Cu 2+ (3d 9 )/Mn 4+ (3d 3 ) ionic pair and traces of corresponding cationic ordered phases. Such a configuration (3d 9 /3d 3 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 CuMnO 2 crednerite is fundamentally important, which may offer guidance to explore the potentials of other ABO 2 -type compounds for functional applications.