Superior-capacity binder-free anode electrode for lithium-ion batteries: Co _x Mn _y Ni _z O nanosheets with metal/oxygen vacancies directly formed on Cu foil

Recently, transition metal oxides have attracted great attention as anode materials for lithium-ion batteries due to their high theoretical capacities. However, their poor electrical conductivity, unstable cycling performance and unclear additional capacity are still great challenges. Herein, CoxMnyNizO (x : y : z = 8 : 0.92 : 0.71) nanosheets corresponding to the cubic CoO phase directly formed on a Cu foil (CMN-CH) were fabricated and directly tested as binder-free anode electrodes for lithium-ion batteries. The unique preparation of the electrode without a binder effectively accelerated the transfer of Li+ ions and electrons. Additionally, much more active sites were exposed to the electrolyte in the absence of additives (binder and conductive carbon). Metal vacancies and oxygen vacancies could be clearly observed in the crystal lattices, which were induced by doping Mn and Ni atoms in the CoO crystal lattices. The prepared CMN-CH electrode demonstrated superior capacities of 1501 mA h g-1 at 5 A g-1 after 1500 cycles and 823 mA h g-1 at 10 A g-1 after 1500 cycles, which are far beyond the theoretical capacity of CoO (716 mA h g-1) and surpass that of most CoO-based composites with carbon materials reported in the literature. The reversible conversion between Co2+ and Co3+ during the cycling process contributed greatly to the reversible capacity. Based on the obtained excellent electrochemical capacities, the prepared CMN-CH has great potential to be used as an anode electrode for lithium-ion batteries.