Bond-Discretization in Nickel Oxide to Boost Capacity for Rechargeable Nickel─Zinc Batteries

Aqueous Ni─Zn batteries are highly attractive for grid-scale energy storage due to their safety, low cost, and environmental friendliness. However, their energy density is limited by the underutilization of nickel-based cathode materials. Here, a bond-discretization strategy in NiO is developed to overcome this challenge. By designing a low-symmetry crystallographic structure with a reduced Ni-Ni coordination number of 9, the NiO cathode achieves a near-theoretical capacity for the one-electron reaction. This structural modification leads to a discretization of Ni─O bonds and achieves a ratio of 50% for weak O─H bonds in the transformation product of Ni(OH)₂, equal to the proportion (50%) of H atoms that needs to be detached from Ni(OH)₂ to NiOOH, which can realize nearly theoretical capacity for the one-electron reaction. The bond-discretization also promotes catalytic activity for oxygen evolution and oxygen reduction reactions, enabling the integration of Ni─Zn batteries with Zn-air batteries. The low-symmetry NiO cathode demonstrates high specific capacity, excellent rate capability, and superior cycling stability, with a full cell paired with a zinc anode achieving a specific energy of 459 Wh kg^-1 at a specific power of 995.7 W kg^-1. This work provides a new bonding-engineering strategy for constructing high-performance energy storage devices.