Nickel-Doping Induced Oxygen Vacancies in MnO₂ Hollow Cube with Enlarged Interlayer Spacing for Stable Sodium Ions Storage

Intrinsic low conductivity, poor structural stability, and narrow interlayer spacing limit the development of MnO₂ in sodium-ion (Na⁺) supercapacitors. This work constructs the hollow cubic Mn-PBA precursor through an ion-exchange process to in situ obtain a hollow cubic H-Ni-MnO₂ composite with Ni doping and oxygen vacancies (O_V) via a self-oxidation strategy. Experiments and theoretical calculations show that the hollow nanostructure and the expanding interlayer spacing induced by Ni doping are beneficial for exposing more reactive sites, synergistically manipulating the Na⁺ transport pathways. COMSOL simulations reveal the hollow cube can buffer the deformation stress generated by the electrochemical reaction, leading to the stable Na⁺ storage ability of the H-Ni-MnO₂ electrode. Meanwhile, Ni doping induces a rich abundance of O_V at the molecular level, triggering intense interfacial electronic interactions and regulating the Na⁺ adsorption energy to improve Na⁺ reaction kinetics. The prepared H-Ni-MnO₂ electrode exhibits superior rate capability (80.6% at 10 A g^-1) and high-rate long-term cycle stability (92.7% retention after 10 000 cycles at 10 A g^-1). In addition, the assembled hybrid supercapacitor demonstrates outstanding energy/power density output and application prospects. This work provides a novel approach to the structure and function design of MnO₂-based materials for advanced Na⁺ storage.