Tuning coordination environment of iron ions to ensure ultra-high pseudocapacitive capability in iron oxide

The mechanism governing the pseudocapacitive lithium storage behavior is one of the most critical unsolved issues in conversion-type anodes for lithium-ion batteries. In this work, we, for the first time, demonstrate that the pseudocapacitive capability of iron oxide-based anodes is closely associated with the electronic structures of iron ions. As proof of concept, the introduction of amorphization, nitrogen doping, and oxygen vacancies reduces the coordination of iron ions and contributes significantly to the pseudocapacitive lithium storage capability of iron oxide, reaching up to 96% of the specific capacity at 1 mV·s⁻¹. Due to the significantly modulated coordination environment, the 3d electrons of Fe(II) are delocalized with increased spin state and the energy band gap is narrowed, accompanied by an upshift of Fermi energy. The redox activity and carrier mobility of iron oxides are substantially increased, which substantially enhance the exchange current density and thereby improve the pseudocapacitive capability of iron oxide.