Hubbard Gap Closure-Induced Dual-Redox Li-Storage Mechanism as the Origin of Anomalously High Capacity and Fast Ion Diffusivity in MOFs-Like Polyoxometalates

MOFs-like polyoxometalate (POMs) electrodes, harvesting combined advantages of interlocking porosity and multi-electron transfer reaction, have already emerged as promising candidates for lithium-ion batteries (LIBs), yet the origins of the underlying redox mechanism in such materials remain a matter of uncertainty. Of critical challenges are the anomalously high storage capacities beyond their theoretical values and the fast ion diffusivity that cannot be satisfactorily comprehended in the theory of crystal lattice. Herein, for the first time we decode t_2g electron occupation-regulated dual-redox Li-storage mechanism as the true origin of extra capacity in POMs electrodes. The lattice and electronic transition of active centers and reaction intermediates were systematically decoupled through density functional theory (DFT) and a suite of structural spectroscopic investigations, such as X-ray absorption near-edge spectroscopy (XANES), soft X-ray absorption spectroscopy (sXAS) and ⁷Li solid-state nuclear magnetic resonance (NMR). Enhanced V-t_2g orbital occupation by Li coordination significantly triggers the Hubbard gap closure and reversible Li deposition/dissolution at surface region. Conjugated V−O-Li configuration at interlayers endow Li⁺ ion pathways along pore walls as the dominant contribution to the low migration barrier and fast diffusivity. As a result, remarkable cycle stability (~100 % capacity retention after 2000 cycles at 1 A g⁻¹), extremely high specific capacity (1200 mAh g⁻¹ at 100 mA g⁻¹) and excellent rate performance (404 mAh g⁻¹ at 8 A g⁻¹) were achieved, providing new understandings on the underlying mechanism of POMs electrodes and pivotal guidance for dual-storage materials.