Nanoscale Borate Coating Network Stabilized Iron Oxide Anode for High-Energy-Density Bipolar Lithium-Ion Batteries

High-capacity metal oxides based on non-toxic earth-abundant elements offer unique opportunities as advanced anodes for lithium-ion batteries (LIBs). But they often suffer from large volumetric expansion, particle pulverization, extensive side reactions, and fast degradations during cycling. Here, an easy synthesis method is reported to construct amorphous borate coating network, which stabilizes conversion-type iron oxide anode for the high-energy-density semi-solid-state bipolar LIBs. The nano-borate coated iron oxide anode has high tap density (1.6 g cm^-3 ), high capacity (710 mAh g^-1 between 0.5 - 3.0 V, vs Li/Li⁺ ), good rate performance (200 mAh g^-1 at 50 C), and excellent cycling stability (≈100% capacity resention over 1,000 cycles at 5 A g^-1 ). When paired with high-voltage cathode LiCoO₂ , it enables Cu current collector-free pouch-type classic and bipolar full cells with high voltage (7.6 V with two stack layers), achieving high energy density (≈350 Wh kg^-1 ), outstanding power density (≈6,700 W kg^-1 ), and extended cycle life (75% capacity retention after 2,000 cycles at 2 C), superior to the state-of-the-art high-power LIBs using Li₄ Ti₅ O₁₂ anode. The design and methodology of the nanoscale polyanion-like coating can be applied to other metal oxides electrode materials, as well as other electrochemical materials and devices.