Bypassing First-Stage Degradation via Preconversion Interface Engineering in Iron Oxalate Anodes

Oxalate-based anodes are attractive high-capacity candidates for lithium-ion batteries, but their practical application is hindered by severe first-stage degradation during early cycling. Here, combined experimental and theoretical analyses suggest that the early-stage irreversibility of iron oxalate is driven by interfacial chemical degradation of oxalate groups, with possible mechanical damage accelerating failure by exposing fresh reactive surfaces. Guided by this mechanism, we develop a preconversion interfacial engineering strategy by thermochemically preforming an FeO_x passivation layer on the surface of iron oxalate and other functional components are incorporated prior to cycling. The resulting FeO_x layer passivates reactive oxalate groups and regulates the interfacial chemistry of the iron oxalate particles. Meanwhile, auxiliary functional components, including the conductive graphite framework and elastic polyacrylonitrile shell, may enhance the charge transport and accommodate volume variation. As a result, the engineered electrode delivers a second-cycle Coulombic efficiency of 92% and 96% capacity retention over the first six cycles, significantly outperforming that of pristine iron oxalate (75% and 68%, respectively). At 0.5 A g^-1, it retains 897 mAh g^-1 after 100 cycles and 1053 mAh g^-1 after 300 cycles. This work establishes a preconversion interfacial passivation strategy for stabilizing chemically reactive oxalate-based conversion anodes.