Spin-Polarized Surface Capacitance Effects Enable Fe₃ O₄ Anode Superior Wide Operation-Temperature Sodium Storage

Fe₃ O₄ is widely investigated as an anode for ambient sodium-ion batteries (SIBs), but its electrochemical properties in the wide operation-temperature range have rarely been studied. Herein, the Fe₃ O₄ nanoparticles, which are well encapsulated by carbon nanolayers, are uniformly dispersed on the graphene basal plane (named Fe₃ O₄ /C@G) to be used as the anode for SIBs. The existence of graphene can reduce the size of Fe₃ O₄ /C nanoparticles from 150 to 80 nm and greatly boost charge transport capability of electrode, resulting in an obvious size decrease of superparamagnetic Fe nanoparticles generated from the conversion reaction from 5 to 2 nm. Importantly, the ultra-small superparamagnetic Fe nanoparticles (≈2 nm) can induce a strong spin-polarized surface capacitance effect at operating temperatures ranging from -40 to 60 °C, thus achieving highly efficient Na-ion transport and storage in a wide operation-temperature range. Consequently, the Fe₃ O₄ /C@G anode shows high capacity, excellent fast-charging capability, and cycling stability ranging from -40 to 60 °C in half/full cells. This work demonstrates the viability of Fe₃ O₄ as anode for wide operation-temperature SIBs and reveals that spin-polarized surface capacitance effects can promote Na-ion storage over a wide operation temperature range.