Fresh sodium storage of FeCoNi alloys confined in biomass carbon revealed by operando magnetometry

Focusing on the commercialization of sodium ion batteries (SIBs), the biggest challenge is discovering proper anode materials with high availability and safety. Among various materials, alloying metal anodes can achieve high-capacity sodium storage. However, transition metals, such as Fe, Co, Ni, etc. are considered to be inactive alloying with Na. Herein, by introducing trimetallic FeCoNi alloy nanoparticles confined in waste wool-derived carbon based on a sulfhydryl domain-limiting strategy, we synthesize porous N, S co-doped biomass carbon-coated trimetallic FeCoNi alloy composites (FeCoNi@NSC). Surprisingly, the optimal FeCoNi@NSC anode displays superior reversible specific capacities of 439.4 mAh·g⁻¹ at 0.1 A·g⁻¹, and an excellent cycling life of up to1000 cycles at 4 A·g⁻¹, which is similar to that of the current reported tin/antimony/germanium/bismuth-based anodes. Besides the capacity contribution of biomass carbon, the combination of in situ magnetometry, high-resolution transmission electron microscopy and electrochemical impedance characterization techniques reveal that the rising capacity originates from spin-polarized surface capacitance, extra capacity of transition metal oxides, and low-voltage capacity of solid electrolyte interphase, as quantificated by monitoring the variation in magnetism of surface capacitance. This work provides a breakthrough concept for novel anode materials of SIBs choices.