Galvanostatic interruption of lithium insertion into magnetite: Evidence of surface layer formation

•Surface layer formation on magnetite nanocrystals was investigated.•Surface layer formation decreases the amount of active material.•The decrease in active material is proportional to crystal surface area.•The surface layer forms through a nucleation and growth process.•Optimal crystal size balances active material loss and mass transport resistance.

Magnetite is a known lithium intercalation material, and the loss of active, nanocrystalline magnetite can be inferred from the open-circuit potential relaxation. Specifically, for current interruption after relatively small amounts of lithium insertion, the potential first increases and then decreases, and the decrease is hypothesized to be due to a formation of a surface layer, which increases the solid-state lithium concentration in the remaining active material. Comparisons of simulation to experiment suggest that the reactions with the electrolyte result in the formation of a thin layer of electrochemically inactive material, which is best described by a nucleation and growth mechanism. Simulations are consistent with experimental results observed for 6, 8 and 32-nm crystals. Furthermore, simulations capture the experimental differences in lithiation behavior between the first and second cycles.