Solid electrolyte interphase layer formation on mesophase graphite electrodes with different electrolytes studied by small-angle neutron scattering

Relative solid electrolyte interphase (SEI) film thickness investigation on graphite electrodes after the first charge state in the lithium-ion battery was successfully demonstrated by the ex-situ small-angle neutron scattering technique. Here, for both the mesophase graphite powder (MGP) and the fine-mesophase graphite powder (FMGP) anodes, with two different particle sizes was analyzed precisely by the Guinier–Porod model. The data revealed a stable, maximum (~tens of nm) bi-layer SEI film formed on MGP anode in ethylene-carbonate/dimethyl-carbonate (EC/DMC) at a capacity of 50 mAh/g and sluggish above 100 mAhg^-1. The SEI formed on FMGP with and without 3 wt% fluoroethylene-carbonate additive in EC/DMC showed the relative thickness greater than that only in ethylene-carbonate/diethyl-carbonate. Lithiation initiated the rapid SEI formation on the graphite surface and achieved a maximum thickness in the cell potential ≤0.2 V, and became thinner when the graphite particle expanded after Li⁺ intercalation. It was observed that the SEI thickness influenced the electrolytes and additives, which might ultimately impact battery performance. Our preliminary results make evident that small-angle neutron scattering could be employed to better understand the complex microstructure solid electrolyte interphase formation and its accurate thickness, on a mesophase graphite anode.