Micromechanisms of solid electrolyte interphase formation on electrochemically cycled graphite electrodes in lithium-ion cells

The microstructural and compositional changes that occurred in the solid electrolyte interphase (SEI) formed on graphite electrodes subjected to voltammetry tests (vs. Li/Li+) at different voltage scan rates were investigated. The microstructure of the SEI layer, characterized using high-resolution transmission electron microscopy, consisted of an amorphous structure incorporating crystalline domains of ∼5–20 nm in size. Evidence of lithium compounds, namely Li2CO3 and Li2O2, and nano-sized graphite fragments was found within these crystalline domains. The morphology and thickness of the SEI depended on the applied voltage scan rate (dV/dt). The variations in the Li+ diffusion coefficient (DLi+) at the electrode/electrolyte interface during the SEI formation process were measured and two regimes were identified depending on the scan rate; for dV/dt ≥ 3.00 mV s−1, DLi+ was 3.13 × 10−8 cm2 s−1. At lower scan rates where DLi+ was low, 0.57 × 10−8 cm2 s−1, a uniform and continuous SEI layer with a tubular morphology was formed whereas at high dV/dt, the SEI formed had a columnar morphology and did not provide a uniform coverage.