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 similar to 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 (D-Li(+)) 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 mVs(-1), D-Li(+) was 3.13 x 10(-8) cm(2)s(-1). At lower scan rates where D-Li(+) was low, 0.57 x 10(-8) cm(2)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. (C) 2012 Elsevier Ltd. All rights reserved.