Electrochemical cycling behaviour of lithium carbonate (Li2CO3) pre-treated graphite anodes - SEI formation and graphite damage mechanisms

Graphite electrode surfaces were treated using a simple process of sedimentation in aqueous solutions containing 0.5 and 1.0 wt.% Li2CO3 with particle sizes of ∼1-2 μm. During the first cycle of voltammetry tests (vs. Li/Li+), the graphite surface was subjected to electrochemical degradation as a result of fracture and removal of near-surface graphite particles. Surface degradation was accompanied by a 0.4% strain in the graphite lattice as determined by in situ Raman spectroscopy. Pre-treated electrodes experienced a capacity drop of 3% in the first cycle, compared to a 40% drop observed in case of untreated graphite electrodes. After testing for 100 cycles, a capacity of 0.54 mAh cm−2 was recorded for the pre-treated electrodes as opposed to a significant drop to 0.11 mAh cm−2 for the untreated graphite. Cross-sectional HR-TEM indicated that the SEI formed on the pre-treated electrodes primarily consisted of Li2CO3 crystals of 14.6 ± 6.9 nm in size distributed within an amorphous matrix. The results suggested that the Li2CO3 enriched SEI formed on the pre-treated electrodes reduced the intensity of solvent co-intercalation induced surface damage. It is proposed that the Li2CO3 enriched SEI facilitated Li+ diffusion and hence improved the capacity retention during long-term cycling.