Dynamics of the6Li/7Li Exchange at a Graphite–Solid Electrolyte Interphase: A Time of Flight–Secondary Ion Mass Spectrometry Study

The isotopic Li-ion exchange occurring in the solid electrolyte interphase (SEI) of a graphite electrode is studied by time of flight secondary ions mass spectrometry analysis (ToF-SIMS) of Li isotopes (6Li and 7Li) and selected fragments. The experiments are conducted under two different configurations: (i) self- diffusion conditions and (ii) galvanostatic cycling. In both configurations, one full cycle is performed in a lithium half-cell containing the graphite electrode in order to form the SEI layer. The SEI is then analyzed by ToF-SIMS by considering characteristic fragments. Isotopic exchange is done by forming the SEI with 7Li, dismantling the cell to get the delithiated graphite electrode and (i) dipping it in an electrolyte containing 6Li or (ii) cycling it in a new cell using an electrolyte containing 6Li as electrolyte ion and as metal in the counter electrode. Self-diffusion experiments reveal that the isotope exchange occurs very rapidly and can be modeled by the cumulative Weibull equation applied to diffusion. By applying the Einstein–Smoluchowski relation to the scale parameter τ (τ = 103 s for the self-diffusion experiment) in this equation, a characteristic length for diffusion can be calculated (Ld = 2.3 nm) assuming D(Li+) = 2.6 × 10–16 cm2·s–1 for the diffusion coefficient in the mineral compact part of the SEI. The Ld obtained by this manner is in good agreement with the expected thickness of the mineral part of the SEI (L = 1–2 nm). When the cell is cycled instead of dipped in the electrolyte, the isotopic exchange is considerably slowed (τ = 7.8 × 103 s) owing to the migration of the anions and the effect of successive charge and discharge. In both configurations, all 7Li ions in the SEI are exchanged with 6Li from the electrolyte meaning that the SEI is working similarly to an ion-exchange resin.