Dynamics Of The Li-6/Li-7 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 (Li-6 and Li-7) 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 Li-7, dismantling the cell to get the delithiated graphite electrode and (i) dipping it in an electrolyte containing Li-6 or (ii) cycling it in a new cell using an electrolyte containing Li-6 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 tau (tau = 103 s for the self-diffusion experiment) in this equation, a characteristic length for diffusion can be calculated (L-d = 2.3 nm) assuming D(Li+) = 2.6 x 10(-16) cm(2).s(-1) for the diffusion coefficient in the mineral compact part of the SEI. The L-d 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 (tau = 7.8 x 10(3) s) owing to the migration of the anions and the effect of successive charge and discharge. In both configurations, all Li-7 ions in the SEI are exchanged with Li-6 from the electrolyte meaning that the SEI is working similarly to an ion-exchange resin.