Silicon is of significant interest for high-energy-density lithium ion batteries (LIB) required for electric vehicle applications but is affected by rapid capacity decay, which is related to large volume changes upon (de-)lithiation. Despite several strategies to overcome the limited lifetime such as nano-sized silicon, the use of silicon oxides instead of (metallurgic) silicon, silicon/graphite composite electrodes, the amount of silicon employed in commercial LIBs is still limited to less than 20wt%.
Nuclear Magnetic Resonance (NMR) techniques have emerged as valuable tool to evaluate the lithiation kinetics of graphite intercalation compounds and lithium silicides, including analysis of lithium plating in LIBs.Particularly, highly reactive and metastable Li15+δSi4 phases that lead to severe capacity fading were previously quantified by 7Li NMR in silicon anodes. Though lithiation kinetics of pure graphite and silicon anodes were analyzed by operando 7Li NMR spectroscopy, an explicitly quantitative NMR protocol to unravel lithiation kinetics of industrially relevant Si/C composites is still pending. Herein, we demonstrate a quantitative operando 7Li NMR protocol for pouch-type LIBs to elucidate mechanistic aspects of degradation of high-capacity (16mAh cm-2) Si/C composites (70wt% µm-size Si). This includes setup of a NMR compatible pressure device, capable of applying up to 1MPa (10bar) of external pressure while (dis-) charging the cells and acquiring 7Li NMR data. Lithiation kinetics of Si/C composites in Li||Si/C and NMC622||Si/C LIBs are unraveled in more detail, depending on the applied external pressure and charging rates. An extended lithiation model is proposed, and invoked as guideline for more reliable protocols of formation and cycling to further boost corresponding lifetimes of Si/C based LIBs with high silicon content.