Prelithiation of silicon/graphite composite anodes: Benefits and mechanisms for long-lasting Li-Ion batteries

Prelithiation of silicon/graphite-based composite anodes is a promising strategy to limit Li-ion battery capacity loss over long cycling. We report on the spontaneous-corrosion-driven-lithiation (SCDL) of lithium metal on the anode surface of a-Si/c-FeSi2/graphite//LiNi0·6Mn0·2Co0·2O2 cells, and compare it to electrochemically-driven-lithiation (EDL). High cyclability performances are achieved, i.e. more than 1760 cycles at 100% depth-of-discharge with less than 11% capacity loss at C/2. To quantify the fate of the lithium supply over prolonged cycling and follow in situ the benefits of prelithiation, we perform simultaneous operando synchrotron wide-angle and small-angle X-ray scattering (WAXS/SAXS). By accessing both the graphite lithiation process (WAXS) and the nanostructural variations of the silicon phase (SAXS), we obtain a complete picture of the lithium repartition during lithiation/delithiation of the prelithiated anode. Results indicate that the additional lithium is stored in the silicon, which acts as a reservoir compensating for lithium trapping. The silicon phase therefore plays an essential role but without the detrimental effect of large volume variations. By design, SCDL leads to a more heterogeneous prelithiation compared to EDL given that only the electrode area directly in contact with the lithium metal is initially prelithiated. However, we show that during cycling the additional lithium is redistributed throughout the electrode by migrating into the silicon and, finally, the two methods yield equivalent systems, with similar electrode morphologies and a much increased cell performance over cycling. Prelithiation could therefore be used for the production of high energy density and highly stable Li-ion batteries, implementing the easily up-scalable SCDL process.