Elucidating ‘Transfer-Lithiation’ from Graphite to Si within Composite Anodes during Pre-Lithiation and Regular Charging

Abstract

Si-based anodes can increase specific energy and energy density of Li ion batteries. However, the volume-induced material stress and capacity loss necessitates only a partial Si utilization within composite anodes, typically with state-of-the-art graphite, so called Si/Gr composites. In this work, various Si nanowires (SiNWs), a promising Si architecture for these composites, are investigated and modified via pre-lithiation. Though, charged pre-lithiated anodes show potentials below 0 V vs. Li|Li⁺ in the initial cycles, they do not show indications for metallic Li, which is likely a hint for a triggered surface Li depletion in course of a continuous “transfer-lithiation” from lithiated Gr to Si, which is indicated by decreasing LiC₆ and increasing Li_xSi_y signals via nuclear magnetic resonance (NMR), X-ray diffraction (XRD) as well as shifts in capacities of respective voltage plateaus during discharge after storage. A relevant contribution of self-discharge is unlikely as shown by a stable open-circuit-voltage during storage in charged state and similar subsequent discharge capacities, being consequently a hint for an intra-electrode capacity shift. The process of transfer lithiation is finally validated via solid-state ⁷Li NMR for varied Si morphology, i. e., amorphous and crystalline, as well as during pre-lithiation with passivated lithium metal powder (PLMP).