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).

Keywords: Li plating; Local element; Pre-lithiation; Si overlithiation; Si-based anodes.

Figure 1

a) Schematic illustration of the synthesis of the different SiNW/Gr composites. SEM figures with varied magnifications for b) SiNW/Gr‐A, c) SiNW/Gr‐B and d) SiNW/Gr‐C. Materials obtained by Au‐catalyst have a straighter (rectilinear) morphology and the increased SiNW diameter size is clearly evident for SiNW/Gr‐B/C, while SiNW/Gr ‐A reveals thin filaments. The morphology of Sn‐derived SiNWs differs, i. e., is more curved and angular.

Figure 2

Charge/discharge potential vs. capacity profiles of a) first and c) second cycle (40 mA g⁻¹; 0.01–1.5 V) for varied SiNWs in SiNW/Gr||Li (−half‐cell) with respective dQ/dV vs. voltage plots in b) and d). e) Charge/discharge cycling, conducted with 231 mA g⁻¹ (≈0.33 C) after formation. f) Corresponding capacity retention.

Figure 3

a) Specific discharge capacities of NCM622 || SiNW/Gr full‐cells as a function of cycle number and b) illustration of the effect of various degrees of electrochemical pre‐lithiation of SiNW/Gr‐A in reference to the specific discharge capacity at 0.1 C (80 mA g⁻¹) with colored markers to demonstrate the pre‐lithiation to different DOPLs (218 mAh g⁻¹ (10 %, red), 295 mAh g⁻¹ (20 %, blue), 373 mAh g⁻¹ (30 %, green), 450 mAh g⁻¹ (40 %, purple) and 528 mAh g⁻¹ (50 %, yellow), respectively).

Figure 4

a) Specific discharge capacities over cycling of NCM622 || SiNW/Gr‐A full‐cells with different DOPLs. Evolution of NE potential in three‐electrode NCM622 || SiNW/Gr‐A full‐cells at DOPL of b) 0 % and c) 50 % and d) ex situ NMR measurements of pre‐lithiated NE and separator after cycling indicating no Li metal (purple circle at 250 ppm ^[39] ).

Figure 5

a) In situ XRD measurements of fully lithiated SiNW/Gr NEs over time, b) corresponding voltage vs. time profiles indicating absence of self‐discharge and the presence of the LiC₆ phase.

Figure 6

(a) Voltage vs. capacity profiles of SiNW/Gr‐A || Li metal cells directly de‐lithiated (black) and de‐lithiated after 100 h (red). The evolution of the discharge indicates less capacity for LiC₆ (i) de‐lithiation after a resting period of 100 h and higher capacity for Si de‐lithiation (ii), i. e., transfer lithiation between lithiated Gr and Si, (b) as schematically shown.

Figure 7

a) Schematic illustration of the proposed ‘transfer‐lithiation’ of Gr with different SiNWs. It is proposed that during the resting period, fully lithiated Gr (LiC₆) transfers Li and further lithiates Si as a result of a local element configuration. Based on the lower diameter and more contact points between small size Si and Gr, the process is faster for smaller diameter of Si wires. b) Powder from fully lithiated Gr electrodes is mixed with i) c‐Si, ii) a‐Si (already cycled for one cycle, i. e., “aged”) and iii) a‐Si lithiated to 100 mV. After mixing static solid‐state ⁷Li NMR experiments are conducted.

Figure 8

Static solid‐state ⁷Li NMR measurements of fully lithiated graphite mixed with a) c‐Si, c) a‐Si, e) Si powder lithiated at 100 mV at B ₀=11.74 T and T=328 K. b,d,f) show the corresponding temporal changes of peak areas as the difference of spectra with the respective ⁷Li NMR spectrum acquired first (at t=0 h). Hence, positive intensity shows an increasing signal, negative intensity a decreasing signal, respectively. Note that the ⁷Li resonance of LiC₆ also shows a characteristic satellite transition powder pattern due to non‐negligible quadrupolar coupling.

Figure 9

a) Schematic illustration of the preparation of SiNW/Gr‐A composite with PLMP for ⁷Li MAS NMR measurements. b) Evolution of the lithiated Gr and lithiated Si signals over time, c) evolution of Li metal from PLMP over time and d) normalized integrated peak areas for each measurement pointing to transfer lithiation.

Figure 10

Incorporating Si in a graphite‐based anode can enhance the energy density, though Li⁺ re‐destribution between graphite and Si in the charged (or pre‐lithiated) state should not be disregarded, as overlithiation of Si can occur. This transfer‐lithiation from lithiated graphite (LiC₆) to Si (a typical local element) is proven, among others, via NMR and galvanostatically after pre‐lithiation and/or storage experiments.