Amorphization Progress of Microscale-Silicon Studied via Operando and in Situ X-Ray Diffraction during Formation and Cycling
Authors: Dickmanns Jonas L. S. , Grossmann Lukas , Bock Moritz , Holst Marc , Pham Thien An , Gilles Ralph , Gasteiger Hubert Andreas 
Published: December 2024

Silicon as an anode active material for lithium-ion batteries has a theoretical gravimetric capacity of 3579 mAh g-1, considering the highest electrochemically lithiated phase of Li3.75Si.1 Hence, silicon has gained increasing interest in the last years to either partially or completely replace graphite in commercial lithium-ion batteries to increase energy density.2 However, the expansion of silicon of up to 300% upon complete lithiation bears challenges such as particle cracking, loss of electrical contact to the active material, and the continuous formation of the solid-electrolyte interphase (SEI). To decrease these detrimental effects, partial lithiation of microscale silicon was introduced, limiting the used capacity of silicon to ~1200 mAh g-1 and thus reducing its overall expansion, rendering ~250 cycles in full-cells possible.3

XRD measurements during the initial lithiation of crystalline silicon revealed that it is irreversibly amorphized in the first cycle when its potential drops below ~170 mV vs. Li+/Li.4 In the case of partial lithiation of microscale silicon, cycling above this potential limit leads to amorphized silicon particles with a remaining crystalline fraction. In this state, the amorphized part forms a lower lithiated phase LixSi (with x~2), which can be reversibly de-/lithiated.5

In this study, the amorphization progress of microscale silicon full-cells using a single-layer pouch cell setupwas investigated operando over the first four formation cycles and in situ after extended cycling. The anodes consisted of 70% silicon (d
50 =4.5 µm, Wacker Chemie AG), 8% LiPAA binder (Sigma Aldrich), 2% conductive carbon (Super C65, Imerys), and 8% graphite (KS6L, Imerys). The cathode consisted of a nickel-rich transition metal oxide cathode active material (LiNi0.83Co0.12Mn0.05O2, BASF SE). As electrolyte, a 1 M LiPF6 in FEC:DEC 2:8 (v:v) was used. As shown in Figure 1, the cycling performance in pouch cells, reaching 80% state-of-health (SOH) after ~250 cycles at 25 °C matches well with the literature.6

Figure 1 highlights the cycles at or after which XRD measurements were performed. To accomplish operando XRD measurements, a custom-made pouch cell holder with glassy carbon windows was used, which allows for an operating compression of 2 bar. In this setup, an XRD pattern could be taken every ~15 min for the first four cycles at 0.1 h-1. Using the XRD analysis method developed previously,5 the XRD data confirm a significant amorphization in the first charge, while no amorphization occurs during the relaxation phase and during discharge of the cell. In the second charge, the amorphization continues only after ~70% of the capacity was transferred. The start of amorphization aligns with the anode onset potential of ~170 mV vs. Li+/Li, in accordance with the literature.4,5 With increasing cycling number, the progress of amorphization per cycle decreases. Although only ~33% of the silicon capacity (i.e., 1200 mAh/g) is reversibly used, the amorphization reaches ~70% after 50 cycles, highlighting the significant particle usage. Continuous cycling up to 500 cycles leads to no significant further amorphization of the silicon.

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Acknowledgements:

The authors acknowledge funding from the German Federal Ministry of Economic Affairs and Climate Action through the project CAESAR (grant number 03EI3046F). We thank BASF SE for the NCM cathode material and Wacker Chemie AG for the silicon anode material. The authors thank the Physics Laboratory of the MLZ for measuring time at the diffractometer as well as B. Veltel and A. Book for the adjustment of the instrument.

Figure 1