In the present study, we evaluate the impact of the delithiation cut-off potential on the silicon particle degradation. Building up on previous studies,5,6 our characterization is concerned with the underlying morphological changes the silicon particles undergo during repeated cycling, which ultimately affect the entire electrode integrity. To test this hypothesis, silicon-graphite composite electrodes with a practical areal capacity of ~1.8 mAh cm‑2 were prepared, consisting of 35 wt% nanometer-sized silicon (~200 nm) and 45 wt% graphite (~20 µm). Vapor grown carbon fibers and lithiated poly(acrylic acid) binder accounted for the remaining 20 wt%.
Utilizing SiG//LiFePO4 Swagelok T-cells with a capacitively oversized positive electrode (~3.5 mAh cm‑2) and a stable reference potential of ~3.45 V vs. Li/Li+, three cut-off conditions of the silicon-graphite electrodes, shown in Figure 1, were investigated. First, the silicon-graphite electrodes were fully lithiated to 0.01 V vs. Li/Li+ and completely delithiated to 1.25 V vs. Li/Li+ (brown curve). Second, we limited either the lithiation cut-off potential to 0.05 V vs. Li/Li+ (red curve) or, alternatively, the delithiation cut-off potential to 0.65 V vs. Li/Li+ (blue curve), with the latter two resulting in a capacity utilization of ~80%.
By differential capacity analysis of selected galvano-static cycles we demonstrate that silicon-graphite electrodes with a limited delithiation cut-off potential of 0.65 V vs. Li/Li+ show less overpotential growth and significant improvement in capacity retention upon cycling. These observations are further supported by impedance spectroscopy at different states-of-charge using a gold-wire reference electrode to obtain individual impedance spectra from the positive and the negative electrode.7
Figure 1. Voltage profiles of silicon-graphite electrodes (5th cycles) operated at different cut-off conditions. The data were obtained from SiG//LiFePO4 Swagelok-T cells with a gold-wire reference electrode that was sandwiched between two glass fiber separators soaked with an electrolyte of LP57 + 5 wt% FEC. Areal SiG capacity: ~1.8 mAh cm-2.
The electrochemical results are complemented by transmission and scanning electron microscopy of the silicon particles and composite electrodes. The structural analysis indicates that the repeated delithiation of silicon particles to highly oxidative potentials (1.25 V vs. Li/Li+) causes dealloying reactions, which result in a roughening of the particle surface and the formation of highly porous structures, leading to significantly increased irreversible capacity losses and electrode polarization. Finally, a comparison of the different cut-off conditions and a proposal for an improved cycling protocol for silicon-graphite electrodes are discussed.
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