Even if silicon (Si) is tested as the alternative to graphite negative electrode, its practical use is still hindered due to a massive volume change during cycling. Repeated volume change imposes a large mechanical stress to Si particles to cause a crack formation and/or pulverization. As a result, Coulombic efficiency is poor due to irreversible charge consumption for electrolyte decomposition and Li trapping. Cycle performance is also poor due to solid electrolyte interphase (SEI) deposition and electrode polarization.¹ Such a volume change also imposes a mechanical stress to the SEI layer covering Si particles. The SEI layer may be damaged by the mechanical stress, resulting in a loss of passivating ability and concomitant electrolyte decomposition.

In this work, as a way to suppress the volume change of Si electrodes, the de-lithiation voltage cutoff was lowered. The expectation here was that the volume change is less severe when cycled within a narrower cut-off range. Less serious SEI damage is expected because mechanical stress is less significant. Meanwhile, the endurance against mechanical stress was examined for the SEI layer derived from fluoroethylene carbonate (FEC), which is known as an effective SEI former.²

Experimental

The 2032 type coin-cells were assembled from nano-sized Si powder and Li metal foil. The used electrolyte was 1.3 M LiPF₆ dissolved in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC). 25 wt. % of FEC was added to examine the additive effect. The galvanostatic charge/discharge test was performed in two different cut-off ranges. The lithiation cut-off voltage was fixed at 0.1 V (vs. Li/Li⁺), whereas the de-lithiation cut-off was varied from 1.5 V to 0.5 V. The morphology of SEI layer was examined by using field-emission scanning electron microscope (FE-SEM). The X-ray photoelectron spectroscopy (XPS) was used to analyze the coverage of SEI layers.

Results and discussion

The XPS data obtained after the first lithiation/de-lithiation show that the Si surface is still covered by SEI layer when the de-lithiation cut-off is 0.5 V, whereas the surface film is removed after the full de-lithiation (1.5 V). This illustrates that the SEI layer is damaged by the mechanical stress being imposed, which is stronger with 1.5 V cut-off due to a larger volume change. The electrolyte continuously decomposes at the newly exposed Si surface to generate a thick SEI layer in the later cycles (Fig. 1a). When the cycling voltage range is narrower (de-lithiation cut-off = 0.5 V), however, the electrolyte decomposition/film deposition in the later cycles is less severe (Fig. 1b) because the initially generated SEI layer is not seriously damaged. The SEI damage is also insignificant even after the full de-lithiation (1.5 V) when FEC is added in the electrolyte. It is seemingly that the SEI layer derived from FEC has a higher endurance against mechanical stress. As a result, it is not damaged in the initial cycles. Additional electrolyte decomposition/film deposition is not serious (Fig. 1c) because the Si electrode is still passivated.  

References

1. J. G. Lee, et al., J. Electrochem. Soc., 2015, 162, A1579.

2. V. Etacheri, et al., Langmuir, 2011, 28, 965.