SEI Formation on Single Crystal Si Electrodes in Organic Carbonate Electrolytes

Silicon, with a theoretical capacity of 4200 mAh, is an attractive alternative to conventional anode materials for lithium ion batteries^[1]. However, silicon like other intermetallic electrodes exhibits poor passivating behavior in organic electrolytes. Moreover, it suffers from a significant volume variation (300%) upon cycling in Li-ion cells, which repeatedly damages the solid-electrolyte interphase (SEI) layer and contributes to the interfacial instability of Si^[2]. The organic electrolyte composition^[3] and surface orientation of silicon crystal^[4] have been reported to play a major role in the SEI formation process, which is critical for battery electrochemical performance and lifetime^[5].

This work focuses on characterization of interfacial phenomena during the SEI formation on Si single crystal electrodes in organic carbonate electrolytes. The SEI formation on the facets (100), (110) and (111) was controlled electrochemically during the first potentiostatic charge in a three electrode cell from 2.5 vs. Li/Li⁺  to 500 mV (before Si-Li alloying) and  10 mV. The effect of four different electrolytes was evaluated: 1 M LiPF₆, EC/DMC (1:1), 1 M LiTFSI in EC/DMC (1:1), 0.5 M LiBOB in EC/DMC (1:1), 1 M LiPF₆/2% LiBOB in EC/DMC (1:1). Composition, structure and morphology of the SEI layer were investigated by a combination of ex situ FTIR, LIBS, AFM and SEM-EDX.

The impact of the electrolyte composition on the kinetics of the SEI formation, its chemical composition and morphology was clearly identified. In LiPF₆- and LiBOB-containing electrolytes, a thick SEI layer is formed with respectively LiF and LiC₂O₄ as main component whereas in LiTFSI-based electrolyte, the formation of a thin unstable SEI layer rich in organic carbonates occurs at lower decomposition potential. This results in remarkable difference during alloying. An extensive fissuration is observed at the silicon surface with LiTFSI while this damage is drastically attenuated with LiPF₆ and LiBOB. Interestingly LiBOB as additive in LiPF₆-based electrolyte leads to an earlier formation of  a passivating surface film at the silicon surface.

Upon polarization, Si single crystal with (100), (110) and (111) orientation displays quantitative and qualitative differences in the electrochemical spectroscopic response. Chemical composition of the SEI layer varied strongly on the different Silicon crystal facet silicon surfaces at potentials >0.5 V and included EC and DEC solvents decomposition products. The SEI on the Si (111) crystal facet is a mixture of organic carbonates whereas the SEI on Si (100) consists mainly of LiF.

At potentials below 0.5 V the SEI undergoes reformation due to Si cracking and structural transition from crystalline to amorphous silicon upon alloying with lithium. This results in similar SEI chemical composition for the three different crystal orientations.

 The correlation between surface activity, composition and stability of the SEI layer, and crystal orientation will be discussed.

Acknowledgement

This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

References

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