For several years, great attention has been paid to silicon as negative electrode material for Li-ion batteries, due to its very high gravimetric capacity (3579 mAh g^-1) in comparison to that of graphite (372 mAh g^-1). However, Si electrodes suffer from poor cyclability due to the large volumetric expansion (up to ~300%) of Si upon its lithiation, resulting in the electrode architecture disintegration, and in the instability of the solid electrolyte interphase (SEI).

We have shown that low-cost and high-performance Si-based electrodes can be obtained by combining (i) the use of ball-milled (nanocrystalline) Si powder resulting in a smoother phase transition; (ii) the processing of the electrode at pH 3 with carboxymethylcellulose (CMC) binder favoring the covalent grafting of the CMC to the Si particles; (iii) the use of fluoroethylene and vinylene carbonates (FEC/VC) electrolyte additives resulting in a more stable SEI.¹

More recently, we have shown that the storage conditions of the Si-based film before assembling in the electrochemical cell has also a major impact on the electrode performance.² In this context, we have elaborated a film maturation process which simply consists of storing the electrode in humid air (85% RH) at room temperature for at least two days. This process has a significant positive impact on the electrode performance as is illustrated in Fig. 1a which compares the cycling performance of ball-milled Si-based electrodes cast on a copper foil with and without a film maturation step. Different mechanisms are proposed to explain why humid storage has such a beneficial impact on the electrode performance. Firstly, humid air may increase the film adherence on the copper current collector by increasing the amount of CuOH groups which can create bonds with the CMC binder. This tends to be supported by the fact that the film maturation step has no significant impact when a carbon layer is deposited on the copper current collector prior to the maturation step as shown on Fig. 1b. Secondly, water molecules from air can react with the film and modify the chemical links between the Si particles and the CMC binder. This tends to be confirmed by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) analysis of the composite electrodes. Indeed, as shown in Fig. 2a, a significant increase of the intensity of the peak centered at 1630 cm⁻¹ assigned to the stretching band of the carboxyl group of CMC is observed for the film stored in humid air, while the intensity of the peak centered at 1750 cm^-1 assigned to ester bond between Si and CMC is decreased. This suggests that during the maturation step, some of the ester bonds between Si particles and CMC binder are converted into less rigid hydrogen bonds as schematized in Fig.2b. This may increase the deformability of the electrode, which can better accommodate volume variations with cycling. Thirdly, the humid air could modify the surface chemistry of silicon particles by increasing the amount of silicon oxide and/or silanols (SiOH). This would increase the amount of bonds between silicon particles and binder, and/or help to stabilise the SEI layer. Various experiments for supporting these different hypotheses will be presented.

References

1. M. Gauthier, D. Mazouzi, D. Reyter, B. Lestriez, P. Moreau, B. Lestriez, D. Guyomard, L. Roué. A low-cost and high-performance Si-based electrode for Li-ion batteries. Energy Environ. Sci. 6 (2013) 2145–2155.
2. C. Real Hernandez, Z. Karkar, D. Guyomard, B. Lestriez, L. Roué. A film maturation process for improving the cycle life of Si-based anodes for Li-ion batteries. Electrochem. Comm. 61 (2015) 102-105.