Increasing the energy density of Li-ion batteries (LiB) is a key issue for transport applications. A promising approach is to replace the currently used graphite in LIB anodes with silicon since its specific capacity is ten times higher than that of graphite. The main challenge is to deal with the large silicon volume expansion induced by its lithiation, which damages the mechanical integrity (electronic network) of the electrode, and produces an unstable solid electrolyte interphase (SEI).

In last years, our group has successfully improved the performance of silicon based anodes by working on various aspects¹. First, our ball-milled silicon offers the right nanostructure to limit Si particle cracking in addition to be produced using an industrially viable process. Second, the use of an acid buffer solution to prepare the electrode ink favors the formation of resilient covalent bonds between the carboxymethylcellulose (CMC) binder and the Si particles. Third, adding fluoroethylene carbonate (FEC) to the electrolyte limits the SEI growth. Lastly, the use of carbon nanoplatelets instead of carbon black as conductive additive insures more durable electronic network, especially for Si-based electrodes with high areal capacities².

Recently, we have developed a film maturation process that further improves the performance of our silicon-based anodes³. This simple process consists in storing the electrode in a humid atmosphere for a few days before drying and cell assembling. As seen in Fig.1, this results in an impressive improvement of the electrode cycle life. Such a cycling performance is remarkable considering the high Si areal mass loading of the electrode (2 mg Si cm^-2).

In this work, we present a comprehensive study that aims at explaining how the maturation process works. On one hand, we have studied the effect of the maturation process on the mechanical properties of our Si-based electrodes using indentation measurement, peeling test, scratch test, in-operando dilatometry and in-operando optical microscopy. Those various experiments confirm that the film adhesion and cohesion strengths are reinforced by the maturation step. On the other hand, we have studied the effect of maturation on the electrochemical performance of various electrode formulations. These results show that maturation works with different types of silicon, binder and conductive agent. They also suggest that the nature of the substrate and the acidic environment play a crucial role in the maturation process. Furthermore, reflectance Fourier transform infrared spectroscopy (ATR-FTIR) and Nuclear Magnetic Resonance (NMR) analyses have been performed in order to evaluate the impact of the maturation step on the chemical bonds between the Si particles and the CMC binder and with the Cu substrate. ). Lastly, focused ion beam scanning electron microscopy (FIB-SEM) tomography shows a better preservation with cycling of the pore and Si particle connectivities for the matured electrode. From the conjunction of these different experiments, a film maturation mechanism is proposed.

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. Z. Karkar, D. Mazouzi, C. Reale Hernandez, D. Guyomard, L. Roué, B. Lestriez. Threshold-like dependence of silicon-based electrode performance on active mass loading and nature of carbon conductive additive. Electrochim. Acta 215 (2016) 276-288.
3. 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.