In situ observation and long-term reactivity of Si/C/CMC composites electrodes for Li-ion batteries

Si/C/CMC composite for electrodes (Na-Carboxy-Methyl-Cellulose) appear today as the most promising strategy in view of substituting carbonaceous materials for silicon as negative active material in Li-ion batteries, hence the need to understand their reaction mechanism. By means of solid state Nuclear Magnetic Resonance spectroscopy, we confirmed that CMC chains can bind to Si via covalent or hydrogen bonding depending upon the pH of the mother suspension. Through coupled in situ Scanning Electron Microscopy and Electrochemical Impedance Spectroscopy observations of such electrodes reacting with Li, we demonstrated the ability of their porosity to buffer the Si swelling up to 1.7-2 Li/Si, further lithiation resulting in internal reorganization with either a definitive break of the covalent CMC-Si bond, or preservation of both the texture and electric wiring in the case of weaker Si-CMC hydrogen bonding thanks to a self-healing process. A relationship between the nature of the Si-CMC bonding and the electrode performance was found with a very positive impact of hydrogen interaction as 100 cycles could be achieved with preservation of the initial texture and excellent retention (3000 mAh/g Si after 100 cycles). Besides, we demonstrated that an alteration in the electrode texture/porosity, by a freeze-drying process, also impacts the electrode reversibility. © 2011 The Electrochemical Society