The failure mechanism of nano-sized Si-based negative electrodes for lithium ion batteries

Understanding the failure mechanism of silicon based negative electrodes for lithium ion batteries is essential for solving the problem of low coulombic efficiency and capacity fading on cycling and to further implement this new very energetic material in commercial cells. To reach this goal, several techniques are used here: post mortem (7)Li MAS NMR and SEM, electrochemical impedance spectroscopy (EIS) and three-electrode-based electrochemical analysis. 7Li MAS NMR analyses of the charged batteries demonstrate that the major part of the lithium lost during the charge of batteries is not trapped in Li(x)Si alloys but instead at the surface of the Si particles, likely as a degradation product of the liquid electrolyte. Observed by SEM, a dead electrode has a thick "SEI'' layer at its surface. EIS and incremental capacity analyses demonstrate that the growth of this layer is responsible for the failure of the electrode through a continuous decrease of its active surface area associated with a rise of the electrode polarization. It is demonstrated that the main cause of capacity fade of Si-based negative electrodes is the liquid electrolyte degradation in the case of nano Si-particles formulated with the carboxymethyl cellulose (CMC) binder. This degradation results in the formation of a blocking layer on the active mass, which further inhibits lithium diffusion through the composite electrode.