Comparative study of sodium polyacrylate and poly(vinylidene fluoride) as binders for high capacity si-graphite composite negative electrodes in Li-ion batteries

To study the effect of different polymer binders on electrochemical performance of the Si composite electrode in rechargeable lithium-ion batteries, sodium polyacrylate (PAANa), sodium carboxymethyl cellulose (CMCNa), and poly(vinylidene fluoride) (PVdF) are utilized as the polymer binders for the preparation of composite electrodes consisting of powdery silicon, graphite, and Ketjen black. The electrodes are examined by cross-sectional observation using a scanning electron microscope after a focused ion beam process, X-ray photoelectron spectroscopy, micro Raman spectroscopy, X-ray diffraction, and a peel test. We report that electrode performance of the Si composites depends on a selection of binders, and PAANa binder remarkably improves the electrochemical lithiation and delithiation performance of the Si-graphite composite electrode compared to that of conventional binders of PVdF and CMCNa. When the electrode is prepared with 30 wt % PAANa binder, the higher initial efficiency is obtained with much improved cyclability. Furthermore, the specific capacity of the electrode reaches 1000 mAh g ^-1 and exceeds 800 mAh g ^-1 of reversible capacity during the 30 continuous cycling test. The PAANa polymer binder increases the mechanical strength and adhesive strength as composite electrodes. Furthermore, the polymer layer reduces the electrolyte decomposition at the Si particles and suppresses the capacity deterioration by volume change and pulverization due to Si-Li alloying, compared with PVdF, leading to the better electrochemical reversibility.