Study On Tin Nanopowder Negative Electrode For Lithium-ion Batteries

Tin has a high theory capacity of994mAh g-1, which is relatively higher than the commercially used graphite anode (372mAh g-1). However, tin anodes usually perform poor cyclic retentions due to the huge volume changes (up to300%) during charge-discharge process. It has been proved that the bulk tin material could easily crack during cycling, which would directly increase the resistance of the electrode. Using nanosized materials or tin-based composites can alleviate this problem to some extent. In addition to the active material, other parts such as a binder and a conductive agent also have significant influences on the electrochemical performance.In this study, Polyvinylidene fluoride (PVDF) and carboxymethyl cellulose (CMC) were used as binders for tin nanopowder anodes for lithium-ion batteries, respectively. We aim to investigate the electrochemical characteristics of the tin nanoparticle by comparing the CMC binder with the conventional polyvinylidene fluoride (PVdF) binder. Cyclic voltammetry (CV), charge-discharge test, scanning electron microscope (SEM) and X-ray diffraction (XRD) were employed to investigate the effects of the binder types and vacuum drying temperature on the electrochemical properties of the tin electrodes. Under the best vacuum drying temperature200℃with the using of binder CMC, the addition of conductive agent acetylene black (AB) and carbon coating on the electrochemical properties of the tin nanoparticle electrode were further discussed.The results show that vacuum drying temperature during the electrode preparation has a significant influence on the electrochemical performance of tin nanopowder electrodes, and in the experimented temperature range from150℃to220℃, the best vacuum drying temperature is200℃for electrodes using both types of binders. After vacuum dried at200℃, the electrode using CMC displayed a higher initial capacity (883mAh g-1) than that of using PVdF (541mAh g-1) and obtained a better cycle performance, which suggests that CMC is a potential binder for tin-based anodes. When acetylene black is added into the electrode using CMC binder, it showed a charge-discharge efficiency of75.8%, which is higher than that of AB-free electrode. Moreover, the discharge capacity after50cycles is293mAh g-1, which is also higher than that of AB-free electrode (132mAh g-1). The TEM test suggests that the good capacity retention in the subsequent cycles may be attributed to the formed branch-like structure on the surface of the tin nanoparticles during cycling. The addition of acetylene black enhances the conductivity of the electrode bulk and the conductivity between the electrode and the electrolyte interface, which may be beniefit to the good cycle performance. Carbon coated tin was prepared by decomposing glucose applying a hydrothermal method, and was further used as the active material for negative electrode (Sn-C/AB) of lithium secondary battery. Charge-discharge test shows that the carbon coated tin electrode with the addition of5wt.%acetylene black as a conductive agent could obtain an initial discharge capacity of967mAh g-1and a discharge capacity of362mAh g-1after50cycles, which is much higher than that of tin electrode (166mAh g-1after50cycles). The coated carbon hinders the agglomeration of tin powder, reduces the irreversible capacity loss of tin; the addition of acetylene black could reduce the impedance between the electrode and the electrolyte and improve the transfer property of lithium ions and the electrons within the electrode, which contributes to the higher initial discharge capacity.