Modification Of SiO Asanode Materials Forlithium Ion Batteries

Silicon-based material has been the most attractive anode material for Li-ionbatteries because of the high content in the crust, high theoretical capacity for lithiumstorage, low discharge potential and excellent safety performance. However, the mostchallenging problem that hinders the commercial application of Si anode material isthe enormous volume expansion during lithium alloy/dealloy cycles. This volumechange results in pulverization of the initial particle morphology and causes the lossof electrical contact between active materials and the electrode framework. And thepoor electronic conductivity of SiO is also an important problem to solve. Sosupressing the volume change and enhancing the cycle performance have been theresearch topics for Si.This paper mainly prepared the SiO/CNxcomposite as anode materials forlithium ion batteries by different methods in which PAN (polyacrylonitrile) was usedas the carbon precursor. In addition, this paper studied the effects of the binder on theelectrochemical properties of the composites by changing the binder. The structureand morphology of the composites were systematically investigated by the X-raydiffraction (XRD), Raman spectroscopy (Raman), Fourier transform infraredspectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) and scanning electronmicroscopy (SEM) respectively. And the cycle stability and electrochemicalperformance were studied by cyclic voltammetry analysis (CV), charge-dischargeperformance test and electrochemical impedance spectroscopy (EIS).Firstly, sol-gel method was used to mix PAN and SiO with the weight ratio of3:7together and the SiO/CNxcomposites were gained after the heat-treatment of PANunder Ar. After mixing with PAN and heat-treatment, the composites showed bettercycle performance. When the heat-treating temperature was500°C, the compositedisplayed the best cycling performance with the discharge capacity of2009.3mAh/ginthe first cycle and the initial coulombic efficiency of64.8%. And the remainingcapacity after50cycles was360mAh/g,which was greatly improved compared to SiO.The sodium alginate was used as the binder instead of the PVDF. When the heat-treating temperature was500°C, the composite displayed the best cyclingperformance with the discharge capacity of2131.3mAh/gin the first cycle and theinitial coulombic efficiency of71.8%. And the remaining capacity after50cycles was646.5mAh/g.The reversible capacity was great improved compared to PVDF.Secondly, PAN and SiO were mixed by ball milling with the weight ratio of3:7and the heat-treatment under Ar atmosphere. The SiO/CNxcomposites gained byballing milling showed better cycling performance because the ball millingguaranteed the better mixture of SiO and PAN and the fully utilization of PAN. Whenthe heat-treating temperature was500°C, the composite displayed the best cyclingperformance with the discharge capacity of1180.3mAh/gin the first cycle and theinitial coulombic efficiency of65.9%. And the reversible capacity after50cycles stillremained490mAh/g. The sodium alginate was used as the binder instead of the PVDF.When the heat-treating temperature was500°C, the composite displayed the bestcycling performance with the discharge capacity of1396.7mAh/gin the first cycle andthe initial coulombic efficiency of72.4%. And the remaining capacity after50cycleswas580.9mAh/g.The reversible capacity was great improved compared to PVDF, butthe decline of the capacity was serious, so we needed to further improved the stabilityand the reversible capacity during the cycling process.Finally,“direct coating” method was used to mix PAN and SiO with the weightratio of3:7together and the SiO/CNxcomposite electrodes without binder weregained after the heat-treatment of PAN under Argon. The particle size of the SiOdecreased to micronano particles by ball milling since the commercially SiO wasmicro particles, we denoted “milled SiO”. The “milled SiO” and PAN were mixeduniformly and then coated on the Cu collector. The composite electrodes were dried invacuum at80°C and heat-treated in tube furnace under Ar, and we got theSiO/CNxcomposite electrodes without binder. During the heat-treating process,N-doped carbon network were coated on the surface of the “milled SiO”, whichimproved the conductivity and cycling performance. When the sample of the washeated at500°C, the composite anode revealed the best cycle stability with the result showing that the cycling performance was stable and optimal at a rate of100mAg1.The initial discharge capacity was as high as2733.7mAh g1with the initialcoulombic efficiency was74.9%, which was kept as927.8mAh g1over100cycles,which was greatly improved compared to SiO.Thecoating carbon layer can not onlyincrease electronic conductivity, but also accommodate part of the volume expansionoccurred during discharge/charge process.