Synthesis And Electrochemical Properties Of Silicon Based Anode Materials For Lithium-ion Batteries

Silicon-based material for lithium-ion batteries is one of the most promising anode materials due to its high theoretical specific capacity of4200mAh g"1and proper potentials for lithium insertion and extraction (<0.5V vs. Li/Li+). However, the huge volume change during the electrochemical charge/discharge cycle processes eventually results in the severe pulverization of the active electrode materials and the breakdown of the electrically conductive network. As a result, it decays the cycling capacity of electrode materials. In order to solve the above problems, we have firstly explored the optimization of the binder content and selection of different binders and conductive agents. Then, a series of silicon/cabon or silicon/graphite/cabon composite materials were synthesized by mechanical mixing, spray drying and high-temperature pyrolysis methods in this thesis. At last, the influence of the additive content on the electrochemical performance also was discussed. It has been demonstrated that those silicon/cabon composite materials can alleviate the huge volume change of active silicon associated with Li insertion/extraction process and improve the electrochemical performance of silicon anode. The main achievements are given as follows:1. By optimizing the binder content in the lithium ion batteries and selecting different types of binders and conductive agents, a favorable condition was obtained to get better cycling performance for the lithium ion batteries using silicon particles as anode materials. The results indicated that the rechargeable capacity and the first coulombic efficiency of silicon based anode material reached optimized values when the binder content was optimized to be20%and content of conductive agent was supposed to be10%respectively. The graphene as conductive agent in the lithium ion batteries can improve the conductivity of the electrode and provide a favorable channel for the transportation of Li ion. The sodium carboxymethylcellulose (CMC) as water-soluble binder can form a self-repairing weak hydrogen bonds with Si particles to improve the structure of the electrode and the cycling stability of the electrode was enhanced.2. The graphite with excellent conductivity was synthesized with Si particles to get the silicon/graphite composites by mechanical mixing. The research results suggested that the initial charge-discharge capacity of the Si/G composites decrease but the capacity retention ratio increase with the graphite content increase. When graphite was added into the anode materials, the theoretical specific capacity of the Si/G composites decreased as expected. On the other hand, the graphite can alleviate the volume changes by silicon during the Li ion insertion/extraction process and therefore improve the cycling performance of the Si/G composites.3. The spray drying process was applied to synthesize the Si/C composite materials using starch as the carbon precursor. The composite material displays an initial reversible specific capacity of1083.2mAh g"1and the capacity stays at200mAh g"1after70cycles under a current density of100mA g"1. Further, the graphite was added into the precursor to successfully fabricate the silicon/graphite/carbon (Si/G/C) composite materials, which had a favorable cycling stability with a capacity of1591.9mAh g"1for first charge cycle and640.4mAh g"1after50cycles. In this Si/G/C composite material, silicon gives the high specific capacity and graphite provides a transport pathway for the ions or electrons while the pyrolytic carbon layer can alleviate the volume changes by silicon during the cycling process. Thus, the cycling stability and electrochemical performance were improved simultaneously in the Si/G/C composite materials for lithium-ion batteries.4. By means of the high-temperature pyrolysis method, we present a versatile strategy to synthesize the Si/C composites as anode material for lithium batteries using polyvinylidene fluoride as the precursor. The Si/C composites showed a reversible specific capacity of735.8mAh g"1and the capacity retention of72.8%after50cycles with a current density of100mA g"1. Furthermore, we chose the starch as the carbon precursor and designed different ratios of Si to starch are used to optimize the compound and the electrochemical properties of the Si/C composites. As an anode material for lithium-ion batteries, the Si/(48wt%)C composites exhibit the best electrochemical properties with the initial reversible specific capacity of781.1mAh g"1and the capacity retention of91.3%after50cycles with a current density of100mA g"1. The improvement could be attributed to the introduction of carbon in the Si/(48wt%)C composites, which can provide a rapid lithium transport pathway, reduce the cell impedance and stabilize the electrode structure during the lithium alloying-dealloying process.5. The improvement in electrochemical cycling properties of the Si/C composites was investigated by taking the vinylene carbonate (VC) as electrolyte additive and the influence of the additive content on the electrochemical performance also was explored. Adding the electrolyte additive can improve the formation potential and the stability of the solid electrolyte interface (SEI), which would enhance the cycling performance of the composites. When the VC content was optimized to be2wt%, the electrochemical cycling properties achieve optimal performance with an initial reversible specific capacity of349.9mAh g"1and a capacity retention ratio of99%after30cycles. However, higher VC content brings a decrease of the enhancement due to the increasing of the thickness and impedance of the SEI induced by the increased decomposition of VC...