The Surface-coating Design Of Si-based Anode Materials For Li-ion Batteries

Advanced lithium ion batteries（LIBs）with improved energy density and long cycle life are in urgent demands as the rapid development of portable electronics and electric vehicles.Developing high capacity Li-storage anode materials is one of the important ways to promote the energy density of LIBs.Silicon has been received considerable attention as a new generation anode material for LIBs because of its extremely high capacity,appropriate lithiation potential and low cost.However,Si anode undergoes a drastic volume change upon lithiation/delithiation process,which unavoidably causes cracking,pulverization,electric disconnection of active particles and the structural instability of the solid electrolyte interphase（SEI）on Si surfaces.These problems have severely hindered the commercial application of silicon-based anode materials.In this thesis,we aimed at developing Si-based anodes with a strong cyclability and high Coulombic efficiencies upon cycling by building a dense and compact surface-coating on nano-Si particles,thus preventing the electrolyte to penetrate into and react with the Si cores.The main contents and results are as follows:1.The surface carbon coating for Si-based anode materials:Based on the assumption that there may exist an interaction between the dangling bonds on the freshly generated Si surface and the carbon source molecules,which benefits to form a dense and integrated carbon coating on Si particles,we proposed a new synthetic method for the Si/C composite by sand-milling the micro-sized Si powders and the carbon precursor together to obtain a homogeneous mixture,and then calcinating the mixture at a high temperature to pyrolyze the carbon precursor.The influence of the sand-milling solvent mediums and carbon sources on the structure and electrochemical performance of the as-synthesized Si/C composites was investigated.The experiment results showed that the Si nanoparticles using DMF as the solvent medium exhibit the best electrochemical performance compared to the sand-milling products in ethanol and n-hexane solvents.Among the three kinds of carbon sources:PF（Phenol-formaldehyde resin）,PMMA（poly（methyl-methacrylate））,and the polymers of MMA（methyl methacrylate）,the Si/C composite synthesized from a PMMA precursor demonstrated the most stable cyclability and the highest Coulombic efficiency upon cycling.This composite can exhibit a Li-storage capacity of 1331.5 m Ah g^-1 at the first cycle and the capacity retention of 73%after 300 cycles.More significantly,it can also exhibit a high Coulombic efficiency of≥99.5%after 150 cycles.2.The conductive polymer coating for Si-based anode materials:Carbon coatings are generally porous and brittle,and therefore easy to fracture or peel off from the nan-Si cores when undergoing a huge volume changes during lithation/delithiation.For this reason,we selected a n-type conductive polymer of polyphenylene（PPP）with a good flexiblity and stretchability as the coating layer to sysnthesize a Si/PPP composite by the ball-milling method.The experimental results showed that the dangling bonds on the Si surfaces genenrated during ball-milling can conjugate with the aromatic rings of the conductive polymers,thus enabling the polymer to tightly bound in a planner-orientation to the nano-Si surfaces.The large conjugated polymer planes can effectively prevent the electrolyte penetrating into the Si cores,thus avoidingf the reconstruction of the SEI films and improving the Coulombic efficiencies.The electrochemical results suggest that the Si/PPP anode can demonstrate a high reversible capacity of 2429 m Ah g^-1,a Coulombic efficiency of 85%for the 1^st cycle,a stable cyclability with 88.3%capacity retention over 500 cycles,and,particularly,a high Coulombic efficiency of99.7%upon extended cycling.Our stucture design offers a new insight for future development of high-capacity and cycle-stable Si anode.