Preparation And Electrochemical Investigation Of Silicon-based Composite Anode Materials For Lithium-ion Batteries

Silicon has the highest theoretical lithium storage capacity and moderatevoltage plateau, making it a promising anode material to replace graphite.However, the large volume effect during the charge/discharge process leads tothe pulverization of the particles and the destruction of the electric conductionnetwork inside the electrode, which limits the commercialization of this kindof material. In order to solve these problems, we set the goal as improving thelithium storage capability of silicon-based anode by designing nano/microcomposite materials. We take advantages of three effective approches togreatly improve the electrochemical performances of the Si-based compositematerials: conductive carbon modification (including carbon wrapping,carbon coating and carbon pinning), porous strcture and inert alloyembedding.The research contents could be seen as follows：(1) Carbon wrapping. A three-dimentional graphene sheets-wrappednano-Si composite was successfully synthesized by a simple spray dryingroute. The composite exhibited a high reversible capacity of1525mAh g-1and improved cycling stability, which could be attributed to the synergistic effect between GS and active Si nanoparticles. The nanosized Si particleswere uniformly dispersed and wrapped in GS matrix, preventing there-stacking of GS. The flexible GS not only constituted a good conductingnetwork, but also accommodated the volume change of Si during cycling.(2) Carbon coating. A microporous carbon coated silicon core-shellnanocomposite was prepared through thermolysis of Si@poly(cyclotriphosphazene-4,4’-sulfonyldiphenol)(PZS) precursor, which wassynthesized via in situ polycondensation of Hexachlorocyclotriphosphazenes(HCCP) and4,4’-Sulfonyldiphenol (BPS). The reversible capacity of theresulting composite was as high as1393mAh g-1with the capacity retentionof90%after50cycles. The improved cycling performance was mainlyattributed to the microporous and uniform carbon shell, which buffered thesevere volume change of active silicon upon cycling and thereby ensured itsstable electrical conductivity.(3) Carbon pinning. Si-MWNT nanocomposite was successfullysynthesized by a CVD process. It presented a high reversible capacity of1592mAh g-1and good cycling stability with the capacity retention of96%after20cycles. This material had the advantage of good electronic conductingnetwork due to the direct scattered growth and pinning of MWNTs on Siparticles. The robust adherence of MWNTs on Si and the excellentconductivity and flexibility of MWNTs could accommodate the severe volume changes of Si upon lithium alloying and de-alloying.(4) Porous structure. A mesoporous silicon-MWNT composite wasprepared via a facile magnesiothermic reduction and one-step CVD process ofMWNT growth and carbon coating. This material showed a high reversiblecapacity of1149mAh g-1with excellent cycling performance and high powercharge/discharge capability, which could retain1087mAh g-1after100cycles(with the capacity retention of95%) and708mAh g-1even after400cycles. Itexhibited a capacity of685mAh g-1at high current density of5A g-1. Theseremarkable performances benefited from the unique heterostructure of highlyporous silicon matrix combined with in situ grown MWNTs connections,which effectively accommodated the large volume variations of silicon andensure a stable electric connection. The amporphous carbon layer couldprotect the active silicon surface and reinforce the stability of the wholecomposite structure.(5) Inert alloy embedding. With Li13Si4alloy, SiCl4and cobalt powder asraw materials, a porous Si/CoSi2/C composite was prepared by a simplemechanochemical reduction method. After carbon coating, the materialexhibited high initial charge capacity and excellent cycling performance,which retained a stable capacity of609mAh g-1even after500cycles, withhigh capacity retention of84.9%. This superior performance was attributed tothe ternary structure of porous silicon, the CoSi2alloy phase, and the carbon coating layer, which accommodated the severe volume changes of silicon andmaintained a stable electric contact. This synthesis route has broad applicationprospect for its high yield, low cost, and good environmental compatibility.In summary, the thesis introduced three effective approaches (carbonmodification, porous structure, inert alloy embedding), designed and preparedSi-based composite materials with various structures, which greatly improvedthe electrochemical performance. These materials technologies enhanced theapplication potential of the silicon-based anodes.