Preparation Of Carbon-Coated Silicon Nanomaterials And Research On The Performance Of Lithium-Ion Batteries

As a star energy storing equipment in the 21st century,LIBs(lithium-ion batteries)have lots of obvious advantages(large specific capacity,pro-environmental,etc.),and have achieved extensive business applications.Anode materials can play a decisive role in the performance of LIBs.At present,graphite is the darling of market and the first choice for commercial anode.However,the market requirement continues to expand with social progresses.Duo to its own limitations(the theoretical limit of capacity is only 372 m Ah g^-1),it is hard for graphite to have enough capacity to satisfy the surge market demand.Therefore,the development of anode with higher cycle capacity has important application value.Silicon has obvious merits(rich reserves,low cost,etc.),especially its theoretical capacity(4200 m Ah g^-1)can meet the market's pursuit of high-capacity anode materials,but silicon has a large degree of volumetric change during deintercalation process(?400%),the electrode structure will eventually be destroyed from the inside out,leading to a sharp drop in electrochemical performance.This article mainly uses different carbon sources and preparation processes to coat nano-silicon to adjust the structure of silicon-carbon hybrids,thereby retarding the volumetric change of silicon during deintercalation process and improving its comprehensive performance.details as follows:(1)Firstly,we prepared two silicon-carbon composites with different structures by a simple and easy-to-mass-produce ball milling method:(1)amorphous carbon-coated silicon mixture(1-BM);(2)crystalline carbon and amorphous carbon coated silicon mixture with hierarchical structure(2-BM).Compared with 1-BM,2-BM has superior ICE(initial coulombic efficiency:89%),and at the same time in terms of cycling(reversible capacity up to 874.5 m Ah g^-1 after 300 cycles)and rate test(380 m Ah g^-1under high rate of 5 C,nearly twice that of 1-BM)also has even better performance.Various characterization results confirmed that the excellent comprehensive performance of 2-BM is mainly profit from the hierarchical coating structure of the crystalline carbon and the amorphous carbon on the silicon.The crystalline carbon effectively avoids excessive dissolution of the electrolyte at the interface,making the SEI(solid electrolyte interphase)film thin and stable,and improving the CE;the porous nature of the amorphous carbon effectively confines the silicon,restricting its volumetric change,and thus the integrity of the electrode structure is effectively protected.(2)Secondly,using inexpensive pitch as the new carbon source,after high-temperature processing,a silicon-carbon composite material(Si@Pitch)with carbon tightly encapsulated nano-silicon was prepared;Si@Pitch and graphite were mixed together to prepare another silicon-carbon composite(Si@Pitch-G);as a comparison,a silicon-carbon hybrid(Si-G)composed of graphite and silicon was also prepared.Electrochemical tests on three silicon-carbon materials indicated that the comprehensive performance of Si@Pitch-G(ICE:92.5%,discharge capacity_{cycle 150}:366.7 m Ah g^-1,capacity retention rate:69.9%)is significantly stronger than Si@Pitch and Si-G.Its remarkable comprehensive performance should be attributed to the synergy of pitch and graphite.(3)Finally,mass production was realized by adjusting the preparation method of Si@Pitch-G,and the raw material ratio of pitch and nano-silicon was changed to further improve electrochemical performance.The mixing method of Si@Pitch and graphite is changed to mechanical ball milling,and the content of pitch carbon is set to three ratios to prepare Si@Pitch@G series materials.Among the three materials,3-Si@Pitch@G(pitch:?4.3 wt%),which has the lowest pitch carbon content,has the best performance in the cycle test(it still has a capacity contribution of 404.4 m Ah g^-1 after 150 cycles).