Preparation And Electrochemical Performance Study Of Novel Bi-based Anode Materials For Lithium-ion Batteries

Lithium ion batteries（LIBs）are widely used in portable electronic devices and hybrid vehicles because of their high energy density,long cycle life and environmental friendliness.In recently,the energy density of LIBs has faced higher demands with the increasing demand for portable devices and the rapid development of hybrid vehicles.As an important part of LIBs,anode material will play an important role in improving the overall energy density of LIBs.However,the commercial graphite anode materials have a specific capacity of only 372m Ah g^-1,which can not meet the demand for high energy density for LIBs.Bismuth（Bi）and its sulfides have attracted much attention due to their non-toxicity,lattice spacing and high theoretical volume specific capacity.However,they both face a massive volume expansion problem（≈215%）during the lithium process,which will result in continued crushing and structural collapse of the material,which in turn will lead to its detachment from the collector,as a result,the Bi-based anode exhibits poor cycling and rate performance.In this paper,the problem of too fast capacity decay of Bi-based anode materials in the process of recycling and how to improve the reversibility of the conversion reaction of Bi-based anode materials in the process of lithium storage are studied.The findings are as follows:（1）A heterostructure Bi₂S₃@Zn S@NC composite was designed as anode material for LIBs.Nitrogen doped carbon can improve the conductivity of Bi₂S₃@Zn S@NC electrode and enhance the transport of Li⁺and electrons.Zn S/NC layer can effectively restrain the volume change of the electrode and prevent the active material from contacting with the electrolyte directly,thus reducing the side reaction.Bi₂S₃@Zn S@NC with very fine nanoparticles can provide more active sites,which leads to high specific capacity of Bi₂S₃@Zn S@NC(886m Ah g^-1after 200 cycles at current density of 200 m A g^-1).The heterostructure of Bi₂S₃and Zn S can shorten the transport path of Li⁺effectively,which makes Bi₂S₃@Zn S@NC electrode have excellent rate performance(425 m Ah g^-1at 5 A g^-1).（2）Bi₂S₃@NC nanocomposite was designed and synthesized by simple solvent reaction,polymerization coating and in situ vulcanization,and a strong Bi-C bond between Bi₂S₃and nitrogen doped carbon layer was constructed in Bi₂S₃@NC nanospheres.In this unique structure,the Bi₂S₃@NC composite has various advantages.On the one hand,the N-doped carbon layer can not only improve the conductivity of Bi₂S₃,but also buffer the volume change.On the other hand,the Bi-C bond can further alleviate the huge stress caused by the volume change and guarantee the charge transfer kinetics.Therefore,when used as a LIBs anode material,Bi₂S₃@NC shows high reversible capacity(775 m Ah g^-1after 100 cycles at a current density of 0.2 A g^-1),excellent rate capacity(high reversible capacity of 412 m Ah g^-1even at 5.0 A g^-1)and long cycle life(reversible capacity of 510 m Ah g^-1after 1000 cycles at current density of 1.0 A g^-1).It provides an effective method for the design and synthesis of transition metal compounds with long cycle stability.（3）The ultra-small Bi nanoparticles encapsulated in double carbon microrods（Bi/C@C MRs）was fabricated by the combination of Bi-MOF precursor and polymer coating process.Thanks to the synergy effects between the inner cross-linked carbon and outer carbon shell,which can effectively buffer the volume variation,improve the conductivity,as well as inhibit the re-agglomeration of Bi nanoparticles.This helps to maintain the structural stability and reaction kinetics during the repeated lithiation/delithiation process,lead to a highly reversible alloying/dealloying process during the cycling process.The Bi/C@C MRs electrode delivers excellent lithium-storage performance,including an ultralong cycle life up to 3000 cycles at1000 m A g^-1with capacity of 506 m Ah g^-1,and an excellent rate performance of 279 m Ah g^-1at an ultrahigh current density of 10000 m A g^-1.