Improved Ageing Resistance And Relavent Mechanisms For The Cathode Materials Of Lithium-ion Batteries Through Fine Tuning Of The Surface Structure

Lithium-ion batteries(LIBs)have wide applicationsin the fields of electric vehicles,hybrid electric vehicles,grid energy storage,and portable electric devices.Cathode materials play crucial roles in LIBs and determine the cost and performance of LIBs.However,the commercialized cathodes are universally suffered from low ageing resistance.Theelectrochemical performance of the cathodes will be deteriorated when exposed to air or eroded by the electrolyte,due to delithiation or dissolution of transition-metal ions.To improve the ageing resistance of the cathodes,we employed a room-temperature-and-pressure chemical vapor deposition(RTP-CVD)technique to in situ grow polyaniline(PANI)and polythiophene(PTh)on the surface of Li Ni0.5Mn0.3Co0.2O2(NMC532)and Li Mn2O4(LMO)particles,respectively,yielding NMC532/PANI and LMO/PTh composite cathodes.Thesamples were characterized with XRD,SEM,EDS,FT-IR,XPS and ICP techniques.In combination with the theoretical computations,the in situ formation mechanism of conductive polymers on the cathodes has been interpreted.Also,the protection effect of conductive polymers against the air-ageing and erosion of the cathodes has been revealed.Further,the effect of conductive polymers on the formation of solid electrolyte interface(SEI)has been demonstrated.The research results are as follows:(1)The molecular dynamics simulation results show that the monomer molecules are preferably captured on the transition-metal arrays during the CVD process.The computation results based on density function theory(DFT)show the transition-metal ions can activate/oxidize the adjacent monomer molecule.Thus,the growth of conductive polymer can be initiated,resulting in the conjugation of surface transition-metal ions with the conductive polymer.These computation results support the RTP-CVD experimental that only trace amount of conductive polymer can effectively modify the cathodes and improve their ageing resistance.(2)The conductive polymers can not only suppress the formation of Li2CO3 during the air-ageing process,but also inhibit the side reactions related to PVDF degradation and transition-metal dissolution,based on the XPS and ICP analysis.(3)The thickness and composition of the SEI are influenced by the conductive polymers on the cathodes during the electrochemical process,and the existence of conductive polymers favors the formation of compact SEI dominated by Li F.(4)NMC532/PANI-100 and LMO/PTh-300 operating at 55 �C exhibit enhanced specific capacity and rate capability but lowered cycling stability,compared with those operating at room temperature.Nevertheless,NMC532/PANI-100 and LMO/PTh-300 operating at 55 �C exhibit superior specific capacity and cycling stability,compared with pristine NMC532 and LMO.(5)The superior specific capacity and rate capability for NMC532/PANI-100 and LMO/PTh-300 operating at 55 �C are attributed to the improved Li+ mobility,and the decreased cycling stability is attributed the structural degradation with respect to the NMC532 and LMO.(6)For NMC532/PANI,1 wt% aniline relative to NMC532 is sufficient for improving the ageing resistance and electrochemical performance of the cathode.As for LMO/PTh,3 wt% thiophene relative to LMO is sufficient for improving the ageing resistance and electrochemical performance of the cathode.This result is assigned to the relatively higher oxidation PThential of thiophene relative to aniline.In other words,thiophene is more difficult to be oxidized than aniline.