Quantum Mechanical Study On The Formation Of Solid Electrolyte Interphase In Lithium-ion Batteries

Lithium-ion batteries（LIBs）are the dominant energy storage device in today’s life.The solid electrolyte interface（SEI）formed by electrolyte reduction and dissociation on the cathode interface is closely related to the cycle life of LIBs.However,the SEI-forming mechanism of complex electrolyte system is still a fundamental research issue that has not been clarified.With the increasing demand for energy,lithium-sulfur batteries（LSBs）,as the promising lithium-ion battery,have been attracting a lot of attention and research.During the battery cycle,the intermediate lithium sulfide（Li₂S_x）produced by the reduction of active material S₈ on the cathode causes shuttle effect.The mechanism of adsorption and catalytic conversion of Li₂S_x on cathode interface is helpful to filter and design suitable cathode materials at a molecular level.To understand the SEI-forming mechanism of different additives on the anode interface,the density functional theory（DFT）B3PW91/6-311++G（2df,2p）level was used for comprehensively exploring the electroreduction mechanism of different electrolyte systems（FEC）Li⁺（X）（X=PC,PF₆^-,PC-PF₆^-and PC₂-PF₆^-）during the reduction decomposition.The reduction potential of（FEC）Li⁺（PF₆^-）is 0.70V,which is in agreement with experimental reduction peak.As the distance increases from PF₆^-in direct contact with Li⁺to the separation of two PC solvent,the LUMO orbital diffuses,the contribution of LUMO orbital decreases from 18.79%to 0.12%,and the effect of reduction capacity becomes weaker and weaker.In addition,it is proved that the interaction between PF₆^-and Li⁺is caused by the electrostatic attraction and van der Waals through the analysis of model（FEC）Li⁺（PF₆^-）.And the bending of PF₆^-towards FEC arouses wide range of van der Waals attraction on the model of separation（FEC）Li⁺（PC）₂-PF₆^-.The different bonding positions of PF₆^-lead to different competitive solvation results in the electrolyte system.On this foundation,four sulfur-containing additives including ethylene sulfite（ES）,1-Prop-1-ene-1,3-sultone（PES）,1,3-propane sultone（PS）and vinyl ethylene sulfite（VES）were simulated at B3PW91/6-311++G（d,p）level subsequently.They were analyzed and compared with fluoroethylene carbonate（FEC）,vinylene carbonate（VC）and vinyl ethylene carbonate（VEC）.The single electron reduction potential of solvent and additive follows the order of ES>VES>PS>PES>FEC>VC>EC>VEC>PC,which proves the sulfur-containing additives（ES,VES,PS and PES）are able to reduce dissociation in preference to other solvents and additives.From our result,ES has the lowest LUMO energy,the largest reduction potential and the lowest binding energy with Li⁺,indicating that ES has the priority of reduction dissociation and does not have the priority of solvation of Li⁺.These results further illustrates ES can form SEI on the anode interface and does not affect the migration of Li⁺in the solvent.This is a strong complement to the SEI-forming mechanism of different functional group additives at a micro-scale.In order to further understand the mechanism of shuttle effect caused by Li₂S_x,the generalized gradient approximation（GGA）DFT,Perdew-Burke-Ernzerh（PBE）,and long range corrected DFT,ωB97XD,were used to investigate the interaction between lithium polysulfides（Li₂S_x,x=2-8）and N-doped graphene for discussing the adsorption mechanism and effect of catalytic conversion.The binding of Li₂Sx to N-doped graphenes is significantly stronger than that to solvents approximately by 0.5-3.0 e V,indicating that N-doped graphenes possess trapping ability towards soluble Li₂S_x.The binding of Li₂S_xto N-doped graphenes follows the order of pyrrolic N>pyridinic N>graphene>graphitic N.From our calculated results,the pyrrolic N exhibits the strongest anchoring effect mainly via forming coordination bond of Li N₃.In addition,van der Waals also plays important role in the binding complexes.According to analysis of micro-nano-scale calculation data（such as charge,bond distance,bond order,electron density difference,ELF,and IGM of Li⁺and N atom）,the strength of Li-N bond was confirmed that the strongest anchoring is to trap Li⁺above the vacancy forming coordination bond instead of a regular covalent bond.Although the strength of Li-N binding has a direct effect on the binding energy,other thermodynamics（free energy,dissociation energy）of catalytic conversion and energy barrier of Li₂S phase transformation show similar capacity.These conclusions explain in detail the behavior mechanism of N-doped graphene restricting Li₂S_x on the cathode surface of LSBs,which is instructive for experiment.