Design,Mechanism And Application Of Ion-Type Electrolyte Additives For Lithium Batteries

For the advantages of high theoretical lithium storage capacity,low density,and low electrode potential,lithium metal is considered as a potential anode material.However,problems of too high reactivity and uncontrolled lithium dendrite growth hinder the large-scale application of lithium metal.Although there have been many works devoting to solve these problems by electrolyte additives,most of the existing works investigated the electrochemical behavior of additive molecules in lithium metal batteries（LMBs）from only a single perspective,which separate the relationship between different mechanisms and ignore their synergy.For this reason,we put forward the“multi-factor principle for electrolyte additive molecule design”,which comprehensively studied the electrochemical behavior of additive molecules,including regulating interfacial film,optimizing Li⁺solvation structure,lithium nucleation formation and inhibition of lithium dendrite growth.This principle is applied to LMBs system,in order to obtain LMBs with high energy density,long cycling life,good rate capability.The main work contents are as follows:（1）In chapter 2,alkyl-triphenyl phosphonium bromide additives were introduced into the electrolyte for LMBs.It was proved that these additives can improve the ionic conductivity of solid electrolyte interphase（SEI）film by introducing Li₃P beneficial component into SEI film;Br^-can coordinate with Li⁺,and repel the solvent molecules in the solvation structure,bring lower desolvation energy barrier of Li⁺.The above two factors are helpful to allow a great deal of Li nucleus can form in a short time,to suppress the growth of Li dendrites.By changing the alkyl chain length,it is proved that different structural cations can affect the coordination number between Br^-and Li⁺,resulting in the different performances of additives.The work in this chapter not only introduced a kind of alkyl-triphenyl phosphine bromide additives,but also proved the influence of cation structure on the performance of electrolyte additives.（2）In chapter 3,potassium perfluorinated sulfonates were used as additives to optimize the electrochemical performance for lithium anode.Both theoretical calculation and experimental results show that these additive molecules can optimize the deposition behavior of lithium metal through regulating structure of SEI film,optimizing dissolved shell structure,cationic electrostatic shield mechanisms.These results prove the correctness of“multi-factor principle for electrolyte additive molecule design”.By changing the chain length of perfluoroalkyl,it was proved that anions with different structures can affect the use efficiency of additives,resulting in the different performances of additives.The work in this chapter not only introduced a kind of potassium perfluorinated sulfonate additives,but also proved the influence of anion structure on the performance of electrolyte additives.（3）In chapter 4,nine kinds of imidazole-based ionic liquids containing different saturated alkyl side chains,different anion types and different unsaturated side chains were selected as additives.The impacts of the above factors on additive properties were studied.Both theoretical calculation and experimental results show that the imidazole-based ionic liquid additive with saturated octyl chain and bis（trifluoromethanesulphonyl）imide anion have the best optimization effect on the lithium deposition behavior,which is the result of the interaction of cation and anion structure.This chapter further proves the correctness of the“multi-factor principle for electrolyte additive molecule design”and the existence of synergistic effects between different additive optimization mechanisms.The work in this chapter not only introduced a kind of room temperature imidazole-based ionic liquid additives,but also proved that there is a synergistic effect between the structures of anion and cation on the performance of electrolyte additives.（4）In Chapter 5,cyclic hexafluoropropyl disulfimide salts were selected as additives to study the influence of different intereaction between cations and anions on the electrochemical behavior for LMBs.Both experimental and theoretical calculation results show that under the action of electric field force,due to the different complexation with Li⁺,the distribution of various anions in battery is different:during charging process,the anions,which have the large interaction with Li⁺,are more likely to distribute at the lithium anode side;whereas the the anions,who have the small interaction with Li⁺,are more likely to distribute at the cathode side.Through this distribution difference,F-rich electrolyte/electrode interphase can be constructed to protect cathode material and Al current collector.The work in this chapter not only introduced a kind of F-rich cyclic hexafluoropropyl disulfimide salt additives,but also proved that the different interaction between various ions has impacts on the electrode/electrolyte interpahse film formation reaction.