Study On The Effects Of Electrolyte Additives On Electrochemical Properties Of Lithium-Ion Batteries

During the charge and discharge of lithium ion batteries,side reactions of the electrolyte lead to the instability of electrode-electrolyte interface and dissolution of transition metals from electrodes.These problems result in increase of battery impedance and attenuation of irreversible capacity,and restrict the development of lithium ion batteries with high specific energy.To solve these problems,this thesis reports our study on the influence of a series of fluorine-containing organic molecules,as the electrolyte additive to the pristine electrolyte of 1 M LiPF₆/EC:EMC=3:7（wt%）,on the electrochemical performance of three lithium ion cathode materials,LiNi_0.8Co_0.1Mn_0.1O₂（NCM811）,LiNi_0.5Mn_1.5O₄（LNMO）and Li₃V₂（PO₄）₃（LVP）.The main research of the thesis is summarized as follows:（1）Based on the density functional theory（DFT）,calculate and compare the HOMO and LUMO energy levels of ethylene carbonate（EC）,ethyl methyl carbonate（EMC）with these of fluorinated organic additives.The additives show higher HOMO energy level and narrower electrochemical windows relative to EC and EMC.Among additive molecules,those containing electron-donating methoxy group（-OCH₃）have higher HOMO energy level than those containing electron-withdrawing trifluoromethyl group（-CF₃）.Linear scanning voltammetry test results show that the selected fluorine-containing organic molecules have lower oxidation decomposition potential than that of the pristine electrolyte,consistent with the theoretical calculation results.Furthermore,and the decomposition current of the additive-free electrolyte contains the decomposition current of the additive electrolyte.（2）Trifluoromethyl-containing additives（T₁,T₂,T₃）are beneficial to reduce the capacity attenuation of the NCM811 material during cycling.Specially,pentafluoro-3’,5’-bis（trifluoromethyl）-1,1’-biphenyl（T₁）shows the best promoting effect.Compared with half-cell using the pristine electrolyte,after 500 cycles of 1 C charge and discharge,the electrolyte with 2 wt%T₁ increases the specific discharge capacity of the material from46 mAh?g^-1 to 100 mAh?g^-1.The capacity retention rate increases from 26.6%to 59.7%,and the charge transfer impedance(R_ct)of the half-cell before and after cycling remains almost unchanged,while the R_ct of the half-cell using pristine electrolyte increases by 119Ωafter cycling.（3）Pentafluoro-4’-trifluoromethyl-1,1’-biphenyl（T₂）and pentafluoro-4’-methoxy-1,1’-biphenyl（T₅）additives have remarkable promotion effects on electrochemical properties of LNMO materials.After 500 cycles of 1 C cycling,compared with the pristine electrolyte,addition of 2 wt%T₂ increases the specific discharge capacity of LNMO from 83.6 mAh?g^-1 to 101.4 mAh?g^-1,and the capacity retention rate promotes from 70.36%to 92.31%;with addition of 2 wt%T₅,the specific capacity of the material remains stable at around 100 mAh?g^-1 after 250 cycles,and stays at 99.3 mAh?g^-1 after500 cycles.（4）The cathode material LVP（Ⅲ）&pH=9 was synthesized by sol-gel method,and its rate performance and long cycle capacity in the voltage range of 3.0 to 4.8 V were tested,to verify the effect of the methoxy-containing additives pentafluoro-4’-methoxy-1,1’-biphenyl（T₅）and tetrafluoro-4,4’-dimethoxy-1,1’-biphenyl（T₆）.At cycling current of 20 C,0.5 wt%T₆ increases the specific discharge capacity of the LVP material from 0to 83.4 mAh?g^-1,comparing with that using the pristine electrolyte.After 500 cycles at 2C current,2 wt%T₅ and 0.5 wt%T₆ increases the capacity retention of the material from82.16%to 92.33%and 90.94%,respectively.The promotion effects of the additives studied in this thesis show certain trends.That is,methoxy-containing additives are suitable for high-voltage cathode materials,and trifluoromethyl-containing additives are suitable for cathode materials of high and low voltages.The plausible promotion mechanism may involve the electrochemical reaction of these fluorine-containing additive molecules on the surface of the positive electrode material,and the product forms a protective layer with high ionic conductivity and reliable stability.The formed protective layer effectively passivates the surface of the positive electrode,which suppresses the continuous occurrence of side reactions at the electrode-electrolyte interface,and improvs the electrochemical performance of lithium ion batteries.