Study Of Influence Of Electrolyte Composition On High-voltage Performance Of Layered Lithium Nickel Cobalt Manganese Oxides

With the development of electric vehicles and hybrid electric vehicles,the lithium ion battery with high specific energy is becoming a hot research spot in recent years.The specific energy for lithium-ion secondary battery is closely related to the working voltage of cathode materials.Therefore,a lot of research work is devoted to the development of cathode material with high working voltage（＞4.5 V vs.Li/Li⁺）.However,the commonly used carbonate based electrolyte is prone to be oxidized and decomposed under the high voltage,resulting in serious capacity decay,and even thermal runaway,which restrict the practical application of high voltage lithium ion battery severely.Therefore,developing the high voltage electrolyte system is regarded as one of the most important means to increase the energy density of lithium ion battery,as well as to improve the battery safety.This thesis aimed at the high voltage electrochemical performance of LiNi_1/3Co_1/3Mn_1/3O₂ and Li_1.2Ni_0.13Co_0.13Mn_0.54O₂cathode,systematically investigate the influence of solvent（fluoroethylene carbonate,FEC）and the additives（phosphorus-,boron-,nitrile-based）on the electrochemical performance of cathode at high voltage,and investigate on the influence mechanism of electrolyte additives combined with electrochemical impedance spectra（EIS）and X-ray photoelectron spectrum（XPS）and other characterization,providing reference for the development of novel high voltage electrolyte system.The influence of FEC as co-solvent on the electrochemical performance of LiNi_1/3Co_1/3Mn_1/3O₂ cathode is investigated under high voltage.For LiNi_1/3Co_1/3Mn_1/3O₂cathode,the effect of FEC as co-solvent with different contents on the cyclic stability and rate capability is investigated.After 100 cycles between 3.0 and 4.6 V（vs.Li/Li⁺）at0.2 C rate,the capacity retention of Li Ni_1/3Co_1/3Mn_1/3O₂ with 1 M LiPF₆ FEC/dimethyl carbonate（DMC）（1/4,vol.%）solution is 82.7%,which is only 41.3%with the 1 M LiPF₆ EC/DMC（1/1,vol.%）electrolyte.In order to analyze the influence reason on the electrochemical performance,a novel infrared transmission electrolytic cell is designed,which can in-situ detect the gas evolution of 1 mol·L^-1LiPF₆ in EC/DMC（1/1,v/v）and FEC/DMC（1/4,v/v）electrolyte systems at different voltages during the first charge process,further verified that the decomposition voltage is improved when EC is replaced by FEC as co-solvent.The effect of FEC on the surface film resistance of LiNi_1/3Co_1/3Mn_1/3O₂ is analyzed by EIS,and the effect of FEC on the high voltage performance of LiNi_1/3Co_1/3Mn_1/3O₂ is analyzed by XPS with the corresponding analysis of surface membrane components changes.It is found that FEC as co-solvent participates in the formation of protective surface films on the cathode,which efficiently suppresses the decomposition of the electrolyte,prevents the severe structure destruction and improves the cycling stability and rate capability of LiNi_1/3Co_1/3Mn_1/3O₂electrode under high voltage.Tris（2,2,2-trifluoroethyl）phosphite（TTFEP）is investigated as an electrolyte additive to improve the electrochemical performance of LiNi_1/3Co_1/3Mn_1/3O₂ cathode at high operating voltage（4.6 V）.The frontier molecular orbital of different solvents in electrolytes and additives is calculated by density functional theory.It is found that TTFEP was oxidated prior to solvent combined with the linear scanning curve（LSV）.Based on the results of electrochemical characterization,it is found that the capacity retention of LiNi_1/3Co_1/3Mn_1/3O₂/Li cell with 1%TTFEP-containing electrolyte reaches up to 85.4%after 100 cycles at 0.5 C(1 C=160 mA g^-1),while that of the cell with the baseline electrolyte（1 M LiPF₆ in EC/DMC electrolyte）only remains 74.2%.Meanwhile,the influence of TTFEP on the self-discharge performance of LiNi_1/3Co_1/3Mn_1/3O₂ cathode and the compatibility with the graphite anode is studied.Further,based on the characterization of EIS,scanning electron microscope（SEM）,X ray diffraction（XRD）and XPS and so on,a protective interphase film was formed on the cathode surface due to the preferential oxidation of TTFEP,which inhibits the electrolyte decomposition and mitigates the cathode structural destruction,leading to the improved electrochemical performance of LiNi_1/3Co_1/3Mn_1/3O₂ cathode at high voltage.The effect mechanism of Tris（2,2,2-trifluoroethyl）borate（TTFEB）additive on the electrochemical performance of LiNi_1/3Co_1/3Mn_1/3O₂ cathode is evaluated.Galvanostatic charge/discharge tests indicate that the cycling performance and rate capability of LiNi_1/3Co_1/3Mn_1/3O₂ cathode are enhanced dramatically with the presence of 0.5%TTFEB in electrolyte,which delivers a high initial discharge capacity of 185.3mAh g^-1at 0.5 C and a high capacity retention of 89.5%after 100 cycles.Moreover,superior discharge capacity of 135.1 mAh g^-1is displayed at high rate of 4 C,while only73.7 mAh g^-1is delivered for the cell without TTFEB.The effects of TTFEB on the LiNi_1/3Co_1/3Mn_1/3O₂ cathode surface are further investigated by EIS,SEM,XRD and XPS.It is found that the enhanced electrochemical performance can mainly be attributed to the formation of thin and uniform film with lower interfacial impedance on cathode surface facilitated by TTFEB,which can alleviate the electrolyte decomposition and prevent of LiNi_1/3Co_1/3Mn_1/3O₂ structural deterioration at high voltage.1,3,6-Hexanetricarbonitrile（HTN）is investigated as an electrolyte additive to improve the electrochemical performance of the Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ cathode at high operating voltage（4.8 V）for the first time.Linear sweep voltammetry（LSV）results indicate that HTN can improve the oxidation potential of the electrolyte.The influences of HTN on the electrochemical behaviors and surface properties of the cathode at high voltage have been investigated by galvanostatic charge/discharge test,electrochemical impedance spectroscopy（EIS）,and ex-situ physical characterizations.Charge-discharge results demonstrate that the capacity retention of the Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ cathode at 0.5 C in 1%HTN-containing electrolyte after 150 cycles is improved to 92.3%,which is much higher than that in the standard electrolyte（ED）.Combined with the theoretical calculation,ICP tests,XRD and XPS analysis,more stable and homogeneous interface film is confirmed to form on the cathode surface with incorporation of HTN,meanwhile,the electrolyte decomposition and the cathode structural destruction are restrained effectively upon cycling at high voltage,leading to improved electrochemical performance of Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ cathode.