Research On Functional Electrolyte System For Layered Lithium/Manganese-rich Oxides Cathode Materials

Lithium-ion batteries have been widely used in mobile phones,laptops,and large-scale energy storage due to their many advantages such as high energy density and long cycle life.In order to meet the increasing application demands,people have put forward higher requirements on the energy density,safety performance and cycle life of lithium-ion batteries.On the one hand,researchers are committed to developing new high energy density electrode materials,such as layered lithium/manganese-rich oxides（LMR）cathode materials,nickel-cobalt-manganese ternary cathode materials and silicon-carbon anode materials.On the other hand,the carbonate-based electrolyte system is easy to decompose under high voltage or high temperature,and does not match the new electrode materials,which seriously affects the performance of lithium-ion batteries.Therefore,it is crucial to construct functional electrolyte system.This thesis mainly designs additives,develops functional electrolytes systems suitable for LMR,and improve the electrochemical performance of LMR/Li half-cells under high voltage and high temperature.The following two aspects of work are carried out in this thesis:Based on theoretical calculations,3-cyano-5-fluorophenylboronic acid（CFBA）was selected as high voltage electrolyte additive to improve the electrochemical performance of LMR/Li half-cells.Adding 0.7 wt%CFBA into the base electrolyte greatly improved the cycling stability of the cell at high voltage.The capacity retention rate of LMR/Li half-cells increased from 25.2%to 88.4%after 200 cycles at a current density of 0.5 C.The specific discharge capacity of the cell reached 170 m Ah·g^-1 at high rate of 3 C.Theoretical calculation and electrochemical analysis shown that CFBA can be preferentially oxidized,inhibit the decomposition of electrolyte and improve the oxidation stability of electrolyte.Known from the analyses of scanning electron microscopy,transmission electron microscope,X-ray photoelectron spectroscopy,binging energy calculation and inductively coupled plasma emission spectroscopy,CFBA can adjust the chemical composition of the LMR surface to form a thin and uniform protective film,combine with harmful HF,inhibit the continuous oxidative decomposition of the electrolyte and the dissolution of transition metals.Based on theoretical calculations,3,5-bis（trifluoromethyl）phenylboronic acid（TFTB）was selected as high temperature electrolyte additive to improve the electrochemical performance of LMR/Li half-cells.Adding 1.0 wt%TFTB to the base electrolyte significantly improved the cycling stability of the cells at room and high temperatures.At a rate of 0.5 C,the discharge specific capacity of the cell with 1.0 wt%TFTB increased from40 m Ah·g^-1 to 222.2 m Ah·g^-1 after 200 cycles at 50°C,and the capacity retention rate increased from 15.1%to 81.8%.Known from the analyses of scanning electron microscopy,X-ray photoelectron spectroscopy and inductively coupled plasma emission spectroscopy,TFTB can enchance the stability of the electrolyte at high temperature,adjust the chemical composition at the electrode-electrolyte interface,inhibit the dissolution of transition metals,and protect the structural integrity of the LMR material.