Design Of Low-temperature Ester Based Electrolyte And Research On Electrochemical Performance Of Lithium Batteries

Lithium batteries with commercial electrolytes and graphite anode have low Li⁺conductivity,poor interfacial charge transfer kinetics,and low stability of solid electrolyte interphase（SEI）at low temperatures,leading to serious issues such as reduced discharge specific capacity and cycle decay.In order to solve the problems of difficult application of electrolytes for lithium batteries and severe lithium evolution from graphite anode at low temperatures,a novel low-temperature ester based electrolyte and molybdenum oxide（MoO_x）/nitrogen doped carbon nanotubes（NCNTs）anode were developed.Combined with the low-temperature electrochemical performance of Li|Li Co O₂（LCO）and MoO_x/NCNTs|LCO lithium batteries,a stable and Li F rich SEI was constructed,and the coupling mechanism between the low coordination number of Li⁺and the solvation structure was analyzed to slow down the growth of lithium dendrites in lithium batteries and the volume expansion of metal oxides during the lithiation and de-lithiation process,so as to achieve the reasonable design of high-performance anode/electrolyte low-temperature lithium battery devices.The main research content and results are as follows:Taking a novel low-temperature ester based electrolyte as the research object,its physical and electrochemical properties and the solvation structure of the electrolyte were studied.Using isobutyl formate（IF）as the diluent,a fluorine sulfur electrolyte system based on lithium difluoroxalate borate,fluoro ethylene carbonate and dimethyl sulfite was proposed,realizing a weak solvation electrolyte structure（coordination number is 0.07）.The low-temperature ester based electrolyte has an ultra-low melting point of-132℃,an ultra-low viscosity of 0.30 Pa·s at-70℃,and a high Li⁺conductivity of 0.40 m S cm^-1.Raman analysis confirmed the weak solvation effect of Li⁺in the electrolyte with IF diluent.Meanwhile,the energy required for Li⁺de-solvation in the low-temperature ester based electrolyte is reduced by 44%compared to commercial electrolyte,which can achieve efficient transmission of Li⁺and is expected to solve the energy supply and storage problems of lithium batteries in low-temperature environments.The electrochemical performance and interface composition of low-temperature ester based electrolytes at different temperatures were studied using lithium metal anode and LCO cathode systems.Li|LCO batteries with low-temperature ester based electrolytes exhibited better cycling stability,rate performance,and charge transfer ability compared to commercial electrolytes at different temperatures.The reversible temperature ranges of Li|LCO batteries with low-temperature ester based electrolytes is-70°C～60°C,which is superior to commercial electrolytes（-20°C～60°C）.At-70°C,the Li|LCO battery with low-temperature ester based electrolyte has a high discharge capacity of 110 m Ah g^-1 after 170 cycles.Through high-temperature gas production testing,it has been proven that the low-temperature ester based electrolyte reduces the gas production content of reducing flammable gases and reduces potential hazards in high-temperature applications.Through X-ray photoelectron spectroscopy,in situ optical microscopy,and time-of-flight secondary ion mass spectrometry testing,it was demonstrated that the weak solvation structure of Li⁺in the low-temperature ester based electrolyte promotes the stable transport of Li⁺at low temperatures.A stable and 10.48%Li F rich SEI was constructed,which has a higher Li⁺conductivity(1.00×10^-5 m S cm^-1)and a larger diffusion coefficient(1.10×10^-21 m² s^-1).The optimization effect of low-temperature ester based electrolyte on electrode/electrolyte interface stability was verified through phase field simulation.The interface compatibility between low-temperature ester based electrolyte and lithium metal anode and commercial LCO cathode enables the ultra-low temperature stable application of high-performance lithium battery devices.In order to improve the safety hazards of low-temperature lithium dendrites in lithium metal batteries,a composite MoO_x/NCNTs anode was prepared by thermal cracking method based on low-temperature ester based electrolyte.The physical and chemical properties such as crystal structure,morphology,specific surface area,conductivity,room temperature and low temperature electrochemical performance,as well as interface composition were studied using high-resolution transmission electron microscopy,X-ray diffraction and Raman testing confirmed that MoO_x/NCNTs are composed of orthorhombic crystal MoO₃,monoclinic crystal MoO₂,and amorphous NCNTs.The addition of NCNTs slowed down the volume expansion during the charging and discharging process and adjusted the valence state of MoO_x,ensuring the integrity of the active substance structure and improving the conductivity of MoO_x/NCNTs anode(0.36 S cm^-1).At room temperature,the MoO_x/NCNTs anode exhibits a high specific capacity,high rate performance,and high cycling stability of 970 m Ah g^-1.At-40°C,MoO_x/NCNTs|LCO lithium-ion batteries based on low-temperature ester based electrolytes exhibited stable cyclic voltammetry electrochemical curves,rate performance,and a high discharge specific capacity of 71 m Ah g^-1.The reaction conversion process of MoO_x/NCNTs anode was determined through in-situ Raman and X-ray photoelectron spectroscopy testing,and it was verified that the low-temperature ester based electrolyte promoted the formation of SEI rich in Li F groups.The calculation of adsorption energy and electron cloud density of MoO_x and NCNTs shows that MoO₂ is conducive to electron transfer and improves the conductivity of materials.