Construction And Electrochemical Performance Of Ultra-high Temperature Electrolyte System Suitable For LiNi_0.5Co_0.2Mn_0.3O₂

The dissolution of transition metals in the electrode materials of lithium ion batteries and the decomposition of lithium hexafluorophosphate?LiPF₆?at ultra-high temperature?75??will aggravate the capacity decay of the battery,so the development of ultra-high temperature electrolyte systems has become a current research hotspot.In this thesis,the ultra-high temperature suitable for LiNi_0.5Co_0.2Mn_0.3O₂?NCM523?is gradually optimized by changing the molar ratio of lithium bisoxalate borate?LiBOB?and lithium tetrafluoroborate?LiBF₄?and adding electrolyte additives.The main research contents are as follows:?1?Mixed lithium salt?LiBOB and LiBF₄?-based ultra-high temperature electrolytes with different molar ratios were prepared,lithium iron phosphate?LiFePO₄,abbreviated as LFP?and graphite were used as electrode materials with stable high-temperature performance and little influence on the high-temperature performance of electrolytes.The electrochemical performance test results of LFP/Li and Li/graphite half-cells at 75?show that when the molar ratio of LiBOB and LiBF₄ is 6:1?denoted as electrolyte B?,the positive and negative half-cells show the best cyclic stability and rate performance and the lowest impedance at ultra-high temperature.The characterization results of scanning electron microscopy?SEM?,transmission electron microscopy?TEM?and X-ray photoelectron spectroscopy?XPS?show that the electrolyte B formed a relatively thin and dense interface film on the surface of both cathode and anode electrode materials,and the compounds containing B-O in the composition of the interface film are beneficial to improve the ultra-high temperature stability of the interface film.Therefore,without considering the influence of the material itself on the ultra-high temperature electrochemical performance of the battery,electrolyte B is the optimal ultra-high temperature electrolyte.?2?In order to study the application universality of ultra-high temperature electrolyte B and improve the shortcomings of NCM523 at high temperature such as Li⁺/Ni²⁺cation mixing,transition metal dissolution and serious loss of active lithium,the electrolyte additive was added to electrolyte B to further optimize the ultra-high temperature electrolyte suitable for NCM523.Add tris?trimethylsilane?borate?TMSB??denoted as B+TMSB?to reduce the small amount of HF and LiF introduced by LiBF4;add 1,3-propylene sulfonate?PES??denoted as B+PES?to reduce interface impedance and gas production.Electrochemical performance tests show that NCM523/Li and Li/graphite half-cells using B+PES electrolyte have more excellent cycle performance and rate performance at 75?.SEM,TEM,XPS,and X-ray diffraction?XRD?analysis show that the additive PES can not only reduce the decomposition of the electrolyte at high temperature and the impedance of the interface film,but also can suppress Li⁺/Ni²⁺cation mixing and material structure destruction.?3?The B+PES was applied to NCM523/graphite full battery,with LiPF₆-EC/DEC and electrolyte B as the comparative electrolytes.The results of electrochemical performance tests show that the cell with electrolyte B+PES has the highest capacity and the smallest capacity decay and volume expansion after high-temperature storage,and the smallest impedance after cycling,indicating that the additive PES can suppress gas production and improve interface stability.The results of SEM,TEM,XRD and inductively coupled plasma-atomic emission spectroscopy?ICP-OES?characterization show that the electrolyte B+PES forms a dense and smooth protective film on the cathode and anode surfaces,which inhibits the increase of battery impedance,reduces the Li⁺/Ni²⁺cation mixing,the dissolution of transition metals and the loss of active lithium,thus improving the cycling stability of the battery.By optimizing the proportion of lithium salt and adding electrolyte additives,an ultra-high temperature electrolyte B+PES suitable for NCM523 is constructed,which not only reduces the dissolution of transition metal of NCM523,the Li⁺/Ni²⁺cation mixing,loss of active lithium and volume expansion of the battery,but also significantly improves its ultra-high temperature cycle stability.This will provide research ideas for the application of lithium ion batteries with NCM523 as the cathode material at ultra-high temperatures.