Investigations On Preparation And Performance Of Safe Electrolyte For Lithium Ion Batteries

Safety problem has become a major issue that inhibits the further development oflithium ion battery. Among all components in lithium ion battery, electrolyte should beresponsible for this unsafe behavior. Developing a safer electrolyte is more urgent thanimproving the power density and energy density in lithium ion battery. Firstly, two differentstructures of niobium disulfide were used as the host for lithium ion intercalation in theorganic liquid electrolyte and normal gel polymer electrolyte, respectively. The structure withbetter performance was chosen for sodium ion intercalation. Then we investigated the newsafe electrolytes of gel polymer electrolyte （GPE）, anti thermal shrinkagable separatorcombined with ionic liquid to form GPE, and solid electrolyte for lithium ion battery.2H NbS₂is a potential electrode material for Li⁺intercalation but its performance ispoor. In order to improve its electrochemical performance,2H Li_0.7NbS₂（space group:P63/mmc） and3R NbS₂（s.g: R3m） were synthesized and characterized as electrode materialsfor a lithium ion battery. Both2H Li_0.7NbS₂and3R NbS₂showed reversiblecharge/discharge reactions based on the Nb（IV）/Nb（III） redox couple. The dischargecapacities were169.5mAh g^-1 for2H LixNbS₂and169.0mAh g^-1 for3R LixNbS₂at0.05Crate and room temperature. After200cycles at1C/1C rate,9%of capacity fade wasobserved for2H LixNbS₂and14%for3R LixNbS₂. It is proven that NbS₂is a goodLi⁺intercalation host regardless of its space group, especially the2H structure exhibitsexcellent electrochemical characterization. Then, two structures of NbS₂were used aselectrodes in the poly（vinylidene fluoride hexafluoropropylene）（PVdF HFP） based gelpolymer electrolyte （GPE）. The cyclic capacity of GPE was almost the same with organicliquid electrolyte in small charge/discharge current, but the rate performance was poor in GPE:the capacity is less half at10C rate compared with that of the liquid electrolyte.The better performance of2H NbS₂structure was chose as an intercalated host for aNa ion battery. Based on the experience of2H Li_0.7NbS₂, the Na0.5NbS₂（space group:P6₃/mmc）, was synthesized instead of2H NbS₂, by a conventional solid state reaction. The2H NaxNbS₂electrode shows a specific capacity of143.6mAh g^-1 in organic liquidelectrolyte and141.2mAh g^-1 in PVdF HFP based GPE, and the voltage profile occurs asignature of Na/vacancy ordering at x=0.5. First principles calculation was applied to revealpossible structures of NaxNbS₂and describe the corresponding electrochemical properties.Although layered NaMS₂systems allow full sodium intercalation, the stronger Na⁺Na⁺intralayer interaction compared to Li⁺Li⁺interaction induces layer gliding and Na ion ordering which can be reflected in the voltage profile.Adding nano Al₂O₃in blending polymer PEO P（VdF HFP） and copolymerP（MMA VAc） co PEGDA to form polymer matrix, the gel polymer electrolytes （GPEs）were developed after coating the polymer matrix to mechanical support. The performances ofthe polymer membranes and the corresponding GPEs are characterized with mechanical test,thermogravimetric analyzer, scanning electron microscopy, cyclic voltammetry,electrochemical impedance spectroscopy, and charge/discharge test. With doping^-10wt.%nano Al₂O₃, the ionic conductivity of both GPEs was3.8×10^-3S cm^-1, but the mechanicalstrength was a little different,14.3MPa for blending polymer PEO P（VdF HFP）; while16.2MPa for P（MMA VAc） co PEGDA. With the same structure of coin cell, Li/GPE/LiCoO₂,the capacity retention is87.2%for PEO P（VdF HFP） after50cycles; andP（MMA VAc） co PEGDA based GPE can keep90.9%of initial capacity after100cycles.A new GPE system was developed by using anti thermal shrinkagablenanoparticles/polymer incorporating with lithium salt LiTFSI in ionic liquid PYR₁₄TFSI. Itseems that nanoparticle of SiO₂exibites better performance than nano Al₂O₃. Roomtemperature lithium ionic conductivity of SiO₂/P（MMA AN VAc）+LiTFSI+PYR₁₄TFSI/VC based GPE is1.2×103S cm1with an oxidative decomposition potential of5.3V （vs. Li/Li⁺）. The battery Li/GPE/LiFePO4shows high initial discharge capacity of143.4mAh g^-1 at0.1C rate, and keeps99.6%capacity after50cycles.Serial of solid electrolytes, Li3xNb1xWxO4（0≤x≤0.1）, Li_2.9Nb_0.9Mo_0.1O4andLi6+yZr2xMxO7（M=Y, In, Zn, Ti） were synthesized by conventional solid state reaction. Thestructure and performance of newly developed solid electrolyte were characterized by powderX ray diffraction, scanning electron spectroscopy, energy dispersive X ray, electrochemicalimpedance spectroscopy, cyclic voltammetry and charge discharge test. The ionicconductivity of Li_2.9Nb_0.9W_0.1O4with vacancy structural solid electrolyte is3.1×10^-4S cm^-1at300℃, which is two orders of magnitude enhancement compared with the parent Li₃NbO₄(1.4×10^-6S cm^-1at300℃). Cyclic voltammetry measurement indicates that doping tungsten（VI） or molybdenum （VI） can be stable in the voltage scanning range of0.05V and4.5V.Insertion of x=0.15Li in the form of interstitial structure Li_6+xZr_2-xM_xO₇can be obtained bysimple solid state synthesis. Electrochemical insertion into Li_6+xZr_1.5Ti_0.5O₇of excess Li is atleast x≈0.17at room temperature. The ionic conductivity of Li_6.15Zr_1.85Y_0.15O₇is almost twoorders of magnitude higher than the parent Li₆Zr₂O₇, thus, interstitial site of lithium ion canimprove the ionic conductivity effectively.