| Since the commercialization of lithium ion battery,it has been widely used in many aspects of people's lives,which has greatly changed the production and lifestyle.In recent years,with the increasing demand of the electric vehicle and energy storage industries,the energy density of lithium ion battery has also been further improved.But at the same time,the fire safety issue of lithium ion battery is also more prominent.The internal components of commercial lithium ion battery are flammable,and the charged electrode materials store higher energy,especially,the high flammability and leakage problems of the organic carbonate liquid electrolyte with low flash point are important factors that cause fire safety accidents.Therefore,the development of intrinsically safe and solid-state electrolytes is one of the fundamental means to reduce the fire risks.In order to solve the problems of flammability and leakage of commercial liquid electrolyte,this thesis carried out the related studies on the synthesis,electrochemical and safety performance of safer SiO2 based ionogel quasi-solid electrolyte,sodium super ionic conductor(NASICON)inorganic solid electrolyte and inorganic-organic composite solid electrolyte.The safety of electrolytes has been significantly improved and all-solid-state batteries with good performance have been obtained eventually.Firstly,the research on SiO2 based ionogel quasi-solid electrolyte was carried out.Tetraethyl orthosilicate(TEOS)as the silicon source,hydrochloric acid as the catalyst,1-butyl-3-methylimidazole tetrafluoroborate([BMIm][BF4])as the ionic liquid and trifluoromethanesulfonic acid Lithium(LiOTf)or lithium bistrifluoromethane-sulfonimide(LiTFSI)as the lithium salt were used to prepare two kinds of SiO2 based ionogel quasi-solid electrolytes by a rapid sol-gel method.This type of electrolyte used silica as the matrix framework and retains some ionic liquids inside,it showed good thermal stability and completely non-flammable performance.The different influence of LiOTf and LiTFSI on the electrochemical performance and thermal stability of the ionogel electrolyte were compared and analyzed.The results showed that both had good thermal stabilities and the thermal decomposition temperature is not lower than 340?.The electrolyte using LiTFSI could not cycle for 50 cycles in the Li/LiFePO4 coin cell,and the coulombic efficiency was about 80%.But for the electrolyte using LiOTf,it exhibited better electrochemical performance.It could maintain a specific capacity of 143 mAh g-1 during 100 cycles,and the coulombic efficiency was close to 100%.By detailed comparison and analysis of the surface morphology of the two kinds of electrolytes,it was found that the ionogel containing LiOTf had a special flower-like structure,which was believed to increase the liquid adsorpted on the surface of the ionogel to improve the interface performance and achieve better electrochemical performance.Then,a series of Li1+xAlxGe2-xP3O12(LAGP)glass ceramic solid electrolytes were synthesized by sol-gel and melt quenching methods.Since this kind of electrolyte was synthesized under the condition of high annealing temperature(?650?),the thermal stability was extremely high and the liquid component was completely eliminated,but the conductivity was the key to determine the electrochemical performance of this type of electrolyte.The effect of annealing temperature(650,750,850,950 and 1050?)and synthesis method on the phase composition,phase structure and the Li+migration performance inside these samples was systematically analyzed by using advanced nuclear magnetic resonance(NMR)technology.The study found that doping Al into LGP mainly forms P(OGe)3(OAl)units,which was beneficial to Li+migration.Furthermore,the melt quenching method was more conducive to the formation of P(OGe)3(OAl)unit,and the AlPO4 impurity content was less at high temperature annealing(>850?),and the room temperature ionic conductivity of the optimal sample(LAGP-MQ-0.5-850)reached 3.45 ×10-4 S cm-1.Finally,in order to apply the LAGP electrolyte with high thermal stability to the battery system,LAGP,LiTFSI and polyethylene oxide(PEO)were further compounded by hot pressing method to prepare the flexible and composite electrolyte with different LAGP content(xLAGP-PL,PL:PEO-LiTFSI).The effect of LAGP content on the thermal stability and electrochemical performance was explored.In terms of thermal stability,the results showed that LAGP could maintain a stable structure in the PL system,the xLAGP-PL electrolytes(x<2)had a thermal stability higher than 300?and the flammability was significantly reduced compared to pure PEO electrolyte.The thermal decomposition temperature of 0.5LAGP-PL electrolyte could reach up to 330?,but higher LAGP content would reduce the thermal stability of the composite electrolyte.In terms of electrochemical performance,the introduction of LAGP could reduce the glass transition temperature(Tg)of PEO,indicating that LAGP could reduce the crystallinity of PEO which helped to improve ionic conductivity.The 0.5LAGP-PL sample had good room temperature ionic conductivity of 3·19×10-4 S cm-1.At the same time,the combination of PEO and LAGP improved the mechanical properties of LAGP,obtaining a flexible electrolyte and optimizing the interface performance.When the 0.5LAGP-PL sample was assembled into a cell(Li/0.5LAGP-PL/LiFePO4),it showed excellent battery performance with up to 300 stable cycles under 0.2 C charge and discharge condition,and the Coulombic efficiency was close to 100%reaching a good level.In summary,the electrolytes prepared in this study showed good electrochemical performance and thermal stability.In terms of electrochemical performance,researches on lithium salts,synthesis methods,and Li+conduction characteristic can optimize the electrolyte interface,improve ionic conductivity,improve cycle stability,and provide a theoretical basis for promoting the application of safe electrolytes in battery systems.In terms of thermal stability,the thermal decomposition temperature of the electrolytes prepared in this study is higher than 300?,which is of great significance for improving the safety of lithium ion batteries. |
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