Study On Functional Separator And Solid-state Electrolyte For High Safety Lithium Battery

In the face of increasingly severe energy and environment problems,it is urgent to energetically develop new nergy vehicles driven by renewable energy for realizing the sustainable development of human society.Owing to the advantages of environmentally friendly and long cycle life,lithium-ion batteries(LIBs)have been regard as the most potiencial energy storage systems to drive new energy electric vehicles.However,in recent years,the development of lithium-ion battery undergos a big obstruction since the frequent spontaneous combustion accidents.Thermal runaway of LIBs is the main cause of safety accidents of new energy electric vehicles.It is found that there are three characteristic temperatures in the thermal runaway process of LIBs:T1,T2 and T3.Wherein,T2 is the trigger temperature of thermal runaway,at which the rate of temperature rise switch from comparatively slow to sharply accelerative,resulting in the combustion and explosion of LIBs.It is significative that makes a thorough comprehension with the triggering mechanism of T2 to improve the safety performance of LIBs.It is generally believed that the T2 temperature is determined by the cask effect of thermal shrinkage of separator,oxygen released from cathode and deposition of lithium metal from anode.In this work,we carry out tripartite research works in term of separator and electrolyte to overcome the safety problems of LIBs,basing on the trigger factors of T2:(1)Design for high thermal dimensional stability seperatorsIn the early days of LIBs,the electrode materials are relatively safe,such as LiMnO2 and LiFePO4.It is the key factor to determine the T2 of LIBs that the separators undergo a large shrinkage at elevated temperature leading to internal short-circuit of LIBs.In this part,we construct a three-dimensional coating layer throughout the whole ceramic separator by in-suit polymerization,connecting the ceramic particles and PE separator as a whole.The developed separators show an excellent thermal stability with no visible shrinkage even at 300 ℃.After reasonable optimization of preparation technology and materials,we successfully develop phenolic modified ceramic-coated separators(CCS@PFR)with both excellent performance and low cost,which have a fantastic commercial-application prospect.The research work of this part can be divieded into three parts as following:1)Process optimization of polydopamine ceramic-coated separatorThe experimental scheme is designed according to the polymerized mechanism of dopamine.The utilization rate and polymerization rate of dopamine was highly improved through optimized preparation process.As a result,the cost of CCS@PDA reduced to 1/6 of the original,and the aging time was decreased from 24h to 2h.The CCS@PDA separator prepared through the optimized preparation process maintains excellent thermal dimensional stability with no visible shrinkage at 200℃ for 30 min.2)Preparation and study of polypyrrole enhanced ceramic-coated sepratorIn this part,the ceramic membrane is modified with polypyrrole instead of dopamine.Through in-situ oxidation polymerization,a three-dimensional polypyrrole coating layer was formed throughout the ceramic-coated membrane,which connected ceramic-coated layer and PE substrate separator as a whole.Therefore,the thermal dimensional stability of separator is greatly improved with no visible shrinkage at 200℃.In addition,we find that polypyrrole can weaken the solvation of Li+with polar solvent molecules,promoting the migration of Li+.As a consequence,the Li+transference number increases from 0.35 to 0.56,which is beneficial to reduce the concentration polarization and can improve the rate performance of battery.3)Preparation and study of phenol formaldehyde resin ceramic-coated sepratorThe CCS@PFR separator which shows excellent thermal stablilty is successfully prepared via a simple soakage process.Compared with CCS@PDA and CCS@PPy,PFR has higher heat resistance and mechanical strength.Therefore,CCS@PFR separator shows higher thermal dimensional stability with no visible shrinkage at 300 ℃.What’s more,the preparation process of CCS@PFR separator is much simpler than CCS@PD A and CCS@PPy since it does not need additional cleaning process after polymerization.With the advantages of excellent thermal stability,low cost and simple preparation process,the CCS@PFR separator can be expected to replace the commercial ceramic-coated separators,and greatly improve the safety of the LIB s(2)Design for flame-retardant separator to isolate the oxygen released from cathode electrodeIn order to solve the insufficient endurance problem of new energy electric vehicles,in recent years,high Ni content NCM cathode materials have gradually replaced LiFePO4 cathode since their higher energy density.However,the high Ni content NCM cathode materials have a poor thermal stability and will release highly reactive oxygen species at a relatievely low temperature.The oxygen species released from cathode will lead to violent exothermic reaction with combustibles in the cell,resulting in combustion and explosion of LIBs.In this part,we further modify flame retardant on CCS@PFR.The flame retardant decomposes at elevated temperature to form a dense covering layer to isolate the contact of oxygen released from cathode and combustibles in the cell,so as to prevent the battery from violent combustion and explosion.Consequently,LiNi0.8Co0.1Mn0.1O2|SiOx-Gr full cells assembled with APPCCS@PFR showed an excellent safety performance without catching fire during a 30 s combustion test and 10 min high temperature test above 300℃.Additionally,3 Ah LiNi0.8Co0.1Mn0.1O2|SiOx-Gr full cells assembled with APP-CCS@PFR successfully survive from the ARC and nail penetration tests without a violent thermal runaway.(3)All solid state lithium metal batteryOwing to the high theroretiacal specific capacity,lithium metal batteries become one of the hot spots of the current researches.However,numerous issues,such as lithium dendriete growth and unstable SEI,bring serious challenges to the development of lithium metal batteries.In this part,by coupling the advantages of organic solid electrolyte and inorganic solid electrolyte,we successfully prepare LLZTO-PVC composite electrolyte with both excellent ionic conductivity of 1.17*10-4 at room temperature and remarkable mechanical strength.The electrochemical window is up to 4.65 V(Vs Li+/Li).Owing to the great wettability of vinylene carbonate momomer with LCO、LLZTO and lithium metal,the interface impedance caused by bad contact of solid state electrolyte and electrode materials is eliminated.The LCO|LLZTO-PVC|Li solid-state batteries prepared by in-situ polymerization with γ ray are able to cycle at 0.2C current rate and 4.3 V high voltage at room temperature.