Study On The Performance Of High-Safety Lithium Metal Batteries Based On Phosphazene Flame-Retardant Electrolytes

The massive use of fossil fuels not only causes serious air pollution,but also leads to serious consequences such as global-warming.Therefore,in the past few decades,green renewable energy such as wind,solar and tidal power has received a lot of attention.However,storage and conversion problems of these energy greatly hinder its development and utilization.Rechargeable lithium-ion batteries are considered to be an efficient energy storage technology and have been successfully used in many fields ranging from portable electronics to electric vehicles.Unfortunately,the theoretical specific capacity of graphite anode currently used in commercial lithium-ion batteries is only 372 mAh g-1,which can not meet the high energy density expectations of sophisticated electronic devices.Lithium metal anode is considered as an ideal anode material for the next-generation of secondary batteries system due to its ultrahigh theoretical capacity(3860 mAh g-1)and the lowest electrochemical potential(-3.04 V vs.standard hydrogen electrode).However,the safety and short cycle life caused by the generation of lithium dendrites in the continuous charge-discharge process hinder the commercial development of lithium metal batteries.In addition,the ester and ether-based electrolytes commonly used in lithium-ion batteries are highly flammable,which also can bring safety risks.Therefore,there is still an urgent problem to suppress the formation of lithium dendrites on the basis of reducing the combustibility of electrolytes.In this work,we introduced phosphazenes as additives to solve the above problems.Electrolyte systems with dual functions were developed for ester and ether-based electrolytes respectively.On the one hand,the combustibility of electrolytes can be reduced,making the electrolyte from flammable to nonflammable.On the other hand,the solid electrolyte interphase(SEI)layer can be formed on the surface of the lithium metal anode,which can suppress the growth of lithium dendrites to improving the electrochemical performance of lithium metal batteries.The main research contents are as follows:(1)The performance of lithium metal batteries based on(trifluoroethoxy)pentafluorocyclotriphosphazene(TFPN)flame-retardant additive was studied.We synthesized a new flame-retardant additive-TFPN.In ester-based electrolyte,flame test reveals that 5%TFPN addition can enable electrolyte non-flammable.Moreover,it is found that TFPN addition can form a LiF-rich SEI layer on lithium metal and suppress the growth of lithium dendrites.With 5 wt%TFPN added electrolyte,the lithium metal batteries with LiNi0.8Co0.1Mn0.1O2 as cathode material show a high reversible capacity and a much improved capacity retention.(2)The performance of lithium metal batteries based on hexafluorocyclotriphosphazene(HFPN)flame-retardant additive was studied.In the ether-based electrolyte,HFPN was introduced as co-solvent.From the flame test,we can get the conclusion that HFPN has a better flame-retardant performance for ether-based electrolyte due to its similar physical and chemical properties.when the concentration of HFPN is 20%,the electrolyte can not be ignited.The 1M LiTFSI/DME-HFPN(v/v,4:1)electrolyte can be used for high voltage cathodes above 4 V.In addition,the introduction of HFPN can form a dense and uniform SEI layer rich in F and N on lithium metal anode,which stabilizes the lithium/electrolyte interface and inhibits the formation of lithium dendrites.The lithium metal batteries with LiCoO2 as cathode(charging to 4.2V)in 1M LiTFSI/DME-HFPN electrolyte show capacity retention of～95%after 100 cycles.