The Application Of Cyclic Ether Polymer Electrolytes In Lithium Batteries And Thermal Stability Analysis

Nowadays,the graphite anode in traditional lithium-ion batteries is unable to meet the demands for high energy density.Using lithium metal(theoretical capacity of 3860 m Ah g^-1)as anode can effectively improve the energy density of batteries.However,the continues interfacial reaction would occur due to the active reducibility of lithium metal and the diffusion of traditional liquid electrolyte,resulting in interfacial resistance increase and considerable consumption of lithium metal and electrolyte.In addition,organic liquid electrolytes are volatile and flammable,which has potential safety hazard when batteries are overheated.The solid electrolytes with low diffusivity can maintain stable interfacial contact with lithium anode,and also has higher thermal stability,which ensures the safety performance of batteries,meanwhile improving the energy density.At present,solid electrolytes are mainly divided into organic and inorganic categories.Compared with inorganic ceramic electrolyte,solid polymer electrolytes（SPEs）have better flexibility and ductility.On the one hand,SPEs can keep close contact with electrodes,adapting to the volume change of which and inhibiting lithium dendrite during cycling;On the other hand,they can promote the preparation of ultra-thin electrolyte membrane to reduce the battery volume,making it closer to practical application.However,SPEs also have some disadvantages.Firstly,the ionic conductivity at room temperature is lower than 10^-4S cm^-1,which cannot meet the needs of practical production and life.Secondly,although SPEs can inhibit lithium dendrite to some extent,this phenomenon is still unavoidable.Finally,Lithium bis（trifluoromethanesulphonyl）imide（LiTFSI）used in SPEs costs much,and it is difficult to degrade in natural,which will lay tremendous burden to environment.Therefore,the preparation of chemically/electrochemically stable SPEs with high-performance and the improvement of recycling of LiTFSI are major focus of the development of solid-state batteries.In this paper,cyclic ether SPEs are prepared:on one hand,the interface of lithium anode is modified to effectively inhibit lithium dendrite,reducing the volume expansion of lithium anode,and improving the cycle efficiency of batteries;On the other hand,the thermal stability of cyclic ether SPEs are analyzed,and the new requirements are put forward to the aspects of the preparation process and working temperature range of this kind of electrolytes.Finally,the thermal decomposition of this kind of SPEs are applied to lithium salt recovery for the first time,effectively reducing the cost of SPEs,and improving the recycling efficiency of LiTFSI.The specific research contents are as follows:（1）Adding stannous fluoride（SnF₂）to the solution of dioxolane（DOL）and LiTFSI can not only initiate ring-opening polymerization of DOL,but also react with lithium metal to form a composite solid-state electrolyte interface（SEI）containing Li/Sn alloy and lithium fluoride（LiF）,uniforming the interfacial electric field and Li⁺flux,suppressing lithium dendrites,and maintaining excellent interface stability.The ionic conductivity of this SPE is 7.2×10^-5S cm^-1at 45 ^oC.The volume expansion rate of deposited lithium is effectively reduced from 90-100%to 22.4-32.7%,improving lithium efficiency and the cycling stability of batteries.（2）The P-DOL SPE rapidly decomposes when heated at 110 ^oC,which would block Li⁺transportation,eventually producing excessive internal resistance and leading to open circuit of the cell.The gas products of P-DOL thermal decomposition include formaldehyde,dioxane,trioxane,and DOL et al.and the volatilization of which would accelerate the decomposition processing.Pure P-DOL is thermally stable up to 300 ^oC,while the addition of SnF₂or LiTFSI would reduce the decomposition temperature and the coexistence of the two makes the decomposition temperature continue to drop to 100 ^oC.Therefore,the P-DOL SPE containing catalyst prepared by in-situ polymerization method shows inferior thermal stability,and the working temperature of which needs to be strictly controlled,otherwise battery failure and safety accidents will happen.The main chain structure of P-THF has superior thermal stability,and the addition of glycidyl2,2,3,3-tetrafluoropropyl ether（TFP）to the polymer precursor could improve the chemical and electrochemical stability of the P-THF SPE.（3）Ring opening polymerization is carried out via SnF₂and SnF₂-LiPF₆catalysts,and TFP is copolymerized with DOL,THF and 1,2-epoxybutane（EOB）to prepare a cyclic ether copolymer electrolytes system.The effect of SnF₂-LiPF₆composite catalyst and SnF₂single catalyst on the dynamic balance of polymerization-depolymerization are investigated,indicating that the increase of polymerization rate will accelerate the depolymerization reaction.Meanwhile,LiPF₆will continue to undergo violent interfacial reaction with lithium metal anode in polyether electrolytes,resulting in a continuous increase in interfacial resistance,until the cell fails.The results show that the removal of the catalyst significantly improves the initial decomposition temperature of the three SPEs,and the Li/Li symmetric batteries can keep stable cycling at 100 ^oC.In addition,the thermal decomposition performance of cyclic ether polymer is applied to the recovery of LiTFSI for the first time:the cyclic ether polymer is heated to decompose at 220°C while keep LiTFSI chemically stable.During heating,the polymer decomposition products are removed from the system,and the residue is washed,filtered and dried after the decomposition is completed.The recovered LiTFSI not only possesses high purity,but also has the same chemical and electrochemical properties compared with commercial LiTFSI.The LiTFSI recovery greatly reduces the cost of polymer preparation,promotes the LiTFSI efficiency,and is conducive to environmental protection and sustainable development of resources.