Investigation Of In-situ Polymerization And Electrochemical Oxidative Decomposition Mechanism Of Electrolytes For Sodium Batteries

The rapid development of industries such as new energy vehicles and consumer electronics has driven the research on energy storage systems.The widely commercialized lithium-ion batteries,encounter development bottlenecks such as high cost of lithium and shortage of lithium resources.The sodium battery has begun to attract the attention of scientific researchers due to its abundant element reserves and low price.It is expected to be used as a supplement to lithium-ion batteries and enrich the choice of energy storage systems.Recently,the research on sodium batteries has developed rapidly.The theoretical capacity of the sodium metal anode reaches 1166 mAh g-1,which exhibits good reversibility in ether-based electrolyte,and the average coulombic efficiency is as high as 99.9%.However,the low boiling point and low oxidation stability of ether electrolytes reduce the safety of sodium metal batteries.In this paper,based on ether electrolytes,a gel polymer electrolyte was constructed by in-situ polymerization to improve its safety.In addition,the oxidative stability of ether electrolyte was explored.The decomposition reaction of the electrolyte solvent and its effect on the electrochemical performance were also investigated.(1)An ether-based gel polymer electrolyte(GPE)is in-situ constructed and successfully applied to Na metal batteries,through Lewis acid-initiated cationic ringopening polymerization of ether solvent 1,3-dioxolane(DOL),with monoglyme(DME)as the plasticizer.The GPE has good thermodynamic and electrochemical stability.The room-temperature ionic conductivity reaches 3.66×10-4 S cm-1,and the transference number of Na+ reaches 0.66.The ether-based GPE has good compatibility with sodium metal anode,significantly reducing the interfacial side reactions between electrolyte and active sodium metal anode.Therefore,the Na|Na symmetrical cell with GPE can be stably cycled for more than 1300 h at a current density of 1.0 mA cm-2.In addition,due to the low fluidity and high mechanical properties of GPE,the Na|TiS2 battery with GPE can effectively inhibit the shuttle of pulverized TiS2 cathode particles during cycling and mitigate its parasitic reaction on the Na metal surface,significantly improving the electrochemical performance of Na metal batteries.The reversible capacity reaches 321 mAh g-1 after more than 1000 cycles at a current density of 200 mA g-1,which is significantly better than that with the liquid electrolyte under the same conditions.At the current density of 50 mA g-1,the reversible capacity reaches 371 mAh g-1 after cycling 500 times,and the cycle life of the battery exceeds 6100 hours.The strategy of in-situ synthesis of GPE supplies a guide for the development of high-safety,low-cost,and long-life sodium batteries.(2)The electrochemical oxidative behavior of NaPF6-diglyme electrolyte was explored.It was found that after the first oxidation reaction platform at 4.5 V,the subsequent cycles showed a stable redox reaction platform at 2.2 V and 2.7 V,and the reversible capacity reached a maximum of 2.2 mAh.The Coulombic efficiency almost reaches 100%,and the battery can be stably cycled more than 200 times.The characterization of the discharge products showed that materials rich in Na,O were formed on the carbon current collector.The XRD pattern of the cathode showed the weak signal of Na2O2·2H2O.Meanwhile,it was found that gas was generated after the battery was disassembled.Therefore,it is preliminarily inferred that diglyme solvent decomposes and oxygen gas may be formed during the oxidation reaction,and participates in the subsequent charge/discharge process as the active material.In this work,the special electrochemical behavior of ether electrolyte was studied.It is found that ether electrolytes decompose and oxygen gas may be formed at high potential,which has good reversibility in sodium battery systems.However,the work still needs a lot of experimental confirmation,which is currently underway.The study is expected to provide a new reference for the exploration of ether-based electrolyte related oxidation mechanism and the design of highly reversible sodium-oxygen batteries.