Research On High Concentration Electrolyte And Analysis Of Eletrochemical Behavior On Electrode-Electrolyte Interface

With the depletion of non-renewable fuels,to get rid of the dependence on fossil fuels,renewable clean energy represented by wind and solar energy are paid great attention.However,the intermittent and unstable properties of this kind of clean energy are difficult to integrate with the existing grid system.In order to overcome this defect,large-scale energy storage systems with secondary batteries as the core technology have gradually attracted widespread attention.In the development path of large-scale energy storage technology,aqueous sodium ion battery is one of the most potential directions.Benefiting from the abundant sodium resources and low redox potential,the advantages of aqueous sodium ion battery in cost and safety performance are particularly prominent.At present,the aqueous solution of sodium salt,as the electrolyte in the aqueous sodium ion battery,has narrow electrochemical stability window,therefore limiting the energy density and further development of aqueous sodium ion batteries.Considering the narrow electrochemical stability window of aqueous sodium ion battery electrolyte,this thesis is based on two strategies of "water-in-salt" and "molecular crowding",to improve the electrochemical stability window of water.The specific research content is as follows:(1)Based on the "water-in-salt" strategy,two low-cost acetates,ammonium acetate(CH3COONH4,NH4Ac)and sodium acetate(CH3COONa,NaAc),are fully dissolved in water with a concentration of 25 M and 5 M,respectively,to prepare a mixed aqueous solution as an electrolyte.Layered manganese dioxide(MnO2/CNTs)with multi-wall carbon nanotubes and carbon-coated sodium titanium phosphate(NaTi2(PO4)3/C,abbreviated as NTP/C)were synthesized as positive and negative electrode materials.The physical and electrochemical properties of aqueous,cathode and anode were tested and assembled into a full battery for related tests.The experimental results show that:the two salts of NH4Ac and NaAc can broaden the electrochemical stability window of water to 3.9 V.MnO2/CNTs and NTP/C have good reversible electrochemical performance in the mixed aqueous solution of acetate.The solution and electrodes were assembled into a full cell,which can achieves a capacity of 74.1 mAh·g-1 at the test condition of 0.1 A·g-1.The cycle performance of full cell is excellent,and the capacity decline per cycle is only 0.06%for 500 cycles.Benefiting from the high ionic conductivity of 28.2 mS/cm,the aqueous full cell has excellent rate performance.(2)Based on "Molecular crowding" strategy,polyethylene glycol(PEG,molecular weight 400)is mixed with water at a mass ratio of 9:1 with addition of 1 M CH3COONa to prepare a local high-concentration aqueous solution as an electrolyte.Na0.44MnO2 and NTP/C were prepared as positive and negative electrode materials,respectively.The electrolyte,positive and negative materials were analyzed using physical and electrochemical methods.The experimental results show that:based on the solution of PEG,water and 1 M CH3COONa,the local concentration of salt can reach 10 M,the electrochemical stability window of water is effectively widened to 3.6 V.And the prepared Na0.44MnO2 and NTP/C materials exhibited good electrochemical performance in this solution.The electrolyte and electrodes are assembled into a button cell for electrochemical performance test.At a current density of 0.05 A·g-1,the full cell can achieves a discharge capacity of 91.8 mAh·g-1.After 100 cycles,the capacity retention rate is 68.6%and the coulombic efficiency can reach 98%.