Aqueous/Hydrogel Electrolyte With High Concentration Lithium Salt For Lithium Rechargeable Batteries

Since the 1990s,lithium-ion batteries have attracted widespread attention due to considerable operating voltage,high-level energy density,and long-term cycle life.However,the low-boiling,flammable and volatile organic solvents as electrolytes for traditional lithium-ion batteries,which caused a series of safety issues such as fire and explosion.One of the promising strategies to address the issue is to employ non-flammable electrolytes as alternatives to the currently used organic carbonate electrolytes,such as solid state or aqueous electrolytes.The aqueous lithium-ion batteries?ALIBs?are considered to be a promising next-generation lithium-ion batteries,due to aqueous electrolytes have the advantages of higher ionic conductivity,safety and environmentally friendly.However,aqueous electrolytes have a narrow electrochemical window??1.23 V?and many side reactions in aqueous eletrolytes,which seriously affect the performance of ALIBs.Basing on this consideration,the works presented in this dissertation will focus on aqueous/hydrogel electrolyte with high concentration lithium salt for lithium rechargeable batteries.Firstly,study the anion-effect on electrochemical widows of aqueous electrolyte;then,clarifies the hydrogen ion is the key factors for polyimide decomposed in aqueous electrolyte,and modifies the saturated Li NO₃ aqueous electrolyte by adding alkali additives to achieve long cycle stability of polyimide;Third,a low water ratio,highly stretchable,self-healing of hydrogel electrolyte film is prepared for flexible lithium rechargeable batteries.The results are detailed as follows:1)Anion-effect on electrochemical widows of aqueous electrolyteFirstly,the saturated?sat.?and 1 mol L^-1?1 M?aqueous electrolytes with Li NO₃?Li BF₄?Li FSI?Li TFSI?Li Cl O₄?Li₂SO₄ and Li BOB are prepared by distill water and lithium salts.Then,the electrochemical stability windows and conductivities are investigated by three-electrode cells,which are assembled with various working electrode?including of stainless steel mesh,aluminum foil and nickel foam?,counter electrode of platinum net and reference electrode of saturated calomel?E=0.2415 V vs.NHE?.The results display that the electrochemical stability window of water could be generally broaden by adding lithium salts.And the values are related to the type of anionic,the concentration of electrolyte,and the material of current collector.The reasons are attributed to the association interactions of anions with H₂O molecules,and the anions adsorption ability on the current collector interface layer also influence the electrochemical stability window of aqueous electrolyte.2)Effects of alkali additives in saturated Li NO₃ aqueous electrolyte for improvement in cycling stability of polyimide anodeIn previous work,the polyimide exhibited a perfect long-term cyclic stability in non-aqueous electrolytes but not in aqueous electrolytes.Through a series of experimental and theoretical calculation,it is found that the hydrogen ions participate in the electrode reaction,which leads to changes in the structural stability of polyimide and accelerates the hydrolysis process of the polyimide.Thus,we proposed to modify the saturated Li NO₃ aqueous electrolyte by adding alkaline additives.The results displayed that the optimized aqueous electrolyte can greatly improve the cycling stability of the polyimide materials.The improvement in cyclic performances is attributed to the effective suppression of H⁺ion accumulation,which is produced by side reaction during cycles.The formation of colloidal Li OH is obtained by introducing a proper amount of KOH into the saturated Li NO₃ electrolyte.The resulting colloidal Li OH not only reduces the concentration of free OH^-ions in the electrolyte but also continuously releases the OH^-ions to react with H⁺ions.Based on the optimized electrolyte,a coin type full cell has been assembled with Li Co O₂ cathode and polyimide anode,it is exhibited a remarkable long-term cycling stability and the capacity without an evident decay,which released a reversible capacity of around 72 m Ah g^-1?calculated by the weight of Li Co O₂?at current density of 1 A g^-1.3)Hydrogel polymer electrolyte film with highly stretchable and self-healing for flexible rechargeable batteriesA hydrogel electrolyte film with highly stretchable,self-healing and self-supporting are prepared by elastic component?PBA-Si?,moisturizing components?CMC?,wetting components?polyether F127?,and hygroscopic lithium salts?Li TFSI?with mass ratio of 2:3:1:6.The hydrogel electrolyte film with low water ratio?<4%?,the ion conductivity is around 1.70 m S cm^-1 at room temperature,and also it is displayed an elongation more than 300%.The self-healing function of hydrogel film is realized by adding deionized water into the fracture section of two pieces films.A flexible rechargeable battery is assembled by LMO-CNT cathode,NTCDA-MDA-CNT anode and flexible hydrogel electrolyte film.The full cell is releasing a discharge specific capacity of 97m Ah g^-1 at current density of 500 m A g^-1,and the capacity still have 71 m Ah g^-1 with 73%retention ratio after 1000 cycles.