Investigation Of Aqueous Lithium Ion Batteries Based On Highly Concentrated Electrolyte

Aqueous lithium ion batteries are considered to be a promising alternative for large-scale energy storage,due to their advantages of high safety,environmental friendliness and low cost.However,the thermodynamic stability window of aqueous electrolyte is only 1.23 V,which causes side reactions such as H₂/O₂ evolution under higher operating voltage,seriously limiting the selection of electrode materials.In addition,most of the electrode materials have the problems of dissolution and structural instability in aqueous electrolyte.These problems lead to the low operating voltage,low energy density and unsatisfactory cycle stability of current reported aqueous batteries.Therefore,in order to improve the operating voltage,energy density and cycle stability of aqueous lithium-ion batteries,an aqueous lithium-ion battery based on high concentration electrolyte was developed.On the one hand,highly concentrated electrolyte can suppress the dissolution of the electrode material into the electrolyte due to the common ion effect,thereby improving the cycle stability of the battery.On the other hand,highly concentrated electrolyte can broaden the electrochemical window of the aqueous lithium-ion battery due to its unique solvation structure and the formation of the SEI film on the surface of the negative electrode,thus we can construct a high voltage and high energy density aqueous lithium ion battery.The main research contents and results of this thesis are as follows:（1）Vanadium-based material with multi valence(V^3+/4+/5+)and high theoretical specific capacity is one of the most attractive electrode materials.However,they have the dissolution problem during long-term cycling in aqueous electrolyte,limiting their application in aqueous lithium ion batteries.The Li VOPO₄ cathode and VO₂ anode were synthesized by sol-gel method and hydrothermal method respectively,and their electrochemical behavior in aqueous electrolyte was investigated.The Li VOPO₄cathode and VO₂ anode showed very poor cycling performance in 1 m Li TFSI electrolyte due to the dissolution of active materials.When cycled in the 20 m Li TFSI electrolyte,the dissolution of active materials was obviously inhibited.The reversible capacity of Li VOPO₄ cathode and VO₂ anode in the first cycle was 128 m Ah g^-1 and137 m Ah g^-1 respectively,and there was no significant capacity decay during the cycle.Because of the special solvation structure of highly concentrated electrolyte,there is almost no free water molecule in the electrolyte,thus inhibiting the dissolution of electrode materials;and the increase of lithium salt concentration can also reduce the dissolution of active materials,based on the common ion effect.In addition,highly concentrated electrolytes have demonstrated its strong ability to decrease the redox activity of water molecule,and the electrolyte salt is decomposed to form a stable solid electrolyte interface（SEI）,the electrochemical stability window of aqueous electrolytes can be extended.The hydrogen evolution and oxygen evolution side reactions are reduced,thereby improving the cycle stability of the electrode material in the aqueous lithium ion battery.（2）An aqueous lithium ion battery was constructed by using Li VOPO₄ cathode,VO₂ anode and 20 m Li TFSI aqueous electrolyte.The operating voltage and energy density of this aqueous lithium ion is 1.4 V and 84.0 Wh kg^-1 respectively,and the capacity retention rate is 84%after 1000 cycles at the current density of 100 m A g^-1.In addition,thanks to the wide liquid range of the highly concentrated electrolyte,the battery can work stably between-20°C and 80°C.For example,the reversible capacity of the battery is 55.8,100.7,129.0 and 130.9 m Ah g^-1 at the temperature of-20,-5,25and 80°C,respectively.And the battery demonstrates good cycling performance at varying temperatures,with almost no capacity decay over 50 cycles.（3）In order to further improve the voltage and energy density of aqueous lithium ion batteries,an organic solvent GBL was added to the highly concentrated electrolyte,which could dissolve more lithium salts and form organic passivation film,thus broadening the electrochemical window of the electrolyte.The experimental results showed that Li Mn₂O₄ cathode and Li₄Ti₅O₁₂ anode exhibited good electrochemical compatibility in Li TFSI·1.4H₂O·0.6GBL electrolyte.We constructed a full cell based on Li Mn₂O₄ cathode and Li₄Ti₅O₁₂ anode.The average discharge voltage platform is2.4 V.At a current rate of 5 C(1 C=160 m A g^-1),the cell delivers a reversible capacity of 137.4 m Ah g^-1 based on the mass of anode material,and capacity retention of 84%over 400 cycles.（4）In view of the electrochemical stability of non-aqueous/aqueous mixed highly concentrated electrolyte（Li TFSI·1.4H₂O·0.6GBL）,we further studied the mechanism of improving the cyclic stability of Li Mn₂O₄ cathode and Li₄Ti₅O₁₂ anode in aqueous lithium ion battery.The experimental results showed that,on the one hand,TFSI anions can be decomposed on the surface of electrodes to form inorganic SEI components rich in Li F;on the other hand,the addition of GBL can assist in the formation of uniform and compact hydrophobic polymer SEI components,thus greatly improving the interface stability between electrode materials and aqueous solutions,further broadening the electrochemical window of aqueous electrolytes,and improving the cycle stability of aqueous lithium ion battery.