Rational Design Of Water/Organic Hybrid Electrolytes And Their Applications In Lithium-ion Batteries

Aqueous lithium-ion batteries using non-flammable aqueous electrolytes are safer battery systems than the lithium-ion batteries with organic electrolytes.However,the electrochemical stability window（ESW）of aqueous electrolytes is as naroow as 1.23 V,which greatly limits the output voltage and energy density of aqueous lithium ion batteries.By using super-concentrated lithium salt,water-in-salt electrolyte successfully expands the ESW of aqueous electrolyte from 1.23 V to 3 V due to the reduced water activity and the formation of an effective solid electrolyte interphase（SEI）.Accordingly,the selection range of electrode materials is broadened,realizing an aqueous lithium-ion battery system with largely improved output voltage and energy density.However,the awkward cathodic limit of the super-concentrated electrolytes is still incapble to support the anodes with low working potential.On the other hand,the heavy use of salt results in high viscosity,poor low-temperature performance and poor wettability.In order to get an expanded ESW at a relatively low concentration,and improve the low temperature performance and wettability of electrolyte,this paper designs a series of water/organic hybrid electrolytes based on the water-in-salt system.The main research contents and results are as follows:（1）Dimethyl carbonate is introduced as a film-forming promotor to construct a more stable SEI.In addtion to participating in the formation of SEI,it is also found that the addition of dimethyl carbonate can further reduce the activity of water.As results,a stable SEI with high content of Li₂CO₃ is obtained,and the ESW is expanded to 3.8 V.The as-assembeld Ti S₂/Li Mn₂O₄ full cell shows an output voltage of 1.5 V and an energy density of 63 Wh/kg.When matched with Li₄Ti₅O₁₂/Li Mn₂O₄ full cell,an elevated output voltage of 2.4 V and a higher energy density of 120 Wh/kg can be obtained.（2）Sulfolane,which has high polarity and high stability,is introduced as a co-solvent to manipulate the Li⁺solvation structure to futher widen the ESW.It is found that sulfolane has a strong affinity and coordination ability with Li⁺,showing competitive coordination with water molecules.As result,the water molecules in the Li⁺solvation layer are reduced,and the possibility of water molecules entering the inner Helmholtz layer is also reduced.Therefore,an ESW of 4.0 V is obtained at an apparent salt concentration of only 5.7 m.The as-constructed Li₄Ti₅O₁₂/Li Mn₂O₄ full cell shows a capacity retention of 70%after 300cycles at 2 C,and the average Coulombic efficiency is as high as 98%.（3）To settle the issues of poor low-temperature performance and high viscosity of the water-in-salt system,diethoxyethane,which has low viscosity and wide liquid range,is introduced as a co-solvent.The viscosity of the resulted water/diethoxyethane hybrid electrolyte is only 59.5 m Pas（25°C）,and it can keep as liquid and shows a high ionic conductivity of 0.089 m S/cm even at-20°C.The as-assembled Li₄Ti₅O₁₂/Li Mn₂O₄ full cell can work normally at-20°C,delivering a capacity utilization of 65%compared with that at room temperature.DFT calculations find that the affinity between Li⁺and diethoxyethane is lower than that with SL,indicating a lower energy barrier for the desolvation process,which is conducive to the rapid exchange of lithium ions at the interface.（4）Considering electrochemical performance,salt concentration and process suitability,a water/sulfolane/fluorinated ether ternary solvent hybrid electrolyte is proposed by introducing fluorinated ether.The introduction of fluorinated ether dilutes the originally compact ionic clusters,reducing the viscosity of the electrolyte significantly,and the polarity is also reduced.As result,the ternary hybrid electrolyte can fully wet the commercial polypropylene separator.The apparent salt concentration of the electrolyte is only 2.3 m,however,it shows the similar solvation structure and electrode/electrolyte interface behavior with super-concentrated electrolytes.The constructed Li₄Ti₅O₁₂/Li Mn₂O₄ pouch full battery demonstrates improved energy density,low temperature performance and rate performance,implying that the electrolyte can well match with the production process of the commecail lithium-ion battery.