Low-cost Mixed Solvent Electrolytes For High-voltage Aqueous Lithium-ion Batteries

At present,due to the continuous breakthrough and wider application of lithium-ion batteries technology,from the aspects of environment friendliness,energy density,cost,development path,etc.,lithium-ion batteries（LIBs）will certainly dominate in electrochemical energy storage applications.However,limited by the fact that conventional LIBs electrolytes employ organic small molecules as solvents,the current safety issues and costs such as fire and explosion of LIBs as energy storage batteries remain the biggest obstacles for large-scale energy storage applications.As one of the key strategies to improve the safety performance of LIBs,it is the most effective solution to replace flammable organic solvents with non-flammable solvents such as water and solid polymers in electrolytes.Aqueous lithium-ion batteries（ALIB）using water as solvents of electrolytes have attracted much attention in the field of batteries safety research due to their low-cost and high safety performance.Nevertheless,the theoretical electrochemical stability window（ESW）of water is still narrow（1.23 V）,which limits the choices of electrode materials for ALIB,thereby limiting the operating voltage and energy density of ALIB.Based on the above problems,this paper studies concentrated electrolytes and mixed-solvent electrolytes through the synergy of co-solvents.The electrode materials are modified by carbon coating and combine with mixed solvent electrolytes to assemble Li4Ti5O12//LiMn2O4（LTO//LMO）full batteries.The electrochemical properties such as reversibility and cycle performance are studied,which provide the basis for the development of high-voltage aqueous lithium-ion batteries and electrochemical energy storage applications with guidance and reference.In order to optimize the low cost and wide electrochemical stability window electrolytes,a mixed solvent LiOAc electrolyte is obtained by adjusting the activity of water molecules and ESW of 13 m LiOAc electrolyte using 1,5-pentanediol（PD）and polyethylene glycol（PEG）as co-solvents.Infrared and Raman spectra characterize the activity of water molecules which have been fully inhibited by the synergy of PD and PEG.A three-electrode battery system is assembled to measure the ESW of the electrolytes by linear sweep voltammetry（LSV）.The LSV test results show that the ESW of the electrolytes is broadened by using co-solvents,which has the widest ESW of 3.1 V as the composition of electrolytes is 2 m LiOAc-PD0.5PEG0.5.Electrode pair of LTO//LMO is used to measure the electrochemical performance of 2 m LiOAc-PD0.5PEG0.5.Cyclic Voltammetry（CV）test shows that LTO and LMO can carry out electrochemical redox reaction in the electrolytes,in which proving the electrode has reversible performance.The results of CV show an average working voltage of 2.3 V,energy density of 61.6 Wh/kg during the first cycle of full cell,and the Columbic efficiency of 89.73%in 100 cycles.Low energy density and charge-discharge efficiency are due to the influence of hydrogen bonding and complexation of co-solvents on the electrochemical activity and Li+conductivity of water molecules in electrolytes.The low cost of fluorine-free mixed solvent LiOAc electrolyte is expected to promote the applications of LIBs in large-scale electrochemical energy storage.In order to further widen the ESW and improve the energy density of ALIB,1,4-dioxane（Diox）is used as a co-solvent to reduce the number of free water molecules at the interface between the electrode and the electrolytes and promote the desolvation of Li+,which can improve the solvation ability while reducing the activity of water molecules and then widen the ESW of the LiTFSI electrolytes.Raman spectra and Nuclear Magnetic Resonance（NMR）results show that a new solvation structure is formed between Li+and O atoms of Diox molecules,and the activity of water molecules is further inhibited.LSV test results show that the electrolytes composition of 4.5 m LiTFSI-Diox has the widest ESW of 4.3 V.Electrode pair of LTO//LMO is used to test the electrochemical performances of full cell of 4.5 m LiTFSI-Diox.The CV test show that LTO and LMO have great reversible cycling ability in the electrolytes,reflecting the excellent Li+transport ability.An average voltage of 2.4 V full cell with energy density of 172 Wh/kg,capacity retention rate of 40%and average Columbic efficiency of 96.72%in 500 cycles at 1 C is obtained.It can be seen that reducing the number of free water molecules at the interface between the electrode and the electrolytes while getting rid of the obstacles of the conduction of Li+is a more effective strategy to widen the ESW of the aqueous electrolytes and improve the energy density of ALIB.