Research Of Wide Temperature Range Hydrogel Sodium Ion Electrolyte

New energy such as solar energy and wind energy have the merits of renewable and environmentally friendly.However,due to the unstable power generation,the new energy power stations cannot use energy efficiently.When electrochemical energy storage facilities are used together with new energy power stations such as wind energy and solar energy,batteries can store excess energy during the peak period of power generation and release the stored energy during the low period of power generation,which can improve the efficiency of energy utilization.At present,the battery systems commonly used in electrochemical energy storage facilities include lead-acid batteries and lithium-ion batteries.However,the low energy density and the corrosive electrolyte of the lead-acid batteries limit its application.And The high cost of lithium-ion batteries and their organic electrolyte will lead to certain safety risks in large-scale integration of batteries.However,aqueous sodium batteries have the merits of low cost,long life,high safety and environmental friendliness,which is suitable for the application in large-scale energy storage facilities.When aqueous battery energy storage facilities are constructed in frigid areas,the cold environment will cause problems such as the freeze of the aqueous electrolyte and the crystallization of the electrolyte salt,resulting in volume expansion of the electrolyte and damage of the battery structure.When aqueous battery energy storage facilities are constructed in tropical areas,the continuous radiation of the sunlight will heat up the battery,leading to a violent water splitting reaction.It will result in a low coulombic efficiency and shortened cycle life,which will seriously affect the performance of the battery.Therefore,it is of great significance to expand the operating temperature range of aqueous sodium ion batteries.The specific research contents and results are as follows:1.Investigation of the low temperature tolerant aqueous sodium ion hydrogel electrolyte.The phase transition process of aqueous electrolyte at low temperature was firstly studied.As the temperature decreases,sodium sulfate will crystallize and water will be frozen.To solve these two problems,a novel Na₂SO₄-Si O₂ hydrogel type electrolyte was proposed using fumed silica as the hydrogel matrix and methyl alcohol as the anti-freezing additives.The electrochemical performance of the electrolyte was characterized by the cyclic voltammetry and the electrochemical impedance spectroscopy;the microscopic morphology of the hydrogel electrolyte was characterized by the scanning electron microscope;and the influence of the amount of methanol on the low temperature resistance of the electrolyte was further studied.We found that as the amount of methanol added increases,the freezing of water in the electrolyte is suppressed,but the solubility of sodium sulfate decreases,resulting in a low conductivity.In the case of ensuring high solubility of sodium sulfate and high electrolyte conductivity,the amount of methanol added is 50%preferably.Finally,the electrolyte can maintain high conductivity at-30°C.The full batteries assembled with Na Ti₂（PO₄）₃ as anode and activated carbon as cathode can maintain a specific capacity of 61.8 m Ah g^-1 at-30°C.The capacity remains stable after 500 cycles.2.Investigation of the electrode-protected film for high temperature resistant aqueous sodium ion battery.Firstly,the reasons for the serious water decomposition reaction and low coulombic efficiency of aqueous batteries at high temperatures are studied.As the temperature increases,the electrochemical window of water becomes narrower and the water decomposition reactions are more likely to occur.The decomposition reaction of water is an irreversible reaction,which will reduce the coulombic efficiency while consuming the water in the electrolyte,shortening the battery life.In this work,a polymer electrode protect film is added on the surface of the electrode to inhibit the contact between the water and the electrode in the premise of conducting sodium ions.In this way,the water cannot gain or lose electrons,thereby inhibiting the occurrence of the water decomposition reactions.The polymer electrode protection is mainly composed of poly（vinylidene fluoride-co-hexafluoropropylene）,poly（ethylene oxide）,and graphene oxide.Morphology of the film was studied by the scanning electron microscope.The electrochemical performance of the whole cell with the electrode protect film was tested by electrochemical impedance spectroscopy and cyclic voltammetry.We found that the addition of PVDF-HFP can inhibit the decomposition of water;the addition of PEO can increase the ionic conductivity of the film;and the addition of appropriate graphene oxide additives can improve the high temperature stability of the film.Finally,the composition of the interface modification film is preferably 89%by mass of PVDF-HFP,9%by mass of PEO,and then 1%by mass of GO.After adding the electrode protect film,the full cell with Li Mn₂O₄ as cathode and carbon coated Na Ti₂（PO₄）₃ as anode can maintain a high coulombic efficiency of more than 99.5%at 70°C,and the capacity remains 84.2%after 1000cycles at a rate of 5 C(1 C=140 m A g^-1).On the one hand,to improve the low temperature performance of the aqueous sodium batteries,we develop a Na₂SO₄-Si O₂ hydrogel type electrolyte using fumed silica as the gel matrix and methyl alcohol as the anti-freezing additive.Due to the inter-molecular bonding between Si O₂ and Na₂SO₄,the precipitation of Na₂SO₄and the continuous growth of the Na₂SO₄ grain in supersaturated electrolyte is highly suppressed at low temperature.In this way,the battery can work at-30℃.On the other hand,to improve the high temperature performance of the aqueous sodium batteries,we develop an electrode-protected film consisted of poly（vinylidene fluoride-co-hexafluoropropylene）（PVDF）,poly（ethylene oxide）（PEO）and graphene oxide（GO）,in which PVDF matrix functions as a water isolation layer.This work greatly broadens the operating temperature range of aqueous sodium batteries,which contributes to the application of aqueous sodium ions batteries in tropical or cold zone.