Exploration Of Low-Cost High-Concentration Aqueous Electrolyte And Its Application In Electrochemical Storage Devices

Aqueous energy storage devices（AESD）have the advantages of low cost,high safety,eco-friendly,fast charge and discharge rate,and good temperature characteristic,which possess intriguing application potential in large-scale energy storage and various special scenarios.However,the application of the AESD faces two bottlenecks:On the one hand,the narrow electrochemical stability window of aqueous based electrolytes,including liquid and quasi-solid state,limits its widely application,especially in the condition where high energy density is required.On the other hand,the stability of the electrode material and the corresponding cell is generally poor due to the high activity of water,including high reactivity and dissolution activity.The development of aqueous electrolytes with a wider stability window and lower activity,and the exploration of suitable electrode materials are the key to construct high-energy and stable aqueous based energy storage/release devices.The work here is aim to the construction of low-cost,high-performance aqueous batteries.The exploration and optimization of aqueous electrolytes is the first and main concern.Further,appropriate electrode materials and systems are designed.Both supercapacitors and batteries are assembled,and their energy storage characteristics are well investigated.The main contents are as follows:（1）A new type of low-cost aqueous electrolyte of high-concentration potassium formate aqueous solution（HCOOK solution with a concentration of 40 mol/kg where“m”stands for mol/kg,mole per kilogram of water）is explored.The electrolyte exhibits good electrochemical performance:its stable potential window is rather wide,up to 4 V using a glassy carbon electrode;its ionic conductivity is as high as 46 m S cm^-1,which makes it suitable for supercapacitors with high power density.Using this electrolyte,commercial activated carbon can work in a voltage of 3.4 V with a specific capacity of 321 F g^-1at a current density of 5 A g^-1loaded on a glassy carbon electrode,but the capacity decays very quickly.Using treated carbon cloth as a self-supporting electrode,its cycle stability is greatly improved.The capacity retention rate of the electrode reaches 87%after 10,000 cycles at 80m A cm^-2.Finally,a symmetrical supercapacitor was assembled.The full cell has a working voltage of 2.8 V.Its capacitance is up to 566.8 m F cm^-2at 20 m A cm^-2,and is still high as418.6 m F cm^-2at a high current of 100 m A cm^-2,showing excellent rate performance.88.3%of the capacitance is retained after 10,000 cycles’cycling at 80 m A cm^-2,showing good stability.In addition,a high energy density of 437μWh cm^-2is achieved at a high power density of 74.0 m W cm^-2.（2）Flower-like VSe₂is synthesized via a hydrothermal method,and its potassium storage behavior in aqueous electrolyte of the 40 mol/kg HCOOK is well studied for the first time.Its performance in the 40 mol/kg HCOOK is significantly better than that in 1 mol/kg KCl and1mol/kg KOH.A capacity of 45.8 m Ah g^-1,33.9%of the initial value is remained after 200cycles’cycling.The main reason for the capacity decay of the VSe₂electrode in the aqueous electrolyte should be the hydrolysis of the intermediate product accompanied by the formation selenium.A gel electrolyte was then prepared through the adding of CMC in the 30 mol/kg HCOOK.The cycle stability of the VSe₂electrode is effectively improved in the gel electrolyte.A capacity of 68.3 m Ah g^-1with a retention rate of 53.2%is remained after cycled in the gel for 200 cycles.Furthermore,using treated carbon cloth as positive electrode and the VSe₂electrode as negative electrode,a K-ion supercapacitor was assembled in the HCOOK gel electrolyte.The voltage can reach 1.8 V.The discharge capacity of the second cycle is 53m Ah g^-1at 1 A g^-1with a capacity retention of 33 m Ah g^-1after 200 cycles.（3）To extend the water decomposition voltage,and inhibit the activity of liquid water,a potassium acetate（KAc）based gel-like electrolyte with high salt water ratio（～1:1.16）and quasi-solid characteristics is developed.Because of the special structure of Ac^-and the unique preparation process,H₂O,Ac^-and K⁺can form polymer-like long chains,so that gel sate is realized without any additives of polymer gelling agents.In addition,the ionic conductivity of the gel can reach 10.9 m S cm^-1at room temperature,and it exhibits excellent temperature adaptability.The quasi-solid state can be maintained even at a high temperature of 90°C;the ion conductivity can still reach 3.4 m S cm^-1at a low temperature of-20°C.The electrochemical stability window of the electrolyte reaches 4 V（on a glassy carbon electrode）due to the high salt water ratio.In this electrolyte,the stability of a Fe Se₂electrode is obviously improved owing to the quasi-solid state and the formation of SEI.The reversible capacity of the Fe Se₂electrode that stores energy through phase inversion Faraday reactions can reach 250 m Ah g^-1and be well maintained at 0.5 A g^-1during 100 cycles’cycling.The capacity is still high as～220 m Ah g^-1at 1 A g^-1and can be maintained even after 250 cycles’cycling.While,the capacity of the electrode decays to nearly zero after 100 cycles in the 30mol/kg KAc electrolyte with liquid state.Furthermore,using commercial Ni（OH）₂as cathode,a Ni-Fe full battery is assembled.Its operating voltage is high as 2.4 V,and the discharge platform can be up to about 1.5 V.The capacity of the full cell can be stabilized at about 130m Ah g^-1after activity（considering the mass of active materials of the two electrodes）,and the capacity can still be high as 90 m Ah g^-1after 500 cycles’cycling.（4）A full cell system of Ni-Zn hybrid ion battery with good electrochemical performance is designed and constructed,using carbon coated zinc oxide,Zn O@C,as anode,Ni（OH）₂as cathode,and the KAc-based gel as electrolyte.In this system,the dissolution problem of Zn O in alkaline electrolyte has been significantly improved under the dual effect of C coating layer and concentrated gel-state electrolyte.The self-dissolution of Zn O@C is only 1.65 wt.%after soaking in the KAc gel for four days.In addition,the formation of zinc dendrites is effectively avoided,and the hydrogen evolution concerned side reactions are largely suppressed.The reversible capacity of the Zn O@C anode is as high as 400 m Ah g^-1after 50 cycles in a three-electrode configuration.The Ni-Zn full battery delivers an initial discharge voltage of 2.3 V and a median voltage of 2.1 V,which is about 0.5 V higher than the traditional Ni-Zn alkaline battery.The capacity of the full battery can reach 315 m Ah g^-1after 50 cycles（only considering the mass of the Zn O@C）.With Zn（OH）₄^2-source in the gel electrolyte,the capacity and the stability of the full cell are largely improved.The capacity is as high as 530m Ah g^-1at a current density of 1 A g^-1after 450 cycles’cycling.More importantly,the reversible capacity of the electrode increases with the increase of the charge and discharge current.The reversible capacity can be up to 640 m Ah g^-1and the Coulombic efficiency is as high as 97.3%at high current densities of 4～8 A g^-1,implying the effectively suppression of hydrogen evolution.