| Rechargeable batteries and supercapacitors based on aqueous electrolytes have the advantages of good safety,low-cost and easy-operation.However,compared with their organic analogues,the water-based electrochemical energy storage devices are seriously restricted by the narrow potential window of water solvent,that not only limits their energy density but also confines the available range of electrochemical reactions.Therefore,exploration of aqueous energy storage devices with wide potential windows as well as high specific capacity is important for their practical applications.Recent researches in aqueous electrolytes are mainly focused on the molecular-level hydration structure of electrolyte salts,while the influence from subatomic-scale neutrons of water solvent has never been considered.Is it possible to extend the electrochemical stability window of aqueous electrolytes by changing the atomic of water molecules?This is still an unknown field at present.In addition,the worldwide increased utilization of renewable energy sources requires large scale but inexpensive grid-level energy storage devices.Although lithium-ion batteries（LIBs）have dominated the market of energy storage for electric vehicles and portable consumer electronics by their high energy densities,it seems impractical to employ this kind of technology with a relatively high cost for gigawatt-level grid energy storages,due to the nonuniformly distrubuted and limited natural lithium source on the earth.Aqueous energy storage devices featured by intrinsic safety and low cost provide an alternative solution for grid-connected stationary energy storage systems.However,the major challenge with lithium-free batteries is their lower energy density compared with LIBs,especially when aqueous electrolytes are used.It is reported that utilization of“water-in-salt”electrolyte（WISE）can effectively increase the energy density of aqueous electrochemical energy storage devices,whereas most of the WISE are prepared by expensive lithium salts.And electrolytes with higher concentrations are restricted by solubility limits.How to make supersolube aqueous electrolytes with non-lithium elements and develop low-cost,high-energy-density lithium-free aqueous batteries is still a great challenge.Based on these questions,several results are obtained in this thesis:1.Discover the“electrochemical effect of isotope”（EEI）of aqueous electrolytes, that the numerically increased neutrons in water solvent extend potential window of aqueous electrolytes.Such an effect is affected by the deuterium content in water solvent,ion specie as well as electrolyte concentration,and is mainly caused by following factors:（i）the lower zero-point energy of deuterium compound and effect of mass on the velocity of passage over the potential-energy barrier results in a higher over-potential for water electrolysis;（ii）the weaker nuclear-electron vibration of deuterium atoms leads to a higher hydration energy between heavy water and electrolyte ions;（iii）the smaller ion product of heavy water reduces the concentration of H+and OH-in aqueous solution,thus slow down the reaction rate of water electrolysis.2.Use the heavy water to make high-voltage aqueous supercapacitors,whose operating voltage was improved by 0.5 V（from 1.8 to 2.3 V）than natural water-based analogues,and corresponding energy density（based on total mass of electrodes）was increased from 17.7 to 27.3 Wh kg-1.The author analyzed the electrochemical performances of heavy water-and natural water-based supercapacitors with varied cutoff voltages by cyclic voltammetry,galvanostatic charge/discharge technologies and the electrochemical impedance spectra.It was found that the employment of heavy water-based electrolyte effectively reduced the diffusion controlled current which was caused by water electrolysis at high voltages,meanwhile enabled excellent rate capability and specific capacitance.3.Discover the water-salt“copolymer”electrolyte（WSCE）which was prepared by co-solubilization of zinc chloride and zinc bromide in water solvent at a high temperature,with the zinc acetate as a“capping agent”.The salt concentration(46 mol kg-1)of WSCE is much higher than that of any saturated aqueous solution based on single component in this system(≤25 mol kg-1).The existence of inorganic oligomers featured by higher molecular mass than any component in the WSCE was proved by the results of Raman spectra and electrospray mass spectrometry,and a unique formation mechanism of electrolyte was proposed: after pre-dissolving superfluous electrolytes in water solvent at a high temperature,the aggregation and grown of salts are“capped”by the acetate groups at room temperature,thus only inorganic oligomers rather than larger crystals are formed,breaking the physical solubility limits of aqueous electrolytes, which is quite different from“salt-in-water”and“water-in-salt”electrolytes, openning a door for“the third kind of aqueous electrolyte”.4.Use the WSCE to construct high-energy-density and inherently safe aqueous dual-ion batteries.During charging/discharging processes,Br-/Br are reversibly oxidized-intercalated into/reduced-extracted from the graphene cathode, meanwhile Zn2+/Zn are deposited/dissolved in anode side.Using the cyclic voltammetry method in a three-electrode system,electrochemical behaviors of WSCE on electrodes and current collector were confirmed,including redox potentials of the cathode and anode.It has been proved that carbon-containing cathode plays a critial role in the reversible oxidation and reduction process of Br-/Br.Comparative experiments showed that the reversibility of electrode reactions increased with the electrolyte concentration.Furthermore,a stage-I intercalation of Br in a macroscopic graphene assembly was observed by in-situ Raman and XRD characterization of graphene aerogel cathode during charge/discharge processes.Such a mechanism provides high capacity(605.7 m Ah g-1)and energy density(908 Wh kg-1)of the aqueous dual-ion batteries. |
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