Cathodes Design And Electrolytes Modification For Aqueous Zinc-Ion Batteries

The establishment of efficient,clean,and renewable energy has become the focus of every country all over the world.Lithium-ion batteries have been widely used in consumer electronics because of their high energy density,long lifespan,and flexible design.However,their application in large-scale energy storage is seriously hindered by the lack of lithium resources in the earth’s crust and safety concerns.Therefore,it is urgent to develop alternative energy storage technologies that are low-cost and environmentally friendly.Among many advanced secondary battery technologies,aqueous zinc-ion batteries have received extensive attention and are very promising as next-generation energy storage systems because of their low cost,high specific energy,high safety,and easy assembly.However,aqueous zinc-ion batteries still face several critical issues before their practical applications.On the one hand,the strong electrostatic interaction between the high charge density Zn2+ and the cathode material accelerates the structure collapse of the host material during cycling,resulting in severe capacity fading;On the other hand,the narrow electrochemical stability window and poor temperature adaptability of aqueous electrolytes can easily lead to hydrogen and oxygen evolution,impairing the cycle stability of the battery.In addition,the high activity of water in the aqueous electrolyte further promotes the dissolution of the cathode material,dendrites growth or corrosion of the zinc anode,and hydrogen evolution,resulting in performance degradation and safety hazards such as short circuit and bulging of the batteries.To resolve the above problems,this thesis focuses on the cathode materials design and the electrolytes modification,studies the performance improvement strategies of aqueous zinc-ion batteries with a comprehensive and systematical research perspective.In terms of cathode materials,we designed and prepared vanadium oxides with stable structure and large interlayer spacing,respectively,which exhibit excellent cycling and rate performance for aqueous zinc-ion batteries.Subsequently,the research experience on the energy storage mechanism of cathode materials and the electrode/electrolyte interface was applied to the construction of electrolytes.A co-solvent type and a molecular crowding type of zinc ion electrolytes was designed respectively,and realized the aqueous zinc-ion batteries with excellent performance and strong practicality.The above research is organized in four sections as described following:（1）VO₂（B）nanoflakes were prepared by hydrothermal method and used as cathode materials for aqueous zinc-ion batteries.VO2（B）has a stable lattice structure and large ion migration channels,thus enabling fast ion insertion/extraction capability and good cycling stability.Through in-depth research on the energy storage mechanism of VO2（B）and the properties of the electrode/electrolyte interface,it was found that the material undergoes a solid-solution reaction during the Zn2+ insertion/extraction processes,accompanied by the reversible formation/decomposition of by-products on the electrode surface.The formation of by-products is an inevitable result of the desolvation of the electrolytes,which promotes the rapid transformation of[Zn（H2O）6]2+ into Zn2+ and thereby promoting the charge transfer or transport at the electrode/electrolyte interface.Further research demonstrated that by-products with different interlayer spacing and ion migration paths would form using the electrolytes with different salts.The by-products（Zn4SO4（OH）6·5H2O）generated in the Zn SO4 electrolytes have moderate interlayer spacing and open zinc ion migration paths,which is more favorable for the interfacial reaction kinetics of the VO2（B）electrode,resulting in excellent electrochemical performance.（2）The pre-intercalated V₂O₅ with non-metal ions and crystalline water,（NH4）0.58V2O5·0.98H2O（NHVO）was prepared by a hydrothermal method,which has an interlayer spacing of 10.9 ? and is used as a cathode for aqueous zinc-ion batteries.The interlayer NH4+ and crystal water play the role of expanding the spacing and stabilizing the structure of NHVO,thereby endowing the material with high specific capacity and alleviating the volume change of NHVO during cycling.Through in-depth research on the energy storage mechanism of NHVO and the properties of the electrode/electrolyte interface,the material undergoes reversible solid-solution reaction and two-phase reaction during electrochemical cycling,accompanied by the reversible formation/decomposition of a by-products（Zn4SO4（OH）6·5H2O）.Benefiting from the large interlayer spacing of NHVO,[Zn（H2O）6]2+ with larger radii can preferentially participate in the electrochemical reaction,so that the intercalation of [Zn（H2O）6]2+ first during the initial discharge process,followed by the intercalation of Zn2+.Due to the desolvation of [Zn（H2O）6]2+,the by-products generated at the electrode/electrolyte interface,which is an important sign to distinguish [Zn（H2O）6]2+ or Zn2+ intercalation.At the same time,the formation of by-products also effectively promoted the desolvation process and accelerated the charge transfer process at the interface.（3）Unraveling a cathode/anode compatible aqueous zinc ion electrolyte with acetonitrile（AN）as a co-solvent.In the optimized electrolyte,there are more anions are present in the first solvation shell of Zn2+.And due to being "bound" by the surrounding AN network,the activity of H2 O molecules in the solvent is greatly reduced.Thus,the electrochemical window of the electrolytes was extended,and the wetting ability of the electrolytes was improved.Meanwhile,the optimized solvation structure can also play a role in alleviating side reactions on the surface of the anode and inhibiting the dissolution of the vanadium-based cathode material.Through systematic research on the energy storage mechanism and the properties of the electrode/electrolyte interface,the results showed that the co-solvent electrolyte could form an in-situ electrode/electrolyte interfacial film on the surface of the cathode/anode after cycling,thereby further regulating the deposition of Zn metal and inhibiting the dissolution and deactivation of the cathode materials.In this electrolyte,the zinc metal anode and the V2O5 cathode both have stable long-life capability,and the operating life of the aqueous zinc-ion battery has been greatly improved.（4）Molecular crowding aqueous zinc ion electrolytes were constructed using molecular crowding agent polyethylene glycol（PEG）and co-solvent dimethyl sulfoxide（DMSO）.Due to the interaction between the molecular crowding agent,cosolvent and water,the hydrogen bond structure of free water is changed and its activity is greatly reduced,which effectively improves the water retention of the electrolytes and avoids the side reactions caused by the presence of water on the surface of the Zn anode and the dissolution of the vanadium-based cathode material.Through in-depth research on the energy storage mechanism of VO2（B）and the properties of the electrode/electrolyte interface,the results showed that in the molecular crowding electrolyte,an in-situ electrode/electrolyte interface film was formed on the surface of the cathode/anode,which further protected the cathode/anode and improved the cycle life of the battery.Meanwhile,the improved electrolyte also shows high thermal stability due to the existence of the high boiling point of PEG and therefore,effectively improves the self-discharge of the battery and the operation ability at high temperatures.In summary,this thesis focuses on the design and development of cathode material and electrolyte for aqueous zinc-ion batteries and successfully constructed vanadiumbased oxide cathode materials with stable structure and high capacity,as well as the aqueous zinc ion electrolyte with long cycle stability and strong environmental adaptability.And the improved materials and electrolytes were applied to the aqueous zinc-ion battery system,which comprehensively improved the overall performance of the battery system.At the same time,the underlying reasons for each strategy to improve the electrochemical performance of the battery are systematically analysed via combining experiments and theoretical calculations,and therefore,this work provides a theoretical basis and realization approach for the subsequent application of aqueous zinc-ion batteries.