Structural Design And Performance Optimization Of New Aqueous Energy Storage Materials For High Energy Density

High-performance electrochemical energy storage devices represented by lithium-ion batteries and supercapacitors are the key to the development of new energy sources.They are widely used in digital information,transportation,military and other fields,and their core lies in electrodes and electrolytes.Due to different energy storage mechanisms,the performance is very different.Limited by the diffusion rate of lithium ions in the bulk phase of the electrode material,the specific power of the lithium ion battery is low,and the lifetime is short.Supercapacitor realizes energy storage by physically adsorbing ions on the surface of the electrode.However,it has a lower specific energy which is limited by the specific surface area.The aqueous-based electrolyte has the advantages of safety,environmentally friendly,and high ionic conductivity compared with the organic electrolyte,but the working voltage is low?<1.5V?.The new energy storage electrode and electrolyte material with high energy,high power and high safety are the hotspots and difficulties in international frontier research.In this thesis,with respect to the key scientific problems that aqueous-based energy storage devices are difficult to obtain with high energy and power density simultaneously,multi-scale material structure design from atom to microstructure and electrochemical performance optimization are carried out:A new and common method for modifying high specific capacitance carbon electrode materials is developed;A high-redox-active dual-ions aqueous electrolyte is found;A new hybrid energy storage system with battery-type electrode materials and capacitive electrode materials is designed;A new hybrid energy storage system with high specific energy,high specific power and high safety performance is achieved.The main research contents and innovative achievements are listed as follows:?1?A new and common method for modifying high specific capacitance carbon electrode materials is developed:Electrochemical oxidation?cyclic voltammetry method,constant voltage method,and constant current method,etc.?is conducted to modify carbon materials.The oxygen-containing functional groups and the pore structure are optimized under the premise that the conductivity is not affected.There are mainly three changes in the following aspects:?a?Increased oxygen content and the conversion of electrochemically inert CO₂-type oxygen-containing functional groups into electrochemically active CO-type oxygen-containing functional groups;?b?Increased hydrophilicity,which is more conducive to energy storage in aqueous electrolytes;?c?Enlarged and connected pores structure due to the reaction of electrochemical oxidation stripping,contributing to the entry and diffusion of electrlyte.Based on the above synergy,the specific capacitance of carbon materials is increased from 172 F/g to320 F/g,increased 86%,and the energy density is increased from 6.0 Wh/kg to 11.2 Wh/kg with undecreased power density.The modified carbon materials also show excellent cycle stability,maintaining 96%of the original capacitance after 10,000 cycles?100%DOD?.The method has universal applicability and can be applied to carbon materials with low electrochemical activity such as graphite,carbon fiber,graphene and activated carbon,and greatly improves the electrochemical performance of these carbon materials.The in-situ electrochemical oxidation modification method is used to increase the energy density of the device by increasing the specific capacity of the carbon material.?2?A high-redox-active dual-ions aqueous electrolyte is found.Aiming at the problem of low energy storage capacity of aqueous electric double layer capacitors,a new idea of aqueous energy storage in which reversible redox reaction occurs at the solid-liquid interface of positive and negative electrodes to generate additional Faraday capacitance is designed.Through theoretical calculation and a large number of experimental verifications,A series of new high specific capacitance energy storage dual ion water electrolytes such as FeBr₃/Na₂SO₄,CuBr₂/HCl and BiBr₃/HCl are found,which greatly increase the specific capacitance.For example,0.05 M BiBr₃ is added to 1.5 M HCl electrolyte.The specific capacitance can be increased from 182 F/g to 1150 F/g,and the energy density is increased from 5.9 Wh/kg to 61.8Wh/kg at 2 A/g,surpassing most of the state-of-the-art work.?3?A new hybrid energy storage system with battery-type electrode materials and capacitive electrode materials is designed.Based on the above-mentioned in-situ electrochemical oxidation of high specific capacitance carbon materials,a new hybrid energy system with high specific energy and high specific power is constructed,in which high specific energy metal element?Cu,Pb,etc.?are preferred as anodes.The new principle of electrochemical energy storage is revealed:In the electrochemical energy storage process,the energy storage of metal anode is through the dissolution and deposition process by redox reaction,owing high energy density.The energy storage of carbon materials cathode is through charge adsorption and Faradic redox reaction,owing high power density.Using lead negative electrode,1 M PbNO₃ aqueous electrolyte,the working voltage reaching 1.38 V,the specific energy is increased from 19.7 Wh/kg?symmetric carbon material supercapacitor with neutral electrolyte?to 82.9 Wh/kg.Using a copper negative electrode,the potential of copper is 0.34 V,and the theoretical specific capacity is 843 mAh/g,which can be used as the negative electrode of aqueous energy storage devices.The electrolyte is 1 M H₂SO₄+0.1 M CuSO₄.The energy density increased from 17 Wh/kg?symmetric carbon material supercapacitor with acid electrolyte?to 45 Wh/kg,without any attenuation after 10,000 cycles test.?4?A new hybrid energy storage system with high specific energy,high specific power and high safety performance is achieved.Based on the above metal anode hybrid energy storage system and high redox-active aqueous electrolyte,a new aqueous energy storage system is designed.Zinc has a potential of-0.76 V and a theoretical specific capacity of 820 mAh/g,energy storage by the dissolution-deposition process by redox reaction,which is an excellent negative electrode material.Zinc sulfate containing redox-active ions(Fe²⁺/Fe³⁺)as electrolyte,modified carbon fiber as a positive electrode energy,energy is stored by a redox dissolution-deposition process of iron ions on the surface of the carbon fiber.The energy storage of the positive electrode occurs at the electrode/electrolyte interface.There is no insertion/extraction process,no effect of volume expansion and contraction,comparing with the cathode material?NiO,MnO₂,etc.?of the zinc-ion battery,ensuring good cycle stability.At a current density of2 A/g,the specific capacitance reaches 352 mAh/g,the discharge platform is 1.11 V,the energy density is 391 Wh/kg,and the power density is 2.2 kW/kg at a current density of 10 A/g.The capacitance still maintains 92.3%after 5000 cyclic tests.