Research On Construction And Mechanism Of Cathode Materials For Li-O₂ Batteries In Complex Atmosphere

With the vigorous development of new-type energy conversion and storage technologies,rechargeable aprotic Li-air batteries have been attracting much more attention.Based on the reversible formation and decomposition of Li₂O₂ products on cathodes,Li-air batteries can provide the ultra-high theoretical specific energy density of 3500 Wh kg^-1,holding great application prospects for next-generation power storage devices.However,Li-air batteries are confronted with many issues and challenges,which are related to the intrinsic properties of battery components and external operating conditions closely.Since Li₂O₂ products are susceptible to H₂O and CO₂ in air and can easily react with them to form Li OH and Li₂CO₃ side products,most of studies on Li-air batteries are operated in dry oxygen atmosphere.Strictly speaking,these Li-air batteries should be referred as Li-O₂ ones.And Li-O₂ batteries have been fully investigated at present,which are faced with problems,including:（1）the limited practical discharge capacity attributed to the clogging electrode channels by Li₂O₂;（2）the high overpotential ascribed to the sluggish reaction kinetics;（3）the short cycling life span resulted from parasitic reactions of the corrosion of carbon materials and the decomposition of electrolytes;and（4）side reactions between air contaminants（H₂O and CO₂）and battery components.To our knowledge,the applications of efficient and stable cathode materials can be promsing to solve above problems effectively,thereby improving the comprehensive performance of Li-O₂batteries.Herein,researches on the construction of cathode materials are conducted with the introduction of solid or soluble catalysts and varieties of in-situ or ex-situ characterization methods are employed to explore the reaction mechanism of Li-O₂batteries under complex atmospheres.Details of this thesis are shown as follows:1.The two-dimensional MoN single crystal nanosheets are prepared to be used as cathode catalysts in Li-O₂ batteries.Due to the high-efficiency bifunctional electrocatalytic activity and good electronic conductivity of MoN,the large electroactive surface area from two-dimensional structures,and the protection from ultra-thin Mo O_x layer,Li-O₂ batteries assembled with MoN cathodes exhibit the enhanced discharge specific capacity of approximately 7400 m Ah g^-1,reduced discharge/charge overpotential of 0.19/0.72 V at the current density of 100 m Ag^-1 and good cycling stability over 79 cycles under fixed specific capacity of 1000 m Ah g^-1.2.The heme-based metal-organic framework nanozymes are synthesized to act as soluble catalysts in Li-O₂ batteries.During discharge/charge processes,the nanozymes can coordinate with superoxide intermediates,functioning as molecular shuttles of superoxide species and electrons between cathodes and products.Compared with pristine Li-O₂ batteries,the overpotentials of those batteries containing nanozymes are reduced about 270 m V and the specific discharge capacity increases to 6750 m Ah g^-1at the current density of 100 m A g^-1.What’s more,because of the suppression of superoxide-related side reactions,Li-O₂ batteries show stable cycling performance.The battery can run over 100 cycles under the limited specific capacity of 1000 m Ah g-¹.3.A self-supporting carbon-free air electrode is constructed to avoid the corrosion of carbon materials in Li-O₂ batteries fundamentally.The Ni foam covering nanostructured villous oxygen-deficient Ni O is fabricated first by the fast laser-scan technique to serve as a stable and activated current collector for Li-O₂ batteries.The introduction of oxygen vacancies plays an important role in improving the electronic conductivity and providing active sites for electrochemistry/chemistry reactions.Then,a small amount of Ru nanoparticles are deposited on the surface of modified current collectors to be employed as cathodes directly.Benefiting from the synergistic effect of oxygen-deficient Ni O and Ru catalysts,Li-O₂ batteries deliver the high discharge capacity,low charge potential and stable cycling performance.A specific discharge capacity of about 2500 m Ah g^-1 and charge potential plateau at 3.76 V are exhibited at the current density of 100 m A g^-1.Under the fixed specific capacity of 500 m Ah g^-1,the battery still holds a low charge plateau below 4.08 V even after 100 cycles.4.The superhydrophilic carbonaceous material of activated graphene is applied as the“H₂O self-trapping”cathode to investigate the influence of water on Li-O₂batteries.The introduction of H₂O promotes the formation of Li OH,which is beneficial to improving the discharge capacity,rate capability and energy efficiency of Li-O₂ batteries.At the large current density of 1 m A cm^-2(equivalent to 2 A g^-1),a specific discharge capacity of 2241 m Ah g^-1 and the ultra-low charge plateau at around 3.3 V are obtained.Ⅰn-situ Raman spectroscopy and online differential electrochemical mass spectrometry tests demonstrate that the ultra-low plateau is mainly ascribed to the oxidation of Li OH.Advantages of activated graphene,like the ultra-large specific surface area,high pore volume,narrow pore size distribution and abundant oxygen-containing functional groups,enable it to adsorb water molecules from air.5.A dinuclear Cu（Ⅰ）complex is introduced as the soluble catalyst in the Li-CO₂ cell to regulate the electrochemical behavior of carbon dioxide on cathode surface.During the discharge process,the Cu（Ⅰ）complex serves as a redox mediator,which can capture CO₂ to form the bridging Cu（ⅠⅠ）compound,thus to convert the direct reduction of CO₂into the reduction of the bridging Cu（ⅠⅠ）compound tactfully.As a result,the discharge products are amorphous Li₂C₂O₄.Besides,the Li-CO₂ cells show dramatically enhanced capacity and reduced discharge/charge overpotential,whose discharge plateau is above 3.0 V and the corresponding energy efficiency exceeds 80%.