Research On Key Issues Of Cathodes And New Strategies For High Specific Energy Li-O₂/Li-CO₂ Battery

High-energy-density,safety,stability,and sustainability rechargeable secondary batteries have been a long-term goal in the field of electrochemical energy storage devices and renewable energy.Among the numerous energy storage systems,lithium-oxygen（Li-O₂）and lithium-carbon dioxide（Li-CO₂）batteries are considered as the most promising next-generation new secondary batteries due to their ultra-high energy density.However,there are still many key scientific problems that hindered the realization of large-scale production and application.Air cathodes where the redox reactions occur are particularly prominent among these issues.A series of problems such as large charge-discharge polarization,low energy efficiency,poor rate performance,and the short cycle life would be caused by sluggish reaction kinetics on the cathode.To effectively improve the battery reaction kinetics,developing and designing high-performance air cathodes and novel field-assisted strategies are considered effective strategies.Based on the above problems,this thesis mainly carried out the following research work:1.From the perspective of the development of air cathodes to improve the reaction kinetics.（1）An all-biomass-derived three-dimensional hierarchical porous air cathode was designed and synthesized.Inspired by the structural diversity in nature,a three-dimensional porous conductive carbon substrate（SC）was prepared by a high-temperature solid-phase method using sugarcane with abundant vertical microchannels as precursors.Subsequently,the air cathode Egg-SC with heteroatom doping was obtained by a one-pot method using eggs rich in nitrogen,phosphorus,iron,and other elements as the doping source.The Egg-SC cathode was assembled into a Li-O₂ battery,and its electrochemical performance was investigated in terms of the polarization,rate,and cycling of the battery.The battery exhibits a specific capacity as high as 8.07 m Ah cm^-2,excellent rate performance,and cycle life of up to294 cycles.At the same time,the deposition and decomposition process of discharge products were deeply studied,and the results showed that the Egg-SC cathode could effectively regulate the deposition and decomposition of discharge products.（2）A highly stable bio-inspired air cathode was firstly designed and synthesized.First,a conductive carbon substrate（Sponge）with a porous fibrous network was obtained by a high-temperature solid-phase method using the natural sponge rich in functional fibrous.Subsequently,highly stable titanium dioxide（TiO₂）arrays were grown in situ on the Sponge by a seed-assisted method to protect Sponge,resulting in a three-dimensional self-supporting porous air cathode TiO₂/Sponge.The TiO₂/Sponge was assembled into a Li-CO₂ battery and its electrochemical performance were investigated.The battery exhibits greatly improved specific capacity and significantly reduced polarization potential,and can be stably cycled for up to 1600 hours under capacity-limiting cycling conditions.In addition,through the structural design of the air cathode,the nanosheet-like discharge products can be rapidly and reversibly formed and decomposed on the surface of the TiO₂ array.2.Developing novel field-assisted strategies to improve the reaction kinetics.（1）The design of traditional air cathodes and catalysts can reduce the overpotential of Li-O₂/Li-CO₂ batteries to a certain extent,but its energy conversion efficiency is still difficult to exceed 85%.Thus,using the energy of other natural fields to compensate for the energy required in the battery reaction process was taken into consideration,thereby improving the energy conversion efficiency.A flexible photo-air cathode TiO₂/CC was firstly constructed by in situ growth of TiO₂ arrays with good optoelectronic properties on carbon cloth（CC）via a seed-assisted method.Benefiting from the semiconducting properties of TiO₂,a light-assisted flexible Li-CO₂ battery was constructed based on TiO₂/CC cathode and investigated its photoelectrochemical performance.Under illumination,the Li-CO₂ battery achieves an ultra-high energy conversion efficiency of 97.9%.For the discharge process,a large amount of photogenerated electricity is concentrated on the surface of the cathode as catalytic active sites,which effectively reduces the potential barrier required for CO₂ reduction.More importantly,the photogenerated holes with strong oxidative properties can assist the decomposition of discharge products,enabling the charging process to be carried out at extremely low potentials.（2）For the first time,a novel non-contact and environmentally friendly external magnetic field tuning method was introduced to suppress the recombination of photogenerated carriers.First,dense and uniform nickel oxide（NiO）nanosheets were grown in situ on the surface of foamed nickel（FNi）by a one-step hydrothermal method to prepare a multifunctional air cathode NiO/FNi with magnetic and semiconducting characteristics.Using the NiO/FNi air cathode,we constructed a magnetic/optical multi-field coupling-assisted Li-O₂ battery for the first time and investigated its electrochemical performance.The battery exhibits an ultra-low charge potential of 2.73 V and an ultra-high energy conversion efficiency of 96.7%.It found that the photogenerated electrons and holes in the NiO/FNi air cathode are subject to the Lorentz force in the magnetic field and move in opposite directions,thus realizing the prolongation of the separation time of photogenerated electrons and holes.In this thesis,based on the key scientific issues of Li-O₂/Li-CO₂ batteries,we propose novel solutions from two perspectives in developing air cathodes and novel field-assisted strategies.The electrochemical performance,reaction kinetics,and regulation of the deposition and decomposition of discharge products were studied by various testing methods,which provided new research ideas for the development of Li-O₂/Li-CO₂ batteries.