Design,construction And Performance Of Porous Cathode For Lithium-air Batteries With High Areal Capacity

With the development and progress of human society,the limited energy density of traditional lithium-ion batteries（LIBs）has been unable to meet the energy requirements of advanced energy storage equipment.Lithium-air batteries（LABs）have become strong candidates for next-generation rechargeable secondary batteries due to their extremely high theoretical energy density(3500 Wh kg^-1).However,some key issues still need to be overcome in the commercialization of lithium-air batteries,and the air cathode with high areal capacity and low charge voltage is one of the most important challenges.The fundamental reason lies in the slow kinetics of oxygen reduction reaction（ORR）and oxygen evolution reaction（OER）（the conversion reaction of O₂ and Li₂O₂）,which will lead to low energy density,poor cycle reversibility,and limited cycle life of batteries and so on.The above problems require related research on the synthesis and preparation of cathode catalyst materials and the design and optimization of cathode structures.Although considerable progress has been made in the development of cathode catalysts,most of the studies reported so far have been carried out at limited active material loading(≤1 mg cm^-2).Air cathodes with low loading can achieve high specific capacity values,but the areal capacities of these batteries are far from meeting the needs of practical applications.To address the aforementioned issues,starting from low-cost transition metal-based catalysts,this paper proposes a catalyst design strategy from single transition metal oxide to heterogeneous composite oxides using various material preparation techniques such as nanotechnology,self-assembly,and composite constrution.This strategy has achieved improved electrochemical performance such as energy conversion efficiency and cycle life of lithium-air batteries.Furthermore,starting with the optimization of the microstructure of the air cathode,a porous structure design approach has been proposed with adjustment strategies from random pores to ordered pores,which enabled the construction of multi-level gas,electrolyte,and ion transfer channels and discharge product storage sites.By promoting the transmission and diffusion dynamics of electrons,lithium ions,and gases within the electrode,the aim of enhancing the cycle stability and cycle life of lithium-air batteries in real external air environments has been achieved.The main research work is as follows:（1）By adjusting the ratio of alloy composition and corrosion conditions,porous micro/nano structure Mn O nanoflowers with high specific surface area were prepared by chemical dealloying technology,which gave full play to the kinetics advantages of ORR and OER of small-sized nanomaterials.This Mn O was used as the catalyst for lithium-air batteries,and economical and environmental-friendly glucose was added as water-soluble porogen during the preparation of the air electrode.Then the micro/meso/macroporous integrated hierarchically porous electrode was obtained by physical de-template method.The multi-level porous structure of the electrode was analyzed by SEM,BET,MIP,Micro-CT and other characterization techniques.The nano-scale pores in the range of 10 to 30 nm and the micron-scale macropores concentrated in the 5-10μm range can give full play to the advantages of different scale electrode pores,which can not only improve the reaction kinetics of ORR and OER,but also promote the rapid transport of substances in the electrode.The existence of hierarchical pores can fully expose enough reactive sites to solve the three-phase reaction interface problem.Thanks to this hierarchical porous structured electrode design strategy,the lithium-air battery using this electrode can cycle for 2000 hours at a high limited capacity of 3.6 and 6 m Ah cm^-2（relative humidity:RH=10-20%）,which is superior to all the previously reported research results of high areal capacity lithium-air batteries.（2）Heterostructure catalysts can usually combine the advantages of the two components in terms of adsorption energy and catalytic reaction activation energy to obtain better performance than single-component catalysts.We designed Fe and Mn composite oxide heterogeneous materials,combining the respective advantages of the two transition metals to achieve synergistic catalysis.The Fe₃O₄@MnO₂nanocomposites were prepared by a one-step in situ synthesis method,and with the synergistic effect of dual transition metals（Fe and Mn）,the lithium-air battery catalyzed by Fe₃O₄@MnO₂ could not only significantly improve the cycling performance of the battery（stable cycle 254 times）,but also effectively reduce the charge/discharge overpotential of the battery during cycle（the first cycle discharge and charge overpotentials were 0.12 and 0.22 V,respectively）.These results suggest that this material can be used as an excellent ORR/OER bifunctional novel catalyst for lithium-air batteries.Furthermore,the issue of ion transport kinetics is a major challenge in designing high areal capacity lithium-air batteries.By introducing a low-temperature decomposing ammonium bicarbonate（NH₄HCO₃）porogen,a temperature gradient from the bottom of the electrode upward is generated in a unidirectional heating mode,and the decomposition of the porogen produces an oriented pore structure perpendicular to the electrode plane.It provides a fast transmission channel for lithium ions and O₂,and provides sufficient storage space for discharge products.Due to these improvements,lithium-air batteries achieve a high discharge capacity of23.01 m Ah cm^-2 in open real air.lithium-air batteries with vertical channel electrodes can also cycle stably for more than 1600 hours（relative humidity:RH=30-50%）and maintain a relatively stable charge/discharge plateau when the limited areal capacity is6 m Ah cm^-2,exhibiting excellent electrochemical performance.