In a lithium-air battery, the positive part is the main region where all reactions take place. The discharge reaction occurs at the gas/solid/liquid three-phase interface of oxygen, positive electrode material and liquid electrolyte, resulting in the formation of solid-state reaction product lithium peroxide. The air electrode is not only a diffusion channel for gases, but also provides space for product storage, and is also a conductive carrier for electrons, so the selection of cathode materials is crucial. The performance of the air electrode directly affects the capacity, cost, life and so on of the lithium-air battery.
Solid Li2O2 is formed during the discharge of lithium-air batteries, and the greater the discharge capacity, the greater the amount of Li2O2 deposited. On the one hand, it will block the electrode surface and reduce the overall conductivity of the electrode. On the other hand, due to the poor electronic conductivity of crystalline Li2O2, it is difficult to degrade during the charging and oxidation process, resulting in an overpotential greater than 1V. The increase of the oxidation potential will aggravate the occurrence of side reactions in the battery system and deteriorate the overall performance of the battery. Based on the special discharge mechanism of lithium-air batteries, air electrode materials generally need to have good electrical conductivity and oxygen reduction characteristics. Most cathode materials have the characteristics of large specific surface area, porous (mesoporous), abundant active sites, and good spatial structure stability. Developing an ideal cathode catalyst to reduce the kinetics of the charging reaction is one of the most effective ways to reduce the high overpotential of charging and discharging and improve the battery cycle efficiency.
Cathode materials are mainly electrode materials with electrocatalytic activity, which can be mainly divided into the following categories.
(1) Porous carbon-based cathode materials. Carbon materials are currently the most widely used and studied cathode materials due to their high electrical conductivity, large specific surface area, porosity, light weight, low cost, strong oxygen adsorption and good oxygen reduction activity. Including: super conductive carbon black (SuperP), activated Carbon, ketjen black, various configurations of carbon nanotubes (single-arm/multi-walled CNTs), nanofibrils, array, graphene, gas diffusion layer (GDL), etc.
The pore size, volume, distribution, and defect sites of carbon-based materials directly affect the kinetics of ORR/OER, thereby affecting the electrochemical performance of batteries. However, the high charging overpotential (>3.5 V) due to oxidizing solid lithium peroxide will accelerate the decomposition of the carbon-based cathode material to generate Li2CO3, thereby reducing the cycle and safe life of the battery.
(2) Non-precious metals and metal compounds. The morphology of metal oxides is easy to control, and the morphology can be regulated by chemical or electrochemical synthesis methods according to the needs of the air electrode used. Metal oxides: Fe2O3, Fe3O4, CuO, CoFe2O4, ITO, SnO2; transition metal nickel, cobalt, manganese-based oxides: α-MnO2, Co3O4, NiCo2O4, γ-MnOOH, MnCo2O4, Ti4O7; TiC, TiSi, CuFe and other micro-nanostructured materials have good electrochemical activity, but metal element compounds have poor conductivity, so they are mostly used in combination with conductive carbon materials.
(3) Precious metals. Noble metal-based air electrodes, such as gold electrodes, have good catalytic activity and excellent electrical conductivity, and the pore structure is easy to design to provide storage space for discharge products, and its better stability, to a large extent inhibits the occurrence of side reactions. Precious metals as ORR/OER catalyst materials have very significant advantages. Typical noble metal catalysts are Au, Pb, Ru, etc. Among them, nanoporous gold (NPG), noble metal Pb, Ru modified carbon-based materials have particularly outstanding catalytic effects. Although noble metals have excellent electrocatalytic activity, they catalyze not only the main reaction but also many side reactions during battery cycling, such as the decomposition of electrolyte and the degradation of carbon-based materials. In addition, due to the expensive price of precious metals, there are practical limitations in application, and the properties of selective catalysis need to be further enhanced.