1. Basic concept of lithium-air battery
Lithium-air batteries, known as “breathable batteries”, have a theoretical specific energy of nearly 11,700W·h/kg (excluding the weight of oxygen in the air), which is comparable in energy density to gasoline.
More importantly, the oxygen in the cathode reaction can theoretically come from the air, and no substances harmful to the environment will be produced in the whole process, which is a completely green process with zero pollution.
Generally, the structure of a lithium-air battery is mainly divided into a metal lithium negative electrode, an electrolyte (non-aqueous proton electrolyte, aqueous electrolyte, mixed electrolyte of water and non-aqueous, all-solid electrolyte), a separator, and an air electrode. The schematic diagram is shown in Figure 1. In the work of studying air batteries, in order to avoid the influence of pollutants (water, carbon dioxide, etc.) in the air on the battery, it is usually used to work in an oxygen environment. Therefore, there is no essential difference between lithium-air batteries, air electrodes and oxygen electrodes (except for special indications). The following contents are collectively referred to as lithium-air batteries.
Working principle (take the organic electrolyte system as an example): During discharge, the metal lithium negative electrode loses electrons and becomes Li+, and Li+ reaches the air positive electrode through diffusion. At this time, oxygen in the air is dissolved in the electrolyte, and O2 gets electrons at the positive electrode to undergo a reduction reaction, and combines with Li+ in the electrolyte to form a discharge product Li2O2. During charging, Li2O2 is oxidized and decomposed to release O2, and Li+ is reduced at the negative electrode to form metallic Li. The electrode reaction expression is as follows:
Positive electrode: Li↔Li++e–
Negative electrode: 2Li++2e–+O2↔Li2O2
Overall response: 2Li+O2↔Li2O2
The above reaction is reversible under certain conditions, which can realize the cyclic charge-discharge of lithium-air battery.
However, due to the influence of solvents, supporting electrolytes, catalysts, additives, electrode potentials, etc., the charge and discharge of lithium-air batteries will be regulated by the equivalent reaction mechanism of surface phase and solution.
2. Separators, polymer binders and current collectors for lithium-air batteries
The widely used lithium-air battery separators are mainly composed of polyethylene (PE), polypropylene (PP), polyurethane (PU), glass fiber separator (GF) and so on. PE separators are commonly used battery separators with high porosity, while PU separators have low porosity. PU can isolate the contact of metal lithium with water and oxygen, and alleviate the occurrence of side reactions to a certain extent. The development of separators with selective permeability is of great significance for stabilizing lithium metal anodes.
(2) Polymer binder
The widely used binders for the preparation of air electrodes in lithium-air batteries mainly include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyethylene (PE), polypropylene (PP), polyethylene oxide (PEO), etc. The main function of the binder is to make the positive electrode active material contact the conductive agent or current collector more closely and prevent it from falling off.
In addition, the above-mentioned organic binder widely used in the electrode preparation process will affect the overall conductivity of the electrode, thereby reducing the electron transport ability. At the same time, the binder will be degraded during the battery reaction process, resulting in the destruction of the overall structure of the electrode, increasing the contact resistance of the electrode active material, electrolyte and oxygen, which directly affects the electrochemical performance of the battery. To this end, the development of non-binder electrode materials is full of potential and challenges.
(3) Current collector
The current collector mostly uses metal electronic conductors, and the load electrode material is in close contact with it, which is an indispensable part of the assembly of rigid and flexible energy storage devices. At present, the widely used air electrode current collectors include metal mesh (metal stainless steel mesh, aluminum mesh, nickel mesh, copper mesh, etc.), metal foam current collectors (foamed copper, foamed nickel, etc.), and non-metallic current collectors (carbon paper), etc.