The electrode can be divided into positive electrode and negative electrode, the positive electrode refers to the end with higher potential, and the negative electrode refers to the end with lower potential. Generally, the positive electrode is the cathode and a reduction reaction occurs when the electrons are obtained, while the negative electrode is the anode and an oxidation reaction occurs when electrons are lost. The electrode can be metallic or non-metallic, as long as it can exchange electrons with the electrolyte solution. In the electrochemical analysis, the electrode is a kind of sensor that converts solution concentration into an electrical signal. Electrode material refers to a substance that can be used as an electrode.
- Embedded cathode materials: Embedded cathode materials mean that metal ions can be reversibly embedded in the cathode materials. In the early days, embedded cathode materials represented by TiS2 and NbSe were widely studied. At present, the research and commercialization of embedded cathode materials are focused on layered compounds represented by LiCoO2, spinel compounds represented by Li2Mn2O4, and olivine compounds represented by LiFePO4.
Figure 1. Schematic diagram of crystal structure of (a) LiCoO2; (b) Li2Mn2O4; (c) LiFePO4
- Converted cathode material: The converted cathode material refers to an electrode material that undergoes a redox reaction with metal ions in the battery during the charge and discharge process, and its structure changes with the breaking and reorganization of the chemical bonds. The converted cathode mainly includes metal fluoride (MF) and metal chloride (MCl), which have suitable working voltage and high theoretical specific capacity.
- Lithium alloy anode material: Lithium can be alloyed with many metal elements such as silicon, germanium, tin and antimony to serve as the negative electrode of lithium batteries. The alloying of lithium can be realized in the process of battery charging, and this kind of alloy negative electrode has a high specific capacity, suitable working potential and good safety performance.
- Metal oxide anode materials: Various metal oxides have been widely used in the study of negative electrode materials because of their high reversible capacity. Based on the reaction mechanism, metal oxide negative electrodes can be divided into embedded metal oxide anodes, conversion metal oxide anodes and alloy metal oxide anodes. For example, transition metal oxides (such as TiO2, WO2) can store Li+ through embedding reactions, and these materials are attractive anode materials because of their low cost and non-toxicity.
Figure 2. Nanometer titanium dioxide
- Carbon anode material: Carbon material is one of the most widely used anode materials for batteries. Among them, graphite is still the preferred anode material for commercial lithium-ion batteries, and Li+ can be embedded and detached between graphene planes. Graphene has a unique two-dimensional monolayer carbon structure, ultra-high specific surface area, good electrical conductivity and voltage window. Besides, graphene has a strong lithium storage capacity, and Li+ can be stored not only on both sides of graphene sheets, but also on the edges, defects and covalent sites of graphene sheets.
Figure 3. Molecular structure of graphene.
- Obrovac, M. N.; Dunlap, R. A.(2001)., "Electrochemical displacement reaction of lithium with metal oxides." Journal of The Electrochemical Society. 148 (6), A576-A588.
- Park, C. M.; Kim, J. H.; Kim, H.(2010)., "Li-alloy based anode materials for Li secondary batteries." Chemical Society Reviews. 39 (8), 3115-3141.