The positive electrode materials of lithium-ion batteries are lithium-containing transition metal oxides and phosphides such as LiCoO2 and LiFePO4, and conductive polymers such as polyacetylene, polyphenylene, polypyrrole, polythiophene, and active polysulfide compounds. Lithium-intercalated compound positive electrode materials are an important component of lithium-ion batteries. Since positive electrode materials constitute a large proportion of lithium-ion batteries, their performance significantly affects battery performance, and their cost directly determines the overall battery cost.
Currently, research on cathode materials mainly focuses on electrode materials such as lithium cobalt oxide and lithium nickel oxide. At the same time, the rise of some new cathode materials (including conductive polymer cathode materials) has also injected new vitality into the development of lithium-ion battery cathode materials. Finding and developing new systems of lithium-ion battery cathode materials with high voltage, high specific capacity and good cycle performance is an important research topic in this field.
LiCoO2 cathode material
LiCoO2 exists in three phases: layered LiCoO2 of the α-NaFeO2 type, spinel-structured LT-LiCoO2, and rock salt phase LiCoO2. In layered LiCoO2, oxygen atoms adopt a distorted cubic close-packed sequence, with cobalt and lithium occupying octahedral positions (3a) and (3b) in the cubic close-packed structure, respectively. In spinel-structured LiCoO2, oxygen atoms are arranged in an ideal cubic close-packed arrangement, with 25% cobalt atoms in the lithium layer and 25% lithium atoms in the cobalt layer. In the rock salt phase lattice, Li+ and Co3+ are randomly arranged, making it impossible to clearly distinguish between the lithium and cobalt layers.
Currently, layered LiCoO2 is widely used in lithium-ion batteries. It boasts advantages such as high operating voltage, stable charge/discharge voltage, suitability for high-current charge/discharge, high specific energy, and good cycle performance. Lithium ions move in two dimensions within the strongly bonded CoO2 layers, resulting in high lithium-ion conductivity and a diffusion coefficient of 10⁻⁹ to 10⁻⁷ cm²·s⁻¹. Its theoretical capacity is 274 mAh·g⁻¹, while its actual specific capacity is around 140 mAh·g⁻¹. Due to its simple manufacturing process and stable electrochemical performance, it was the first cathode material to be commercialized.
LiNiO2 cathode material
Ideal LiNiO2 crystals possess an α-NaFeO2-type layered structure similar to LiCoO2. The theoretical capacity of LiNiO2 is 275 mAh/g, while actual capacities have reached 190-210 mAh/g. Compared to LiCoO2, LiNiO2 has advantages in price and reserves. However, many problems remain in the practical production and application of LiNiO2. Therefore, extensive research has been conducted on the synthesis methods and doping modification of LiNiO2.
The difficulties in synthesizing LiNiO2, structural phase transitions, and poor thermal stability are all rooted in its intrinsic structure. Elemental doping of LiNiO2 to improve its structure is an effective means to enhance its specific capacity, cycle performance, and stability.
Li-Mn-O cathode materials
Due to its abundant resources, low price, and non-toxic and pollution-free nature, manganese is considered the most promising cathode material for lithium-ion batteries. Li-Mn-O cathode materials exist in two types: spinel-type LiMn2O4 and layered LiMnO2.
Spinel-type LiMn2O4 has advantages such as good safety and ease of synthesis, making it one of the most studied cathode materials for lithium-ion batteries. However, LiMn2O4 exhibits the John-Teller effect, which easily leads to structural distortion during charge and discharge, causing rapid capacity decay, especially under high-temperature operating conditions.
LiFePO4 cathode material
LiFePO4 cathode material is a novel type of cathode material for lithium-ion batteries. Due to the abundance, low price, and non-toxicity of iron resources, LiFePO4 is a promising lithium-ion battery cathode material. Its high energy density, low price, and excellent safety make it particularly suitable for power batteries. Its emergence represents a major breakthrough in lithium-ion battery materials and has become a hot research topic worldwide.
Conductive polymer cathode material
In lithium-ion batteries, in addition to metal oxides as positive electrode materials, conductive polymers can also be used as positive electrode materials.
Currently researched polymer cathode materials for lithium-ion batteries include polyacetylene, polyphenylene, polypyrrole, and polythiophene, which achieve electrochemical processes through the doping and dedoping of anions. However, these conductive polymers generally have low volumetric capacity density, and the reaction system requires a large electrolyte volume, making it difficult to obtain high energy density.
DMcT, as a cathode material for lithium-ion batteries, has advantages in specific energy, but its electrochemical oxidation-reduction rate at room temperature is relatively slow, so it cannot meet the requirements of high-current discharge of batteries.
