The cathode materials for lithium-ion batteries are lithium-containing transition metal oxides and phosphides such as LiCoO2 and LiFePO4, as well as conductive polymers such as polyacetylene, polyphenylene, polypyrrole, polythiophene, and active polysulfide compounds.
Lithium-intercalated compound cathode materials are an important component of lithium-ion batteries. Cathode materials account for a large proportion of lithium-ion batteries (the mass ratio of positive to negative electrode materials is 3:1 to 4:1), therefore, the performance of cathode materials will greatly affect the performance of the battery, and their cost will directly determine the cost of the battery.
1. 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 due to its advantages such as high operating voltage, stable charge and discharge voltage, suitability for high-current charge and discharge, high specific energy, and good cycle performance.
2. LiNiO2 cathode material
Ideally, 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 both price and availability.
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.
3. 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.
4. 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 cathode material for lithium-ion batteries.
LiFePO4 has an olivine-type structure with space group Pnmb. In this structure, the Fe3+/Fe2+ voltage relative to metallic lithium is 3.4V, with a theoretical specific capacity of 170 mAh/g. Furthermore, the oxidation of LiFePO4 to FePO4- during charging reduces its volume, which can compensate for the volume expansion of the carbon anode and improve the volume utilization rate of lithium-ion batteries. However, LiFePO4 has a relatively high resistivity and low electrode material utilization; therefore, research has primarily focused on addressing its conductivity issue.
5. Conductive polymer cathode materials
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.
