LiMPO4 (M=Co, Ni) is a typical high-voltage olivine-like crystal structure material. Among them, LiCoPO4 has a discharge potential of 4.8V and LiNiPO4 has a discharge potential of 5.2V, and their theoretical capacities are close to 170mAh/g.
5.2V is currently the highest charge and discharge voltage. Because an electrolyte that can match it has not yet been developed, there are no reports on the performance of lithium batteries using LiNiPO4 cathode materials. There are more reports on LiCoPO4 materials, but the cycle charge and discharge performance of LiCoPO4 materials under the existing electrolyte system is very poor.
The olivine-like crystal structure exhibits three side effects under high voltage conditions: 1. The cathode material reacts with the electrolyte to form a solid-liquid interface layer; 2. The electrolyte partially dissolves Co ions, significantly deteriorating the cycle charge-discharge performance of lithium batteries; 3. Poor electrical conductivity. Therefore, it is necessary to employ three methods to improve the performance of the high-voltage olivine-like crystal structure.
1. Nanoparticles. Using nanoscale particles for active materials results in shorter lithium-ion and electron transport and diffusion paths compared to micron-scale particles.
2. Doping. Unlike spinel-like crystal structures, which can be doped with both cations and anions, olivine-like crystal structures are only doped with cations to improve the conductivity of the cathode material.
3. Coating. Amorphous carbon coatings can form interconnected high-speed electron transport channels, thereby improving performance, especially the initial discharge capacity and charge-discharge cycle performance.
Overall, LiMPO4 (M=Co, Ni) with an olivine-like crystal structure has a higher theoretical capacity than that with a spinel-like crystal structure. However, due to its low conductivity and poor cycle performance, the research and development of lithium-ion batteries using this cathode material has not been as fruitful as that of lithium-ion batteries with a spinel-like crystal structure. To enter the industrialization stage, more advanced performance improvement technologies are needed, which mainly refers to power lithium batteries for electric vehicles.
