Since the late 1990s, research on olivine-type lithium iron phosphate (LiFePO4) cathode materials has attracted widespread attention from researchers. It is expected to become the preferred alternative to LiCoO2 cathode material for next-generation lithium-ion batteries, especially as a cathode material for power lithium-ion batteries.
The synthesis methods of olivine-type lithium iron phosphate include coprecipitation, solid-phase synthesis, hydrothermal/solvent synthesis, sol/gel synthesis, microwave synthesis, and other methods.
Solid-phase synthesis was the earliest method used for the synthesis of lithium iron phosphate. It typically uses lithium carbonate and lithium hydroxide as lithium sources, and a homogeneous mixture of organic iron salts such as ferrous acetate and ferrous oxalate, as well as ammonium dihydrogen phosphate, as starting materials. The mixture is synthesized at high temperature after pre-calcination and grinding.
The coprecipitation method for preparing ultrafine oxides has a long history. The specific process involves dissolving suitable raw materials, adding other compounds to precipitate the product, and then drying and calcining it to obtain the final product. Due to the uniform dispersion of the raw materials during dissolution, the precursors for coprecipitation can be synthesized at low temperatures. However, due to the inherent characteristics of the coprecipitation method, the precursor precipitation often occurs instantaneously, making it difficult to control the proportions of each element. After calcination, non-stoichiometric properties of the elements in the product may occur.
Microwave synthesis is a method for preparing ceramic materials that has been developed in recent years, and some researchers have already applied it to the preparation of lithium iron phosphate.
The sol-gel method is a relatively common and frequently used method. However, it is not commonly used to prepare LiFePO4, mainly because LiFePO4 has special requirements for the atmosphere during the synthesis process.
Hydrothermal synthesis refers to synthesis carried out at temperatures of 100-1000 degrees Celsius and pressures of 1 MPa-1 GPa using chemical reactions of substances in aqueous solutions. Under subcritical and supercritical hydrothermal conditions, the reactivity is enhanced because the reactions occur at the molecular level, allowing hydrothermal reactions to replace certain high-temperature solid-state reactions. Furthermore, since the homogeneous and heterogeneous nucleation mechanisms of hydrothermal reactions differ from the diffusion mechanisms of solid-state reactions, it is possible to create novel compounds and materials that cannot be prepared by other methods.
Hydrothermal synthesis is also a commonly used method for preparing lithium iron phosphate. Compared with high-temperature solid-phase methods, hydrothermal methods for LiFePO4 have advantages such as high product purity, uniform phase, good dispersibility, small particle size, and simple operation.
The solvothermal method is a development of hydrothermal reactions. This process is relatively simple and easy to control, and in a closed system, it can effectively prevent the volatilization of toxic substances and prepare air-sensitive precursors. Furthermore, the formation of phases, particle size, and morphology can be controlled, and the products exhibit good dispersibility.
Under solvothermal conditions, the properties of the solvent (density, viscosity, dispersion) interact and vary greatly, differing significantly from those under normal conditions. Consequently, the solubility, dispersion, and chemical reactivity of the reactants (usually solids) are greatly enhanced. This allows the reaction to occur at lower temperatures.
