Mechanism of Low-Temperature Solid-State Reaction
Most solid-state reactions are difficult to proceed at low temperatures. However, certain low-melting molecular solids, hydrated inorganic compounds, and many organic materials can form solid complexes that undergo low-temperature solid-state reaction even at room temperature or 0°C. Research shows that the presence of crystallization water in these compounds does not change the reaction direction or limit but helps lower the reaction temperature and accelerate the reaction rate.
Particle size is generally less dependent on grinding time but is influenced by reaction conditions and preparation methods. Conversely, particle shape is closely related to the duration of grinding. Thoroughly grinding reactants into fine, uniform powders increases the surface area dramatically, which is crucial for shortening reaction time and promoting reaction occurrence in low-temperature solid-state reaction.
Studies on the reaction mechanism of low-temperature solid-state reaction have proposed four main stages: diffusion, reaction, nucleation, and growth, each potentially determining the overall reaction rate. In the solid state, trace crystallization water provides microenvironments that accelerate reactions, allowing particles to collide and nucleate quickly. However, slow diffusion of ions through various phases, particularly the product phase, limits crystal growth. According to crystallography principles, fast nucleation with slow crystal growth produces small grains, while slower nucleation with faster growth yields larger grains. This explains why low-temperature solid-state reaction often results in fine-grained products.
When synthesizing lithium cobalt oxide (Li-CoO₂) cathode materials using low-temperature solid-state reaction, reactants often contain crystallization water. The presence of trace water reduces reaction temperature, accelerates the reaction, and promotes fine particle formation. Adequate grinding under low-temperature conditions ensures uniform mixing of reactants at the microscopic level, shortens lithium ion diffusion distance during calcination, and results in particles with uniform size distribution.