Dehydration is a lithium-ion battery materials eternal topic, both positive and negative material production, or electrode production process have to face the problem of dehydration. FePO4 precursor material is both LiFePO4 material, but also can be used alone as the positive electrode material, so the problem of dehydration FePO4 material that we can not avoid the problem.
Generally iron phosphate dehydration process is divided into two parts: the first one, mainly to take off some of the material in the free water which is very easy to remove, the temperature is lower.
Second, the crystallization of the material off the water, the water molecules with the iron phosphate materials chemically combined manner, it requires a higher activation energy - that is, higher temperatures, complete removal of the water to this part, but the study reaction kinetics of the process is not a lot.
Preparation of iron phosphate commonly used ferric sulfate or other soluble ferric iron source, phosphoric acid or phosphate as the phosphorus source, NaOH as PH regulator, by co-precipitation methods.
Actual production is generally controlled between 1.6-2.0 PH, PH is too high when it may precipitate Fe (OH) 3 impurities, and PH value is too low will cause the precipitation of Fe3 + incomplete. The precipitate is filtered and washed after high temperature sintering is required, this process is mainly for two purposes, first off FePO4˙2H20 material in the water, and secondly to make FePO4 crystal material fully developed, in order to ensure a complete crystalline material type.
TG was found in the range of 50-223 ℃, FePO4˙2H20 material appeared in 20.23% of weight loss, which is mainly FePO4˙2H20 material two crystal water is removed, and then as the temperature rises, FePO4 material does not continue to appear weightless, so the dehydration process is mainly done in this process.
In 736 ℃ appeared an exothermic peak, and there was no loss of quality, which indicates that at this temperature, the material FePO4 crystalline transition occurs subsequent XRD diffraction analysis also found that, at 700 ℃ under synthetic material FePO4 diffraction peak is wide, part of the characteristic peak does not appear, at this temperature synthetic crystalline material FePO4 incomplete development of poor crystallinity.
The temperature was raised to 800 ℃, all characteristic peaks were appeared, but the characteristic peak intensity is still low, wide, indicating at the same temperature, crystal growth is still not complete, when the firing temperature is raised to 900 ℃, can pay attention to this time appears not only all the peaks, a characteristic peak of hexagonal (206) / (302) has also been completely separated, indicating FePO4 crystalline material well-developed.
At 900 ℃, FePO4 prepared material belonging to the hexagonal lattice parameters a = 0.50330nm, b = 0.50330nm, c = 1.12470nm, having α- quartz structure which is conducive to the lithium ion material embedded into FePO4 .
Kinetic studies on the dehydration FePO4 not much, FePO4 material dehydration mechanism and kinetics study has important implications for the development of the production process iron phosphate material.
The use of TG-DTG-DTA thermal analysis to study the mechanism and kinetics of dehydration FePO4˙4H20 materials, the study found FePO4˙4H20 material at 200 ℃, there were two DTG DTA exothermic peak and peak rate of weight loss, dehydration process is a two-step reaction, calculations show that the reaction is a D4-Fn-step reaction in which the activation energy of the reaction is D4 79.62KJ / mol, activation energy of the reaction Fn 103.04KJ / mol.
The study found no effect on the heating rate in the dehydration reaction mass, so long as the heating temperature reaches the appropriate temperature can be sufficiently removed FePO4 water. This has important implications for the sintering process of iron phosphate.