Britain to achieve continuous hydrothermal production of high-performance lithium iron phosphate

Lithium iron phosphate was first synthesized by the Goodenough team in the United States. Lithium iron phosphate material is a very environmentally friendly lithium-ion battery cathode material, which uses less raw material than the traditional LiCoO2 material and has much less environmental toxicity Thermal stability should be significantly higher than the other cathode materials, making the lithium iron phosphate material safety is significantly better than other materials. At present, the main technological route of synthesizing lithium iron phosphate in the industry is to synthesize iron phosphate by the co-precipitation method using soluble iron salt and phosphoric acid as raw materials, and then mixing the iron phosphate and the lithium carbonate by a ball mill process, and then granulating by spray and finally using High temperature solid + thermal reduction method combined lithium iron phosphate. The advantage of this method is that the technology is mature, the morphology of the product is easy to control, and the disadvantages are also obvious. For example, high energy consumption and difficulty in synthesizing lithium iron phosphate with a nano particle size. Lithium iron phosphate material due to the low conductivity of the electronic conductivity (10-9-10-8S · cm-1) and lithium ion conductivity (10-17-10-12cm2 · S-1) lower rate performance is poor, the current The solution is: 1) surface carbon coating; 2) particle nanocrystallization; 3) metal ion doping (such as Nb, V, Mg, etc.), but at present the current process is difficult to achieve particle nanocrystallization and metal ion doping Hydrothermal method is a good method for synthesizing nanomaterials. Because of its reaction characteristics, trace metals can be uniformly incorporated into materials, but the biggest challenge that traditional hydrothermal methods face Yield is too low, it is difficult to achieve industrial production. Recently, a continuous hydrothermal method for the synthesis of high-performance lithium iron phosphate materials was developed by Ian D. Johnson et al. At University College London. In order to improve the performance of lithium iron phosphate material under high rate discharge, Ian D. Johnson et al. Adopted the method of Nb element doping and the continuous hydrothermal method (CHFS) was used to synthesize high-performance Nb-doped lithium iron phosphate material . In this method, supercritical water is used to dissolve water-soluble metal salts, which greatly accelerates the reaction. The greatest advantage of this method is that it can synthesize a large amount of nanomaterials and can reach the number of kilograms per day under laboratory conditions level. The specific method Ian D. Johnson uses is as follows. The raw materials used in this method are mainly FeSO4 · 7H2O, LiOH · H2O, ammonium niobate oxalate hydrate, phosphoric acid and deionized water. All raw materials used need pre-degassing with nitrogen. The key point of the method is how to use the hydrothermal method to synthesize lithium iron phosphate material continuously. First, ammonium niobate oxalate hydrate and FeSO4 · 7H2O are dissolved in deionized water for a total of 0.25 mol, and 0.65 mol fructose is added, 0.375 mol of phosphoric acid was added to a 1/4-inch cup followed by the addition of 0.8625 mol of a LiOH.H2O aqueous solution which was then mixed in a confined jet mixer with a supercritical water jet at a temperature of 450 ° C and a pressure of 240 bar Critical water contact to obtain Nb-doped nano-LFP powder, followed by high-temperature treatment of the powder so that the surface of the organic carbonation occurred, thus completing the carbon coating process. EDS analysis shows that a small amount of Nb is uniformly distributed in the lithium iron phosphate, and the tiny primary particles in the material are agglomerated to form a semicircular secondary particle with a diameter of less than 100 nm and a rhombic secondary particle with a slightly larger diameter. Electrochemical experiments show that the lithium ion diffusion efficiency of the lithium iron phosphate materials with different Nb doping content is calculated by Randles-Sevcik formula because the introduction of Nb element significantly improves the lithium ion diffusion rate. When the Nb element doping The diffusion efficiencies were 1.0 ×, 1.5% and 2.0%, which were 2.0 × 10-10, 2.2 × 10-10 and 1.9 × 10-10cm2 · S-1, respectively, but both were higher than pure lithium iron phosphate Of 1.0 × 10 -10 cm 2 · S-1. It was also found in the rate performance test that the addition of Nb element significantly improves the rate performance of the material. For example, at a rate of 10 C, Nb-doped lithium iron phosphate can still obtain a specific capacity of 110 mAh / g, while pure lithium iron phosphate The material generally has a specific capacity of about 70-90 mAh / g, but when the amount of Nb doping exceeds 1%, the improvement effect becomes insignificant. Therefore, it is recommended that the amount of Nb be controlled at about 1%. The yield of this method under laboratory conditions can reach 0.25kg / h. Compared with the traditional hydrothermal method, the yield of the method has been greatly improved, and the continuous production has been realized, the utilization rate of the equipment has been increased, and the equipment has great advantages The potential for industrialization.