Synthesis Of Lithium-ion Battery Cathode Material LiFePO₄ And Anode Material Li₄Ti₅O₁₂ From Natural Ilmenite

With the growing shortage of global energy and resources, the development of new energy and comprehensive utilization of mineral resources have become the focus of worldwide attention. In this paper, we propose a new idea to synthesize lithium-ion battery cathode material LiFePO₄ and anode material Li₄Ti₅O₁₂ from natural ilmenite （or waste）. Meanwhile, the physical and chemical processes and theories of the ilmenite metallurgy and materials preparation are studied in detail.The elements of ilmenite are selectively leached by mechanical activation and hydrochloric acid leaching at atmospheric pressure. The results show that mechanical activation can refine the grain size and increase the surface roughness of ilmenite particles, which result in the increase of the specific surface area. The mechanical activation can also disrupt the integrity of ilmenite crystal grains, and induce the formation of a large number of lattice defects, which lead to the lattice expansion. All the above actions could strengthen the leaching of ilmenite. Under the optimal conditions, the leaching ratio of Ti is only 1.07%, and Si is almost not leached, while the leaching ratios of Fe, Mg, Al, Mn and Ca are all more than 95.5%. As a result, Ti and Si are still in the slag, and the other elements are enriched in the lixivium. In addition, the synthetic rutile with the grade of over 90 wt.% is obtained by calcinating the above titanium-slag directly.For the first time, titanium is leached from the titanium-slag by using H₂O2 as coordination agent. Under the optimal conditions, the leaching ratio of Ti reaches 98.9%. A peroxo-titania compound is prepared by heating the coordination lixivium, but the compound contains a small amount of Si and its particle size is as large as 2～5μm. However, by adding the template agent NaOH before heating, the particle agglomeration is prevented and a nano-sized needlelike-spherical peroxo-titania compound is obtained, which does not contain Si. Subsequently, the linear （100～200 nm long） and rod-shaped （200～500 nm long,20～40 nm wide） TiO2 are obtained by calcinating the compound at 400～800℃, and the purity of TiO2 reaches 99.3%. Furthermore, when the template agent NaOH is substitued by LiOH, a nano-sized platelike peroxo-titania compound is obtained, and the Si is also removed. For the first time, peroxo-titanic compound is used as the precursor of Li₄Ti₅O₁₂ anode material. The results reveal that the Li₄Ti₅O₁₂ samples prepared from the needlelike-spherical and platelike peroxo-titania compounds exhibit excellent performance with the initial charge capacities of 158.5 and 161.6 mAh·g-1 at 0.1 C rate, and excellent rate performance and cycling performance.The initial precipitation pH values of the elements in ilmenite lixivium are separately calculated according to the solubility product principle. It is found that the initial precipitation pH values of Fe, Al and Ti are 0.318,0.728 and 0.784, while those of Mg, Mn and Ca are all greater than 3.4. It is the first time that FePO₄·2H₂O is prepared from the ilmenite lixivium. The results indicate that only small amounts of Al and Ti precipitate into FePO₄·2H₂O particles under the conditions of pH=2.0 and P/Fe=1.1, while the other elements, Mg, Mn and Ca still remain in the solution. Then, Ti-Al doped LiFePO₄ is prepared from the FePO₄·2H₂O precursor. The LiFePO₄ sample shows a well crystallized, single olivine-type phase, and its particle size is 50～500 nm. Electrochemical test results show that the sample exhibits the first discharge capacity of 151.3,140.1 and 122.9 mAh·g-1 at 1 C,2 C and 5 C rate, respectively, and a capacity retention of 99.2%,99.8%and 95.9% after 100 cycles.For the first time, the influences of Ti-Al co-doping on the structure and properties of LiFePO₄ are investigated, and the mechanism of Ti, Al and Ti-Al doping are specifically studied. XRD and Rietveld-refine results indicate that appropriate amounts of Ti, Al and Ti-Al doping do not obviously change the structure of LiFePO₄. When the doping level is low, Ti atoms tend to occupy Li sites and Al atoms prefer to occupy Fe sites, but at higher doping levels, Ti and Al atoms will occupy both Li and Fe sites, and impurity phases might be appear. SEM shows that small amounts of Ti or Al doping could limit the agglomeration of LiFePO₄ particles, but the excessive increase in the doping amount of Al will promote the particles aggregation. The Al/Ti ratio has little influence on the morphology of Ti-Al doped LiFePO₄ when the total doping content is constant （2 mol%）. HRTEM shows that all the LiFePO₄ samples are wrapped with amorphous nano-carbon films, and the particles are connected with nano-carbon nets. All the LiFePO₄ crystal lattices are clear but contain lattice defects which are caused by the cation doping. For the Al, Ti single doped LiFePO₄, electrode kinetics tests show that the lithium-ion diffusion coefficient and exchange current density first increase as the doping quantity rises, and then decrease. However, the kinetic parameters of Ti-Al doped LiFePO₄ samples vary little as the Al/Ti ratio changes when the total doping content is constant. Electrochemical tests indicate that appropriate amounts of Ti, Al and Ti-Al doping can significantly improve the electrochemical performance of LiFePO₄. For the Ti-Al dual doped samples, the electrochemical properties show little change with the variation of Al/Ti ratio （2 mol% total doping content）.For the first time, the FePO₄·2H₂O which contains small amounts of Ti, Al and Ca is prepared from the byproduct of titanium dioxide （FeSO₄·7H₂O waste slag）. Then the multi-metal doped LiFePO₄ is prepared with the as-obtained FePO₄-2H₂O as raw material. XRD and Rietveld-refine results indicate that the LiFePO₄ sample is well crystallized, single olivine-type phase, and multi-metal doping results in the formation of Li vacancies in crystal lattices. The sample exhibits a first discharge capacity of 161,145 and 112 mAh·g-1 at 0.1 C,1 C and 5 C rate, respectively, and shows excellent cycling performance. This method provides a new approach for hydrate-sulfuric titanium dioxide enterprises, which can deal with the large amount of FeSO₄-7H₂O residue.