Preparation Of LiFePO₄as Lithium-ion Battery Cathode Materials

Abstract:Since Goodenough discovered the ithium-ion insertion/desertion properties of lithium iron phosphate(LFP) in1997, LFP has attracted much attention and recognition as lithium ion battery (LIBs) cathode materials. The theoretical capacity of lithium iron phosphate battery is about170mAh·g-1, and the charge/discharge platforms is about3.45V (vs. Li+/Li), supplying a stable output potential and high specific energy density. The resources to produce LFP are abundant in nature which is without harmful and toxic effects to the environment, and fit for large-scale application and low production cost. Although the lithium resource is short nowadays, the abundance in earth is large. After researched for many years, the LFP have great potentials to be used to replace LiCoO2for LIBs cathode materials as practical applications, especially for power battery of electric vehicles. In this thesis, the modified chemical lithiation route to prepare LFP (named solid lithiation method) as cathode material was proposed. This method is high efficient and low cost compared to other methods. The main contents of the thesis are as followings:(1) Using liquid precipitation method, hydrate iron phosphate was prepared from FeCl3and (NH4)2HPO4, then LiI was used as reducing agent and lithium resource to obtain amorphous hydrate lithium iron phosphate under300℃. Rising the temperature to above400℃for2h, the amorphous hydrate lithium iron phosphate began to lost crystal water and crystallize to olivine-type LFP. In this work, the reaction mechanism and the experimental conditons were investigated. The crystallized product was gained under700℃calcination. The results showed that the specific capacity of the product was140.1mAh·g-1, reaching82.4%of the theoretical specific capacity with a discharge platform at3.43V. At the high charge rate of10C, the specific capacity was83.1mAh·g-1, reaching48.8%of the theoratical specific capacity.(2) LFP/C composite products were prepared through two steps. Firstly, amorphous hydrate lithium iron phosphate was prepared under 300℃, then above amorphous intermediates was coated by carbon through decompositing of glucose under300℃. the final product was crystallized under700℃calcination. This as-prepared products showed the specific capacity at0.1C of159.4mAh·g-1, reaching82.4%of the theoretical specific capacity with a discharge platform at3.41V, the difference of platform voltage is about40mV. At the10C charge rate the specific capacity is117.6mAh·g-1, reaching69.2%of the theoretical specific capacity, which is41.5%higher than pure LFP.(3) Lithium iron phosphate/graphene (LFP/G) composites were prepared via introducing graphene to modify the LFP particles. First, using co-precipitation method to make graphene oxider and hydrate iron phosphate composites, then the composites were reduced by Lil to gain the LFP/G composites, which calcined under700℃. The experimental results showed that the graphene is tightly adhered on the surface of crystallized LFP particles. The particle size of LFP is ca.1μm, the size of graphene is1～3×1～3μm2with2-3pieces of sheet structure. This LFP/G composites showed its specific capacity at0.1C of163.2mAh·g-1, reaching96%of the theoretical specific capacity with a discharge platform at3.45V, the difference of platform voltage is about40mV. At10C charge rate the specific capacity is63mAh·g-1.