Study On Process Of Li⁺ Insert Into LiFePO₄ In Aqueous Electrolyte

LiFePO4 has neat and orderly olivine-type structure, is orthogonal crystal system, spatial point group is Pnmb, is a slightly distorted close-packed structure of six-dimensional. FeO6 octahedral and PO4 tetrahedra constitute to the space frame of crystsla. P occupies the tetrahedral position, Fe and Li are filled in the gap of the octahedral, Li has a non-dimensional mobility, it can reversibily insert/extract during charge-discharge process. During chage process, Li+ embedded from the center of oxygen atom octahedral, and vice versa during discharge process.In this paper, use scanning cyclic voltammetry(CV) method to study insert/extract Li+ process of LiFePO4 in LiNO3 aqueous solution, the scanning curve is a complete,closed curves which have redoxation/oxidation peaks, the peaks are sharp, indicate that LiFePO4 has good electrochemical reversibility in aqueous solution during insertion and extraction process. After several cycles, the capacity decrease.In this paper, use several different kinds of conductive materials as collector, such as stainless steel wire meshes,Pt; use different aqueous solutions,such as LiNO3,Li2SO4 aqueous solutions, put LiFePO4 in aqueous solutions that have Li+ for cyclic voltammetry test, experiments show that different conductive materials as collector and different aqueous solutions have slight affect to insert/extract Li+ process of LiFePO4 in LiNO3 aqueous solution. These studies confirm that LiFePO4 is a material to insert Li+ in aqueous solution.Put LiFePO4 into LiNO3 solution for potentiostatic Li+ insertion, then put LiFePO4 into deionized water for potentiostatic Li+ extraction, use UV spectrophotometry method to determinat the absorption of Li+, contrast to standard concentration(g/L)~absorbanc curve, according to equation A = abc,calculate out the concentration of Li+, according to maximum insertion Li+ capacity in LiFePO4, calculate the insertion Li+ efficiency of the LiFePO4 in the first time. The efficiency of first insertion Li+ is 35.30%, the insertion Li+ capacity at first time is12.80mg/g. the insertion Li+ efficiency is 25.50% in the second time. The resort shows that LiFePO4 is a material to insert Li+ in aqueous solution. However, it is inefficient.Repeat these operations mentioned above, insert/extract Li+ for several times to one same LiFePO4 material, the insertion capacity is sharply decrease after several insert/extract Li+ process. May be on one hand, prolonged immersion in the aqueous solution, LiFePO4 dissolve in aqueous solution, on the other hand, LiFePO4 and collector contacts is not tightly enough, it is difficult to charge transfer.Use LiFePO4 to insert Li+ form aqueous solution with low concentration of Li+, released the embedded Li+ into deionized water, released into the same deionized water during every insert/extract Li+ process. Collect Li+ form low concentration aqueous to become high concentration aqueous which has Li+.In this thesis, use UV spectrophotometry to determinate the concentration of Li+ in aqueous solution. Adding 1.0mL 0.02% thoron as the chromogenic agent;adding 7.0mL acetone as chromogenic medium; adding 20.00% KOH 0.2mL,in order to adjust to high strong alkaline aqueous; adding deionized water to the volume of 10.0mL, test wavelengthλis 486nm, complex time is 40min, then test the absorption of Li+.