Synthesis Of LiFePO₄/C Cathode Material By Sol-gel Method And Its Modification Research

The Li Fe PO₄ could be a cathode material due to its peculiar olivine crystal structure which give a condition of Li+ insertion/extraction. It has many advantages : a wide range of raw material source and very cheap,a high working voltage, safe and non-toxic, a big theoretical specific capacity, green and good thermal stability. The Li Fe PO₄ has been preliminary commercialized and has a nice prospects as a cathode of battery. But, also because the structure and composition of Li Fe PO₄ itself,which cause a low electrical conductivity and a bad lithium ion diffusion capacity between the crystal particles. These limits its application prospects, there are many means to improve the electrochemical performance of Li Fe PO₄ as the battery anode materials, doping modification was the most commonly used and effective method.The effect of different doping amount of graphene and the doping at different stage for the property of Li Fe PO₄ was concretely researched. Then, at the base of optimized doping amount and stage of graphenen,adding Mg2+,and through a series of tests to determine the effect of Li Fe PO₄ with the mix doping modification. At last,we conducted a Exploratory experiment: choose several kinds of cheaper metal ion from rare earth metal elements as the dopant,and tested the performance of Li Fe PO₄.In this work, we aimed at further optimize the specific capacity of Li Fe PO₄, improve the cycle and ratio performance and so on, to promote the commercial development of Li-ion battery, and the mix of doping provides a new thought and method for the modification of Li Fe PO₄. The main research results and the content is as follows:At the first section of the paper, the four kinds of lanthanum rare earth metal elements Tm2O3, Sm2O3,Ce O2 and Gd2O3 used as the dopant,adding to the Li Fe PO₄/C whit the doping amount of 1wt%, 2wt%, 3wt%, 5wt%respectively. The results showed that:the Li Fe PO₄/C whit doping 1wt%Gd3+,1wt%Ce4+ and 2wt%Ce4+ will raise the electrochemical performance, the specific capacity of Li Fe PO₄ / C effect of doping 1wt%Gd3+ was the best, it could reach 135.7 m Ah/g at 0.1C and 85.5 m Ah/g at 10 C, after the 50 times charge and discharge cycles the specific capacity can keep 93.3% of the initial specific capacity, and has good crystal structure and microstructure.Adding different amount of graphene at the stage of sol,first bali-milling and second ball-milling,respectively. The crystalline structures, morphologies, compositions, tap densities and electrochemical performances of the Li Fe PO₄/C were investigated with XRD, SEM, Raman, tapping apparatus and battery tester. The result showed that : the specific capacity increased with the doping amount of graphene,but the tap density decreased with the doping amount of graphene; the increment of the specific capacity and rate performance is the largest when doping amount between 1.5wt% and 2wt% and the best adding stage was the stage of second ball-milling. At the stage of second ball-milling,the specific capacity of Li Fe PO₄/C with 2wt% graphene was 151.3m Ah/g at 0.1C and 122.4m Ah/g at 10 C,after 50 charge-discharge cycles at 0.1C the attenuation of specific capacity was only 2.13%, the tap density was 1.17 g/cm3 and the capacitance per unit volume could reach up to 177.0 m Ah/cm3; the specific capacity of Li Fe PO₄/C with 5wt% graphene was 159.6m Ah/g, but after 50 charge-discharge cycles at 0.1C the attenuation of specific capacity was 6.23%, the tap density was 0.94 g/cm3 and the capacitance per unit volume was only 150.0m Ah/cm3. From the effectiveness, economic and practical, tap density,etc,the Li Fe PO₄/C which added 2wt% graphene at he stage of second ball-milling was the finest. On the basis of the finest,adding different amount of Mg2+ at the stage of sol.Finally, the result showed that the optimized doping amount was 2wt%, the specific capacity of Li Fe PO₄/C with 2wt% graphene and 2wt% Mg2+ was 162.7m Ah/g at 0.1C and 128.2m Ah/g at 10 C,after 50 charge-discharge cycles at 0.1C the attenuation of specific capacity was 2.23%, the tap density was 1.20g/cm3 and the capacitance per unit volume could reach up to 195.2 m Ah/cm3.