Synthesis, Structure And Performance Of LiFePO₄/Li_1.3Al _0.3Ti_1.7(PO₄)₃ /C

Olivine lithium iron phosphate (LiFePO₄) is inexpensive, nontoxic and environmentally benign. Moreover, it has high theoretical specific capacity, excellent safety properties and cycling performances, which provides advantages for this cathode material in the application in the power sources of electric vehicles. However, there are two drawbacks preventing LiFePO₄ to be put into commercially used. One is its low electronic and ionic conductivity, which leads to the poor rate capability. The second problem is the low tap-density, which results in a low volumetric specific capacity. Tremendous efforts have so far been devoted to improve the electronic conductivity of LiFePO₄ and tap-density .several effective ways have been proposed. However, the problem concerning the ionic conductivity of this material remains to be solved. In this paper, two methods (one-step method and two-step method) were used to increase the ionic conductivity of lithium iron phosphate lithium by doped ion conductor Li_1.3Al_0.3Ti_1.7(PO₄)₃. Because Preparation conditions has important implications for performance and structures of materials, this article through a series of synthetic conditions such as calcination temperature, ionic conductor doping amont and sintering time were used to to improve the properties of the material. The main results and conclusions were listed as following:(1) Studied the influences of the calcining temperature, ionic conductors Li_1.3Al_0.3Ti_1.7(PO₄) doping amont and the calcine time on the performance and structure of the LiFePO₄ prepared by one-step method, and studied the material properties and structure prepared under optimal conditions by one-step method.The results showed that with increasing of the calcination temperature, ionic conductors Li_1.3Al_0.3Ti_1.7(PO₄) doping amont or calcination time increases, the specific discharge capacity increases gradually then reduces; When the calcining temperature was 650°C, ionic conductors Li_1.3Al_0.3Ti_1.7(PO₄) doping amont 3% and calcination time 25h, the material discharge capacity was biggest, 143.3mAh/g; Under this condition, the obtained material was complete Crystal structure, mainly assumed spherical or spherical pellet ,particle size about 60nm,a reunion phenomenon; the discharge capacity was 143.3 mAh/g and the 122.7 mAh/g at 0.1C and 1.0C, respectively. After 25 cycles, the composite cathode retains 96.8 % of the first cycle discharge capacity at 1.0C.(2) Studied the influences of the calcining temperature, ionic conductors Li_1.3Al_0.3Ti_1.7(PO₄) doping amont and the calcine time on the performance and structure of the LiFePO₄ prepared by two-step method, and studied the material properties and structure prepared under optimal conditions by two-step method.The results showed that with increasing of the calcination temperature, ionic conductors Li_1.3Al_0.3Ti_1.7(PO₄) doping amont or calcination time increases, the specific discharge capacity increases gradually then reduces; When the calcining temperature was 650°C, ionic conductors Li_1.3Al_0.3Ti_1.7(PO₄) doping amont 2% and calcination time 25h, the material discharge capacity was biggest, 157.7mAh/g; Under this condition, the obtained material was complete Crystal structure, mainly assumed spherical or spherical pellet ,particle size about 50nm,a not reunion phenomenon;the discharge capacity was 157.7 mAh/g and the 137.9 mAh/g at 0.1C and 1.0C, respectively. After 25 cycles, the composite cathode retains 97.9 % of the first cycle discharge capacity at 1.0C.(3) Studied the difference of the structure and properties of the samples by the one-step method and two-step method under optimal conditions. The results show that no significant difference between crystal structure , infrared spectra and morphology of samples prepared by one-step method and two-step method were detected, but the samples prepared by two-step no agglomeration.The discharge capacity of samples prepared by two-step method at 0.1C and 1C increased by 10% and 12.4% in samples prepared by one-step method ,respectively. the discharge platform 0.02V and 0.2V,respectively; After 25 cycles,capacity retention of samples prepared by two-step method at 1C and 2C increased by 1.1 and 0.8 percentage points in samples prepared by one-step method ,respectively.