The Preparation And Electrochemical Properties Of LiFePO₄ And Co-doped LiMn₂O₄

The cathode material R&D is a key part for the development of high-powered lithium ion batteries. Olivine-type LiFePO₄ offers benefits such as a quite large theoretical capacity, flat charge/discharge curve, good structural and thermal stability, excellent cycle performance, inexpensive cost as well as environmental benignity. Recently, LiFePO₄ has been deep studied as a promising replacement for LiCoO₂. However, one of the main problems is that it suffers from poor electronic conductivity and low Li+ ions diffusion across the two-phase boundary between LiFePO₄ and FePO₄, therefore, its electrochemical properties and commercial applications are limited. In this dissertation, we tried to improve the electrochemical properties focusing on the study of synthesis routes, surface modifacition and Ti(â…£) ion doping. Moreover, the pseudocapacitance of LiFePO₄ and LiMn_2-xCo_xO₄ (0≤x≤0.4) in some sulfate aqueous eletrolytes was investigated. Generally, they are as follows:In the first chapter, the current survey on the lithium ion batteries and supercapacitors cathode materials, especially the LiFePO₄ material was detailly introduced. Meantime, the meaning of this dissertation and some scientific problems needed to be solved were brought forward.In the second chapter, considering the structural similatity between the reactant and object-product, FePO₄ was used as a raw marerial to prepare LiFePO₄/C. Comparison for the two carbon sources between acetylene black and sucrose, the results indicated that the latter can more uniformly coat on the surface or distribute between the particles of LiFePO₄, which played a vital role in controlling the size of particles, increasing the electronic conductivity and improving the electrochemical properties.In the third chapter, LiFePO₄/C cathode material was synthesized by a simple microwave-assisted process and further calcined treatment , which was a timesaving and efficient method. During the synthesizing process, NH4FePO₄·H₂O, which has a similar crystal structure with LiFePO₄, was utilized as iron(â…¡) and phosphate sources simultaneously. The results showed that further calcined treatment can improved the electrochemical performance of the microwave synthesis product.Ti(â…£)-doped LiFePO₄/C was prepared by the above synthesis process. Charge/discharge tests indicated that the reversible capacity, cycling performance and rate capability of the product were obviously improved when the doping content was 0.02. According to the principle of solid-state chemistry, one of the improvement may be attributed to the occupancy of Ti(â…£) ions in Li+ ions sites to form Ti...Li impurity defects and V,Li lithium vacancy defects. The defects can improve the kinetics of Li+ ions diffusion and increase the conductivity of the material.In the fourth chapter, LiFePO₄/C composite had been prepared by using tri-n-butyl phosphate (TBP) as a multi-function reactant. The results indicated that the product prepared at 600 oC for 15 h had an average size of 100 nm with narrow distribution and regular global shape. The charge/discharge tests for this sample exhibited the first discharge capacity of up to 158 mAh/g at the current density of 0.1 mA cm^-2. After 100th cycle, the specific discharge capacity was 153 mAh g^-1 with only the loss of 3.3%. The effects of TBP on synthesis process and possible reaction mechanism were detailly discussed.In the fifth chapter, we studied the pseudocapacitance performance of two type simulative supercapacitors, which used LiFePO₄ or spinel LiMn2-xCoxO4 (0≤x≤0.4) as cathode, active carbon as anode and sulfate aqueous solution as electrolyte. The effects of different electrolytes and current densities on the pseudocapacitance of the materials were studied. The results showed that in 1 mol/L Li₂SO₄, at low current density, LiFePO₄ exhibited obvious capacitance characteristic with good cycle performance, however, when the current density increased to 1 mA/cm², the polarization became very serious. LiMn_1.9Co_0.1O₄ had a higher capacity and more excellent rate capability than LiFePO₄ in 1 mol/L Li₂SO₄.The sixth chapter summarized the main conclusion of the thesis. Moreover, some new ideas and expectation of writer's were also included.