| As a new energy storage and energy conversion system, Lithium ion battery has been widely used in mobile phones, digital cameras and other portable electronic devices due to the fact that it has high open-circuit voltage, high energy density, no memory effect, low self-discharge phenomena, wide operating temperature range and so on. With increasing improvement of electrical performance and advances in assembly management and technology of battery, lithium-ion batteries, which have large specific capacity, small size and high energy density, has been used in electric vehicles, hybrid vehicles and other medium-sized energy storage devices. This trend has becoming a main stream. However, nowadays lithium-ion battery still does not reach the goal that obtaining a high energy density and meanwhile a high power density. Therefore, the researches for development of high-performance cathode materials are very necessary and urgent. LiMnPO4 is a new type of cathode material for lithium-ion battery. Based on its advantages, like non-toxic, resource-rich, high open-circuit voltage, high energy density and good thermal stability, LiMnPO4 has been regarded as one of the best candidates for the cathode materials of a new generation power battery. However, the conductivity and ion mobility rate of pure LiMnPO4 are not satisfactory, which leads to a poor capability. Conventional preparation technologies are complicated and do not benefit industrial production. To overcome these problems, the main objectives and outcomes of the researches in this paper are:(1) LiMnP04 was prepared using hydrothermal method and solvothermal method based on modification of crystal growth with surfactant and organic solvent. The effects of four types surfactants, which are citric acid, sodium dodecyl benzene sulfonate (SDBS), hexadecyl trimethyl ammonium bromide (CTAB), didodecyl dimethyl ammonium bromide (DDAB), on morphology of LiMnPO4 crystals were investigated. The results show that the modifications of citric acid, CTAB and DDAB can induce preferential growth in (010) crystal planes of LiMnPO4, but electrochemical performance of LiMnPO4 cathode materials is poor because of its larger particle size. The effects of the mixed solvents, such as ethanol/water, ethylene glycol-water and glycerol/water, on profile of LiMnPO4 crystals were investigated. The results show that ethylene glycol/water system can be used to prepare LiMnPO4 crystals with preferential growth in (010) planes. As a cathode material, this type LiMnPO4 has a superior discharge capacity at low rate, but the high-rate discharge capacity fades obviously and coulombic efficiency is relatively low.(2) LiMnPO4/C cathode materials were prepared by one-step solid state reaction assisted with ball milling. Investigation were carried out for the electrochemical performance of LiMnPO4/C cathode materials with MnO2, Mn2O3 and mixing manganese oxide (marked as M&M) respectively, while sucrose is as a carbon source. The results show that the crystallinity of M&M-LMP/C prepared with precursors M&M are high; the content of Mn2+ are relative large; the initial capacity is 153mAh/g at C/20 rate. M&M-LMP/C shows a good voltage platform at low rate. The discharge voltage platform at 1 C rate is still higher than 3.8 V (vs Li+/Li), and the specific discharge capacity is more than 120 mAh/g.Other things being equal, the sintering temperatures were carried out using M&M as the precursor. The results show that with increasing of sintering temperature, the particle size of LiMnPO4/C and the degree of crystallinity increase, while the content of carbon decreases. Increasing sintering temperature will strengthen the carbothermal reduction that is conducive for Mn4+ and Mn3+ to be reduced to Mn2+, to promote the electrochemical performance. For LiMnPO4/C cathode material synthesized at 600℃, due to its high crystallinity, small particle size and the appropriate distribution of conductive carbon, capacity retention of 94% is achieved over 200 cycles at at 1C.(3) With the method that porous Mn2O3 precursors with vacuum impregnation and in-situ coating assisted solid state reaction, LiMnPO4 crystals with preferential growth in (010) planes were prepared.The microstructures of the Mn2O3 nanoplates prepared with solvothermal method and solid phase pyrolysis method were investigated. The results show that Mn2O3 nanoplates prepared by solvothermal method have microporous structure, the specific surface area is 22.8 m2/g, the pore size shows a broad distribution; Mn2O3 prepared by solid phase pyrolysis method is nonporous.A comparative study of distribution of conductive carbon was carried out. L-Mn2O3-LMP/C was synthesized by porous Mn2O3 precursors with negative pressure immersion and in-situ coating. S-Mn2O3-LMP/C was synthesized by nonporous Mn2O3 with ball milling coating method. The results show that the three-dimensional continuous conductive carbons with negative pressure immersion and in-situ coating evenly distributed between the LiMnPO4/C nanoplates, the carbon content is approximately 8.16 wt%; conductive carbons with ball milling coating are maldistribution, the carbon content is 6.93 wt%. L-Mn2O3-LMP/C and S-Mn2O3-LMP/C are olivine structure. L-Mn2O3-LMP/C has a preferential growth of the (010) lattice plane, while S-Mn2O3-LMP/C’s has a preferential growth of the (100) lattice plane. The electrochemical results show that L-Mn2O3-LMP/C has an excellent rate capability and cycle stability, the discharge capacity is 157.3 mAh/g, 122.6 mAh/g and 105.8 mAh/g at C/20,1 C and 2 C, respectively. After 100 cycles at 1 C, about 99.3% of the capacity is maintained. S-Mn2O3-LMP/C cathode materials have a better low-rate performance, but with the increase of the discharge current, specific discharge capacity significantly decrease, capacity retention is less than 90% after 100 cycles at 1 C.Although LiMnP04 nanoplates with preferential growth of the (010) lattice plane were synthesized by hydrothermal method and solvent method, the cathode materials have poor cycling performance. LiMnPO4/C cathode materials with high crystallinity, small particle size, preferential growth of (010) lattice plane, uniform carbon coating, excellent electrochemical performance were prepared via the method that porous Mn2O3 precursors with vacuum impregnation and in-situ coating assisted solid state reaction. This process is favourable for scale-up production. |
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