| Cathode materials is one of the main contents for the lithium battery, and is also critical for the development of battery technology. Olivine lithium iron phosphate (LiFePO4) is considered as a promising cathode material for high power-density lithium ion battery due to its high capacity, long cycle life, environmental friendly, low cost, and safety consideration. However, the instinct drawbacks of olivine structure induce a poor rate performance, resulting from the low lithium ion diffusion rate and low electronic conductivity. At room temperature, bare LiFePO4 is an insulating with an electrical conductivity of about 10-9 to 10-10S·cm-1, which is much lower than those of LiCoO2 and LiMn2O4. The electrical conductivity and electrical conductivity of bare LiMnPO4 is lower than of LiFePO4. Carbon coating on the LiMPO4 (M=Mn, Fe) particles is one of the most important techniques to improve its electrochemical performance with respect to the specific capacity, rate performance, and cycling life. Carbon can not only increase the electronic conductivity on the surface of LiMPO4 particles, but also limit the growth of the particle and alleviates to aggregation of LiMPO4 particles. In addition, carbon can play the role of a reducing agent, avoiding the oxidation of Fe2+ to Fe3+ during sintering, and thus simplify the atmosphere requirement during synthesis. In recent years, more attentions are taken to the preparation of carbon coated cathode materials by using MOFs as precursors in situ pyrolysis method. Lots of composite materials have been prepared via MOFs as the precursors, exhibiting good electrochemical property. Nevertheless, such MOFs are high-cost and difficult to synthesize, which limits their potential application as cathode materials. Organic-inorganic layered compounds are another kind of potential precursors for preparing new cathode materials, but the corresponding researches are still few. Therefore, we focus on the synthesis of LiMPO4/C based on these compounds, and investigate their electrochemical properties as cathode materials. The main results are summarized as follows:1. LiFePO4/C was synthesized using the layered organic-inorganic iron phosphate FePO4-Diaminooctane as the precursor. The lithium sources was optimized to improve the electrochemical properties of LiFePO4/C. LiFePO4 nanoparticles embedded in a nanoporous carbon matrix was synthesized by the precursor and lithium nitrate. Its discharge capacity at 0.1 C and 10 C rates were 150 mAh·g-1 and 80 mAh·g-1, respectively. It also exhibits an excellent cycling stability with a capacity retention of 96.2% over 500 charge/discharge cycles at 1 C rate.2. LiFePO4/C was synthesized using the layered FePO4-Diaminohexane as the precursor. The 3D carbon network was not only coated on LiFePO4 particles, but also bridges the gaps among many different particles. Its first discharge capacity at 0.1 C rate were 147 mAh·g-1. Its discharge capacity at 10 C and 30 C rates were as highly as 110 mAh·g-1 and 90 mAh·g-1, respectively.3.LiMnPO4/C was synthesized using the layered organic-inorganic manganese phosphate as the precursor through in situ pyrolysis. In order to improve the electrochemical properties of LiMnPO4/C, we optimized the sintering temperature, sintering time, the sources of amine and lithium sources. Under the optimal conditions, LiMnPO4/C nano-structure material shows good electrochemical properties. It can deliver the capacity of 121 mAh·g-1 at C/20 rates. |
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