| Olivine-type phosphate LiMPO4(M=Mn,Fe) have been proven to be one of most promising cathode materials with their advantages in low cost, environmental friendliness, high safety and reliability as well as excellent cycle performance. The electrochemical performance of LiMPO4has been significantly improved via carbon coating and particle nanosizing. However, the electrochemical properties of LiFePO4at low temperature remain to enhance, the discharge capacity and rate performance of LiMPO4cannot meet the demands for commercial application. In addition, there is still no unified description on the lithiate/dilithiate mechanism of LiMPO4when charged/discharged. In order to analyze the effects of morphology and microstructure of materials on the electrochemical properties and the intrinsic factors that limit the electrochemical properties of LiMPO4, the diffusion behavior of Li in the LiFeO4is studied in the charge/discharge process. And the impacts of nano particles on the electrochemical performance are also thoroughly discussed. The high performance LiMPO4nanoparticls are synthesized by ion-exchang process, which make full use of the crystal differences between NH4MPO4·H2O and LiMPO4. The LiFePO4/C samples with excellent properties are prepared using ethyne as "gaseous carbon source" and the advantages of "gaseous carbon source" in the solid reaction are clarified.1The diffusion behavior of Li in LiFeO4is thoroughly studied by an ex-situ analysis method. We find the "stack type" lithiate/delithiate mechanism along the b axis of olivine structure during charge/discharge process and then build the lithiate/delithiate model of LiFePO4. In this work, an ex-situ test method is designed, which can be applied to study the distribution of Li in the electrode materials. Combined with electrochemical lithiate/delithiate methode and electron energy loss spectroscopy, the distributions of Li in the LiFePO4with different charge states are studied and we find that lithium inserts in the materials or extracts from the materials along b axis direction as the "stack way". It means that the diffusion of Li-ion is characterized by the "first in-last out, last in-first out", while the phase boundary of T/H phase always migrate along the b axis as the’outside-in’way. The equations for electrode potential in the electrolyte and relations of electric current and overpotential (i-η) are built. Based on these relations, we systematically analyze the changes of overpotential, phase boundary of T/H and the composition of phases during the charge/discharge process, and put forward a full description of the diffusion behavior of Li in the T/H phase. In addition, the effects of morphology and microstructure of LiFePO4on the electrochemical properties are clearly clarified by the full use of charge/discharge model and i-η relations.2A new "from up to down" method to synthesize the LiMP04nano particles is designed and developed via ion-exchange reaction, which relies on the crystal similarity and differences between NH4MPO4·H2O and LiMPO4. The foundation of the ion-exchange reaction is the crystal similarity between the two kind of compounds and the precursor is tore into nano particles by the substantial interior stress produced by the differences between two crystal structures. LiFePO4and LiMnPO4nano particles are synthesized by the optimized ion exchange reaction. LiMPO4nano particles inherit the lattice orientation relationship of their former bodies and display order arrangement along the a plane of olivine-type. As a result, there are a large quantities of space among nano particles. Carbon coated LiFePO4/c samples show excellent electrochemical performance, whose reversible capacities reach161mAh/g and128mAh/g at0.1C and10C.3C2H2is adopted as gaseous carbon source and carbon coated LiFePO4/C is prepared by the solid reaction. The roles that gaseous carbon source plays in the solid reaction are analyzed. We find that gaseous carbon source can reach the interior of precursor and then carbon is precipitated when Fe3+is reduced during the synthesis of LiFePO4, resulting in uniform and complete carbon coatings. This process can effectively inhibit the overgrowth of LiFePO4particles. The size of LiFePO4particles prepared by the C2H2gaseous carbon source is about100nm, and can be well coated by carbon. The porous LiFePO4particles aggregated from LiFePO4nanoparticles exhibits high reversible capacity at different rate. And the excellent electrochemical properties of LiFePO4particles can also be characterized by their high discharge capacity, which can remain at143mAh/g after1000cycle at1C. |
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