Study On Synthesis And Modification Of Polyanion Fe-based Cathode Material For Lithium Ion Batteries

Polyanion iron-base cathode materials, LiFePO4 and Li2FeSiO4 are seen as highly promising candidates for large-scale Li-ion battery applications due to the merits of abundant raw materials, environmental friendliness, excellent cyclic stability, and high safety. However, the intrinsic crystal structure of polyanion iron-base cathode materials results in poor conductivity and electrochemical inertness, which seriously hinder their application in large scale. In order to improve the electrochemical performance, the serial research work is focused on synthesis methods and modification technique of iron-base cathode materials.In-situ carbon-coated LiFePO4 cathode material was prepared by ball-milling activation and subsequent solid state reaction, with LiH2PO4 and FeC2O4·2H2O as starting materials, and polyvinyl alcohol (PVA) as carbon source. The influences of synthetic conditions on the crystal structure, physical property and electrochemical performance of LiFePO4/C composites were investigated in detail. And the structure of carbon in the composites was studied by Raman spectrum, which indicated the sintering temperature played the significant role on the graphitization degree of carbon pyorlyzed from PVA. Under the optimum conditions, the residual pyorlyzed carbon in situ formed a well conductive nano-film on the surface of LiFePO4 grain, and the obtained LiFePO4/C cathode delivered reversible discharge specific capacity of 155.4 mAh/g at 0.1C rate and 137.6 mAh/g at 1C rate, with tap density of 0.87 g/cm3. On the basis of carbon coating modification, Mn-ion-doping LiFe0.99Mn0.01P04/C composites were prepared with lower carbon content, displaying good cyclic capability and higher reversible discharge specific capacity of 158.9 mAh/g at 0.1C rate and 140.1 mAh/g at 1C rate and 130 mAh/g at 2C rate, as well as higher tap density of 1.02 g/cm3.Homogeneous distribution of [Fe3(PO4)2·8H2O+Li3PO4] precursor with the high reacting activity was prepared by mechano-chemical liquid phase activation technique, with LiH2PO4 and reduction iron powder as starting materials, and PVA as carbon source. The reaction course between LiH2PO4 and reduction iron powder under mechanical activation is analyzed. PVA pyrolyzed during the formation of LiFePO4 synchronously, as a result, carbon-coated LiFePO4 composites were synthesized in situ by one-step solid state reaction of as-prepared precursor. The optimum results showed that, uniform in-situ carbon coating (5nm or so) was formed on the surface of LiFePO4 crystalline. Consequently, well-crystallized LiFePO4/C composites with homogeneous fine particle size were obtained, which had the discharge specific capacity of 156.8 mAh/g at 0.1C rate and 140.7 mAh/g at 1C rate and 130.2 mAh/g at 2C rate, and tap density of 1.12 g/cm3. In order to further enhance the electrochemical performance of LiFePO4/C, LiFe0.99M0.01P04/C (M=Mn2+,V3+,Si4+) composites cathode were prepared by carbon coating combination with Fe-site doping, then the rate performance and cyclic stability of LiFePO4 was improved significantly. It was found that the sample LiFe0.99Si0.01P04/C displayed 137.1 mAh/g at 3C discharge rate and maintained 136 mAh/g after 40th cycles. The modification effect of aliovalent doping on Fe site was discussed from the viewpoint of defect chemistry. The dopant substituting on the Fe sites can create lithium vacancies in olivine LiFePO4 lattice by charge-compensation mechanism. The existences of lithium vacancy may expand Li+diffusion channels in the structure, weaken the bound of oxygen to lithium, and increase lithium mobility. And the analyses of electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) also indicated that aliovalent doping on Fe site improved conduction property of LiFePO4, reduced charge transfer impedance and Li ion diffusion resistance in the electrode process, increased phase transformation kinetics during cycling, and enhanced reversibility of LiFePO4 electrode.Pure LiFePO4 materials with different morphology were prepared by mechanochemical-activation-assisted polyol synthesis processing, using different raw materials, and a low temperature approach for efficient preparation of LiFePO4 cathode was explored. LiFePO4 rods with good crystallization were synthesized in boiling tetraethyleneglycol (TEG) medium by re fluxing processing, using rod-shaped [Fe3(PO4)2·8H2O +Li3PO4] as precursor. In order to improve the electrical conductivity, carbon coating modification on the pure LiFePO4 was carried out with PVA as carbon source. The prepared LiFePO4/C composite delivered discharge capacity of 139.8 mAh/g at 0.1 C rate and 129.5 mAh/g at 2C rate. Highly crystalline LiFePO4 ultrafine particles were synthesized by a polyol reduction processing without post annealing, with TEG acting as solvent and reductant, FePO4 and Li2CO3 as the starting material. Carbon-coating LiFePO4 composites were prepared by calcining the mixture of as-prepared pure LiFePO4 and PVA. And the obtained composites delivered discharge capacity of 157.3 mAh/g at 0.1C rate and 136.2 mAh/g at 2C rate, displaying good rate capability and cyclic stability.Microwave heating technique was introduced into the synthesis of Li2FeSi04/C cathode materials. Li2FeSiO4/C cathode materials were synthesized by microwave solid-state processing, with Li2CO3 FeC2O4·2H2O,and nano-SiO2 as the starting materials, PVA and super-P carbon as the carbon sources. Under the selective microwave synthesis system, super-P carbon powder and pyrolyzed amorphous carbon not only effectively provided the high temperature to induce the complete reaction but also formed the conductive network to enhance the electronic conductivity. The prepared Li2FeSiO4/C composite had discharge capacity of 124.2 mAh/g at 0.05C rate and 102.3 mAh/g at 0.5C rate at 60℃. To improve preparation route of precursor, the homogeneous distribution of FeOOH/SiO2 was prepared from FeCl24H2O and Na2SiO3"9H2O by low-heating solid-state reaction. Then Li2FeSiO4/C composites were prepared by microwave carbothermal reduction method, with PVA and super-P carbon as reductants. Under optimum conditions, highly pure Li2FeSiO4/C material with uniform and fine particle size is obtained, showing discharge capacity of 129.6 mAh/g at 0.05 C rate and 107.5 mAh/g at 0.5Crate at 60℃.