Preparation And Modification Of Novel Lithiun Ion Battery Anode Material MgFe₂O₄

Lithium ion battery has the advantages of high specific energy, high open circuit voltage, long cycle life,low self-discharge rate, wide working temperature range, no memory effect and environment friendly. As anew generation green energy, it can be used in portable electronic products, electric vehicles and militaryfield etc. Anode materials, as one of the key materials, affect the performance of lithium ion battery greatly.At present, graphite carbon materials have been commercialized as anode materials. Although carbonmaterials has made great progress in cycling stability and security than metallic lithium, it still can notavoid the dangerous of lithium accumulation on the anode surface to form lithium dendrite, causingpotential safety problems because of the similar intercalation potential of carbon materials to lithium metal.On the other hand, during the first charge/discharge process, solid electrolyte interphase （Solid ElectrolyteInterface Film, agent SEI film） will form on the surface of carbon materials, causing relatively largeirreversible capacity loss, and increasing the impedance between electrode and electrolyte interface, whichmakes it difficult for the Li+intercalation and deintercalation. Therefore, development of novel anodematerials with good security, high specific capacity, long cycling life has become the focus at present.Because of the merits of cheap and environment friendly, binary and ternary oxide of iron such as Fe₂O₃,Fe₃O₄, FePO4, ZnFe₂O₄, CoFe₂O₄and MgFe₂O₄etc, have become a promising anode material. MgFe₂O₄,with a theoretical capacity of1072mAh/g, and higher voltage platform than that of carbon electrode, islikely to be used as anode material. But there are very few reports about that. The only several reports showthat its cyclic stability is very poor. So, in this work, sol-gel auto-combustion method was used tosynthesize MgFe₂O₄. Effects of carbon or metal oxide coating on the electrochemical performance of itwere discussed. The main research contents are as follows:1. A brief history of the development of lithium-ion battery and its components, features, requirementsfor anode material and its research situation were reviewed. Further more, methods for synthesis andcharacterization of MgFe₂O₄materials and the existing questions were analyzed. Based on the aboveanalysis, the content of this research was determined.2. The MgFe₂O₄nano-crystals were synthesized using sol-gel auto-combustion method, with lowmelting point nitrate as raw materials and citric acid as complex reagent. Effects of sintering temperature and time on the structure, morphology and electrochemical performance of MgFe₂O₄were discussed. Theoptimized experimental conditions are as follows: sintering at500℃for2h. Under this condition, theMgFe₂O₄material shows relatively higher crystallinity, more uniform morphology and betterelectrochemical performance. Further more, its electrochemical properties were investigated at differentactive material ratio. Results show that the electrode with a ratio of40:40:20, which is the ratio of activematerial: super-P carbon （SP）: polyvinylidene fluoride （PVDF）, presents relatively superior performancewith the initial discharge capacity of1123mAh·g^-1. And after50cycles, it still maintains at635mAh·g^-1,which is nearly double that of the other two electrodes with active material ratio of60:25:15and80:15:5.3. Using glucose, β-Cyclodextrin, organic glycine as carbon sources, carbon-coated MgFe₂O₄materialswas synthesized. Effects of carbon source and its dosage on the structure, morphology and electrochemicalperformance of MgFe₂O₄were discussed. Results show that the specific capacities of the three samplesreach1699mAh·g^-1,1282mAh·g^-1and1616mAh·g^-1（0.1C） at25oC, respectively. And after50cycles, itstill maintains742mAh·g^-1,745mAh·g^-1and915mAh·g^-1, higher than the un-coated one. In summary,selection of glycine as a carbon source,15%coated MgFe₂O₄/C material for the synthesis of performancehas been improved.4. Metal oxides, TiO₂or Al₂O₃coated MgFe₂O₄was synthesized. Effects of oxide and its dosage on thestructure, morphology and electrochemical performance of MgFe₂O₄were discussed. Results show that thespecific capacities of the two samples reach1068mAh·g^-1and911mAh·g^-1（0.1C） at25oC, respectively.And after50cycles, it still maintains633mAh·g^-1and491mAh·g^-1respectively. In summary, the capacityhasn’t been improved for the0.5%Al₂O₃coated MgFe₂O₄materials, but its cycling stability was inhancedto some extent.