Synthesis Of LiFePO₄/C Cathode Materials By Carbothermal Method Using Fe₂O₃as Raw Material

One of the world-concerned problems today is energy problem. With the development of the society, the demand of energy is growing fast. We need to explore new renewable and clean energy sources. In addition, the environmental problems caused by the coal and oil cannot be ignored.So, lithium ion secondary battery is a kind of clean energy which should be widely used.Olivine-type LiFePO4is gradually becoming one of the ideal cathode materials for lithium batteries due to its high theoretical capacity of170mAh/g, a flat discharge potential of3.4V and excellent thermal stability and chemical safety. Despite these obvious advantages, the biggest drawback is its low ionic and electronic conductivity. In order to overcome its instinct obstacles, many methods have been adopted such as doping with metal cations, coating carbon and reducing the particle size.In our paper,based on the traditional solid-state synthesis method, we used Fe2O3as iron source which is more inexpensive and stable and adopted sucrose as reducing and conductive agent to synthesize LiFeO4/C cathode materials through a carbothermal method. Firstly,we investigated on the morphology and crystalline phases of some different kinds of commercial Fe2O3.and then we attempted to synthesize a-Fe2O3through hydrothermal method and spray pyrolysis method. We found proper iron source for solid-state reaction through comparison.Common a-Fe2O3,nano α-Fe2O3and high purity a-Fe2O3are proved to be the proper iron sources. The primary particle size of a-Fe2O3with D5o=5.7μm, The primary particle size of high purity a-Fe2O3is around100-200nmwith D50=1.15μm。 hydrothermal method can attain a-Fe2O3around100-200nm, spray pyrolysis method can attain a-Fe2O3around several hundred nanometers. But both the methods are either too complicated or time concuming which are not suitable for the solid-state reaction.In the following research, we tried to do some tentative explorations on the preparation process. The effect of lithium source, iron source,sintering temperature, carbon content,calcining time on the electrochemical performance of LiFePO4/C had been studied.The preliminary result shows that LiFePO4/C synthesized at740℃for20h using nano-sized Fe2O3and LiH2PO4as raw materials shows good electrochemistry performance at low discharge rates. Meanwhile,we intended to shrink the sintering time due to the advantages of small particle size and large surface area of nanoscale Fe2O3and inhibit the growth and agglomeration of the particles in order to improve the performance at high rates.We took advantage of both nano-sized Fe2O3and high-purity Fe2O3to synthesize LiFePO4/C cathode materials by one-step fast carbothermal method. The results showed that the precursor using nano-sized Fe2O3as iron source which was directly calcined at740℃for5h under N2atmosphere could attain nano-structured LiFePO4(d<100nm). Its discharge capacity were156mAh/g at0.1C and130mAh/g at10C,respectively.In addition,The precursor using high-purity Fe2O3as iron source which was calcined at750℃for5h in a N2＋5%H2atmosphere also exhibited the discharge capacity of155mAh/g at0.1C and120mAh/g at10C.Both the two kinds of Fe2O3can synthesize pure olivine-type LiFePO4/C materials in a short time which show a perfect cycle performance at high discharge rates.Our method is convenient, environmentally friendly and energy-saving which is a appropriate way for large-scale industrialization.