Synthesis Of Cathode Material LiFePO₄ Of Lithium Ion Batteries By Sol-Gel Method

LiFePO₄ is considered as a promising cathode material for lithium-ion batteries due to its merits such as low cost, environmental friendliness and good cycling capability. In this paper, the research and development on LiFePO₄ as a cathode material are reviewed systematically, and the aspects that prevent the real application of LiFePO₄ as a cathode material for lithium-ion batteries are also pointed out. In the aims of developing an available technique to prepare LiFePO₄, Sol-Gel method was adopted. And the effects of fabrication parameters including sintering atmospheres, precursor concentrations, iron source precursors and chelating agents on the microstructure and electrochemical properties of LiFePO₄ were investigated. The microstructure and phase compositions of the LiFePO₄ materials were studied by SEM and XRD, etc. The mechanism of the effect of iron precursors and chelating agents on the electrochemical properties of LiFePO₄ was also discussed.The results showed that LiFePO₄ could not be successfully synthesized at a sintering atmosphere of nitrogen only. However, when an additive of 5vol.%H2 mixed into the nitrogen atmosphere, the product was composed of a major phase of LiFePO₄ and a minor phase of Fe₂P. A small amount of hydrogen in the sintering atmosphere could successfully prevent the formation of Fe³+ compounds, being a key factor on the synthesis of pure LiFePO₄.In the condition of the Fe(NO₃)3-9H2O being used as iron source and ethylene glycol as chelating agent, the effects of hydrogen content in the sintering atmosphere on the microstructure and electrochemical properties of LiFePO₄ were studied. The results showed that Fe2P formed in N2+5¹5%H2 atmospheres. Further, the content of Fe2P increased with increasing H2 content, and excessive Fe2P led to decreasing of the discharge capacity. The suitable amount of Fe2P in LiFePO4 products, which was of 3.29wt.%, was obtained in N2+10%H2 atmosphere, and the corresponding discharge capacity was 75mAh/g at a discharge rate of 0.1C. Furthermore, under this atmosphere, a suitable precursors concentration of 2/3M was also obtained in an examining concentration ranging 1/3-1M, for getting a relative high discharge capacity, which was of 110mAh at a rate of 0.01 C. Low precursors concentration led the agglomeration of LiFePO4 particles, thus lowered the discharge capacity.Fe2P also formed when FeC2O4H2O was used as iron source and ethylene glycol as chelating agent under a sintering atmosphere containing small amount of H2.However, Fe2P disappeared when the H2 content increased to 15vol.%, the product was only composed of LiFePC^, and a discharge capacity of 145mAh/g was reached at 0.1 C rate. In this condition, the suitable precursors concentration for getting relative high discharge capacity was also 2/3M, and low precursors concentration also worsened the discharge capacity of LiFePO,*.Ferric compound and ferrous compound caused different results on the microstructure and electrochemical properties of the LiFePO4 products when they were used as iron source. In the case of ethylene glycol being used as chelating agent, the carbon contents of LiFePO4product were 3.3wt.% and 6.4wt.%, respectively, for ferric compound and ferrous compound, the corresponding discharge capacities were 145mAh/g and 75mAh/g at rate of 0.1C. In the case of citric acid was used as chelating agent, the carbon contents of LiFePO4 product were 6.0wt.% and 10.1wt.%, respectively, for ferric compound and ferrous compound, the corresponding discharge capacities were 120mAh/g and lOOmAh/g at rate of 0.1 C. The results showed that LiFePCU fabricated from ferrous source exhibited higher discharge capacity than that from ferric source.Additionally, the influences of stability of complexing compounds on the electrochemical properties, carbon contents and the microstructure of LiFePCU products were discussed by the theory of coordination chemistry. The stability of ferrous complex compounds was low so that they were easily decomposed, favoring the crystallization of LiFePC^ particles and low carbon content, which favors electrochemical capacities of active material. Vice versa, ferric complex compounds were difficult to decompose due to their high stability, network structure with high carbon content frequently presented in those materials, preventing diffusion of lithium ion and thus worsening the specific capacities.