Developmentof New Fe-bases Materials for Lithium secondary battery

Abstract

LiFeO2 has many advantages against many layered cathode materials because it is nontoxic and contains the most abundant metal in the world. Many groups tried to develop various kinds of LiFeO2 forms with a good battery performance. However, almost LiFeO2 materials were obtained using a complex reaction and the cycle characteristics were unsatisfactory as a practical cathode material. Therefore, we report here a new orthorhombic LiFeO2 and LixFeyOz with improved cycling performance using a solid-state reaction at low temperature (150-200oC). LiFeO2 and LixFeyOz were synthesized sing LiOhH2O, r-FeOOH, and a-FeOOH by conventional solid-state method. The powder X-ray diffractioin using CuKa radiation was employed to identify the crystalline phase of the syntesized material. The cycling was performed at a current density of 0.1 mA/cm2 with a cut off voltage of 1.5-4.5 V. LiFeO2 was synthesized at low temperature using the solid state method. It was composed of orthorhombic LiFeO2 and small amount of spinel LiFe5O8 phases. A Li/LiFeO2 cells showed not only a fairly high initial discharge capacity of over 150 mAh/g but also a good cycle retention rate at low current density of 0.1mA/cm2. We first report that the orthorhombic LiFeO2 underwent a structural change to the spinel phase during the charge/discharge process, which resulted in the capacity decline during cycling. Lithium iron oxide were also synthesized by the solid state reaction at various temperatures (200-800oC). The lithium iron oxide obtained at 200oC showed an nano crystalline phase with three kinds of structure, LiFe5O8, Li5FeO4, and a trace cubic of a-LiFeO2. This material presented not only a high initial discharge capacity (215 mAh/g), but also an excellent cycling retention from the 11th to the 50th cycle (95%). On the other hand, the powder calcined at 800iC showed a well developed single a-LiFeO2 phase and it exhibited a very poor cycling performance below 5 mAh/g at the same test condition. Furthermore, we found a new result that anao crystalline lithium iron oxides obtained at low temperature also partially transformed into the LiFeO2 phase. It might contribute to the stabilization of host structure as a role of buffer in lithium iron oxide system.