Lithium secondary battery using potassium-β-ferrite as a new cathode active material
Abstract The possibility of K + -β-ferrite as a cathode active material for lithium secondary battery was investigated. Discharge–charge characteristics based on the lithium insertion and extraction of K + -β-ferrite have been studied by constructing an electrochemical cell Li|1 M LiClO 4 PC|K 1.33 Fe 11 O 17 (K + -β-ferrite). The cyclic voltammograms for K + -β-ferrite were unchanged after 10 cycles of operation. The OCV (open circuit voltage) before discharge was 3.2 V vs. Li/Li + . The discharge potential maintained 2.3 V vs. Li/Li + up to ca. 200 mAh/g at 60°C under a constant current density of 0.1 mA/cm 2 . According to the chemical analysis, K + -β-ferrite at each discharge step certainly contained lithium, in agreement with the lithium content calculated by current integration. Lithium content y in K 1+ x Li y Fe 11 O 17 was increased up to 11. After discharge, the spinel blocks were disordered, whereas the layer structure still remained in the XRD pattern. Lithium ions seem to mainly enter into γ-Fe 2 O 3 layers (spinel blocks) of K + -β-ferrite, because alkali layers could practically contain only 1.5 alkali ions for Fe 11 O 17 . The cell voltage in charge increased up to 4.5 V vs. Li/Li + at 25°C. The potential during charge was lowered to 3.7 V by raising the temperature to 60°C. The XRD pattern of the cathode material at every step of charge was similar to that at 200 mAh/g of discharge. Degradation of discharge–charge characteristics was not observed after 10 cycles of operation.