The possibility of K+-beta-ferrite as a cathode active material for lithium
secondary battery was investigated. Discharge-charge characteristics based
on the lithium insertion and extraction of K+-beta-ferrite have been studi
ed by constructing an electrochemical cell Li/1 M LiClO4 PC/K1.33Fe11O17 (K
+-beta-ferrite). The cyclic voltammograms for K+-beta-ferrite were unchange
d after 10 cycles of operation. The OCV (open circuit voltage) before disch
arge 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 degrees C under a constant current density of
0.1 mA/cm(2). According to the chemical analysis, K+-beta-ferrite at each
discharge step certainly contained lithium, in agreement with the lithium c
ontent calculated by current integration. Lithium content y in K1+xLiyFe11O
17 was increased up to 11. After discharge, the spinel blocks were disorder
ed, whereas the layer structure still remained in the XRD pattern. Lithium
ions seem to mainly enter into gamma-Fe2O3 layers (spinel blocks) of K+-bet
a-ferrite, because alkali layers could practically contain only 1.5 alkali
ions for Fe11O17. The cell voltage in charge increased up to 4.5 V vs. Li/L
i+ at 25 degrees C. The potential during charge was lowered to 3.7 V by rai
sing the temperature to 60 degrees C. The XRD pattern of the cathode materi
al at every step of charge was similar to that at 200 mAh/g of discharge. D
egradation of discharge-charge characteristics was not observed after 10 cy
cles of operation. (C) 1998 Elsevier Science B.V. All rights reserved.