Synthesis, characterization, and electrochemical properties of amorphous CrO2-delta (0<=delta<=0.5) cathodes

Amorphous chromium oxides CrO2-delta (0 less than or equal to delta less than or equal to 0.5) have been synthesized by a reduction of K2CrO4 with KBH4 in aqueous solutions. The samples have been characterized by x-ray diffraction, differential scanning calorimetry, thermogravimetric analysis, transmission electron microscopy, and iodometric titration. The oxidation state of Cr and the total oxygen content (2-delta) generally decrease with increasing volume of KBH4 at a given pH or with increasing pH at a given volume of KBH4. The amorphous CrO2-delta samples lose oxygen sharply at 400 < T < 600 degrees C to give crystalline Cr2O3; the temperature at which oxygen loss occurs increases from about 400 degrees C for delta approximate to 0 to about 600 degrees C for delta approximate to 0.5. The capacity of the amorphous CrO2-delta cathodes decreases with increasing delta due to a decreasing oxidation state of Cr. Amorphous CrO2 shows a maximum initial capacity of 180 mAh/g in the range 3.3 to 2.3 V for a current density of 0.1 mA/cm(2). Although the capacity declines during the initial cycles, it becomes stable after about 20 cycles, for example, at about 90 mAh/g for 0.5 mA/cm(2).