On the other hand, as shown by a solid line in FIG. 1, it is found that each of a charge curve and a discharge curve of the composite oxide Li2Na1.75Ti5.75Nb0.25O14 has, as majority excluding the initial stage and the last stage of the charge and the discharge, a portion in which a variation in the potential accompanied with a change in the capacity is large. Specifically, it is found from the charge curve in the Li insertion direction that when the charge is started from a potential of 1.50 V (vs. Li/Li+) and reaches 90% of the total capacity, the potential of the composite oxide Li2Na1.75Ti5.75Nb0.25O14 becomes about 1.15 V (vs. Li/Li+); in other words, the composite oxide Li2Na1.75Ti5.75Nb0.25O14 exhibits a variation in the potential of about 0.35 V during the charge. Similarly, it is found from the discharge curve in the Li extraction direction that when the discharge is started from a potential of 1.15 V (vs. Li/Li+) and reaches 90% of the total capacity, the capacity of the composite oxide Li2Na1.75Ti5.75Nb0.25O14 becomes about 1.50 V (vs. Li/Li+), and there is a variation in the potential of about 0.35 V during the discharge. Thus, each of the charge and discharge curves of the composite oxide Li2Na1.75Ti5.75Nb0.25O14 contains, as majority, a portion in which a variation in the potential accompanied with a change in the capacity which is larger than that in the potential flat portion contained in the charge-and-discharge curve of the composite oxide Li2Na2Ti6O14, i.e., a portion having a larger gradient than the potential flat portion.
