Improved cyclability of Na-birnessite partially substituted by cobalt

We measured the electrode performance of Na-birnessite, which is layered manganese dioxide with sodium ions and water in the interlayer, for use in rechargeable lithium batteries. We focused on the effects of water on the electrode performance. X-ray diffraction (XRD) measurement and thermogravimetric (TG) analysis indicated that the weight loss on heating the material at 100 °C corresponded to the removal of surface water with no structural change, while that at 150 °C was associated with the removal of lattice water, leading to the layer shrinkage. According to this result, we prepared three kinds of electrode samples with different drying conditions, and examined the effect on the electrode performance. Furthermore, to improve the cyclability, we synthesized samples in which cobalt was substituted for some of the manganese. Even with a large substitution ratio of over 30%, we obtained the same initial capacity as the non-substituted sample, and the cyclability was improved. We assume that the substituted cobalt fixed sodium ions in the interlayer in the form of pillars that stabilized the layer structure.