Doping effects on structure and electrode performance of K-birnessite-type manganese dioxides for rechargeable lithium battery
Introduction
Manganese oxides have been studied for electrochemical energy storage due to being a low toxic and inexpensive material which form lamellar, spinel and tunnel frameworks [1], [2], [3]. The electrochemically active materials, which have a working potential for Li insertion/deinsertion around 3–4 V vs. Li, are strong candidates for lithium (ion) batteries. As one of the manganese oxides, the birnessite-type manganese oxide is a hydration-layered manganese oxide, which forms MnO2 sheets of MnO6 octahedra assembled in layers and has cations of pillar (mainly alkaline ion) and lattice water in the interlayer space. Up to now, the syntheses of the birnessite are achieved by various methods such as a calcination reaction [4], [5], [6], [7], [8], [9], [10], [11], [12], sol–gel synthesis [12], [13], [14], [15], hydrothermal synthesis [16], [17], etc. The structural details of potassium-containing birnessite were investigated as previously reported [4], [5], [6], [7], [8]. These materials show a working potential around 3 V vs. Li and high discharge capacities during initial cycling, though there is the problem of structural collapse resulting in discharge capacity fading [5], [15]. In order to improve the battery performance, Franger et al. [15] and Tsuda et al. [13] reported the enhancement of the cycle performance by Co doping in birnessite.
Previously, we reported the synthesis of a birnessite by utilizing the thermal decomposition of KMnO4 and its electrochemical performance in Li cells [9], [10]. In the present paper, we report that potassium birnessites made by thermal decomposition reaction of KMnO4 mixed with Co, Ni, Al, Mn, and K nitrates, and we investigated the relation between the synthetic conditions and electrochemical performance.
Section snippets
Experimental
The layered manganese dioxides doped with Al, Ni, or Co were obtained by the calcinations of mixtures (about 0.5 g) of KMnO4 and Al, Ni, or Co nitrate, respectively, which were dissolved in several ml of distilled water, at 600 °C (50 °C h−1) for 5 h in air. The calcined sample was ultrasonically treated for 5 min in a sufficient amount of water. The precipitate in the ultrasonically treated solution was then separated by filtration, washed with water until neutral, and dried at 80 °C for 24 h in air.
Results and discussion
Fig. 1 shows the TG curve of the KMnO4 powder. Below 230 °C, the weight of KMnO4 is constant. At temperatures higher than 230 °C, the weight quickly decreased to 85.5%, and then the weight did not change up to 600 °C. The weight loss around 250 °C is due to the thermal decomposition of KMnO4 as previously reported [11]. The weight loss is expressed by the following equation.5KMnO4 → 3MnO2 + K2MnO4 + K3MnO4 + 3O2Based on this thermal decomposition, the weight loss due to the O2 evolution is estimated to be
Conclusion
The potassium birnessites doped with Co, Ni, or Al were successfully obtained from KMnO4 mixed with these nitrates by a calcination synthesis. Among the doped compounds, Co-25 birnessite has the highest discharge capacity of 200 mAh g−1 and capacity retention of about 80% for 20 cycles. It was found that the electrochemical performance of birnessite doped with appropriate amount of cobalt was improved due to the change in the stacking structure and the decrease in the charge transfer resistance.
Acknowledgements
The authors thank Dr. T. Sasaki and Mr. K. Saka for their helpful assistance in the experimental work. This study was partially supported by the New Energy and Industrial Technology Development Organization (NEDO), Japan, Yazaki Memorial Foundation for Science and Technology, and Iwatani Naoji Foundation's Research Grant.
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