Spray pyrolysis and electrochemical performance of Na 0.44 MnO 2 for sodium-ion battery cathodes
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Research Letter
Spray pyrolysis and electrochemical performance of Na0.44MnO2 for sodium-ion battery cathodes Kuan-Yu Shen, Miklos Lengyel, Louis Wang, and Richard L. Axelbaum, Department of Energy, Environmental and Chemical Engineering, Washington University in St. Louis, One Brookings Drive, St. Louis, MO 63130, USA Address all correspondence to R.L. Axelbaum at [email protected] (Received 27 November 2016; accepted 23 January 2017)
Abstract In this study, we investigate spray pyrolysis as an approach to synthesis of tunnel structure sodium manganese oxide, as it is a cost-effective and scalable technology. The powders synthesized with Na/Mn ratio of 0.50 displayed a pure tunnel structure, and demonstrated the best electrochemical performance, with a discharge capacity of 115 mAh/g. The material also showed good cycleability and rate capability. Noticeable decay in performance was seen in materials with Na/Mn ratios other than 0.50, indicating that this material is sensitive to minor compositional deviations. This study has demonstrated that spray pyrolysis is a promising synthesis method for this material.
Introduction Demand for large-scale energy storage solutions has been increasing as society reduces its dependence on fossil fuels. Lithium-ion batteries have been the dominant choice for energy storage for decades; however, there are concerns related to the limited availability of lithium, which could potentially increase costs and curtail large-scale implementation.[1,2] As a promising alternative, sodium-ion batteries have attracted significant attention in recent years. Sodium is one of the most abundant elements on earth and its related chemicals are significantly cheaper than the corresponding lithium ones.[3,4] Sodium is also the second-lightest and smallest alkali metal, next to lithium, thus minimizing the sacrifice in energy density, and allowing for more appealing economics. The potential markets for sodium-ion batteries include applications where cycle life and cost are more critical factors than energy density, such as gridscale energy storage for smart grid applications.[2–4] Among all the sodium-ion battery cathode materials, Na0.44MnO2 (Na4Mn9O18) is particularly attractive due to its wide tunnel crystal structure, which is suitable for sodium intercalation. The manganese ions in Na0.44MnO2 are dispersed in octahedral sites (MnO6) and a square-pyramidal environment (MnO5).[5] The lattice forms two double and one triple octahedral chains, which results in two different types of three-dimensional tunnels. Two active sodium sites are situated in large S-shape tunnels, while the other inactive site is found in smaller tunnels. When fully charged, only the inactive site in smaller tunnels is filled with sodium, which leads to a theoretical capacity of 121 mAh/g. In 2007, Sauvage et al. studied Na0.44MnO2 and recorded at least five biphasic phenomena between 2 and 3.8 V at a slow rate of
1/200 C.[5] Hosono et al. improved the cycleability and retained 100 mAh/g at 0.42 C after 20 cycles.[6] Zhao et a
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