Ordered and Disordered Hybrid Tunnel Structured Manganese Oxides for Na-Ion Batteries

Thursday, 1 June 2017: 13:54
Grand Salon C - Section 13 (Hilton New Orleans Riverside)
B. Byles and E. Pomerantseva (Drexel University)
Through modification of previously developed hydrothermal techniques, we synthesize two tunnel structured manganese oxides with novel hybrid tunnel configurations and investigate their behavior in Na-ion batteries. Tunnel manganese oxides represent a promising class of materials for Na-ion batteries due to their low cost, low toxicity, and high electrochemical activity. The two materials studied in this work contain unique combinations of structural tunnels composed of varying amounts of MnO6 octahedra surrounding stabilizing Na+ ions, resulting in numerous different tunnel dimensions within individual nanostructures. Through cross-sectional TEM analysis, the ordered phase was found to consist of a systematic combination of 2x2, 2x3, and 2x4 octahedra tunnels. The disordered phase, on the other hand, was found to consist of a more random and nonuniform distribution of not only 2xn (n=2, 3, 4, 5, 6) tunnels, but also larger 3x3 octahedra tunnels. Moreover, although both materials were synthesized with a nanowire morphology, the disordered phase is found to be composed of nanowires with smaller diameter.

We investigate these two materials as cathodes for Na-ion batteries and find that upon first discharge, the ordered phase exhibits a capacity nearly 300 mAh g-1, while the disordered phase exhibits a capacity of just below 200 mAh g-1. Upon further cycling, however, the disordered phase maintains a greater amount of its initial capacity, indicating that although the better defined tunnel structure may be advantageous for greater initial ion intercalation capacity, the disordered phase shows a greater degree of electrochemical stability. The rate performance of these two materials is discussed in relationship to the different diffusion pathways present in the materials, as well as the structural stability of the two phases upon repeated ions insertion and deinsertion. The performance of these materials is also compared to tunnel manganese oxides with pure tunnel configurations, and the advantage of having such a hybrid tunnel configuration is also emphasized. This work highlights the performance of two similar novel cathode materials for Na-ion batteries, as well as the relationship between the degree of structural order in manganese oxides and their electrochemical behavior in Na-ion batteries.