The search for low cost and environmental friendly intercalation cathode materials offering high power density in rechargeable ion-exchange batteries is driven by the limitations of the existing Li-ion technology. At present, the use of the lithium metal as a negative electrode is restricted to the use of specific polymer electrolytes¹ which hinder the formation of the dendrites. Therefore, the graphite negative electrodes are employed. However, they reduce the theoretical capacity density from 2046 mAh cm^-3 to ~850 mAh cm^-3 and drastically increase costs. Here, the application of multivalent battery technology that pairs an intercalation cathode with the metal electrode thus allowing for higher energy density and lower costs, is desired².

Among the candidates, Mg metal that possesses high volumetric specific capacity of 3833 mAh cm^-3, exhibits no dendrite growth on deposition², is safe to handle in ambient atmosphere and largely available, is of special interest³. Various materials have shown the initial promise for multivalent intercalation, including Chevrel phase Mo₆S₆, layered V₂O₅, graphitic fluoride, etc. However, owing to the limited mobility of Mg ions and possible concurrent insertion of water and/or protons, the cycling stability of these host materials has been shown insufficient.

The promising candidates for the cathode materials that display higher voltages than Chevrel phases are variants of manganese oxide. In this study we have chosen a MgMn₂O₄ spinel and (Mg_xNa_y)Mn₂O₄ birnessite phases. These materials crystal structures employ different mechanisms for keeping the stability upon cycling and allow for Mg de/insertion, which was performed in magnesium nitrite aqueous electrolyte. The aim of the study was to investigate the possible Mg insertion mechanisms in both materials prior assembling a battery by correlating the cyclic voltammetry (CV) results with the structural and compositional changes of these cathode materials by S/TEM at the atomic level upon increasing number of cycles.

 The spinel phase was prepared by the delithiation of the commercially available LiMn₂O₄ spinel in 0,1 M Mg(NO₃)₂ aqueous electrolyte and its following magnesiation. The Mg containing birnessite phase was synthesized by the rout described by Aronson and coworkers via Na-birnessite phase⁴. The STEM-EDX confirmed partial exchange of Na over Mg with the Mg occupying fully the smaller particles and only the outer shells of the larger particles. Same behavior was observed in the spinel material, where the small particles had a higher Mg content than the large particles (above 200 nm). Both materials were then put through the CV tests to explore the Mg de/insertion mechanisms. Plots in Fig 1 (a,c)  show that both spinel and birnessite structures can reversibly insert the Mg ions. The complete stabilization of the CV curve was observed in both materials at around the 20^th cycle (Fig. 1 a,c). STEM-ABF images (Fig. 1 b,d) were taken from the material after the third cycle, when the initial changes of structure due to the Mg de/insertion took place. The ABF technique allowed for the visualization of the lighter Mg and O atoms that can be vaguely seen in case of spinel structure. The ABF imaging of birnessite, in its turn, confirmed the presence of extra O atoms belonging to the crystal water interlayer that has been reported to play a crucial role in the layered cathode materials by enhancing the ion diffusion as well as suppressing the Mn²⁺ dissolution⁵.

¹ L. Damen, J. Hassoun, M. Mastragostino, B. Scrosati, J. Power Sources 195, 6902, (2010)

² H. D. Yoo, I. Shterenberg, Y. Gofer, G. Gershinsky, N. Pour, D. Aurbach, Energy Environ. Sci. 6, 2265, (2013)

³ J. Muldoon, C. B. Bucur, T. Gregory, Chem. Rev. 114, 11683, (2014)

⁴ B. J. Aronson, A. K. Kinser, S. Passerini, W. H. Smyrl and A. Stein, Chem. Mater. 11, 949, (1999)

⁵ K. W. Nam, S. Kim, E. Yang, Y. Jung, E. Levi, D. Aurbach and J. W. Choi, Chem. Mater. 27, 3721 (2015)

Figures:

Figure 1. Cyclic voltammetry plots of (a) MgxMn2O4 spinel and (c) (MgxNay)Mn2O4 birnessite materials showing the successful reversible Mg insertion. The insets on both graphs are CTEM pictures of the corresponding investigated material. STEM-ABF images with overlayed atomic structure were taken from (b) spinel phase and (d) birnessite phase upon 3rd cycle.

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EMC Abstracts - https://emc-proceedings.com/abstract/study-of-the-mg-insertion-in-mn-based-spinel-and-birnessite-structures-upon-electrochemical-cycling/