P2 Layered Na Transition Metal Oxides: A Synthetic Precursor to New O2 xLi2MnO3•(1-x)LiMO2 Ion-Exchanged ‘Layered-layered’ Cathode Materials

It is now firmly established that Li- and Mn-rich layered compounds form domains of Li₂MnO₃ that are essentially epitaxially matched with coincident LiMO₂ (M=transition metals) layered structures when synthesized at high temperatures from either co-precipitated precursors (homogeneous initial distribution) or bulk starting material oxides or carbonates at the correct stoichiometric ratios.  Since monoclinic Li₂MnO₃ and hexagonal LiMO₂ individually are O3 oxygen stacked, the result of melding L₂MnO₃ domains with LiMO₂ thus yields a ‘layered-layered’ composite which in turn is also O3 oxygen stacking.  In reality it is likely that some stacking faults are prevalent but this is extremely hard to quantitate.  Recently it has been revealed that a xLi₂MnO₃•(1-x)LiMO₂conversion to spinel with cycling is feasible principally resulting in voltage fade [1].  Since spinel is also O3 stacked it is reasonable that layer gliding during the transformation is curtailed and only a direct movement of cations within the fixed oxygen anion array occurs.

In an effort to preclude the O3 layered to O3 spinel conversion during cycling, it was surmised that a precursor P2 layered structure should be implemented in the synthetic route.  Because of the high thermodynamic barrier to break existing M-O bonds, a P2 configuration will not convert to O3 (and thus spinel) within the course of cycling or during the synthetic step, this approach seems encouraging.

Direct reaction of M precursors with Li-salts typically forms O3 stacked compounds, however, if one is to replace most of the Li in the reactants with the proper stoichiometry of Na then the product can be guided to P2 only.  Subsequent Na exchange with Li during the next step directs an O2 (or O4, O6 mixed composites) layer stacked product [2].

The ion-exchanged (IEx) synthetically derived O2 IEx-xLi₂MnO₃•(1-x)LiMO₂ material which has the (TODAHE5050) composition: IEx-0.5Li₂MnO₃•0.5Li(Ni_0.375Mn_0.375Co_0.25)O₂ features capacities in excess of 220 mAhg^-1(in Figure 1), but more importantly, the voltage fade is curtailed.  This presentation will highlight the synthesis, materials chemistry and its relation to structure-function-property relationships.

 References

1. D. Mohanty, S. Kalnaus, R. A. Meisner, K. J. Rhodes, J. Li, E.A. Payzant, D. L. Wood III, C. Daniel, J. Power Sources, 229, 239-248 (2013)
2. D. Kim, S.-H. Kang, M. Balasubramanian, C. S. Johnson,. Electrochem Commun., 12, 1618-1621 (2010) 

Acknowledgments

Funding from the Department of Energy under Contract DE-AC02-06CH11357 is gratefully acknowledged.  We would also like to acknowledge Peter Faguy and David Howell for their support and the funding support from Department of Energy, Energy Efficiency and Renewable Energy, Office of Vehicle Technologies.

 The submitted document has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357. The U.S. Government retains for itself, and others acting on its behalf, a paid-up nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government.