Structural Evolution of Spinel Phases in ‘Layered-Layered-Spinel’ Composite Cathodes
Authors: Lee Eungje , Ren Yang , Ahn Docheon , Park Joong Sun , Mohanty Debasish , Huq Ashfia , Croy Jason R. , Thackeray Michael M. 
Published: February 2020

The structural integration of a spinel (S) component into a ‘layered-layered’ (L-L) composite electrode system has been demonstrated as a promising strategy in the development of stable, high-energy cathodes for lithium-ion batteries.[1-3] This approach to materials design is based on the premise that the 3-D structure of a spinel component can provide additional binding between the 2-D layers of a ‘layered-layered’ structure and increase the structural robustness at higher levels of delithiation (i.e., states-of-charge). In addition, the 3-D nature of the spinel structure allows for more facile lithium-ion diffusion. The ‘layered-layered-spinel’ (L-L-S) composite cathodes can be synthesized by reducing the amount of lithium in the precursor materials normally used to prepare stoichiometric, ‘layered-layered’ cathodes. For example, reducing the lithium content of a stoichiometric 0.5Li2MnO3•0.5LiMn0.5Mn0.5O2 L-L composite structure (alternatively, Li1.5Mn0.75Ni0.25O2.5), in which the Mn:Ni ratio is 3:1, drives the formation of (1-x)[0.5Li2MnO3•0.5LiMn0.5Mn0.5O2]•x[LiMn1.5Ni0.5O4] L-L-S composite compounds.[1,4] However, several critical questions remain unanswered: (1) At what level of lithium deficiency do transition-metal cations prefer to occupy spinel-like configurations (e.g., lithium-layer sites) and prompt the formation of localized, spinel and spinel-like domains? (2) How do different Mn:Ni ratios in parent L-L compounds affect the phase evolution of integrated S components? (3) What is the role of Co in the synthesis and performance of L-L-S composite structures? Therefore, in an attempt to better understand the structural complexity of L-L-S systems, this presentation will report on the evolution of spinel components in several, model, L-L-S compounds prepared by solid-state synthesis routes with different degrees of lithium deficiencies. Results of a structural investigation, conducted by combined Rietveld refinement of neutron and synchrotron X-ray diffraction data, revealing the significant effects of Ni:Mn ratios and Co contents on the structures of these composite materials will be discussed.

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Acknowledgment

Support from the Vehicle Technologies Program, Hybrid and Electric Systems, in particular, David Howell, Peter Faguy, and Tien Duong at the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, is gratefully acknowledged. 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.