Energy-Density Optimization in Lithium-Rich Layered-Oxide Cathode Materials
Authors: Benedek Roy , Iddir Hakim 
Published: February 2020

Practical utilization of the high capacities available in lithium-rich layered cathode materials (in excess of 250 mAh/g) has been hampered by the phenomenon of voltage fade (VF). Experiment and simulation have revealed structural transformations [1] that occur in the plateau region of the first charge and set the stage for the VF that occurs in subsequent cycling. We consider in this presentation what guidance, if any, our present understanding of VF provides for achieving high-energy-density VF-free operation of the lithium-rich layered materials. It is convenient to view the starting material as a mixture of Li2MnO3-like and LiMO2-like domains [2]. In the simplest picture, Li is removed (at low energies) from the LiMO2 domains during the first charge, and then (at higher energies) from the Li2MnO3 domains. After delithiating the LiMO2 domains (at which point the ratio of Li ions removed to transition metal ions, p
max(LiMO2), is only slightly below unity), the onset of the voltage “plateau” is reached; extension of the charge through the full extent of the voltage plateau [p
max(Li2MnO3) ~ 2] then creates a high driving force for the displacement of ions from the oxygen sublattice to form O-O bonds [1]. The defects in the oxygen sublattice lower the barriers for Mn migration from the M layer, a precursor, e.g., to spinel formation. Against this background, we suggest that the most likely regime in which to seek VF-free operation is to reduce the driving force of oxygen-sublattice disintegration by restricting the charge in the voltage plateau to p
max(Li2MnO3) ~ 1 (at which delithiation of both LiMO2 and Li2MnO3 domains are comparable). Experiment and simulation show [1] that the oxygen sublattice in Li2MnO3 domains remains stable in the bulk at this level of delithiation. With the oxygen sublattice stable, Mn ions cannot migrate, and the original framework structure, with its high energy density, remains intact.    Stabilization of oxygen in the bulk still leaves the problem of atomic rearrangement at surfaces [3], which may occur even at low delithiation levels (p
max(Li2MnO3) << 1), and contribute to VF. To achieve VF-free operation, therefore, it is crucial to block the surface reaction channel. One suggestion is to coat the lithium-rich primary particles with the high-voltage spinel LiNi1/2Mn3/2O4 (4), which is stable at high voltages.   

 In summary, we hypothesize that stable, VF-free, operation requires (a) restricting the charge to voltages at which the oxygen sublattice in the bulk is stable, and (b)  blocking the surface reaction channel, by a coating or gradient structure.  Approaches that focus exclusively on mitigating VF by optimization of the bulk composition, we believe, will not yield a material with suitably low VF, in the absence of a means to block the surface reaction channel. Should (b) be realized, we envision low VF operation with p
max(LiMO2) ~ p
max(Li2MnO3) ~ 1, and a capacity slightly above 200.  Although not a quantum leap, this would represent an improvement over the best non-Li-excess systems.

[1] J. R. Croy, H. Iddir, K. G. Gallagher, C. S. Johnson, R. Benedek, and M. Balasubramanian, Phys. Chem. Chem. Phys. 17, 24382 (2015).                                             

[2] H. Iddir, B. Key, F. Dogan, J. T. Russell, B. R. Long, J. Bareño, J. R. Croy, and R. Benedek, J. Mater. Chem. 3, 11471 (2015).                                                              

[3] K. J. Carroll, D. Qian, C. Fell, S. Calvin, and G. M. Veith, M. Chi, L. Baggetto, and Y. S. Meng, Phys. Chem. Chem. Phys. 15, 11128 (2013).                                        

[4] Y. Chen, K. Xie, C. Zheng, Z. Ma, and Z. Chen, ACS Appl. Mater. Interfaces 6, 16888 (2014)
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 Funding from the Office of Vehicle Technologies, U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, is gratefully acknowledged, as arecomputer time allocations at the Fusion Computer Facility, Argonne National Laboratory and at EMSL Pacific Northwest National Laboratory. This research used resources of the National Energy Research Scientific Computing Center, which is supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

The submitted abstract 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.