Long-Range and Local Structure in the Layered Oxide Li_1.2Co_0.4Mn_0.4O₂

Abstract

The layered oxides being considered as intercalation compounds for lithium batteries display significant differences between the long-range crystal structure and local arrangements around individual atoms. These differences are important, because the local atomic environments affect Li-ion transport and, hence, the oxide’s rate capability, by determining activation barrier energies, by blocking or opening Li-diffusion pathways, etc. Traditional diffraction methods provide key information on the average crystal structure. However, no single experimental technique can unequivocally determine the average long-range crystal structure and the distribution of local environments over crystallographic distances while retaining atomic-scale resolution. Therefore, in this study, we have employed a combination of diffraction, microscopy, and spectroscopy techniques to investigate the long-range (∼1 μm) and local structure (≤1 nm) of Li1.2Co0.4Mn0.4O2, which is a model compound for layered oxides being considered for transportation applications. We find that Li1.2Co0.4Mn0.4O2 contains mostly Mn4+ in Li2MnO3-like atomic environments and Co3+ in LiCoO2-like atomic environments, which are intimately mixed over length scales of ≥2−3 nm, resulting in a Li1.2Co0.4Mn0.4O2 crystallite composition that appears homogeneous over the long-range. In addition, we observed a quasi-random distribution of locally monoclinic structures, topotaxially integrated within a rhombohedral-NaFeO2 framework. Based on these observations, we propose a dendritic microstructure model for Li1.2Co0.4Mn0.4O2 consisting of well integrated LiCoO2- and Li2MnO3-like structures.

Advanced analytical electron microscopy for lithium-ion batteries

“…On the basis of this fact, Jarvis et al 23 reported that their Li-excess layered material is a solid solution. By contrast, Bareno et al 24 found a locally Li 2 MnO 3 -like region within the parent rhombohedral Li-excess layered material structure, and Boulineau et al 25 observed the coexistence of the two phases, R3m and C2/m, with the same nominal composition. The precise structural determination in these microscopy studies reconciled the long-standing debate.…”

Raman Microscopy of Lithium-Manganese-Rich Transition Metal Oxide Cathodes

“…While the exact structure of lithium-rich TM oxides has been the subject of much debate, a number of recent studies conclude that the material forms as the composite of these two distinct phases with nanoscale domains of Li 2 MnO 3 and LiMO 2 character. [1][2][3][4][5][6] The intense interest in lithium excess cathodes stems from the potential to deliver very large reversible capacities (>200 mAh g −1 ) when charged beyond 4.5 V vs. Li 0 /Li + . 7 Part of the capacity derives from electrochemical activation of the Li 2 MnO 3 component at voltages beyond 4.4 V. Two mechanisms have been proposed for this activation.…”

“…9,[17][18][19][20][21][22][23][24][25] In particular, several groups have applied Raman microscopy to understand the structure and structural evolution of lithium-rich, manganese-rich TM oxides of nickel, manganese, and cobalt (hereafter LMR-NMC). 18,19,24,[26][27][28][29][30][31] Raman microscopy offers high spatial resolution (< 1μm 3 ), large field of view, and chemical specificity, making it an ideal tool to investigate both the pristine material and composite battery electrodes. 32,33 Nonetheless, there are significant differences in the spectra reported for LMR-NMC oxides, as well as ambiguity in the interpretation of the vibrational modes.…”

Nanoscale Voltage Enhancement at Cathode Interfaces in Li-Ion Batteries

“…Moreover, interfaces inevitably occur for cathode particles at their surface, either with a solid electrolyte interphase (SEI) or frequently a protective coating layer, the latter of which is typically comprised of insulating metal oxides or fluorides [7][8][9][10][11][12][13][14][15] or sometimes an electrochemically active material (e.g., Li-Ni-PO 4 coating layer) 16 .…”