Abstract
Among lithium battery cathode materials, Li1.2Ni0.13Mn0.54Co0.13O2 (LR-NMC) has a high theoretical capacity, but suffers from voltage and capacity fade during cycling. This is partially ascribed to transition metal cation migration, which involves the local transformation of the honeycomb layered structure to spinel-like nano-domains. Determination of the honeycomb layered/spinel phase ratio from powder X-ray diffraction data is hindered by the nanoscale of the functional material and the domains, diverse types of twinning, stacking faults, and the possible presence of the rock salt phase. Determining the phase ratio from transmission electron microscopy imaging can only be done for thin regions near the surfaces of the crystals, and the intense beam that is needed for imaging induces the same transformation to spinel as cycling does. In this article, it is demonstrated that the low electron dose sufficient for electron diffraction allows the collection of data without inducing a phase transformation. Using calculated electron diffraction patterns, we demonstrate that it is possible to determine the volume ratio of the different phases in the particles using a pair-wise comparison of the intensities of the reflections. Using this method, the volume ratio of spinel structure to honeycomb layered structure is determined for a submicron sized crystal from experimental three-dimensional electron diffraction (3D ED) and precession electron diffraction (PED) data. Both twinning and the possible presence of the rock salt phase are taken into account. After 150 charge–discharge cycles, 4% of the volume in LR-NMC particles was transformed irreversibly from the honeycomb layered structure to the spinel structure. The proposed method would be applicable to other multi-phase materials as well.
Highlights
Battery cathode materials based on Li1+δ (Ni1−x−y Mnx Coy )1+δ O2 are widely investigated in various compositions for the different advantages that specific Li:Ni:Mn:Co ratios can give in capacity and power density
We demonstrate the difference in sensitivity to the electron beam between pristine and charged LR-NMC type materials, demonstrate that it is possible to obtain electron diffraction patterns without introducing extra spinel, and provide a strategy to determine the ratio between the different phases present in a particle from 3D ED when least squares refinement does not give a solution due to a complex microstructure
While the effect of the electron beam on the spinel content in Li1.2 Ni0.13 Mn0.54 Co0.13 O2 (LR-NMC)-type materials has been well documented for transmission electron microscopy (TEM) imaging techniques [8,9,10], no literature is available on the effect of electron diffraction on the amount of spinel
Summary
Introduction iationsBattery cathode materials based on Li1+δ (Ni1−x−y Mnx Coy )1+δ O2 are widely investigated in various compositions for the different advantages that specific Li:Ni:Mn:Co ratios can give in capacity and power density. Voltage fade and capacity fade are not yet optimally subdued, partially due to the creation of secondary phases during cycling [1,2,3,4]. Their parent structure is a layered LiCoO2 type, and in the case of lithium-rich compounds (δ > 0)—such as the Li1.2 Ni0.13 Mn0.54 Co0.13 O2 (LR-NMC) studied in this paper—there is a honeycomb order between the lithium atoms and the transition metal atoms in the transition metal layers (Figure 1a). There can be a further transformation to a rock salt type structure with disorder between the Licensee MDPI, Basel, Switzerland.
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