⁶Li NMR Studies of Cation Disorder and Transition Metal Ordering in Li[Ni_1/3Mn_1/3Co_1/3]O₂ Using Ultrafast Magic Angle Spinning

Abstract

Studies of Li[Ni 1/3 Mn 1/3 Co 1/3 ]O 2 prepared under six different conditions are compared using highresolution solid-state 6 Li NMR. Differing degrees of cation disorder are established via integration of the NMR resonances, and this quantification of cation disorder is compared with Rietveld refinements of powder X-ray and neutron diffraction studies. Chemical shift trends to high frequency with decreasing degrees of disorder are established among this family of samples and explained according to the orbital overlap experienced by Li nuclei in the two environments: within the lithium layers and exchanged with nickel into the transition metal layers. Finally, an interesting case of local transition metal charge ordering is observed. Three unique environments are described, which can be accounted for based on electroneutrality arguments, and the known clustering of Ni 2+ and Mn 4+ . This effect has not been detected in these materials by other methods including neutron and X-ray diffraction. Thus, the local ordering, which is observed in the dominant NMR resonance of Li in its own layers is thought to be pervasive (affecting the majority of the NMR nuclei), but very local, so as to be seen only by techniques such as NMR which probe immediate neighborhoods.

Surface Structure, Morphology, and Stability of Li(Ni1/3Mn1/3Co1/3)O2 Cathode Material

“…This ordering has been found to be the most favorable using solid state NMR. 30 This structure was also confirmed by DFT calculations of an initial set of 30 lowest energy configurations determined based on electrostatic interactions only (not shown). The ordering can be explained based on the local charge balance.…”

Local structure and electrochemistry of LiNi y Mn y Co1 − 2y O2 electrode materials for Li-ion batteries

“…For an ideal LiMO 2 crystal, ionic configuration is regulated as Li at the 3a site (0,0,0), M at the 3b site (0,0, 1 = 2 ), and O at the 6c site (0,0, 1 = 4 ) [10]. According to previous experiments (i.e., X-ray absorption near edge structure, nuclear magnetic resonance, X-ray photoelectron spectroscopy), the oxidation states of Ni, Co, and Mn in the pristine material are proved to be +2, +3, and +4, respectively [11][12][13].…”

Study of the local structure of LiNi0.33+δMn0.33+δCo0.33−2δO2 (0.025≤δ≤0.075) oxides

“…The shifts and the full width at half maximum of the resonances are reported in Table 5. According to the study reported by Cahill et al [36] the ratio of the smaller peak to that of the larger peak gives the degree of cation disorder. The values of the cation disorder 0.84% and 2.30% for ı = 0.025, and 0.075, respectively, are in good agreement with the values determined by Rietveld refinement of their X-ray powders diffraction patterns.…”

“…The other decomposition bands are attributed to the various interactions Li O Ni and Li O Mn. Moreover, the set resonances at lower frequency correspond to the Li present in the Li oxide layers, while those at higher frequency are assigned to Li located in the transition metal layers [36]. The first set cannot be seen in the figure, because the first resonance is located at ca.…”