Understanding the Role of Ni in Stabilizing the Lithium-Rich High-Capacity Cathode Material Li[Ni_xLi_(1–2x)/3Mn_(2–x)/3]O₂ (0 ≤ x ≤ 0.5)

Abstract

The lithium-rich high-capacity cathode material Li[Ni x Li (1−2x)/3 Mn (2−x)/3 ]O 2 was investigated by X-ray absorption spectroscopy (XAS) to understand the role of Ni in the lithiumrich layered line in the pseudoternary system. The electronic structural changes that occur at different charged states were examined with soft XAS to understand the role of the secondary transition metal in controlling the stability. The Mn L II,III -edges reveal a relationship between Ni and the degree of structural transformation after the first cycle. Close examination of Li 2 MnO 3 shows that Mn changes its oxidation state slightly during charging; however, there is a more dramatic change upon reduction from 4+ to a value close to 3+ at discharge, where it participates in the subsequent charge-compensation mechanism. The Ni L II,III -edges reveal that nickel acts as a stabilizer preventing the complete transformation of Mn by substitution reduction that is thought to couple with the anionic redox couple during the extended oxygen-activation plateau. The O K-edge reveals that loss of stability might be related to diminished d−sp hybridization occurring after oxygen activation, which is improved by the incorporation of nickel as a stabilizing agent. These findings clarify the complex relationships among nickel, manganese, and oxygen within lithium-rich high-capacity cathode materials in controlling stability while simultaneously obtaining high capacities exceeding 250 mA•h/g.

Coupling between oxygen redox and cation migration explains unusual electrochemistry in lithium-rich layered oxides

“…5. Thus, changes in the average TM spectroscopy do not offer insight into the nature of O redox, as previously proposed 12 . However, the nature of the O redox XAS signature can provide such insight, as changes in the O K edge can reflect how the O 2p –character states are altered during the redox process 58,61–63 .…”

“…By contrast, when charging through the voltage plateau (4.35 V, ‘1’ → 4.60 V, ‘2’), the TM L 3 -edge spectra change minimally: the Co and Mn L 3 edges exhibit some broadening, and there is a small increase of the low energy peak at the Ni L 3 edge, suggesting a surprising reduction of the Ni during charge 12,30,31 . We later return to this important observation.…”

“…Previous XAS results 6,7,10,12–14,30–32 mainly utilized electron and fluorescence based detection modes (Fig. 2b), which are distorted by surface contributions, self-absorption, and peak broadening effects and show only a weak and broad feature at the same energy.…”

“…Ceder and coworkers, meanwhile, have predicted that O–O dimerization should only occur in the presence of d 0 and d 10 cations due to the rotational freedom of the O 2p orbitals, with a localized hole O 2– /O – mechanism prevailing elsewhere 23 . Bruce and coworkers recently reported evidence for the localized O 2– /O – mechanism in Li- and Mn- rich Ni/Mn/Co layered oxides (LMR-NMC) based on O K edge XAS 6,7 , yet similar observations have led others to conclude a 2O 2– /O 2 n– redox couple in the same system 12,13 . Thus, a clear consensus on which mechanism prevails in which materials is lacking.…”

“…For example, Li-rich surface and near-surface regions often exhibit oxygen evolution and reconstruction during delithiation 24–28 , yet surface-sensitive X-ray photoelectron spectroscopy (XPS) is often used to infer the nature of bulk anion redox 5,9,11,29 . Likewise, correlations between spatially averaged X-ray absorption spectra (XAS) are often used to assess hybridization 6,7,10,12–14,30–32 , which assumes that redox chemistry occurs uniformly. The relatively subtle evolution in the spectra during anion redox, combined with their sensitivity to probing depth (a few nm for quantitative XPS and total-electron-yield (TEY) XAS) and detection mode (e.g., self-absorption distortions in fluorescence-yield (FY) XAS) has contributed to the conflicting proposed mechanisms for oxygen redox in certain materials and, consequently, explanations for its role in determining capacity and electrochemical stability 15,21,22,32,33 .…”