Electronic Structure of Lithium Nickel Oxides by Electron Energy Loss Spectroscopy

Abstract

The electronic structures of NiO, LiNiO2, and NiO2 are studied by the electron energy loss spectroscopy at Ni L(2,3), Ni M(2,3), and O K edges. The Ni L(2,3) edge spectra suggest that the formal charge of nickel is +2 in NiO, +3 with a low-spin state in LiNiO2, and +4 with a low-spin state in NiO2. This is well confirmed by first-principles calculations. The Ni M(2,3) edge spectra show similar chemical shifts to those of the Ni L(2,3) edge. Superposition of the Li K edge spectrum, however, hinders further analysis. Although the formal charge of oxygen is -2 in all the three phases, the O K edge spectra indicate a more remarkable difference in the electronic structure of the oxygen in NiO2 than that in either NiO or LiNiO2. The spectra suggest that lithium extraction from LiNiO2 reinforces the covalent bonding between the oxygen and nickel atoms and causes a notable reduction in electron density at the oxygen atoms.

Surface properties of LiCoO2, LiNiO2 and LiNi1−xCoxO2

“…Following the same reasoning for LiNiO 2 , however, requires the formation of Ni 3+ , an uncommon state for solid state nickel oxides and one that is often stabilized by the formation of defects or hydroxides that produce mixed Ni 2+ /Ni 3+ oxidation states. Indeed, two general descriptions have emerged in the literature in relation to the chemical nature of nickel in LiNiO 2 , one analogous to that found for LiCoO 2 in which the oxidation state for the nickel is Ni 3+ [23], [24], [25], [26], [27] and [28] and a second one that relies on localized Ni 2+ -O − pairs in which charge is transferred from a neighboring lattice oxygen onto the nickel to preserve, or at least more closely approximate, the favored 2 + state [29], [30], [31], [32], [33] and [34]. We present here a study of surface composition of LiCoO 2 , LiNiO 2 and LiNi 0.5 Co 0.5 O 2 which suggests that, at least in the near-surface region, the nickel-containing lithium metal oxide is stabilized by dilithiation to produce a nickel cation with an average electron density closer to that of the more favored Ni 2+ state.…”

Determination of the mechanism and extent of surface degradation in Ni-based cathode materials after repeated electrochemical cycling

“…[17][18][19] Here, we use only the L 3 /L 2 ratio of each TM in the compounds in order to express the changes in oxidation states, since the intrinsic energy resolution of the electron beam (0.8-1.0 eV) may lead to misinterpretations when EELS results are analyzed based on the shift of the onset of the edges in the EEL spectra. The white-line ratio (L 3 /L 2 ) decreases with oxidation, but increases with reduction for the Mn, 19 Co, 20 and Ni ions; 21 thus, white-line ratios are an appropriate indicator of valence changes of these TM ions. Figure 2(c) summarizes the O K, Mn, Co, and Ni L 2,3 EEL spectra taken from the cycled NMC433 particle, using the ∆E and L 3 /L 2 indexes of TMs described above.…”

Ab-initio multiplet calculation of oxygen vacancy effect on Ti-L2,3 electron energy loss near edge structures of BaTiO3

“…14 Recently, Ikeno et al developed an ab initio multiplet calculation code for the L 2,3 edges of TMs. [14][15][16][17][18][19] In this paper, the Ti-L 2,3 ELNES of BaTiO 3 are theoretically investigated by the ab initio multiplet method. Moreover, the effects of the oxygen vacancy on the spectrum and the ability of Ti-L 2,3 ELNES to detect oxygen vacancy are discussed.…”