An In Situ X‐Ray Absorption Spectroscopic Study of Charged Li ( 1 − z ) Ni ( 1 + z )  O 2 Cathode Material

Abstract

We have measured in situ the Ni K‐edge X‐ray absorption spectra of Lifalse(1−zfalse)Nifalse(1+zfalse)O2 cathode material charged in a nonaqueous cell. The material was charged to various states of charge (i.e., Li content) which corresponded to x = 0.0, 0.12, 0.24, 0.37, 0.49, and 0.86 in Lifalse(1−x−zfalse)Nifalse(1+zfalse)O2 . We have determined variations in the Ni‒O and Ni‒Ni coordination numbers, bond lengths, and local disorders as well as the Ni K‐edge energies as a function of Li content. We show that in the pristine state, the composition of the material can be described by the formula Li0.86Ni1.14O2 (i.e., x = 0 and z = 0.14). That is, the material consists of Ni2+ (25%) and Ni3+ (75%) with half the Ni2+ atoms residing in Li sites and the other half in the NiO2 slabs. Upon charging, initially Ni2+ is oxidized to Ni3+ up to a state of charge which corresponds to x = 2 z . Upon further charging to states corresponding to 2z <x≤1 − z, Ni3+ is oxidized to Ni4+ with fractions being dependent on the values of x and z. Analysis of the edge energies for NiO, stoichiometric LiNiO2 , and KNiIO6 as reference compounds for Ni2+ , Ni3+ , and Ni4+ , respectively, shows a quadratic dependence for edge energy vs. oxidation state. This type of correlation is consistent with variations observed in earlier studies for some Mn reference compounds in the same range of oxidation states. Oxidation‐state determination of Ni in Lifalse(1−x−zfalse)Nifalse(1+zfalse)O2 as a function of state of charge (i.e., Li content or x) on the basis of edge energies yielded results which are in excellent agreement with oxidation state determinations made on the basis of the mole fractions for Ni2+ , Ni3+ , and Ni4+ extracted from extended X‐ray absorption fine structure spectra. © 1999 The Electrochemical Society. All rights reserved.

The Science of Electrode Materials for Lithium Batteries

“…This is consistent with recent theoretical work [36][37][38]. Not all experimental work has been interpreted in this way [39][40][41][42][43], but measurements of the O K-edge do show such behavior [34,[44][45][46][47]. From measurements of the Ni and Co L 2,3 edges, we found that there may be a small change in 3d occupany at Ni atoms, but not at Co atoms in our samples of LiNi 0.8 Co 0.2 O 2 [34].…”

“…* We find no evidence of special structures at compositions x=0.5 and x=0.75 as reported by Delacourt, et al [41], and they have no evidence that the new materials at these compositions are anything other than disordered phases [42].…”

Surface properties of LiCoO2, LiNiO2 and LiNi1−xCoxO2

“…LiCoO 2 does, however, suffer from relatively high cost [10] and environmental processing and disposal problems [11]. For this reason, the related materials LiNiO 2 and LiNi 1−x Co x O 2 have been studied as potential substitutes for the lithium ion anode [12], [13], [14], [15], [16], [17] and [18]. LiCoO 2 and "hexagonal" LiNiO 2 can be idealized as cubic (rocksalt) structures in which planes of the much smaller lithium ions alternate with cobalt or nickel ions along the 〈 1 1 1〉 direction ( Fig.…”

White lines andd-band occupancy for the3dtransition-metal oxides and lithium transition-metal oxides

“…Other research has been driven by the success of the lithiated transition-metal oxides as cathodes in rechargeable lithium batteries. A number of x-ray absorption and EELS experiments have been performed to investigate charge compensation during electrochemical cycling of lithium 2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19 17 and it is unclear how much charge is transferred to each transition-metal ion. It is our contention that changes in the transitionmetal valence can be measured quantitatively using the transition-metal white lines.…”