Microscopy and Spectroscopy of Lithium Nickel Oxide-Based Particles Used in High Power Lithium-Ion Cells

Abstract

Structural and electronic investigations were conducted on lithium nickel oxide-based particles used in positive electrodes of 18650-type high-power Li-ion cells. K-edge X-ray absorption spectroscopy ͑XAS͒ revealed trivalent Ni and Co ions in the bulk LiNi 0.8 Co 0.2 O 2 powder used to prepare the high power electrode laminates. Using oxygen K-edge XAS, high resolution electron microscopy, nanoprobe diffraction, and electron energy-loss spectroscopy, we identified a Ͻ5 nm thick modified layer on the surface of the oxide particles, which results from the loss of Ni and Li ordering in the layered R3 m structure. This structural change was accompanied by oxygen loss and a lowering of the Ni-and Co-oxidation states in the surface layer. Growth of this surface layer may contribute to the impedance rise observed during accelerated aging of these Li-ion cells.The Li x Ni 1Ϫx O solid solutions form over a lithium concentration range of 0 р x р 0.5. 1-3 When lithium is added to NiO (x ϭ 0), conventional wisdom states that the Li ϩ charge compensation is achieved through the creation of Ni ϩ3 ions. The crystal structure of these solid solutions changes with x. For low x (0 р x р 0.3), lithium cations substitute randomly for nickel cations in the NiO lattice ͑NaCl-type structure͒. For x Ͼ 0.3, ordering of the Li and Ni ions starts in alternating cubic ͑111͒ planes culminating at x ϭ 0.5 in layered LiNiO 2 (␣-NaFeO 2 structure͒. The oxidation state of Ni has long been believed to be ϩ3 in LiNiO 2 -based compounds.Recent electronic structure investigations have, however, sparked a debate on the Ni oxidation states. Several spectroscopic and theoretical studies have concluded that hole states in Li x Ni 1Ϫx O compounds are localized predominantly at oxygen rather than at nickel, and that the charge compensating states are better described as O Ϫ rather than Ni ϩ3 ; that is, the nickel ions retain their ϩ2 character for all values of x. [4][5][6][7][8] In contrast, other researchers have affirmed the traditional view that Ni ϩ3 is present in LiNiO 2 -based compounds. 9-13 The charge-compensation debate achieves additional complexity when lithium is intercalated and deintercalated from these compounds. Several theoretical and ex situ experimental studies have indicated that the majority of the electron charge is accommodated by the oxygen anions during Li-ion cell cycling, [14][15][16][17][18][19] whereas other in situ experimental studies have indicated that the charge compensation occurs predominantly at the nickel sites. [11][12][13] An understanding of nickel oxidation states and electron charge compensation processes is critical for determining mechanisms that limit the calendar life and thermal abuse characteristics of high power Li-ion cells being developed for hybrid electric vehicle ͑HEV͒ applications as part of the Advanced Technology Development program in the United States. [20][21][22][23][24][25] In this program, the capacity and pulse power capabilities of ϳ1 Ah capacity, 18650-type ͑18 mm diam, 65 mm long͒ cells,...

Surface properties of LiCoO2, LiNiO2 and LiNi1−xCoxO2

“…Rather, it appears harder to form a bulk solid solution of the mixed transition metal oxide LiNi 0.5 Co 0.5 O 2 and conditions necessary to promote a homogeneous, crystalline bulk may be too harsh to preserve the stoichiometry of the surface. TEM structural studies of the closely related material LiNi 0.8 Co 0.2 O 2 [25] fi nd an approximately 5 nm thick surface region with substantial nickel and lithium disordering of the layered R3m bulk structure. Instead of stabilizing the substrate properties of nickel-containing lithium oxides to more closely resemble LiCoO 2 , LiNi 0.5 Co 0.5 O 2 appears to be closer in surface oxygen and lithium chemical environment to LiNiO 2 as measured by XPS and to have inherited problems of lithium depletion of this material.…”

“…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.…”

Editors' Choice—Growth of Ambient Induced Surface Impurity Species on Layered Positive Electrode Materials and Impact on Electrochemical Performance

“…6,[19][20][21][22] It has been proposed that the transition metal reduction from a valence state of +3 to +2 produces active oxygen species which concurrently catalyzes the formation of * Electrochemical Society Member.…”

XAFS Investigations of LiNi0.45Mn0.45Co0.1−yAlyO2Positive Electrode Materials

“…This can entail either a decrease in the 3a Ni concentration (via migration to the transition metal layer) or removal of the 3a Ni into a NiO-like layer on the particle surfaces, for instance. 45,46 Some studies have suggested that a majority (~75%) of the 3a…”