Oxygen Vacancies and Intermediate Spin Trivalent Cobalt Ions in Lithium-Overstoichiometric LiCoO2 (original) (raw)
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Journal of Power Sources, 2003
X-ray, IRS, XPS, EDRS and magnetic measurements were used to study Li 1þx CoO 2 (0 < x ! 0:1) samples prepared by conventional ceramic method. It was shown that nonstoichiometric Li 1þx CoO 2 are characterized by homogeneous crystal structure with statistically distributed vacancies in the cobalt and oxygen layers and the increased CoO bond covalency. The excess lithium results not in the reduction of Co 3þ ions, but in the appearance of a new state of oxygen ions different from cell oxide, with higher value of binding energy (BE), i.e. with smaller electronic density. Acid treatment of Li 1þx CoO 2 leads to the appearance of delocalized (itinerant) electrons. The electronic state of cobalt ions does not change noticeably whereas the additional oxygen state increases significantly, thus, evidencing that oxygen ions do compensate for the charge upon chemical delithiation. The structure of nonstoichiometric samples appeared to be more stable upon this process.
The Journal of Physical Chemistry C, 2013
LiCoO 2 , one of the major positive electrode materials for Li-ion batteries, can be synthesized with excess Li. Previous experimental work suggested the existence of intermediate spin (IS) Co 3+ ions in square-based pyramids to account for the defect in this material. We present here a theoretical study based on density functional theory (DFT) calculations together with an X-ray absorption spectroscopy (XAS) experimental study. In the theoretical study, a hypothetical Li 4 Co 2 O 5 material, where all the Co ions are in pyramids, was initially considered as a model material. Using DFT+U, the intermediate spin state of the Co 3+ ions is found stable for U values around 1.5 eV. The crystal and electronic structures are studied in detail, showing that the defect must actually be considered as a pair of such square-based pyramids, and that Co−Co bonding can explain the position of Co in the basal plane. Using a supercell corresponding to more diluted defects (as in the actual material), the calculations show that the IS state is also stabilized. In order to investigate experimentally the change in the electronic structure in the Li-overstoichiometric LiCoO 2 , we used X-ray absorption near edge structure (XANES) spectroscopy and propose an interpretation of the O Kedge spectra based on the DFT+U calculations, that fully supports the presence of pairs of intermediate spin state Co 3+ defects in Lioverstoichiometric LiCoO 2 .
Frontiers of Materials Science, 2015
Pure, layered compounds of overlithiated Li 1+x Ni 0.8 Co 0.2 O 2 (x = 0.05 and 0.1) were successfully prepared by a modified combustion method. XRD studies showed that cell parameters of the material decreased with increasing the lithium content. SEM revealed that the morphology of particles changed from rounded polyhedral-like crystallites to sharp-edged polyhedral crystals with more doped lithium. EDX showed that the stoichiometries of Ni and Co agrees with calculated synthesized values. Electrochemical studies revealed the overlithiated samples have improved capacities as well as cycling behavior. The sample with x = 0.05 shows the best performance with a specific capacity of 113.29 mA•h•g-1 and the best capacity retention of 92.2% over 10 cycles. XPS results showed that the binding energy of Li 1s is decreased for the Li doped samples with the smallest value for the x = 0.05 sample, implying that Li + ions can be extracted more easily from Li 1.05 Ni 0.8 Co 0.2 O 2 than the other stoichiometries accounting for the improved performance of the material. Considerations of core level XPS peaks for transition metals reveal the existence in several oxidation states. However, the percentage of the +3 oxidation state of transition metals for the when x = 0.1 is the highest and the availability for charge transition from the +3 to +4 state of the transition metal during deintercalation is more readily available. KEYWORDS: overlithiation; LiNi 0.8 Co 0.2 O 2 ; interstitial doped; Li 1.05 Ni 0.8 Co 0.2 O 2 ; Li 1.1 Ni 0.8 Co 0.2 O 2 Contents 1 Introduction Lithium cobalt oxide has been widely used as a cathode
Structure and electrochemistry of lithium cobalt oxide synthesised at 400°C
Materials Research Bulletin, 1992
October 25, 1991; Communicated by J.B. Goodenough) A novel LiCoO 2 compound has been prepared by the reaction of Li2CO 3 and CoCO 3 at 400°C. Unlike the well-known LiCoO 2 structure that is synthesised at higher temperature (900°C) and contains Li + and Co 3+ ions in discrete layers between planes of close-packed oxygen ions, the structure of LiCoO 2 (400"C) has approximately 6% cobalt within the lithium layers. The electrochemical properties of LiCoO 2 (400°C) differ significantly from LiCoO 2 (900"C). Whereas electrochemical extraction from LixCoO 2 (900°C) in room-temperature lithium cells takes place as a single-phase reaction above 3.9V for x<__0.9, electrochemical extraction from LixCoO 2 (400°C) occurs as a two-phase reaction at an open-circuit voltage of 3.61V for 0.1<x<0.95. Because Li~CoO z (400°C) is a less oxidizing material than its hightemperature analogue, it is expected to be more stable in many of the organic-based electrolytes that are currently employed in lithium cells.
Chemistry of Materials, 2005
Iron substitution was attempted by direct solid-state synthesis in stoichiometric LiCoO 2 and lithiumoverstoichiometric "Li 1.1 CoO 2 ". Iron substitution was not obtained in stoichiometric LiCo 0.98 Fe 0.02 O 2 samples, consistent with the fact that the size of Fe 3+ ions is significantly larger than that of Co 3+ ions in the octahedral site. In contrast, up to 8 atom % iron could be substituted in the lithium-overstoichiometric "Li 1.1 CoO 2 " samples with an actual composition of Li 1.04 Co 0.96 O 1.96 , which could be rationalized by considering the structural defect model proposed previously by some of us. In the defect model, lithiumoverstoichiometric samples consist of excess Li + replacing Co 3+ charge-compensated by an oxygen vacancy in the cobalt layers, which creates two adjacent square-based pyramids containing intermediate-spin (IS) Co 3+ ([Li] interslab [Co 3+(LS) 1-3t Co 3+(IS) 2t Li t ] slab [O 2-t ], with t) 0.04 and 0.08 IS Co 3+ and LS) low spin). 7 Li MAS NMR showed that the signals associated with intermediate-spin Co 3+ decreased but were not completely suppressed upon iron substitution, even for the 8 atom % Fe-substituted sample, with no new signal appearing in iron-substituted lithium-overstoichiometric samples. Moreover, the values of Mössbauer parameters, isomer shift 0.249 mm‚s-1 and quadrupolar splitting 0.4 mm‚s-1 , revealed that high-spin Fe 3+ was present in the square-pyramidal sites in the 2 atom % iron-substituted lithium-overstoichiometric sample. These results lent further support for the nature of the defects proposed previously for lithiumoverstoichiometric "Li 1.1 CoO 2 ".
Degree of covalency of LiCoO2: X-ray emission and photoelectron study
Solid State Communications, 1996
The layered compound LiCoO2 was studied by means of X-ray emission (CoLc~, OKec spectra) and X-ray photoelectron spectroscopy. The data were compared with those of CoO. Strong covalency of LiCoO2 was found and its value was estimated. The measurements show that LiCoOz must be determined as a charge-transfer insulator where the band gap is formed by dSL-states.
Journal of Physical Chemistry C, 2008
In the present work, the intercalation of lithium ions in the Li x CoO 2 host material was investigated by means of the discharge curves and voltage-composition profile with the main goal of establishing a close correlation between the voltage-composition patterns and the structure of the Li x CoO 2 prepared at low and high temperatures. As a result, the structures of the Li x CoO 2 prepared at low and high temperatures were investigated by a careful analysis of synchrotron X-ray powder diffraction (including a quantitative Rietveld refinement), which showed the presence of two majority phases for Li x CoO 2 prepared at low temperature, that is, spinel and layered phases. On the basis of the information provided by the quantitative structural analysis, the voltagecomposition profiles were interpreted by considering at least two ionic modes of charges, one in which lithium ions occupy sites in the spinel structure and the other in which lithium ions occupy the sites of the layered structure. It was found that at least two modes (two types of site to charged) of charge were related to the layered structure during ionic charging of the host. Finally, and curiously, it was found that the decreasing of the charge capacity as a function of charge-discharge cycles is mainly related to a type of site located in the layered structure (or sites related to layered phase transition, i.e. order-disorder transition from hexagonal to monoclinic symmetry).
Chemistry of Materials, 2013
Stoichiometric lithium cobalt oxide LiCoO 2 is known to exhibit several structural phase transitions with x in Li x CoO 2 at ambient temperature (T); e.g., an initial rhombohedral (R3̅ m) phase transforms into a monoclinic (C2/m) phase at x ∼ 0.5. In contrast, lithium overstoichiometric (Li) 3b [Li δ Co 1−δ ] 3a O 2−δ with δ ≥ ∼0.02, where δ is the Li + ions at the 3a (Co) site, maintains the R3̅ m symmetry until x ∼ 0.5 in Li x (Li δ Co 1−δ)O 2−δ at ambient T, and this is the reason why such material has been widely used in commercial lithium ion batteries. We performed X-ray diffraction measurements in the T range between 100 and 300 K for the lithium overstoichiometric Li x (Li 0.02 Co 0.98)O 1.98 samples with x = 1, 0.56, and 0.51 to understand the factors that govern the structural changes in Li x (Li δ Co 1−δ)O 2−δ with δ ≥ 0. Both x = 0.56 and 0.51 samples exhibit a structural phase transition from the high-T R3̅ m phase to the low-T C2/m phase at 250 K (=T s1). Furthermore, these samples indicate another structural phase transition at 170 K (=T s2); although their crystal structures still have the C2/m symmetry, the degree of monoclinic distortion starts to decrease below T s2 , associated with a magnetic anomaly and a freezing of the Li + ions at the 3b site. Because the two structural phase transitions of T s1 (=330 K) and T s2 (=150 K) are also observed for the stoichiometric Li x CoO 2 compound with x ∼ 0.5, the C2/m phase in Li x (Li δ Co 1−δ)O 2−δ is found to appear in the limited x and T ranges. The characteristics and possible origin of T s1 and T s2 for both stoichiometric Li x CoO 2 and lithium overstoichiometric Li x (Li 0.02 Co 0.98)O 1.98 samples are discussed.