Phase transitions and structural mechanisms of the formation of LiCoO2 polymorphic modifications (original) (raw)
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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.
Structural Stability of LiCoO2 at 400°C
Journal of Solid State Chemistry, 2002
The relative stability of the lithiated-spinel structure, Li 2 [Co 2 ]O 4 , at 4001C to the layered LiCoO 2 structure has been investigated. ''Low-temperature'' LT-LiCoO 2 samples were synthesized at 4001C by the solid-state reaction of Li 2 CO 3 with CoCO 3 (or Co 3 O 4) for various times between 10 min and 232 days. Least-squares refinements of X-ray powder diffraction patterns were used to determine the fractions of lithiated-spinel Li 2 [Co 2 ]O 4 and layered LiCoO 2 in the samples. X-ray powder diffraction and transmission electron microscope data show that Li 2 [Co 2 ]O 4 nucleates from an intermediate Li x Co 1Àx [Co 2 ]O 4 spinel product before transforming very slowly to layered LiCoO 2. The experimental data confirm the theoretical prediction that layered LiCoO 2 is thermodynamically more stable than the lithiated-spinel structure at 4001C and support the arguments that a non-ideal cation distribution in Li 2 [Co 2 ]O 4 , non-stoichiometry and kinetic factors restrict the transformation of the lithiated-spinel structure to layered LiCoO 2 at this temperature.
Structural Features of Low-Temperature LiCoO2and Acid-Delithiated Products
Journal of Solid State Chemistry, 1998
X-ray diffraction and transmission electron microscopy were used to study the structural features of low-temperature LiCoO 2 (LT-LiCoO 2) samples prepared at 400°C either by a simple solid-state reaction or via a sol-gel process. Single-crystal electron diffraction analysis showed that both a lithiated-spinel Li 2 [Co 2 ]O 4 (Fd3m) and a layered-type structure (R3 m) were present in LT-LiCoO 2 samples but that the lithiated-spinel structure was the major phase. Electron diffraction analysis also indicated that some crystallites in the LT-LiCoO 2 samples had a cation distribution in the spinel notation, +(Li 16؊4x) 16c [Li 4x ] 16d , layer1 +(Co 16؊4x) 16d [Co 4x ] 16c , layer2 O 32 (0 < x < 1) that was intermediate between the ideal layered (x ؍ 1) and ideal lithiated-spinel (x ؍ 0) structures. Electron diffraction confirmed that acid-delithiation of LT-LiCoO 2 resulted in a lithium-deficient spinel, Li 0.8 [Co 2 ]O 4 , with lithium ions on the tetrahedral sites of the spinel structure. The structural features of LT-LiCoO 2 and the aciddelithiated Li 0.4 CoO 2 products provide reasons for the poor electrochemical properties of Li/LT-LiCoO 2 cells and are consistent with earlier studies.
First-Principles Investigation of Phase Stability in the O2-LiCoO 2 System
Chemistry of Materials, 2003
A first-principles investigation of the phase stability in the O2-LiCoO 2 system is performed to better understand the unusual layered phases obtained upon Li deintercalation (i.e., T # 2 and O6). First-principles pseudopotential calculations within the local density approximation and thermodynamic models extracted from these calculations both show that two tetrahedral sites for the Li ions need to be considered in the T # 2 structure for qualitative agreement with experiment to be obtained. Only when both tetrahedral sites in T # 2 are considered is the experimentally observed two-phase O2/T # 2 region predicted. This indicates that this structural phase transformation is induced by enhanced configurational entropy in the T # 2 phase and not by a metal-insulator transition as was previously proposed. We also predict that two ordered compounds are stable at room temperature: Li 1/4 CoO 2 in the O2 structure and Li 1/3 CoO 2 in the O6 structure. We show that the formation of the O6 phase is not related to Li staging. (1) Delmas, C.; Braconnier, J. J.; Hagenmuller, P. Mater. Res. Bull. 1982, 17, 117. (2) Carlier, D.; Saadoune, I.; Suard, E.; Croguennec, L.; Ménétrier, M.; Delmas, C. Solid State
Structural Study of the T#2-LixCoO2 (0.52 < x ≤ 0.72) Phase
Inorganic Chemistry, 2004
The metastable O2-LiCoO 2 phase undergoes several reversible phase transitions upon lithium deintercalation. The first transition leads to an unusual oxygen stacking in such layered compounds. This stacking is found to be stable for 0.52 < x e 0.72 in Li x CoO 2 and is called T # 2. We studied this phase from a structural viewpoint using X-ray and neutron diffraction (ab initio method). The new stacking derives from the O2 one by gliding every second CoO 2 slab by (1 / 3 , 1 / 6 , 0). The lithium ions are found to occupy very distorted tetrahedral sites in this structure. We also discuss the possibility of this T # 2 phase to exhibit stacking faults, whose amount depends on the method used to prepare this deintercalated phase.
Disordering and Electronic State of Cobalt Ions in Mechanochemically Synthesized LiCoO2
Journal of Solid State Chemistry, 2002
Mechanical activation (MA) combined with heat treatment at moderate temperatures was used to prepare disordered and highly dispersed LiCoO 2 starting from the mixtures of various cobalt precursors (CoOOH, Co(OH) 2 , and Co) and LiOH. Xray powder diffraction and IR spectroscopy were used to investigate the phase composition and the crystal structure of as-prepared samples, while the electronic state of cobalt ions was characterized by diffuse reflectance electron spectroscopy. MA of the LiOH+CoOOH mixture led to the formation of LT-LiCoO 2 with a cubic spinel-related structure. Heat treatment at 6001C of the latter resulted in the formation of HT-LiCoO 2 with a hexagonal layered structure similar to ceramic LiCoO 2 . However, as-prepared HT-LiCoO 2 is characterized by Co 3+ O 6 octahedra less perfect than those of ceramic LiCoO 2 . All MA-LiCoO 2 samples are exclusively described by localized d electrons. # 2002 Elsevier Science (USA)
A reinvestigation of the structures of lithium-cobalt-oxides with neutron-diffraction data
Materials Research Bulletin, 1993
The structures of LT-LiCoO 2 (synthesised by reaction of LizCO a and CoCO a at 40(Y'C) and its delithiated product LT-Lio.4CoO 2 have been reinvestigated by neutron powder diffraction. Despite an unusually close similarity between diffraction profiles that makes it difficult to determine whether the structures are layered or spinel-like, the data confirm that the preferred structure of the LT-LiCoO 2 sample made for this study is one that has a cobalt distribution which is intermediate between an ideal layered and an ideal lithiated spinel structure. On the other hand, refinement of the data of LT-Li0.4CoO 2 prepared by reacting LT-LiCoO 2 with acid shows, unequivocally, that a spineltype structure is formed. These structures are discussed in relation to previously reported electrochemical data obtained from Li/LT-LiCoO 2 cells.
Solid State Ionics, 2003
Thermal decomposition of freeze-dried salt precursors leads to the formation of low-temperature (LT) modification of LiCoO 2 at 350 -450 jC. The conversion rate of LT into high-temperature (HT) modification at 850 jC depends greatly on the anion composition of salt precursors and correlates quite well with the appearance of second step at thermogravimetric curves of their thermal decomposition related to the solid-state reaction between Li 2 CO 3 and Co 3 O 4 . Relationship between the appearance of Co 3 O 4 and preferential formation of LT/HT polymorphs at reduced temperatures is discussed. The consecutive formation of LT and HT modifications during solid-state reaction between Li 2 CO 3 and Co 3 O 4 at T>800 jC was observed. LiCoO 2 cathode materials with the domination of LT polymorph demonstrated a better initial discharge capacity while a greater amount of HT modification is accompanied by better reversibility of charge -discharge processes. D
Inorganic Chemistry, 2013
We report the first in-situ time-resolved X-ray diffraction investigation in conjunction with a non-isothermal kinetic study using the model-free isoconversional kinetic method to determine the formation mechanism for the solid-state synthesis of electrochemically active LiCoO 2 from Li 2 CO 3 and Co 3 O 4. Detailed information on the phase evolution as well as thermal events during the heating process was clearly observed, explained, and supported. This investigation provides structural as well as kinetic evidence for a multistep reaction and proposes the first plausible formation mechanism for the solid-state synthesis of LiCoO 2 .
First-principles investigation of phase stability in Li x CoO 2
In this work, the phase diagram of Li x CoO 2 is calculated from first principles for x ranging from 0 to 1. Our calculations indicate that there is a tendency for Li ordering at x 1 2 in agreement with experiment J. N. Reimers and J. R. Dahn, J. Electrochem. Soc. 139, 2091 1992. At low Li concentration, we find that a staged compound is stable in which the Li ions selectively segregate to every other Li plane leaving the remaining Li planes vacant. We do not find the two-phase region observed at high Li concentration and speculate that this two-phase region is caused by the metal-insulator transition that occurs at concentrations slightly below x 1. S0163-18299806529-1