First-principles investigation of phase stability in Li x CoO 2 (original) (raw)

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

First-Principles Evidence for Stage Ordering in li x Co0 2

We ha\~e investigated phase stability in the laye~ed Li"CoO~ intercalation compound for x < 0.4 from first princi les. By combmm §; a lattice model description of the Li-vacancy configurational degrees of freedom with first-rincr les ~sffudopotentlal calculat.lOns, we hav~ calculated the free. energy of the material as a function of Li concentratio~ in tgree I erent. host structures ..(I) the rhombohedral form of LI"COO~, (ii) the hexagonal form of CoO, and (iii) a staze II com­ pound 01 LI,CoO~ in which the host structure can be considered as a hybrid of the rhombohedral and hexagonal host structures. The Iirst-principles .free energies indicate .that the ~tage I~ compound is the most stable of the thr~e phases for

Ab INITIO CALCULATION OF THE LixCoO2 PHASE DIAGRAM

MRS Proceedings, 1997

ABSTRACTThe electrochemical properties of the layered intercalation compound LiCoO2 used as a cathode in Li batteries have been investigated extensively in the past 15 years. Despite this research, little is known about the nature and thermodynamic driving forces for the phase transformations that occur as the Li concentration is varied. In this work, the phase diagram of LixCoO2 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 [1]. 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 find that the two phase region observed at high Li concentration is not due to Li ordering and speculate that it is driven by a metal-insulator transition which occurs at concentrations slightly below x < 1.

Electronic phase diagram of the layered cobalt oxide system LixCoO2 (0.0≤x≤1.0)

Physical Review B, 2009

Here we report the magnetic properties of the layered cobalt oxide system, Li x CoO 2 , in the whole range of Li composition, 0 Յ x Յ 1. Based on dc-magnetic-susceptibility data, combined with results of 59 Co nuclear magnetic resonance ͑NMR͒ and nuclear quadrupole resonance ͑NQR͒ observations, the electronic phase diagram of Li x CoO 2 has been established. As in the related material Na x CoO 2 , a magnetic critical point is found to exist between x = 0.35 and 0.40, which separates the Pauli-paramagnetic and Curie-Weiss metals. In the Pauli-paramagnetic regime ͑x Յ 0.35͒, the antiferromagnetic spin correlations systematically increase with decreasing x. Nevertheless, CoO 2 , the x = 0 end member is a noncorrelated metal in the whole temperature range studied. In the Curie-Weiss regime ͑x Ն 0.40͒, on the other hand, various phase transitions are observed. For x = 0.40, a susceptibility hump is seen at 30 K, suggesting the onset of static antiferromagnetic order. A magnetic jump, which is likely to be triggered by charge ordering, is clearly observed at T t Ϸ 175 K in samples with x = 0.50 ͑=1 / 2͒ and 0.67 ͑=2 / 3͒, while only a tiny kink appears at T Ϸ 210 K in the sample with an intermediate Li composition, x = 0.60. Thus, the phase diagram of the Li x CoO 2 system is complex and the electronic properties are sensitively influenced by the Li content ͑x͒.

Impact of Lithium-Ion Ordering on Surface Electronic States of LixCoO2

Physical review letters, 2013

Li x CoO 2 exhibits intriguing electronic properties due to a strong electron correlation and complex interplay between Co and Li ions. However, fundamental understanding of the nanoscale distribution of Li ions and its effect on the electronic properties remains unclear. We use scanning tunneling microscopy and density functional theory to elucidate the degree of Li x CoO 2 surface electronic state modification that can be achieved by Li ordering. The surface Li ions are highly mobile and preferentially form a (1× 1) hexagonal lattice, whereas the surface CoO 2 layer shows metallic and insulating phases, indicating the coexistence of ordered and disordered Li ions in the subsurface layer. These results provide evidence of novel electronic properties produced by spatially inhomogeneous Li-ordering patterns.

Cation and vacancy ordering in Li x CoO 2

Using a combination of first-principles total energies, a cluster expansion technique, and Monte Carlo simulations, we have studied the Li/Co ordering in LiCoO 2 and Li-vacancy/Co ordering in the CoO 2. We find: i A ground-state search of the space of substitutional cation configurations yields the CuPt structure as the lowest-energy state in the octahedral system LiCoO 2 and CoO 2), in agreement with the experimentally observed phase. ii Finite-temperature calculations predict that the solid-state order-disorder transitions for LiCoO 2 and CoO 2 occur at temperatures (5100 K and 4400 K, respectively much higher than melting , thus making these transitions experimentally inaccessible. iii The energy of the reaction E tot (,LiCoO 2)E tot (,CoO 2)E tot (Li, bcc) gives the average battery voltage V ¯ of a Li x CoO 2 /Li cell for the cathode in the structure. Searching the space of configurations for large average voltages, we find that CuPt a monolayer 111 superlattice has a high voltage (V ¯ 3.78 V), but that this could be increased by cation randomization (V ¯ 3.99 V), by partial disordering (V ¯ 3.86 V), or by forming a two-layer Li 2 Co 2 O 4 superlattice along 111 (V ¯ 4.90 V). S0163-18299800904-7

Synthesis and Properties of CoO 2 , the x = 0 End Member of the Li x CoO 2 and Na x CoO 2 Systems

Chemistry of Materials, 2007

We report here the synthesis of single-phase bulk samples of CoO 2 , the x = 0 end member of the A x CoO 2 systems (A = Li, Na), from a pristine LiCoO 2 sample using an electrochemical technique to completely de-intercalate lithium. Thus, synthesized CoO 2 samples were found to be oxygen-stoichiometric and possess a crystal structure consisting of stacked triangular-lattice CoO 2 layers only. The magnetic susceptibility of the CoO 2 sample was revealed to be relatively large in its initial value and then level off as the temperature increases, suggesting that CoO 2 is a Pauli-paramagnetic metal with itinerant electrons.

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.