Crystal structure and microstructure of some La(2/3-x)Li 3xTiO3 oxides: an example of the complementary use of electron diffraction and microscopy and synchrotron X-ray diffraction to study complex materials

Three representative oxides of the La(2/3)(-)(x)()Li(3)(x)()TiO(3) system have been studied by selected area electron diffraction (SAED), high-resolution transmission electron microscopy (HRTEM), and powder synchrotron X-ray diffraction. HRTEM showed that the materials have a complex microstructure. The SAED and HRTEM results have allowed us to propose a model to refine the crystal structure of these oxides that also accounts for their microstructure. The materials have a perovskite-related structure with a diagonal unit cell ( radical 2a(p) x radical 2a(p) x 2a(p)) as a consequence of the tilting of the TiO(6) octahedra. Ordering of lanthanum and lithium ions and vacancies along the 2a(p)-axis, as well as displacements of titanium ions from the center of the octahedra, have been determined. The size and shape of the domains have been obtained from the synchrotron X-ray diffraction data; in addition, other extended defects such as strains and compositional fluctuations have been detected.     

On the influence of the vacancy distribution on the structure and ionic conductivity of A-site-deficient Li(x)Sr(x)La(2/3-x)TiO3 perovskites

The crystal structure and dielectric properties of slowly cooled A-site-deficient perovskites Li(x)Sr(x)La(2/3-x)□(1/3-x)TiO(3) (0.04 ≤ x ≤ 0.33) have been investigated by powder X-ray diffraction (XRD), impedance spectroscopy, and (7)Li NMR techniques. In this series, nominal vacancies decrease with Li content, but the total amount of A-site vacancies, n(t) = Li + □, participating in conduction processes remains basically constant. Rietveld analysis of the XRD patterns showed a change of symmetry from orthorhombic to tetragonal when the lithium and strontium contents increased above x = 0.08 and from tetragonal to cubic above x = 0.16. Structural modifications are mainly due to the cation vacancy ordering along the c axis, which disappear gradually when the lithium content increases. In agreement with the structural information, two lithium signals with different quadrupole constants are detected in (7)Li NMR spectra of orthorhombic/tetragonal phases, which have been associated with lithium in two crystallographic z/c = 0 and 1/2 planes of perovskites. In cubic samples, only a single narrow component, indicative of mobile species, was detected. Lithium motion was thermally activated, with activation energies going from 0.35 to 0.38 eV. Evolution of the bulk dc-conductivity preexponential factors along the series showed a maximum that has been first related to the dependence of lithium hopping on the lithium and vacancy concentrations. Finally, changes in the vacancy ordering, produced along the series, affect the dimensionality of the conductivity, indicating that not only the amount of vacancies but also its distribution are relevant.     

Synthesis, X-ray and neutron diffraction characterization, and ionic conduction properties of a new oxothiomolybdate Li3[Mo8S8O8(OH)8[HWO5(H2O)]] x 18H2O

The new oxothiomolybdate anion [Mo8S8O8(OH)8[HWO5(H2O)]]3- (denoted HMo8W3-) has been synthesized in aqueous solution by an acido-basic condensation reaction. Four (Mo(V)2S2O2) building blocks are connected through hydroxo bridges around a central [W(VI)O6] octahedron. X-ray and neutron diffraction studies have been performed on single crystals of the lithium salt Li3[Mo8S8O8(OH)8[HWO5(H2O)]] x 18H2O (Li3HMo8W x 18H2O) in an aqueous grown from HMo8W3- solution of LiCl (1 M). The neutron diffraction experiment enabled us to locate both the protons and the lithium ions. In the structure of Li3HMo8W x 18H20, ring-shaped anions interleaved by a cluster of disordered hydrogen-bonded water molecules stack on top of each other along lithium pillars. The lithium columns are formed by alternating edge-sharing octahedra and tetrahedra, with one lithium site in four being totally vacant. Ionic conductivity measurements on pressed pellets have shown that Li3HMo8W x 18H2O is a good ionic conductor at room temperature (sigma = 10(-5) S cm(-1)), but the ionic conductivity on single crystals is smaller by two orders of magnitude and is isotropic; this suggests the main path of conduction involves surface protons rather than lithium ions of the bulk.     

Structural and thermochemical chemisorption of CO2 on Li(4+x)(Si(1-x)Al(x))O4 and Li(4-x)(Si(1-x)V(x))O4 solid solutions

Different Li(4)SiO(4) solid solutions containing aluminum (Li(4+x)(Si(1-x)Al(x))O(4)) or vanadium (Li(4-x)(Si(1-x)V(x))O(4)) were prepared by solid state reactions. Samples were characterized by X-ray diffraction and solid state nuclear magnetic resonance. Then, samples were tested as CO(2) captors. Characterization results show that both, aluminum and vanadium ions, occupy silicon sites into the Li(4)SiO(4) lattice. Thus, the dissolution of aluminum is compensated by Li(1+) interstitials, while the dissolution of vanadium leads to lithium vacancies formation. Finally, the CO(2) capture evaluation shows that the aluminum presence into the Li(4)SiO(4) structure highly improves the CO(2) chemisorption, and on the contrary, vanadium addition inhibits it. The differences observed between the CO(2) chemisorption processes are mainly correlated to the different lithium secondary phases produced in each case and their corresponding diffusion properties.     

Electronically driven structural distortions in lithium intercalates of the n = 2 Ruddlesden-Popper-type host Y2Ti2O5S2: synthesis, structure, and properties of LixY2Ti2O5S2 (0 < x < 2)

Lithium intercalation into the oxide slabs of the cation-deficient n = 2 Ruddlesden-Popper oxysulfide Y(2)Ti(2)O(5)S(2) to produce Li(x)Y(2)Ti(2)O(5)S(2) (0 < x < 2) is described. Neutron powder diffraction measurements reveal that at low levels of lithium intercalation into Y(2)Ti(2)O(5)S(2), the tetragonal symmetry of the host is retained: Li(0.30(5))Y(2)Ti(2)O(5)S(2), I4/mmm, a = 3.80002(2) A, c = 22.6396(2) A, Z = 2. The lithium ion occupies a site coordinated by four oxide ions in an approximately square planar geometry in the perovskite-like oxide slabs of the structure. At higher levels of lithium intercalation, the symmetry of the cell is lowered to orthorhombic: Li(0.99(5))Y(2)Ti(2)O(5)S(2), Immm, a = 3.82697(3) A, b = 3.91378(3) A, c = 22.2718(2) A, Z = 2, with ordering of Li(+) ions over two inequivalent sites. At still higher levels of lithium intercalation, tetragonal symmetry is regained: Li(1.52(5))Y(2)Ti(2)O(5)S(2), I4/mmm, a = 3.91443(4) A, c = 22.0669(3) A, Z = 2. A phase gap exists close to the transition from the tetragonal to orthorhombic structures (0.6 < x < 0.8). The changes in symmetry of the system with electron count may be considered analogous to a cooperative electronically driven Jahn-Teller type distortion. Magnetic susceptibility and resistivity measurements are consistent with metallic properties for x > 1, and the two-phase region is identified as coincident with an insulator to metal transition.     

Coupled ion/electron hopping in Li(x)NiO2: a 7Li NMR study

Deintercalated "Li(x)NiO2" materials (x = 0.25, 0.33, 0.50, 0.58, and 0.65) were obtained using the electrochemical route from the Li0.985Ni1.015O2 and Li0.993Ni1.007O2 compounds. Refinements of X-ray diffraction data using the Rietveld method show a good agreement with the phase diagram of the Li(x)NiO2 system studied earlier in this laboratory. Electronic conductivity measurements show a thermally activated electron-hopping process for the deintercalated Li0.5NiO2 phase. In the Li(x)NiO2 materials investigated (x = 0.25, 0.33, 0.50, and 0.58), 7Li NMR shows mobility effects leading to an exchanged signal at room temperature. A clear tendency for Li to be surrounded mainly by Ni3+ ions with the 180 degree configuration is observed, particularly, for strongly deintercalated materials with smaller Li+ and Ni3+ contents, even upon heating, when this mobility becomes very fast in the NMR time scale. This suggests that Li/vacancy hopping does occur on the NMR time scale but that Ni3+/Ni4+ hopping does not occur independently. The position of Li seems to govern the oxidation state of the Ni around it at any time; the electrons follow the Li ions to satisfy local electroneutrality and minimal energy configuration. The observed NMR shifts are compatible with the Li/vacancy and Ni3+/Ni4+ ordering patterns calculated by Arroyo y de Dompablo et al. for x = 0.25 and x = 0.50, but not for x = 0.33 and x = 0.58.     

NMR study of Li+ ion dynamics in the perovskite Li(3x)La(1/3-x)NbO3

(7)Li and (6)Li nuclear magnetic resonance (NMR) experiments are carried out on the perovskite Li(3x)La(1/3-x)NbO(3). The results are compared to those obtained on the titanate Li(3x)La(2/3-x)TiO3 (LLTO) in order to investigate the effect, on the lithium ion dynamics, of the total substitution of Nb(5+) for Ti(4+) in the B-site of the ABO(3) perovskites. The XRD patterns analysis reveals that this substitution leads to a change in the distribution of the La(3+) ions in the structure. La(3+) ions distribution is very important, in regard to ionic conductivity, because these immobile ions can be considered as obstacles for the long-range Li+ motion. If compared to the titanates, the compounds of the niobate solid solution have a bigger unit cell volume, a smaller number of La(3+) ions, and a higher number of vacancies. These should favor the motion of the mobile ions into the structure. This is not experimentally observed. Therefore, the interactions between the mobile species and their environment greatly influence their mobility. (7)Li and (6)Li NMR relaxation time experiments reveal that the Li relaxation mechanism is not dominated by quadrupolar interaction. (7)Li NMR spectra reveal the presence of different Li+ ion sites. Some Li+ ions reside in an isotropic environment with no distortion, some others reside in weakly distorted environments. T(1), T(1)(rho), and T(2) experiments allow us to evidence two motions of Li+. As in LLTO, T(1) probes a fast motion of the Li+ ions inside the A-cage of the perovskite structure and T(1)(rho) a slow motion of these ions from A-cage to A-cage. At variance with what has been observed in LLTO, these different Li+ ions can be differentiated through the spin-lattice relaxation times, T(1) and T(1)(rho), as well as through the transverse relaxation time, T(2).     

Ti4+离子掺杂对Li3V2(PO4)3晶体结构与性能的影响

采用溶胶凝胶/碳热还原法合成了锂离子电池正极材料Li3V2(PO4)3及其掺Ti化合物Li3-2x(V1-xTix)2-(PO4)3. 电化学测试结果表明, 经Ti4+离子掺杂后材料的充放电性能及循环性能明显提高. 与纯相Li3V2(PO4)3在3.58、3.67和4.08 V出现三个平台相比, 掺杂后材料的前两个平台发生简并且平台趋于模糊的倾斜状态. 这种趋势随掺杂量的增大而增强. 差热分析(DTA)表明掺杂生成了稳定的酌相产物. 采用X射线衍射和Rietveld方法表征了化合物的晶体结构, 结果表明, 三个不同位置Li的不完全占据导致晶体中产生阳离子空穴, 使材料在常温下的离子电导率提高了3个数量级. 锂离子混排提高了样品的电导率和充放电比容量.     

Li(+) ion conductivity in rock salt-structured nickel-doped Li(3)NbO(4)

Two mechanisms of doping Li(3)NbO(4), which has an ordered, rock salt superstructure, have been established. In the "stoichiometric mechanism", the overall cation-to-anion ratio is maintained at 1:1 by means of the substitution 3Li(+) + Nb(5+) --> 4Ni(2+). In the "vacancy mechanism", Li(+) ion vacancies are created by means of the substitution 2Li(+) --> Ni(2+). Solid solution ranges have been determined for both mechanisms and a partial phase diagram constructed for the stoichiometric join. On the vacancy join, the substitution mechanism has been confirmed by powder neutron diffraction; associated with lithium vacancy creation, a dramatic increase in Li(+) ion conductivity occurs with increasing Ni content, reaching a value of 5 x 10(-4) Omega(-1) cm(-1) at 300 degrees C for composition x= 0.1 in the formula Li(3-2x)Ni(x)NbO(4). This is the first example of high Li(+) ion conductivity in complex oxides with rock salt-related structures.     

Relationship between the electrochemical behavior and Li arrangement in Li(x)M(y)Mn(2-y)O4 (M = Co, Cr) with spinel structure

The relationship between the electrochemical behavior and the arrangement of lithium/vacancies has been investigated with electrochemical Li removal in Li(x)M(y)Mn(2-y)O4 (x < or = 1.0, 0.0 < or = y < or = 0.3, M = Co, Cr). It was shown that the electrochemical removal proceeds via two voltage regions: (1) approximately 3.9 V at x > or = approximately 0.5 and (2) approximately 4.2 V at x < or = approximately 0.5. To understand the stepwise behavior, entropy measurement of reaction, DeltaS(obs), was performed by using the electrochemical methods. The changes of the sign in deltaS(obs) from negative to positive at the composition x approximately 0.50 in Li(x)M(y)Mn(2-y)O4 indicated that the ordered arrangement of Li/vacancies was formed with electrochemical Li removal. Moreover, such an ordering was suppressed by the substitution of Co3+ and Cr3+ for Mn3+. To clarify the nature and origin of Li/vacancy ordering, the Monte Carlo simulation was performed in view of Coulombic interaction. The simulation reproduced the formation of a new phase arising from Li/vacancy ordering at x = 0.50 in Li(x)Mn2O4. In addition, the ordered arrangement of Li/vacancy at x = 0.5 was perturbed by the trivalent M3+ replacement in spinel structure due to the local clustering of Li+ around M3+. Consequently, the electrochemical behavior in spinel LiMn2O4 was deeply related to the Coulombic interactions, proved by the fact that experimentally observed changes in entropy agreed well with Monte Carlo simulation based on the Coulombic interaction.     

Structural and electrical characterisation of Li(2)O : TiO(2) : SnO(2) : P(2)O(5) electrolyte glass

Glasses of general formula 50Li(2)O : xSnO(2) : (10 -x)TiO(2) : 40P(2)O(5)(0.0 < or = x < or = 10) were investigated by differential scanning calorimetry, X-ray diffraction and ac impedance, (31)P solid-state NMR and IR spectroscopies. Three isotropic resonances can be identified in the (31)P NMR spectra, which have been assigned to various phosphate species. Analysis of the ratios of integrated intensities in the (31)P spectra leads to models for the Ti and Sn coordination environments. Both TiO(2) and SnO(2) are found to be predominantly network forming with Ti and Sn proposed to be in five- and four-coordinate environments respectively. Analysis of ac impedance spectra collected at low temperatures reveals two forms of permittivity dispersion, viz: high frequency conductivity dispersion and Cole-Cole type relaxation of permittivity. The activation energy of the relaxation frequency of the permittivity dispersion is equal to that of the dc conductivity, which is consistent with cooperative motion of lithium ions. The results also suggest that the observed increase in conductivity with temperature appears to be mainly due to an increase in mobility rather than increase in carrier concentration.     

Crystal and magnetic structure of the system Li0.5+0.5xFe2.5-1.5xTixO4 (x=0.16, 0.44, and 0.72)

The influence of thermal treatment on the structure of lithium titanium ferrite spinels of general formula Li(0.5+0.5x)Fe(2.5-1.5x)Ti(x)O4 (x=0.16, 0.44, and 0.72) has been studied by X-ray and neutron diffraction and analysed by the Rietveld method. The results allow us to conclude the presence of an ordered phase (space group P4(3)32) at room temperature for the sample with x=0.16; this phase does not appear for the remaining compositions. The magnetic properties evidence a ferrimagnetic ordering in the structures when a random cation distribution (space group Fd3 m) is obtained. An exhaustive study carried out by neutron diffraction measurements on these samples shows a different behaviour when the titanium content is increased, concluding that the lower substituted phase (x=0.16) exhibits a Neel's collinear ferrimagnetic structure, with the higher substituted structures being noncollinear (x=0.44 and 0.72).     

Electronic and local structural changes in Li(2+x)Ti3O7 ramsdellite compounds upon electrochemical Li-ion insertion reactions by X-ray absorption spectroscopy

Electronic and local structural changes in ramsdellite-type Li(2+x)Ti3O7 compound were investigated by X-ray absorption spectroscopy (XAS) measurements. Upon electrochemical Li-ion insertions, the host lattice with ramsdellite structure is retained, indicated by X-ray powder diffraction. Ti K-edge extended X-ray absorption fine structure (EXAFS) analysis shows, however, slight local structural distortions around Ti ions. The energy shifts and the changes in the peak intensity of Ti K-edge and Ti L-edge XAS reveal the reducing oxidation states of Ti ions as the amount of electrochemically-inserted Li-ion increases. Equally important, oxide ions have a significant effect on the electronic transfer process, suggested by O K-edge XAS. These results on electronic structural changes were interpreted using the Zaanen-Sawatzky-Allen scheme.     

Electrochemical property: Structure relationships in monoclinic Li(3-y)V2(PO4)3

Monoclinic lithium vanadium phosphate, alpha-Li(3)V(2)(PO(4))(3), is a highly promising material proposed as a cathode for lithium-ion batteries. It possesses both good ion mobility and high lithium capacity because of its ability to reversibly extract all three lithium ions from the lattice. Here, using a combination of neutron diffraction and (7)Li MAS NMR studies, we are able to correlate the structural features in the series of single-phase materials Li(3-y)V(2)(PO(4))(3) with the electrochemical voltage-composition profile. A combination of charge ordering on the vanadium sites and lithium ordering/disordering among lattice sites is responsible for the features in the electrochemical curve, including the observed hysteresis. Importantly, this work highlights the importance of ion-ion interactions in determining phase transitions in these materials.     

Polarization Rotation in the Monoclinic Perovskite BiCo(1-x) Fe(x) O(3)

The monoclinic perovskite BiCo(1-x) Fe(x) O(3) (x≈0.7) undergoes a second-order structural transition from tetragonal to monoclinic, which is accompanied by a rotation of the polarization vector from the [001] to [111] directions of a pseudo cubic cell. The crystal structure, determined by electron diffraction and powder synchrotron X-ray diffraction, was the same as that of Pb(Ti(1-x) Zr(x) )O(3) at the morphotropic phase boundary.     

Li1.3Ti1.7Al0.3(PO4)3与Na+的离子交换

锂离子传导材料Li_1.3Ti_1.7Al_0.3(PO₄)₃是具有NASICON结构的功能材料, 与Na⁺进行离子交换具有选择性高的特性. 研究了在不同温度条件下NaCl和LiCl水溶液中Li_1.3Ti_1.7Al_0.3(PO₄)₃上的Na/Li离子交换行为. 实验结果表明, 升高温度能显著提高Li_1.3Ti_1.7Al_0.3(PO₄)₃的Na/Li交换反应速率, 提高LiCl中杂质Na的分离效果.     

Changes in electronic structure upon lithium insertion into the A-site deficient perovskite type oxides (Li,La)TiO3

Investigation on variation of the electronic structure accompanying the electrochemical lithium insertion into the perovskite type oxide, (Li,La)TiO3, has been carried out by X-ray absorption spectroscopy (XAS). During the electrochemical lithium insertion, titanium ion reduced its oxidation state from Ti4+ to Ti3+, while La3+ does not contribute to the reduction reaction resulting from Ti K-edge and La L3-edge XAS, respectively. Furthermore, O K-edge XAS showed marked spectral changes with electrochemical lithium insertion, indicating the electronic structure around oxide ion affected by lithium insertion reaction. From the XAS measurement, we have concluded the variation observed in O K-edge XAS was related to the strong interaction with inserted Li ion. To confirm this, first-principles band calculations were performed for the perovskite structure before and after electrochemical lithium insertion. The calculated results showed that the electron originated from inserted Li transferred to neighboring oxide ion locally as well as to Ti ion. This may be due to local neutralization effect of Li to reduce the electrostatic interaction in the crystal.     

Study of Surface Reaction of Spinel Li(4)Ti(5)O(12) during the First Lithium Insertion and Extraction Processes Using Atomic Force Microscopy and Analytical Transmission Electron Microscopy

Spinel lithium titanate (Li(4)Ti(5)O(12), LTO) is a promising anode material for a lithium ion battery because of its excellent properties such as high rate charge-discharge capability and life cycle stability, which were understood from the viewpoint of bulk properties such as small lattice volume changes by lithium insertion. However, the detailed surface reaction of lithium insertion and extraction has not yet been studied despite its importance to understand the mechanism of an electrochemical reaction. In this paper, we apply both atomic force microscopy (AFM) and transmission electron microscopy (TEM) to investigate the changes in the atomic and electronic structures of the Li(4)Ti(5)O(12) surface during the charge-discharged (lithium insertion and extraction) processes. The AFM observation revealed that irreversible structural changes of an atomically flat Li(4)Ti(5)O(12) surface occurs at the early stage of the first lithium insertion process, which induces the reduction of charge transfer resistance at the electrolyte/Li(4)Ti(5)O(12) interface. The TEM observation clarified that cubic rock-salt crystal layers with a half lattice size of the original spinel structure are epitaxially formed after the first charge-discharge cycle. Electron energy loss spectroscopy (EELS) observation revealed that the formed surface layer should be α-Li(2)TiO(3). Although the transformation of Li(4)Ti(5)O(12) to Li(7)Ti(5)O(12) is well-known as the lithium insertion reaction of the bulk phase, the generation of surface product layers should be inevitable in real charge-discharge processes and may play an effective role in the stable electrode performance as a solid-electrolyte interphase (SEI).     

Influence of vacancy ordering on the percolative behavior of (Li(1)(-x)Na(x))(3y)La(2/3-y)TiO(3) perovskites

Influence of the vacancy concentration on the Li conductivity of the (Li(1-x)Na(x))(0.2)La(0.6)TiO(3) and (Li(1-x)Na(x)(0.5)La(0.5)TiO(3) perovskite series, with 0 < or = x < 1, has been investigated by neutron diffraction (ND), impedance spectroscopy (IS), nuclear magnetic resonance (NMR), and Monte Carlo simulations. In both series, Li(+) ions occupy unit cell faces, but Na(+) ions are located at A sites of the perovskite. From this fact, the amount of vacant A sites that participate in Li conductivity is given by the expression n(v) = [Li] + square, where square is the nominal vacancy concentration. Substitution of Li by Na decreases the amount of vacancies, reducing drastically the Li conductivity when n(v) approaches the percolation threshold of the perovskite conduction network. In disordered (Li(1-x)Na(x))(0.5)La(0.5)TiO(3) perovskites, the percolation threshold is 0.31; however, in ordered (Li(1-x)Na(x))(0.2)La(0.6)TiO(3) perovskites, this parameter changes to 0.26. Near the percolation threshold, the amount of mobile Li species deduced by (7)Li NMR spectroscopy is lower than that derived from structural formulas but higher than deduced from dc conductivity measurements. Conductivity values have been explained by Monte Carlo simulations, which assume a random walk for Li ions in the conduction network of the perovskite. In these simulations, distribution of vacancies conforms to structural models deduced from ND experiments.     

Local electronic structure of layered Li(x)Ni0.5Mn0.5O2 and Li(x)Ni(1/3)Mn(1/3)Co(1/3)O2

Samples of Li(x)Ni0.5Mn0.5O2 and Li(x)Ni(1/3)Mn(1/3)Co(1/3)O2 were prepared as active materials in electrochemical half-cells and were cycled electrochemically to obtain different values of Li concentration, x. Absorption edges of Ni, Mn, Co, and O in these materials of differing x were measured by electron energy loss spectrometry (EELS) in a transmission electron microscope to determine the changes in local electronic structure caused by delithiation. The work was supported by electronic structure calculations with the VASP pseudopotential package, the full-potential linear augmented plane wave code WIEN2K, and atomic multiplet calculations that took account of the electronic effects from local octahedral symmetry. A valence change from Ni2+ to Ni4+ with delithiation would have caused a 3 eV shift in energy of the intense white line at the Ni L3 edge, but the measured shift was less than 1.2 eV. The intensities of the "white lines" at the Ni L-edges did not change enough to account for a substantial change of Ni valence. No changes were detectable at the Mn and Co L-edges after delithiation either. Both EELS and the computational efforts showed that most of the charge compensation for Li+ takes place at hybridized O 2p states, not at Ni atoms.