Dual lithium insertion and conversion mechanisms in a titanium-based mixed-anion nanocomposite

The electrochemical reaction of lithium with a vacancy-containing titanium hydroxyfluoride was studied. On the basis of pair distribution function analysis, NMR, and X-ray photoelectron spectroscopy, we propose that the material undergoes partitioning upon initial discharge to form a nanostructured composite containing crystalline Li(x)TiO(2), surrounded by a Ti(0) and LiF layer. The Ti(0) is reoxidized upon reversible charging to an amorphous TiF(3) phase via a conversion reaction. The crystalline Li(x)TiO(2) is involved in an insertion reaction. The resulting composite electrode, Ti(0)-LiF/Li(x)TiO(2) ? TiF(3)/ Li(y)TiO(2), allows reaction of more than one Li per Ti, providing a route to higher capacities while improving the energy efficiency compared to pure conversion chemistries.     

Influence of mesoporosity on lithium-ion storage capacity and rate performance of nanostructured TiO2(B)

Here, we present the Li(+) insertion behavior of mesoporous ordered TiO(2)(B) nanoparticles (meso-TiO(2)(B)). Using presynthesized 4 nm TiO(2)(B) nanoparticles as building blocks and a commercially available ethylene glycol-propylene glycol block copolymer (P123) as a structure-directing agent, we were able to produce mesoporous structures of high-purity TiO(2)(B) with nanocrystallinity and mesopore channels ranging from 10 to 20 nm in diameter. We compared the Li(+) insertion properties of nontemplated TiO(2)(B) nanoparticles (nano-TiO(2)(B)) to meso-TiO(2)(B) via voltammetry and galvanostatic cycling and found significant increases in overall Li(+) insertion capacity for the latter. While nano-TiO(2)(B) and meso-TiO(2)(B) both show surface charging (pseudocapacitive) Li(+) insertion behavior, meso-TiO(2)(B) exhibits a higher overall capacity especially at high charge rates. We attribute this effect to higher electrode/electrolyte contact area as well as the improved electron and ion transport in meso-TiO(2)(B). In this study, we have demonstrated the influence of both nanostructuring and mesoporosity on Li(+) insertion behavior by rationally controlling the overall architecture of the TiO(2)(B) materials.     

Lithium ion effect on electron injection from a photoexcited coumarin derivative into a TiO2 nanocrystalline film investigated by visible-to-IR ultrafast spectroscopy

The dynamics of ultrafast electron injection from a coumarin derivative (NKX-2311), which is an efficient photosensitizer for dye-sensitized solar cells, into the conduction band of TiO(2) nanocrystalline films have been investigated by means of femtosecond transient absorption spectroscopy in a wide wavelength range from 600 nm to 10 mum. In the absence of Li(+) ions, electron injection into the TiO(2) conduction band occurred in about 300 fs. In the presence of Li(+) ions, however, electron injection occurred within approximately 100 fs, and the oxidized dye generated was found to interact with nearby Li(+) ions. Possible positions of Li(+) ion attachment to the dye molecule were examined by means of semiempirical molecular orbital calculations. The electron injection efficiency was found to increase by a factor of 1.37 in the presence of Li(+) ions. The effects of Li(+) ions on the energy of the TiO(2) conduction band and the electronic interaction between the dye molecule and Li(+) ions are discussed, and the major cause for the acceleration of electron injection was suggested to be a conduction-band shift of TiO(2).     

Li+掺杂纳米TiO2的制备及其光催化性能研究

采用溶胶-凝胶法和浸渍法制备了Li⁺掺杂纳米TiO₂光催化剂,并用XRD和TEM等技术进行了表征;用pH值漂移法测量了催化剂的零电位pH值(pHpzc).结果表明,500℃煅烧制得的催化剂均为锐钛矿相;Li⁺的掺杂抑制了TiO₂粒子的生长,提高了催化剂的分散性;催化剂的零电位pH值为6.6—8.1,其值取决于Li⁺的浓度和掺杂方式.分别以紫外光和太阳光为光源,孔雀石绿和甲基橙为降解物评价了催化剂的光催化活性;并用气相色谱测试了污染物降解产生的CO₂的含量.结果显示,对孔雀石绿的降解,浸渍法和溶胶-凝胶法掺Li⁺都能有效提高TiO₂的光催化活性,但浸渍法比溶胶-凝胶法效果更好,催化活性最高的为浸渍法制备的5%(摩尔分数)Li⁺掺杂TiO₂,其在紫外光和太阳光下的光催化活性分别比纯TiO₂提高了6—8倍和9—10倍;对甲基橙的降解,除溶胶-凝胶法制备的3%(摩尔分数)Li⁺掺杂TiO₂能稍提高光催化活性外,其它Li⁺的掺杂都不同程度降低了TiO₂的光催化活性;随污染物降解率的增加,最终降解产物CO₂的含量增加.实验结果表明,Li⁺掺杂改变了催化剂表面的电荷状态从而改变了催化剂的零电位pH值是造成催化剂降解不同污染物具有不同催化活性的主要原因.     

Two phase morphology limits lithium diffusion in TiO(2)(anatase): a (7)Li MAS NMR study.

7Li magic angle spinning solid-state nuclear magnetic resonance is applied to investigate the lithium local environment and lithium ion mobility in tetragonal anatase TiO(2) and orthorhombic lithium titanate Li(0.6)TiO(2). Upon lithium insertion, an increasing fraction of the material changes its crystallographic structure from anatase TiO(2) to lithium titanate Li(0.6)TiO(2). Phase separation occurs, and as a result, the Li-rich lithium titanate phase is coexisting with the Li-poor TiO(2) phase containing only small Li amounts approximately equal to 0.01. In both the anatase and the lithium titanate lattice, Li is found to be hopping over the available sites with activation energies of 0.2 and 0.09 eV, respectively. This leads to rapid microscopic diffusion rates at room temperature (D(micr) = 4.7 x 10(-12) cm(2)s(-1) in anatase and D(micr) = 1.3 x 10(-11) cm(2)s(-1) in lithium titanate). However, macroscopic intercalation data show activation energies of approximately 0.5 eV and smaller diffusion coefficients. We suggest that the diffusion through the phase boundary is determining the activation energy of the overall diffusion and the overall diffusion rate itself. The chemical shift of lithium in anatase is independent of temperature up to approximately 250 K but decreases at higher temperatures, reflecting a change in the 3d conduction electron densities. The Li mobility becomes prominent from this same temperature showing that such electronic effects possibly facilitate the mobility.     

The effect of concentration on Li diffusivity and conductivity in rutile TiO2

Li transport characteristics are studied by means of density functional theory (DFT) and molecular dynamics (MD) simulations in order to investigate concentration effects on Li chemical diffusivity and conductivity in TiO(2) rutile. Our MD simulations predict one-dimensional diffusion of Li ions via jumps between the octahedral sites along the channels parallel to the c-axis. The diffusion barrier and diffusion coefficient (at room temperature) for the isolated Li, determined by means of DFT calculations, correspond to 60 meV and 9.1 × 10(-6) cm(2) s(-1), respectively. Such a small barrier suggests rapid mass transport along the channels. MD simulations are performed to evaluate the concentration dependent diffusivity profiles. The changes in Li energetics and dynamics are studied as a function of Li content, which is varied primarily between 10% and 50%. In addition, we consider a couple of compositions over 50% although this is above the intercalation limit. Our results suggest that Li diffusivity is strongly dependent on the Li?∶?TiO(2) ratio, and it decreases with increasing Li concentration. For instance, at room temperature, we find Li diffusivity for high concentrations (50% Li) to be three orders of magnitude slower than that for lower concentrations (10% Li). Our analyses on the energetics and dynamics suggest that the changes in the diffusivities originate from successive increases in the barriers with increasing concentration. The decrease in diffusivity as a function of increasing Li content is attributed to the fact that additional Li ions successively block the energetically preferred vacant sites along the channels. Our analyses also show that increasing Li concentration enhances the Li-Li repulsion within the channels, and as a result, diffusion is hindered. We also compare concentration-dependent diffusivities for Li diffusion in anatase, rutile and amorphous TiO(2). Interestingly, we find differing concentration dependence of the diffusivity in these chemically identical but structurally non-equivalent TiO(2) polymorphs. Our study suggests that these differences result from intrinsic structural characteristics of TiO(2) polymorphs, which ultimately contribute to intercalation limit, diffusivity, ionic conductivity, and the electrochemical performance in energy storage applications.     

Charge accumulation and polarization in titanium dioxide electrodes

Nanocrystalline TiO(2) electrodes were studied spectroelectrochemically by observing the simultaneous relaxation of the current and absorbance after applying a voltage step. The absorbance behaved differently in two time regimes: (1) ionic polarization in the oxide electrode, in which charged ions, such as Ti(3+) sites and/or interstitial Ti(4+) sites, move in response to the applied electric field, and (2) the diffusion of Li(+) ions into the TiO(2). These two behaviors were analyzed with equivalent circuit models. Li(+) ions reduce the resistance of the TiO(2) by approximately 90%, increase the capacitance by approximately 350%, and decrease the inductance by approximately 30%. Voltage cycling produces a buildup of intercalated Li(+) ions, lessening the electrode's response to the potential step, and causing it to become a more efficient inductor. The potential distribution in the nanoparticles is described by using a dielectric model in which roughly half the applied potential is dropped across the interface with a Li(+)-ion-containing electrolyte.     

STUDY ON OXID TIVE COUPLING OF METHANE Ⅱ.CATALY TIC PERFORMANCE AND CHARACTERIZATION OF TiO_2-BASED CATALYSTS

The catalytic performance of Mn/TiO_2,La-Mn/TiO_2,Li-La-Mn/TiO_2 etc for the oxidetive coupling of methane(OCM)was investignted.Thecatalysta were cheracterised with X-ray diffraction(XRD)and X-rayphotoelectron spectroscopy(XPS).The results reveal that catalyst Li-La-Mn/TiO_2 exhibits high activity and C_2 selectivity;Ti in this catalyst exists asTi~( 4) state;Li can promote the formation of lanthanum tituate via theinteraction between La and TiO_2;the formed La_2Ti_2O_7 and La_4Ti_9O_(24) aredistributed in the inner surface layer and Mn exists in outer surface layer in lowvalence states.The high activity and C_2 selectivity of catalyst Li-La-Mn/TiO_2are intimately related to the valence states of Mn,Li,La and theirdistribution on the catulyst surface layer.     

Li-ion-conductive Li2TiO3-coated Li[Li0.2Mn0.51Ni0.19Co0.1]O2 for high-performance cathode material in lithium-ion battery

Journal of Solid State Electrochemistry - Li2TiO3 is used as a novel coating material to modify Li(Li0.2Mn0.51Ni0.19Co0.1)O2 electrode to enhance the electrochemical performance of the host...     

Synthesis of hierarchically mesoporous anatase spheres and their application in lithium batteries

Hierarchically mesoporous TiO2 (anatase) sub-micron spheres with uniform particle size exhibiting high Li storage capacity and good cycling performance have been successfully prepared in a large quantity by using TiO2-CdSO4 composite as intermediate.     

Investigation of cation-induced degradation of dye-sensitized solar cells for a new strategy to long-term stability

Current-voltage characteristics, electron lifetimes (tau), and electron diffusion coefficients (D) of dye-sensitized TiO2 solar cells (DSCs) composed of liquid electrolytes were repeatedly measured over a period of time. It was found that the energy conversion efficiency of the DSCs using electrolytes composed of Li+ or tetrabutylammonium cation as the counter charges of I-/I3- redox couples decreased with the lapse of time. On the other hand, such a decrease was not observed for the DSC consisting of 1,2-dimethyl-3-propylimidazolium cation or of Li+ coupled with the addition of tert-butylpyridine. The decrease of the efficiency was in accordance with a decreased electron lifetime. The notable decrease in the presence of Li+ is probably caused by the excess amount of Li+ adsorption on the TiO2 surface.     

Li+共掺杂对Er3+-Yb3+∶TiO2紫外、可见和红外发光同步增强研究

采用溶胶-凝胶(Sol-gel)法制备了Li+共掺杂的Er3+-Yb3+∶TiO2粉末。976 nm激光激发下在波长350~1700 nm范围内观察到了紫外、蓝色、绿色和红色上转换发光和红外下转换发光。随着Li+共掺杂浓度由0增大到20mol%,Er3+-Yb3+∶TiO2的紫外、可见和红外发光强度同步增强。低Li+共掺杂浓度引起的Li+固溶以及高Li+共掺杂浓度引起的相变过程相继破坏了Er3+的晶体场对称性,导致紫外、可见和红外发光显著增强。结果表明共掺杂Li+是一种提高Er3+掺杂材料发光性能的有效方法。     

Li+共掺杂对Er3+-Yb3+:TiO2紫外、可见和红外发光同步增强研究

采用溶胶-凝胶(Sol-gel)法制备了Li+共掺杂的Er3+-Yb3+:TiO2粉末.976 nm激光激发下在波长350～1700nm范围内观察到了紫外、蓝色、绿色和红色上转换发光和红外下转换发光.随着Li+共掺杂浓度由0增大到20mol%,Er3+-Yb3+:TiO2的紫外、可见和红外发光强度同步增强.低Li+共掺杂浓度引起的Li+固溶以及高Li+共掺杂浓度引起的相变过程相继破坏了Er3+的晶体场对称性,导致紫外、可见和红外发光显著增强.结果表明共掺杂Li+是一种提高Er3+掺杂材料发光性能的有效方法.     

Multiple Li positions inside oxygen octahedra in lithiated TiO2 anatase

Intercalation of Li in TiO2 anatase results in a phase separation in a Li-poor and a Li-rich phase. The local lithium configuration in the coexisting crystallographic phases is resolved by detailed analysis of neutron diffraction data. In each of the phases, two distinct positions within the octahedral interstices are found, with a temperature-dependent occupancy. A combination of quasi-elastic neutron scattering and force field molecular dynamics simulations shows that Li is hopping on a picosecond time scale between the two sites in the octahedral interstices. The results also suggest a specific Li arrangement along the crystallographic a direction, albeit without long range order. It is likely that multiple discrete Li sites within a distorted oxygen octahedron occur not only in intercalated TiO2 anatase but also in other (transition metal) oxides.     

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.     

Enhancement of intercalation properties of V2O5 film by TiO2 addition

Although it is well-known that TiO2 incorporation can greatly improve the cyclic stability of V2O5, the influences of TiO2 addition on the Li+ intercalation properties of V2O5 remain an issue of debate in literature. In this paper, we report on a systematic investigation of the preparation and intercalation properties of V2O5-TiO2 mixture films. The present work demonstrates that high Li+ intercalation rates and capacity in V2O5 films are achievable with TiO2 addition. For example, the addition of 20 mol % Ti into V2O5 polycrystalline demonstrated an approximated 100% improvement in Li+ intercalation performance as compared to single V2O5 electrodes. Such enhancement in intercalation properties of V2O5 films with TiO2 addition was attributed to changes in microstructure, crystallinity, and also a possible lattice structure and interaction force between adjacent layers in V2O5.     

Defect chemistry, surface structures, and lithium insertion in anatase TiO2

Atomistic simulation techniques are used to investigate the defect properties of anatase TiO(2) and Li(x)TiO(2) both in the bulk and at the surfaces. Interatomic potential parameters are derived that reproduce the lattice constants of anatase, and the energies of bulk defects and surface structures are calculated. Reduction of anatase involving interstitial Ti is found to be the most favorable defect reaction in the bulk, with a lower energy than either Frenkel or Schottky reactions. The binding energies of selected defect clusters are also presented: for the Ti(3+)-Li(+) defect cluster, the binding energy is found to be approximately 0.5 eV, suggesting that intercalated Li ions stabilize conduction band electrons. The Li ion migration path is found to run between octahedral sites, with an activation energy of 0.45-0.65 eV for mole fractions of lithium in Li(x)TiO(2) of x < or = 0.1. The calculated surface energies are used to predict the crystal morphology, which is found to be a truncated bipyramid in which only the (101) and (001) surfaces are expressed, in accord with the available microscopy data. Calculations of defect energies at the (101) surface suggest that single Ti(3+) defects and neutral Ti(3+)-Li(+) pairs tend to segregate to the surface.     

Combining the pair distribution function and computational methods to understand lithium insertion in Brookite (TiO2)

X-ray pair distribution function (PDF) methods and first-principles calculations have been combined to probe the structure of electrochemically lithiated TiO(2) Brookite. Traditional powder diffraction studies suggest that Brookite amorphizes upon lithium insertion, with the Bragg reflections disappearing. However, PDF analysis indicates that the TiO(2) framework connectivity is maintained throughout lithium intercalation, with expansions along the a and b axes. The Li(+) ions within the framework are poorly observed in the X-ray PDF, which is dominated by contributions from the more strongly scattering Ti and O atoms. First-principles calculations were used to identify energetically favorable Li(+) sites within the Brookite lattice and to develop a complete structural model of the lithiated material. This model replicates the local structure and decreased intermediate range order observed in the PDF data. The analysis suggests that local structural distortions of the TiO(2) lattice accommodate lithium in five-coordinate sites. This structural model is consistent with the observed electrochemical behavior.     

The influence of size on phase morphology and Li-ion mobility in nanosized lithiated anatase TiO2

Sustainable energy storage in the form of Li-ion batteries requires new and advanced materials in particular with a higher power density. Nanostructuring appears to be a promising strategy, in which the higher power density in nanosized materials is related to the dramatically shortened Li-ion diffusion paths. However, nanosizing materials also changes intrinsic material properties, which influence both ionic and electronic conductivity. In this work neutron diffraction is used to show that in addition to these two aspects, nanostructuring changes the phase behavior and morphology. Lithiated 40-nm TiO(2) anatase crystallites become single phase, either having the Li-poor original anatase phase, or the Li-rich Li-titanate phase, in contrast to microsized crystallites where these two phases coexist in equilibrium within one crystal particle. In addition, Li(x)TiO(2) compositions occur with stoichiometries that are not stable in micron-sized crystallites, indicating enhanced solid solution behavior. Reduced conduction electron densities at the sites of the Li ions are observed by NMR spectroscopy. This is accompanied by reduced spontaneous Li-ion mobility, suggesting a correlation between the electron density at the Li-ion site and the Li-ion mobility. The present results show that in the case of lithiated anatase TiO(2), significant effects on phase composition, morphology, and electronic configurations are induced, as well as slower intracrystallite Li diffusion.     

Pulsed laser–deposited Li2TiO3 thin film electrodes for energy storage

Journal of Solid State Electrochemistry - Li2TiO3 (LTO) is a promising Ti-based material showing interesting electrochemical performance, good structural stability, cost-effectiveness, and...