The Electrochemical and Structural Changes of Phosphorus-Doped TiO2 through In Situ Raman and In Situ X-Ray Diffraction Analysis

Doping is a widely employed technique to enhance the functionality of lithium-ion battery materials, tailoring their performance for specific applications. In our study, we employed in situ Raman and in situ X-ray diffraction (XRD) spectroscopic techniques to examine the structural alterations and electrochemical behavior of phosphorus-doped titanium dioxide (TiO2) nanoparticles. This investigation revealed several notable changes: an increase in structural defects, enhanced ionic and electronic conductivity, and a reduction in crystallite size. These alterations facilitated higher lithiation rates and led to the first observed appearance of LiTiO2 in the Raman spectra due to anatase lithiation, resulting in a reversible double-phase transition during the charging and discharging processes. Furthermore, doping with 2, 5, and 10 wt % phosphorus resulted in an initial increase in specific capacity compared to undoped TiO2. However, higher doping levels were associated with diminished capacity retention, pinpointing an optimal doping level for phosphorus. These results underscore the critical role of in situ characterization techniques in understanding doping effects, thereby advancing the performance of anode materials, particularly TiO2, in lithium-ion batteries.

Retarding the Shuttling Ions in the Electrochromic TiO2 with Extensive Crystallographic Imperfections

To understand the role of structure imperfections on the performance of electrochromic transition metal oxide (ETMO) is challenging for the design of efficient smart windows. Herein, we investigate the performance evolution with tunable crystallographic imperfections for rutile TiO₂ nanowire film (TNF). Structure imperfections, originating mainly from the copious oxygen deficiency, are apt to cumulatively retard the shuttling ions, resulting in the response rate for raw TNF being less than the half that of TNF annealed at 500 °C. We describe ion accommodation sites as a convolution of normal site and abnormal site, in which the normal site performs reversible coloration but the abnormal site contributes only to charge storage, which gives a rationale for the non-linear coloration and rate capability loss. These findings give a clear picture of the ion shuttling process, which is insightful for enhancing the electrochromic performance via structure reprogramming.

Substantial Na-Ion Storage at High Current Rates: Redox-Pseudocapacitance through Sodium Oxide Formation

Batteries and supercapacitors, both governed by electrochemical processes, operate by different electrochemical mechanisms which determine their characteristic energy and power densities. Battery materials store large amounts of energy by ion intercalation. Electrical double-layer capacitors store charge through surface-controlled ion adsorption which leads to high power and rapid charging, but much smaller amounts of energy stored. Pseudocapacitive materials offer the promise to combine these properties by storing charge through surface-controlled, battery-like redox reactions but at high rates approaching those of electrochemical double-layer capacitors. This work compares the pseudo-capacitive charge storage characteristics of self-organized titanium dioxide (TiO_2-x) nanotubes (NTs) to flat TiO_2-x surface films to further elucidate the proposed charge storage mechanism within the formed surface films. By comparing TiO_2-x NTs to flat TiO_2-x surface films, having distinctively different oxide mass and surface area ratios, it is shown that NaO₂ and Na₂O₂ formation, which constitutes the active surface film material, is governed by the metal oxide bulk. Our results corroborate that oxygen diffusion from the lattice oxide is key to NaO₂ and Na₂O₂ formation.

A Review on Recent Advancements of Ni-NiO Nanocomposite as an Anode for High-Performance Lithium-Ion Battery

Recently, lithium-ion batteries (LIBs) have been widely employed in automobiles, mining operations, space applications, marine vessels and submarines, and defense or military applications. As an anode, commercial carbon or carbon-based materials have some critical issues such as insufficient charge capacity and power density, low working voltage, deadweight formation, short-circuiting tendency initiated from dendrite formation, device warming up, etc., which have led to a search for carbon alternatives. Transition metal oxides (TMOs) such as NiO as an anode can be used as a substitute for carbon material. However, NiO has some limitations such as low coulombic efficiency, low cycle stability, and poor ionic conductivity. These limitations can be overcome through the use of different nanostructures. This present study reviews the integration of the electrochemical performance of binder involved nanocomposite of NiO as an anode of a LIB. This review article aims to epitomize the synthesis and characterization parameters such as specific discharge/charge capacity, cycle stability, rate performance, and cycle ability of a nanocomposite anode. An overview of possible future advances in NiO nanocomposites is also proposed.

TiO2(B) nanosheets modified Li4Ti5O12microsphere anode for high-rate lithium-ion batteries

With the increasing applications of Lithium-ion batteries in heavy equipment and engineering machinery, the requirements of rate capability are continuously growing. The high-rate performance of Li₄Ti₅O₁₂(LTO) needs to be further improved. In this paper, we synthesized LTO microsphere-TiO₂(B) nanosheets (LTO-TOB) composite by using a solvothermal method and subsequent calcination. LTO-TOB composite combines the merits of TiO₂(B) and LTO, resulting in excellent high-rate capability (144.8, 139.3 and 124.4 mAh g^-1at 20 C, 30 C and 50 C) and superior cycling stability (98.9% capability retention after 500 cycles at 5 C). Its excellent electrochemical properties root in the large surface area, high grain-boundary density and pseudocapacitive effect of LTO-TOB. This work reveals that LTO-TOB composite can be a potential anode for high power and energy density lithium-ion batteries.

TiO2 Nanofiber-Modified Lithium Metal Composite Anode for Solid-State Lithium Batteries

Solid-state lithium metal batteries (SSLMBs), using lithium metal as the anode and garnet-structured Li_6.5La₃Zr_1.5Ta_0.5O₁₂ (LLZTO) as the electrolyte, are attractive and promising due to their high energy density and safety. However, the interface contact between the lithium metal and LLZTO is a major obstacle to the performance of SSLMBs. Here, we successfully improve the interface wettability by introducing one-dimensional (1D) TiO₂ nanofibers into the lithium metal to obtain a Li-lithiated TiO₂ composite anode (Li-TiO₂). When 10 wt % TiO₂ nanofibers are added, the formed composite anode offers a seamless interface contact with LLZTO and enables an interfacial resistance of 27 Ω cm², which is much smaller than 374 Ω cm² of pristine lithium metal. Due to the enhanced interface wettability, the symmetric Li-TiO₂|LLZTO|Li-TiO₂ cell upgrades the critical current density to 2.2 mA cm^-2 and endures stable cycling over 550 h. Furthermore, by coupling the Li-TiO₂ composite anode with the LiFePO₄ cathode, the full cell shows stable cycling performance. This work proves the role of TiO₂ nanofibers in enhancing the interface contact between the garnet electrolyte and the lithium metal anode and improving the performance of SSLMBs and provides an effective approach with 1D additives for solving the interface issues.

Dynamics of Lithium Insertion in Electrochromic Titanium Dioxide Nanocrystal Ensembles

Nanocrystalline anatase TiO₂ is a robust model anode for Li insertion in batteries. The influence of nanocrystal size on the equilibrium potential and kinetics of Li insertion is investigated with in operando spectroelectrochemistry of thin film electrodes. Distinct visible and infrared responses correlate with Li insertion and electron accumulation, respectively, and these optical signals are used to deconvolute bulk Li insertion from other electrochemical responses, such as double-layer capacitance, pseudocapacitance, and electrolyte leakage. Electrochemical titration and phase-field simulations reveal that a difference in surface energies between anatase and lithiated phases of TiO₂ systematically tunes the Li-insertion potentials with the particle size. However, the particle size does not affect the kinetics of Li insertion in ensemble electrodes. Rather, the Li-insertion rates depend on the applied overpotential, electrolyte concentration, and initial state of charge. We conclude that Li diffusivity and phase propagation are not rate limiting during Li insertion in TiO₂ nanocrystals. Both of these processes occur rapidly once the transformation between the low-Li anatase and high-Li orthorhombic phases begins in a particle. Instead, discontinuous kinetics of Li accumulation in TiO₂ particles prior to the phase transformations limits (dis)charging rates. We demonstrate a practical means to deconvolute the nonequilibrium charging behavior in nanocrystalline electrodes through a combination of colloidal synthesis, phase field simulations, and spectroelectrochemistry.

Rational design of layered oxide materials for sodium-ion batteries

Sodium-ion batteries have captured widespread attention for grid-scale energy storage owing to the natural abundance of sodium. The performance of such batteries is limited by available electrode materials, especially for sodium-ion layered oxides, motivating the exploration of high compositional diversity. How the composition determines the structural chemistry is decisive for the electrochemical performance but very challenging to predict, especially for complex compositions. We introduce the "cationic potential" that captures the key interactions of layered materials and makes it possible to predict the stacking structures. This is demonstrated through the rational design and preparation of layered electrode materials with improved performance. As the stacking structure determines the functional properties, this methodology offers a solution toward the design of alkali metal layered oxides.

TiO2 Nanotube Layers Decorated with Al2O3/MoS2/Al2O3 as Anode for Li-ion Microbatteries with Enhanced Cycling Stability

TiO₂ nanotube layers (TNTs) decorated with Al₂O₃/MoS₂/Al₂O₃ are investigated as a negative electrode for 3D Li-ion microbatteries. Homogenous nanosheets decoration of MoS₂, sandwiched between Al₂O₃ coatings within self-supporting TNTs was carried out using atomic layer deposition (ALD) process. The structure, morphology, and electrochemical performance of the Al₂O₃/MoS₂/Al₂O₃-decorated TNTs were studied using scanning transmission electron microscopy, energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and chronopotentiometry. Al₂O₃/MoS₂/Al₂O₃-decorated TNTs deliver an areal capacity almost three times higher than that obtained for MoS₂-decorated TNTs and as-prepared TNTs after 100 cycles at 1C. Moreover, stable and high discharge capacity (414 µAh cm^-2) has been obtained after 200 cycles even at very fast kinetics (3C).

In Situ Tuning of Defects and Phase Transition in Titanium Dioxide by Lithiothermic Reduction

Defects engineering of oxides plays a vital role in tuning their physicochemical and electronic properties and thereby determining their potential applications. However, the safe and controllable production of effective defects in the oxides is still challenging. Here, we report a facile one-pot solid lithiothermic reduction approach to generate graded oxygen defects in TiO₂ nanoparticles. Various levels of lithium reduction are systematically studied, and meanwhile, a distinct phase transition from anatase TiO₂ to cubic Li_xTiO₂ is observed with the increasing lithium ratio. The structure and evolution of surface defects and bulk phase transition are investigated in detail. Afterward, we demonstrate their applications in carbon dioxide photoreduction and photothermal imaging. The slightly reduced TiO₂ with effective oxygen defects affords a highly broadened solar spectrum absorption and yields significantly enhanced visible photocatalytic activity in CO₂ conversion, which is further revealed by theoretical calculations. The highly reduced TiO₂ with obvious phase transition shows enhanced solar absorption and achieves high photo-thermal-conversion efficacy, showing huge potential in photo-thermal-related applications. The lithiothermic reduction is a general and effective approach to produce defects and induce phase transition in oxides, which can be used in multiple applications.

Evidence for Fast Lithium-Ion Diffusion and Charge-Transfer Reactions in Amorphous TiO x Nanotubes: Insights for High-Rate Electrochemical Energy Storage

The charge-storage kinetics of amorphous TiO _x nanotube electrodes formed by anodizing three-dimensional porous Ti scaffolds are reported. The resultant electrodes demonstrated not only superior storage capacities and rate capability to anatase TiO _x nanotube electrodes but also improved areal capacities (324 μAh cm^-2 at 50 μA cm^-2 and 182 μAh cm^-2 at 5 mA cm^-2) and cycling stability (over 2000 cycles) over previously reported TiO _x nanotube electrodes using planar current collectors. Amorphous TiO _x exhibits very different electrochemical storage behavior from its anatase counterpart as the majority of its storage capacity can be attributed to capacitive-like processes with more than 74 and 95% relative contributions being attained at 0.05 and 1 mV s^-1, respectively. The kinetic analysis revealed that the insertion/extraction process of Li⁺ in amorphous TiO _x is significantly faster than in anatase structure and controlled by both solid-state diffusion and interfacial charge-transfer kinetics. It is concluded that the large capacitive contribution in amorphous TiO _x originates from its highly defective and loosely packed structure and lack of long-range ordering, which facilitate not only a significantly faster Li⁺ diffusion process (diffusion coefficients of 2 × 10^-14 to 3 × 10^-13 cm² s^-1) but also more facile interfacial charge-transfer kinetics than anatase TiO _x.

Tracking the Chemical and Structural Evolution of the TiS2 Electrode in the Lithium-Ion Cell Using Operando X-ray Absorption Spectroscopy

As the lightest and cheapest transition metal dichalcogenide, TiS₂ possesses great potential as an electrode material for lithium batteries due to the advantages of high energy density storage capability, fast ion diffusion rate, and low volume expansion. Despite the extensive investigation of its electrochemical properties, the fundamental discharge-charge reaction mechanism of the TiS₂ electrode is still elusive. Here, by a combination of ex situ and operando X-ray absorption spectroscopy with density functional theory calculations, we have clearly elucidated the evolution of the structural and chemical properties of TiS₂ during the discharge-charge processes. The lithium intercalation reaction is highly reversible and both Ti and sulfur are involved in the redox reaction during the discharge and charge processes. In contrast, the conversion reaction of TiS₂ is partially reversible in the first cycle. However, Ti-O related compounds are developed during electrochemical cycling over extended cycles, which results in the decrease of the conversion reaction reversibility and the rapid capacity fading. In addition, the solid electrolyte interphase formed on the electrode surface is found to be highly dynamic in the initial cycles and then gradually becomes more stable upon further cycling. Such understanding is important for the future design and optimization of TiS₂ based electrodes for lithium batteries.

The electrochemical storage mechanism in oxy-hydroxyfluorinated anatase for sodium-ion batteries

Replacing lithium ions with sodium ions as the charge carriers in rechargeable batteries can induce noticeable differences in the electrochemical storage mechanisms.

Reversible magnesium and aluminium ions insertion in cation-deficient anatase TiO2

In contrast to monovalent lithium or sodium ions, the reversible insertion of multivalent ions such as Mg²⁺ and Al³⁺ into electrode materials remains an elusive goal. Here, we demonstrate a new strategy to achieve reversible Mg²⁺ and Al³⁺ insertion in anatase TiO₂, achieved through aliovalent doping, to introduce a large number of titanium vacancies that act as intercalation sites. We present a broad range of experimental and theoretical characterizations that show a preferential insertion of multivalent ions into titanium vacancies, allowing a much greater capacity to be obtained compared to pure TiO₂. This result highlights the possibility to use the chemistry of defects to unlock the electrochemical activity of known materials, providing a new strategy for the chemical design of materials for practical multivalent batteries.

Investigation of amorphous to crystalline phase transition of sodium titanate by X-ray absorption spectroscopy and scanning transmission X-ray microscopy

Nanostructured sodium titanate has great potential for various applications such as sodium-ion batteries, photocatalysts, and waste treatment. Understanding the phase-transition mechanism in sodium titanate after annealing is fundamentally important to tune the structure, morphology, and property for targeted applications. In this work, we adopted amorphous sodium titanate grown on carbon nanotubes by an atomic layer deposition technique as a reference and used X-ray absorption spectroscopy (XAS) and scanning transmission X-ray microscopy (STXM), as well as a high-temperature in situ X-ray diffraction (XRD) technique, to elucidate the phase-transition mechanism of sodium titanate from amorphous to crystalline upon annealing from 25 °C to 900 °C. XAS and XRD analysis disclosed that anatase TiO2 first formed in the matrix of amorphous sodium titanate at 500 °C and then recrystallized into Na0.23TiO2 at 700 °C and 900 °C. XAS studies also revealed that the Ti atoms in sodium titanate were oxidized during the annealing process and reached an oxidation state about 3.8+ for Na0.23TiO2. The elevated annealing temperature increased the coordination number of Ti atoms and the crystallinity of sodium titanate. STXM chemical map provided spatial information and visualized evidence on the phase transition among amorphous sodium titanate, anatase TiO2, and Na0.23TiO2 in the samples annealed at intermediate temperatures (500 °C and 700 °C). This work provides a comprehensive understanding on the evolution of sodium titanate, in terms of crystal structure, electronic structure, chemical environment, and morphology, under different post annealing conditions.

Preferentially Oriented TiO2 Nanotubes as Anode Material for Li-Ion Batteries: Insight into Li-Ion Storage and Lithiation Kinetics

Self-organized TiO₂ nanotubes (NTs) with a preferential orientation along the [001] direction are anodically grown by controlling the water content in the fluoride-containing electrolyte. The intrinsic kinetic and thermodynamic properties of the Li intercalation process in the preferentially oriented (PO) TiO₂ NTs and in a randomly oriented (RO) TiO₂ NT reference are determined by combining complementary electrochemical methods, including electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and galvanostatic cycling. PO TiO₂ NTs demonstrate an enhanced performance as anode material in Li-ion batteries due to faster interfacial Li insertion/extraction kinetics. It is shown that the thermodynamic properties, which describe the ability of the host material to intercalate Li ions, have a negligible influence on the superior performance of PO NTs. This work presents a straightforward approach for gaining important insight into the influence of the crystallographic orientation on lithiation/delithiation characteristics of nanostructured TiO₂ based anode materials for Li-ion batteries. The introduced methodology has high potential for the evaluation of battery materials in terms of their lithiation/delithiation thermodynamics and kinetics in general.

Mesoporous Titania Microspheres with Highly Tunable Pores as an Anode Material for Lithium Ion Batteries

Mesoporous titania microspheres (MTMs) have been employed in many applications, including (photo)catalysis as well as energy conversion and storage. Their morphology offers a hierarchical structural design motif that lends itself to being incorporated into established large-scale fabrication processes. Despite the fact that device performance hinges on the precise morphological characteristics of these materials, control over the detailed mesopore structure and the tunability of the pore size remains a challenge. Especially the accessibility of a wide range of mesopore sizes by the same synthesis method is desirable, as this would allow for a comparative study of the relationship between structural features and performance. Here, we report a method that combines sol-gel chemistry with polymer micro- and macrophase separation to synthesize porous titania spheres with diameters in the micrometer range. The as-prepared MTMs exhibit well-defined, accessible porosities with mesopore sizes adjustable by the choice of the polymers. When applied as an anode material in lithium ion batteries (LIBs), the MTMs demonstrate excellent performance. The influence of the pore size and an in situ carbon coating on charge transport and storage is examined, providing important insights for the optimization of structured titania anodes in LIBs. Our synthesis strategy presents a facile one-pot approach that can be applied to different structure-directing agents and inorganic materials, thus further extending its scope of application.

Tracking areal lithium densities from neutron activation - quantitative Li determination in self-organized TiO2 nanotube anode materials for Li-ion batteries

Nanostructuring of electrode materials is a promising approach to enhance the performance of next-generation, high-energy density lithium (Li)-ion batteries. Various experimental and theoretical approaches allow for a detailed understanding of solid-state or surface-controlled reactions that occur in nanoscaled electrode materials. While most techniques which are suitable for nanomaterial investigations are restricted to analysis widths of the order of Å to some nm, they do not allow for characterization over the length scales of interest for electrode design, which is typically in the order of mm. In this work, three different self-organized anodic titania nanotube arrays, comprising as-grown amorphous titania nanotubes, carburized anatase titania nanotubes, and silicon coated carburized anatase titania nanotubes, have been synthesized and studied as model composite anodes for use in Li-ion batteries. Their 2D areal Li densities have been successfully reconstructed with a sub-millimeter spatial resolution over lateral electrode dimensions of 20 mm exploiting the ⁶Li(n,α)³H reaction, in spite of the extremely small areal Li densities (10-20 μg cm^-2 Li) in the nanotubular active material. While the average areal Li densities recorded via triton analysis are found to be in good agreement with the electrochemically measured charges during lithiation, triton analysis revealed, for certain nanotube arrays, areas with a significantly higher Li content ('hot spots') compared to the average. In summary, the presented technique is shown to be extremely well suited for analysis of the lithiation behavior of nanostructured electrode materials with very low Li concentrations. Furthermore, identification of lithiation anomalies is easily possible, which allows for fundamental studies and thus for further advancement of nanostructured Li-ion battery electrodes.

Hierarchical flower-like NiCo2O4@TiO2hetero-nanosheets as anodes for lithium ion batteries

Flower-like NiCo₂O₄consisting of nanosheets are synthesized by hydrothermal technique and subsequently surface-modified with a TiO₂ultrathin layer by a hydrolysis process at low temperature.

Changing the dehydrogenation pathway of LiBH4–MgH2via nanosized lithiated TiO2

Nanosized lithiated titanium oxide (Li_xTiO₂) noticeably improves the kinetic behaviour of 2LiBH₄ + MgH₂.

Li intercalation mechanisms in CaTi5O11, a bronze-B derived compound

A first-principles study was performed to elucidate the electrochemical properties of CaTi₅O₁₁, a recently discovered compound that is a crystallographic variant of TiO₂(B) and that shows promise as an anode material for Li-ion batteries. The crystal structure of CaTi₅O₁₁ was further refined and two symmetrically distinct interstitial sites that can accommodate Li at positive voltage were identified. A statistical mechanics study relying on density functional theory (DFT) calculations predicted that interstitial Li in CaTi₅O₁₁ forms a solid solution with Li insertion resulting in a sloping voltage profile. Li diffusion within CaTi₅O₁₁ was found to be highly anisotropic with low barrier diffusion pathways forming one-dimensional channels parallel to the c axis.

Monodisperse mesoporous anatase beads as high performance and safer anodes for lithium ion batteries

To achieve high efficiency lithium ion batteries (LIBs), an effective active material is important. In this regard, monodisperse mesoporous titania beads (MMTBs) featuring well interconnected nanoparticles were synthesised, and their mesoporous properties were tuned to study how these affect the electrochemical performance in LIBs. Two pore diameters of 15 and 25 nm, three bead diameters of 360, 800 and 2100 nm, and various annealing temperatures (from 300 to 650 °C) were investigated. The electrochemical results showed that while the pore size does not significantly influence the electrochemical behaviour, the specific surface area and the nanocrystal size affect the performance. Also, there is an optimum annealing temperature that enhances electron transfer across the titania bead structure. The carbon content employed in the electrode was varied, showing that the bead diameter strongly influences the minimal content of the conductive carbon required to fabricate the electrode. As a general rule, the smaller the bead diameter, the more carbon was required in the electrode. A large energy capacity and high current rate performance were achieved on the MMTBs featuring high surface area, nano-sized anatase crystals and well-sintered connections between the nanocrystals. The high stability of these mesoporous structures was demonstrated by charge/discharge cycling up to 500 cycles. Devices constructed with the MMTBs retained more than 80% of the initial capacity, indicating an excellent performance.

Surface Engineering and Design Strategy for Surface-Amorphized TiO2@Graphene Hybrids for High Power Li-Ion Battery Electrodes

Surface amorphization provides unprecedented opportunities for altering and tuning material properties. Surface-amorphized TiO2@graphene synthesized using a designed low temperature-phase transformation technique exhibits significantly improved rate capability compared to well-crystallized TiO2@graphene and bare TiO2 electrodes. These improvements facilitates lithium-ion transport in both insertion and extraction processes and enhance electrolyte absorption capability.

A density functional tight binding study of acetic acid adsorption on crystalline and amorphous surfaces of titania

We present a comparative density functional tight binding study of an organic molecule attachment to TiO2 via a carboxylic group, with the example of acetic acid. For the first time, binding to low-energy surfaces of crystalline anatase (101), rutile (110) and (B)-TiO2 (001), as well as to the surface of amorphous (a-) TiO2 is compared with the same computational setup. On all surfaces, bidentate configurations are identified as providing the strongest adsorption energy, Eads = -1.93, -2.49 and -1.09 eV for anatase, rutile and (B)-TiO2, respectively. For monodentate configurations, the strongest Eads = -1.06, -1.11 and -0.86 eV for anatase, rutile and (B)-TiO2, respectively. Multiple monodentate and bidentate configurations are identified on a-TiO2 with a distribution of adsorption energies and with the lowest energy configuration having stronger bonding than that of the crystalline counterparts, with Eads up to -4.92 eV for bidentate and -1.83 eV for monodentate adsorption. Amorphous TiO2 can therefore be used to achieve strong anchoring of organic molecules, such as dyes, that bind via a -COOH group. While the presence of the surface leads to a contraction of the band gap vs. the bulk, molecular adsorption caused no appreciable effect on the band structure around the gap in any of the systems.

A fractal-like electrode based on double-wall nanotubes of anatase exhibiting improved electrochemical behaviour in both lithium and sodium batteries

An anatase nanotube array has been prepared with a special morphology: two concentric walls and a very small central cavity.

Understanding Li diffusion in Li-intercalation compounds

Intercalation compounds, used as electrodes in Li-ion batteries, are a fascinating class of materials that exhibit a wide variety of electronic, crystallographic, thermodynamic, and kinetic properties. With open structures that allow for the easy insertion and removal of Li ions, the properties of these materials strongly depend on the interplay of the host chemistry and crystal structure, the Li concentration, and electrode particle morphology. The large variations in Li concentration within electrodes during each charge and discharge cycle of a Li battery are often accompanied by phase transformations. These transformations include order-disorder transitions, two-phase reactions that require the passage of an interface through the electrode particles, and structural phase transitions, in which the host undergoes a crystallographic change. Although the chemistry of an electrode material determines the voltage range in which it is electrochemically active, the crystal structure of the compound often plays a crucial role in determining the shape of the voltage profile as a function of Li concentration. While the relationship between the voltage profile and crystal structure of transition metal oxide and sulfide intercalation compounds is well characterized, far less is known about the kinetic behavior of these materials. For example, because these processes are especially difficult to isolate experimentally, solid-state Li diffusion, phase transformation mechanisms, and interface reactions remain poorly understood. In this respect, first-principles statistical mechanical approaches can elucidate the effect of chemistry and crystal structure on kinetic properties. In this Account, we review the key factors that govern Li diffusion in intercalation compounds and illustrate how the complexity of Li diffusion mechanisms correlates with the crystal structure of the compound. A variety of important diffusion mechanisms and associated migration barriers are sensitive to the overall Li concentration, resulting in diffusion coefficients that can vary by several orders of magnitude with changes in the lithium content. Vacancy clusters, groupings of vacancies within the crystal lattice, provide a common mechanism that mediates Li diffusion in important intercalation compounds. This mechanism emerges from specific crystallographic features of the host and results in a strong decrease of the Li diffusion coefficient as Li is added to an already Li rich host. Other crystallographic and electronic factors, such as the proximity of transition metal ions to activated states of hops and the occurrence of electronically induced distortions, can result in a strong dependence of the Li mobility on the overall Li concentration. The insights obtained from fundamental studies of ionic diffusion in electrode materials will be instrumental for physical chemists, chemical engineers, synthetic chemists, and materials and device designers who are developing these technologies.

TiO2 anatase nanoparticle networks: synthesis, structure, and electrochemical performance

Nanocrystalline anatase TiO(2) materials with different specific surface areas and pore size distributions are prepared via sol-gel and miniemulsion routes in the presence of surfactants. The samples are characterized by X-ray diffraction, nitrogen sorption, transmission electron microscopy, and electrochemical measurements. The materials show a pure anatase phase with average crystallite size of about 10 nm. The nitrogen sorption analysis reveals specific surface areas ranging from 25 to 150 m(2) g(-1) . It is demonstrated that the electrochemical performance of this material strongly depends on morphology. The mesoporous TiO(2) samples exhibit excellent high rate capabilities and good cycling stability.