Antifluorite-type lithium chromium oxide nitrides: synthesis, structure, order, and electrochemical properties

Antifluorite-type lithium chromium oxide nitrides were prepared by solid-state reaction of Li(3)N, Li(2)O, and Cr(2)N. Depending on the reaction time and starting Li/Cr and O/Cr ratios, either an ordered or a disordered phase (or mixtures of both) is obtained. The formation of the former is favored by short reaction times and low Cr/O ratios whereas the formation of the latter is favored by higher Cr/O ratios and longer reaction times. The two phases were characterized, and the first one was confirmed to be the already reported Li(14)Cr(2)N(8)O phase, whereas the stoichiometry of the second is Li(10)CrN(4)O(2). Interestingly, even if both contain cationic vacancies in the structure, electrochemical lithium intercalation could only be achieved for Li(10)CrN(4)O(2). This phase exhibits a reversible capacity of 160 mAh/g very stable upon cycling. Bond valence and first-principles DFT calculations were carried out to understand the absence of lithium insertion in Li(14)Cr(2)N(8)O. Li-Li repulsion and destabilization of the tetrahedral CrN(4) units induced by occupation of the potential sites, as well as the absence of energetically favorable pathways for transport of the ions to these sites, are suggested to be the reasons.     

Role of anionic vacancies in lithium fluoride in the catalytic oxidation of carbon monoxide on the surface of the Au/LiF/Mo(110) system

The oxidation of carbon monoxide molecules on the surface of gold nanoclusters formed on a lithium fluoride film has been investigated by IR spectroscopy and temperature-programed reaction. The CO oxidation rate is much higher in the case of gold clusters produced on the surface of a LiF film enriched with anionic vacancies (F-centers). This is due to the fact that the gold clusters bonded to F-centers of the substrate differ in electronic state from the clusters that are not bonded to F-centers.     

Activation diffusion of oxygen under conditions of the metal-semiconductor phase transition in vanadium dioxide

Density functional theory is used to calculate the energies of formation of oxygen vacancies and migration of oxygen in the monoclinic and rutile phases of vanadium dioxide. The results are compared to estimates of the parameters of activation diffusion of oxygen using data from the electron-beam modification of thin film structures of vanadium dioxide and their subsequent reduction in the temperature range of 20–100°C. It is shown that diffusion in both phases of vanadium dioxide has a preferential direction of oxygen migration along axis а in the monoclinic phase and axis с in the rutile phase. The difference between the rate of oxygen vacancy generation upon electron-beam exposure above and below the temperature of metal–semiconductor phase transition is explained by the jump (~150%) in the activation energy of oxygen diffusion upon the structural transition of rutile–monoclinic phase. The mobility of oxygen (oxygen vacancies) correspondingly changes by more than an order of magnitude.     

In situ Fe XAFS of reversible lithium insertion in a flexible metal organic framework material

X-ray absorption fine structure (XAFS) analysis of the Fe K-edge during lithium insertion and extraction into the metal organic framework material MIL-53 (Fe^III(OH, F)bdc; bdc = benzene-1,4-dicarboxylate) reveals changes in local atomic environment about iron during the process. The average oxidation state of iron is reduced upon lithium insertion, as evidenced by the edge shift of the XANES spectra, and this is accompanied by a lengthening of Fe–O bonds, seen in the EXAFS. In contrast, the EXAFS analysis shows that the closest Fe–Fe distance remains approximately constant during the insertion and extraction of lithium, consistent with a distortion of the structure due to its flexible nature. The process is reversible upon lithium extraction, proving the redox-active flexibility of the framework.     

Formation of lithium fluoride/metal nanocomposites for energy storage through solid state reduction of metal fluorides

In order to utilize high energy metal fluoride electrode materials as direct replacement electrode materials for lithium ion batteries in the future, a methodology to prelithiate the cathode or anode must be developed. Herein, we introduce the use of a solid state Li₃N route to achieve the lithiation and mechanoreduction of metal fluoride based nanocomposites. The resulting prelithiation was found to be effective with the formation of xLiF:Me structures of very fine nanodimensions analogous to what is found by electrochemical lithiation. Physical and electrochemical properties of these nanocomposites for the bismuth and iron lithium fluoride systems are reported.     

Conversion reaction mechanisms in lithium ion batteries: study of the binary metal fluoride electrodes

Materials that undergo a conversion reaction with lithium (e.g., metal fluorides MF(2): M = Fe, Cu, ...) often accommodate more than one Li atom per transition-metal cation, and are promising candidates for high-capacity cathodes for lithium ion batteries. However, little is known about the mechanisms involved in the conversion process, the origins of the large polarization during electrochemical cycling, and why some materials are reversible (e.g., FeF(2)) while others are not (e.g., CuF(2)). In this study, we investigated the conversion reaction of binary metal fluorides, FeF(2) and CuF(2), using a series of local and bulk probes to better understand the mechanisms underlying their contrasting electrochemical behavior. X-ray pair-distribution-function and magnetization measurements were used to determine changes in short-range ordering, particle size and microstructure, while high-resolution transmission electron microscopy (TEM) and electron energy-loss spectroscopy (EELS) were used to measure the atomic-level structure of individual particles and map the phase distribution in the initial and fully lithiated electrodes. Both FeF(2) and CuF(2) react with lithium via a direct conversion process with no intercalation step, but there are differences in the conversion process and final phase distribution. During the reaction of Li(+) with FeF(2), small metallic iron nanoparticles (<5 nm in diameter) nucleate in close proximity to the converted LiF phase, as a result of the low diffusivity of iron. The iron nanoparticles are interconnected and form a bicontinuous network, which provides a pathway for local electron transport through the insulating LiF phase. In addition, the massive interface formed between nanoscale solid phases provides a pathway for ionic transport during the conversion process. These results offer the first experimental evidence explaining the origins of the high lithium reversibility in FeF(2). In contrast to FeF(2), no continuous Cu network was observed in the lithiated CuF(2); rather, the converted Cu segregates to large particles (5-12 nm in diameter) during the first discharge, which may be partially responsible for the lack of reversibility in the CuF(2) electrode.     

Recent Advances and Perspectives in Lithium−Sulfur Pouch Cells

Lithium–sulfur batteries (LSBs) are considered one of the most promising candidates for next-generation energy storage owing to their large energy capacity. Tremendous effort has been devoted to overcoming the inherent problems of LSBs to facilitate their commercialization, such as polysulfide shuttling and dendritic lithium growth. Pouch cells present additional challenges for LSBs as they require greater electrode active material utilization, a lower electrolyte–sulfur ratio, and more mechanically robust electrode architectures to ensure long-term cycling stability. In this review, the critical challenges facing practical Li–S pouch cells that dictate their energy density and long-term cyclability are summarized. Strategies and perspectives for every major pouch cell component—cathode/anode active materials and electrode construction, separator design, and electrolyte—are discussed with emphasis placed on approaches aimed at improving the reversible electrochemical conversion of sulfur and lithium anode protection for high-energy Li–S pouch cells.     

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.     

On the use of the reverse micelles synthesis of nanomaterials for lithium-ion batteries

The reverse micelles procedure is a convenient route for the preparation of nanomaterials. Chemical reactions in aqueous media are carried out within a restricted volume, limited by the array of surfactant molecules. The versatility of this technique allows its use in the preparation of different electrode materials for lithium-ion batteries. The thermolysis of the reagents in aqueous solution in the micellar volume by contact with hot kerosene allows the preparation of LiCoO₂, LiMn₂O₄, and LiNi_0.5Mn_1.5O₄ fine powders with good electrochemical behavior. The conversion electrode material Co₃O₄ was prepared with controlled particle size and microstructure by a precipitation reaction in the micellar volume. The electrochemical response found in lithium cells was excellent after optimizing the annealing procedure. Cobalt and iron oxalate nanoribbons and submicrometric rhombic particles of manganese carbonate have been prepared by the reverse micelles procedure and partially behave as conversion oxide electrodes. The electrochemical reaction with lithium of these new oxysalt materials takes place by a different conversion reaction than the corresponding oxide, and a surface capacitive contribution has also been detected.     

Three-phase junction for modulating electron–hole migration in anatase–rutile photocatalysts

The heterophase solid–solid junction as an important type of structure unit has wide applications for its special mechanics and electronic properties. Here we present a first three-phase atomic model for the anatase–rutile TiO₂ heterophase junction and determine its optical and electronic properties, which leads to resolution of the long-standing puzzles on the enhanced photocatalytic activity of anatase–rutile photocatalysts. By using a set of novel theoretical methods, including crystal phase transition pathway sampling, interfacial strain analysis and first principles thermodynamics evaluation of holes and electrons, we identify an unusual structurally ordered three-phase junction, a layer-by-layer “T-shaped” anatase/TiO₂-II/rutile junction, for linking anatase with rutile. The intermediate TiO₂-II phase, although predicted to be only a few atomic layers thick in contact with anatase, is critical to alleviate the interfacial strain and to modulate photoactivity. We demonstrate that the three-phase junction acts as a single-way valve allowing the photogenerated hole transfer from anatase to rutile but frustrating the photoelectron flow in the opposite direction, which otherwise cannot be achieved by an anatase–rutile direct junction. This new model clarifies the roles of anatase, rutile and the phase junction in achieving high photoactivity synergistically and provides the theoretical basis for the design of better photocatalysts by exploiting multi-phase junctions.     

New doped Li-M-Mn-O (M = Al, Fe, Ni) spinels as cathodes for rechargeable 3 V lithium batteries

The electrochemical behaviour of new doped Li-M-Mn-O (M = Al, Fe, Ni) spinel oxides in liquid electrolyte lithium cells was studied. The insertion electrode materials were obtained by heating stoichiometric amounts of thoroughly mixed LiOH and M _x Mn_{1− x} CO₃ (M = Fe, Ni; x = 0.08−0.15) or Al _x Mn_{1− x} (CO₃) (OH) _y , in the case of Al, at 380 °C in air for 20 h. The transition metal-doped samples, particularly those containing Ni or obtained at low temperatures, where the resulting spinel was cation-deficient and highly disordered, exhibited the best cycling performance in the potential window 3.3−2.3 V. Cell capacity was retained by 80% after 200 cycles. Capacity fading was observed on increasing the firing temperature, together with improved crystallinity and the disappearance of cation vacancies. This impaired electrochemical behaviour is ascribed to a Jahn-Teller effect, which induces an X-ray-detectable cubic-tetragonal phase transition upon lithium insertion. The phase transition was undetectable in the low-temperature samples. The influence of the Jahn-Teller distortion is thus seemingly lessened by a highly disordered structure. Received: 25 November 1997 / Accepted: 28 January 1998     

Features of the Adsorption of Molecular Oxygen at the Anionic Vacancies on the (110) Face of the Surface of Rutile

The electronic structure of clusters modelling the adsorption complexes of the oxygen molecule on single isolated and adjacent anionic vacancies of the (110) face on the surface of rutile was studied. It was concluded that two types of such adsorption complexes exist on the defective surface of rutile.     

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.     

Single-atom catalysts for high-energy rechargeable batteries

Clean and sustainable electrochemical energy storage has attracted extensive attention. It remains a great challenge to achieve next-generation rechargeable battery systems with high energy density, good rate capability, excellent cycling stability, efficient active material utilization, and high coulombic efficiency. Many catalysts have been explored to promote electrochemical reactions during the charge and discharge process. Among reported catalysts, single-atom catalysts (SACs) have attracted extensive attention due to their maximum atom utilization efficiency, homogenous active centres, and unique reaction mechanisms. In this perspective, we summarize the recent advances of the synthesis methods for SACs and highlight the recent progress of SACs for a new generation of rechargeable batteries, including lithium/sodium metal batteries, lithium/sodium–sulfur batteries, lithium–oxygen batteries, and zinc–air batteries. The challenges and perspectives for the future development of SACs are discussed to shed light on the future research of SACs for boosting the performances of rechargeable batteries.

Single-atom catalysts are reviewed, aiming to achieve optimized properties to boost electrochemical performances of high-energy batteries.     

57Fe Mössbauer Spectroscopy and Electron Microscopy Study of Metal Extraction from CuFe2O4 Electrodes in Lithium Cells

Active CuFe(2)O(4) electrode materials for lithium cells are produced by thermal decomposition of a citrate precursor. The precipitation of the metal citrate is carried out by a freeze-drying procedure. A tetragonally distorted spinel structure is prepared by the decomposition of a citrate precursor. Samples free of impurities are obtained depending on the annealing temperature. The sample heated at 800 degrees C performed at 470 mAh g(-1) after 50 cycles. Electron microscopy is used as the ultimate technique to monitor the morphological changes upon the reversible conversion reaction. Detachment of metallic particles from the starting material, the formation of a polymeric organic film, and the subsequent removal on charging are discussed as determining factors in the electrochemical behaviour of this oxide as an electrode versus lithium. The growth of metallic iron aggregates is inferred from the (57)Fe M?ssbauer spectra.     

Single-ion magnetism in the extended solid-state: insights from X-ray absorption and emission spectroscopy

Large single-ion magnetic anisotropy is observed in lithium nitride doped with iron. The iron sites are two-coordinate, putting iron doped lithium nitride amongst a growing number of two coordinate transition metal single-ion magnets (SIMs). Uniquely, the relaxation times to magnetisation reversal are over two orders of magnitude longer in iron doped lithium nitride than other 3d-metal SIMs, and comparable with high-performance lanthanide-based SIMs. To understand the origin of these enhanced magnetic properties a detailed characterisation of electronic structure is presented. Access to dopant electronic structure calls for atomic specific techniques, hence a combination of detailed single-crystal X-ray absorption and emission spectroscopies are applied. Together K-edge, L_2,3-edge and Kβ X-ray spectroscopies probe local geometry and electronic structure, identifying iron doped lithium nitride to be a prototype, solid-state SIM, clean of stoichiometric vacancies where Fe lattice sites are geometrically equivalent. Extended X-ray absorption fine structure and angular dependent single-crystal X-ray absorption near edge spectroscopy measurements determine Fe^I dopant ions to be linearly coordinated, occupying a D_6h symmetry pocket. The dopant engages in strong 3dπ-bonding, resulting in an exceptionally short Fe–N bond length (1.873(7) Å) and rigorous linearity. It is proposed that this structure protects dopant sites from Renner–Teller vibronic coupling and pseudo Jahn–Teller distortions, enhancing magnetic properties with respect to molecular-based linear complexes. The Fe ligand field is quantified by L_2,3-edge XAS from which the energy reduction of 3d_z² due to strong 4s mixing is deduced. Quantification of magnetic anisotropy barriers in low concentration dopant sites is inhibited by many established methods, including far-infrared and neutron scattering. We deduce variable temperature L₃-edge XAS can be applied to quantify the J = 7/2 magnetic anisotropy barrier, 34.80 meV (∼280 cm⁻¹), that corresponds with Orbach relaxation via the first excited, M_J = ±5/2 doublet. The results demonstrate that dopant sites within solid-state host lattices could offer a viable alternative to rare-earth bulk magnets and high-performance SIMs, where the host matrix can be tailored to impose high symmetry and control lattice induced relaxation effects.

Taking advantage of synchrotron light source methods, we present the geometric and electronic structure of iron doped in lithium nitride.     

Pyrogenic iron(III)-doped TiO2 nanopowders synthesized in RF thermal plasma: phase formation, defect structure, band gap, and magnetic properties

Iron(III)-doped TiO(2) nanopowders, with controlled iron to titanium atomic ratios (R(Fe/Ti)) ranging from nominal 0 to 20%, were synthesized using oxidative pyrolysis of liquid-feed metallorganic precursors in a radiation-frequency (RF) thermal plasma. The valence of iron doped in the TiO(2), phase formation, defect structures, band gaps, and magnetic properties of the resultant nanopowders were systematically investigated using M?ssbauer spectroscopy, XRD, Raman spectroscopy, TEM/HRTEM, UV-vis spectroscopy, and measurements of magnetic properties. The iron doped in TiO(2) was trivalent (3+) in a high-spin state as determined by the isomer shift and quadrupole splitting from the M?ssbauer spectra. No other phases except anatase and rutile TiO(2) were identified in the resultant nanopowders. Interestingly, thermodynamically metastable anatase predominated in the undoped TiO(2) nanopowders, which can be explained from a kinetic point of view based on classical homogeneous nucleation theory. With iron doping, the formation of rutile was strongly promoted because rutile is more tolerant than anatase to the defects such as oxygen vacancies resulting from the substitution of Fe(3+) for Ti(4+) in TiO(2). The concentration of oxygen vacancies reached a maximum at R(Fe/Ti) = 2% above which excessive oxygen vacancies tended to concentrate. As a result of this concentration, an extended defect like crystallographic shear (CS) structure was established. With iron doping, red shift of the absorption edges occurred in addition to the d-d electron transition of iron in the visible light region. The as-prepared iron-doped TiO(2) nanopowders were paramagnetic in nature at room temperature.     

The Role of Iron–Carbon Galvanic Contact in the Formation of Dispersed Iron Hydr(oxides) in Water and Electrolyte Solutions

The comparative analysis of phase formation on the iron surface in aqueous medium in the presence and absence of iron–carbon (coke) galvanic contact was carried out. The role of galvanic contact in phase formation processes was determined. It was shown that, in the presence of galvanic contact almost complete oxidation of iron ions on the surface of an iron half-element and a rather efficient stationary formation of dispersed phases serving as sorbents of heavy metals from solutions take place. The effect of anionic composition of solution on the parameters of phase formation was studied. It was established that maximal amount of iron–oxygen-containing phases is formed in zinc chloride solution. The presence of sulfate and nitrate ions in solution decreased significantly the rate of phase formation in iron–carbon galvanic contact.     

Lithium Storage in Heat‐Treated SnF2/Polyacrylonitrile Anode

Tin(II) fluoride (SnF₂) has a high Li‐storage capacity because it stores lithium first by a conversion reaction and then by a Li/Sn alloying/dealloying reaction. A polyacrylonitrile (PAN)‐bound SnF₂ electrode was heat‐treated to enhance the integral electrical contact and the mechanical strength through its cross‐linked framework. The heat‐treated SnF₂ electrode showed reversible capacities of 1047 mAh g^?1 in the first cycle and 902 mAh g^?1 after 100 cycles. Part of the excess capacity is due to lithium storage at the Sn/LiF interface, and the other part is assumed to correspond to the presence of reduced SnF₂ with protons released during the thermal cross‐linking of PAN.     

Atomistic insights into the conversion reaction in iron fluoride: a dynamically adaptive force field approach

Nanoscale metal fluorides are promising candidates for high capacity lithium ion batteries, in which a conversion reaction upon exposure to Li ions enables access to the multiple valence states of the metal cation. However, little is known about the molecular mechanisms and the reaction pathways in conversion that relate to the need for nanoscale starting materials. To address this reaction and the controversial role of intercalation in a promising conversion material, FeF(2), a dynamically adaptive force field that allows for a change in ion charge during reactions is applied in molecular dynamics simulations. Results provide the atomistic view of this conversion reaction that forms nanocrystals of LiF and Fe(0) and addresses the important controversy regarding intercalation. Simulations of Li(+) exposure on the low energy FeF(2) (001) and (110) surfaces show that the reaction initiates at the surface and iron clusters as well as crystalline LiF are formed, sometimes via an amorphous Li-F. Li intercalation is also observed as a function of surface orientation and rate of exposure to the Li, with different behavior on (001) and (110) surfaces. Intercalation along [001] rapid transport channels is accompanied by a slight reduction of charge density on multiple nearby Fe ions per Li ion until enough Li saturates a region and causes the nearby Fe to lose sufficient charge to become destabilized and form the nanocluster Fe(0). The resultant nanostructures are fully consistent with postconversion TEM observations, and the simulations provide the solution to the controversy regarding intercalation versus conversion and the atomistic rationale for the need for nanoscale metal fluoride starting particles in conversion cathodes.