Hydrogen absorption enhancement of nanocrystalline Li3N/Li2C2 composite

A nanocrystalline composite of lithium nitride and lithium carbide was synthesized through melt infiltration of lithium metal into the mesopores of carbon aerogels followed by nitrogenation with nitrogen gas. The structure, surface properties, and morphology of the prepared samples were examined by XRD, N₂ adsorption at 77 K, FE-SEM, FE-TEM, and TPD/MS. It was found that some of the lithium metal reacted with the carbon to form lithium carbide, and some of the lithium metal was transformed into lithium nitride by nitrogenation, yielding a composite of lithium nitride and lithium carbide. Relative to the bulk lithium nitride, the lithium nitride in the composite showed a significantly enhanced sequential hydrogen absorption capacity and a lowered temperature of hydrogenation/dehydrogenation.     

Mixed-metal Li3N-based systems for hydrogen storage: Li3AlN2 and Li3FeN2

The hydrogen storage systems Li₃AlN₂ and Li₃FeN₂ were synthesized mechanochemically by two different routes. In each case an intermediate material formed after milling, which transformed into Li₃MN₂ (M = Al or Fe) upon annealing. The synthesis route had a measurable effect on the hydrogen storage properties of the material: Li₃AlN₂ prepared from hydrogenous starting materials (LiNH₂ and LiAlH₄) performed better than that synthesized from non-hydrogenous materials (Li₃N and AlN). For both Li₃AlN₂ materials, the hydrogen storage capacity and the absorption kinetics improved significantly upon cycling. Ti-doped Li₃AlN₂ synthesized from LiNH₂ and LiAlH₄ showed the best hydrogen storage characteristics of all, with the best kinetics for hydrogen uptake and release, and the highest hydrogen storage capacity of 3.2 wt.%, of which about half was reversible. Meanwhile, Li₃FeN₂ synthesized from Li₃N and Fe displayed similar kinetics to that synthesized from Li₃N and Fe_xN (2 ≤ x ≤ 4), but demonstrated lower gravimetric hydrogen storage capacities. Li₃FeN₂ displayed a hydrogen uptake capacity of 2.7 wt.%, of which about 1.5 wt.% was reversible. For both Li₃AlN₂ and Li₃FeN₂, doping with TiCl₃ resulted in enhancement of hydrogen absorption kinetics. This represents the first study of a ternary lithium-transition metal nitride system for hydrogen storage.     

Hydrogen storage behaviour of Li3N doped with Li2O and Na2O

Mixtures of Li₂O/Li₃N and Na₂O/Li₃N have been investigated for hydrogen storage. When Li₃N is doped with ca. 5 mol% Li₂O and annealed, both binary compounds exist as separate phases as evident from powder X-ray diffraction. Li₂O acts as a spectator in the hydrogen storage reactions and there is no evidence of enhanced Li⁺ or H⁺ mobility. Na₂O (5 mol%) interacts more strongly with Li₃N, leading to the generation of an unidentified phase, which also appears to play no part in the hydrogen storage reactions of the composite system. We conclude that addition of these levels of Li₂O or Na₂O to Li₃N followed by annealing does not improve the hydrogen storage properties of Li₃N.     

Synthesis and performance of carbon-coated Li3V2(PO4)3 cathode materials by a low temperature solid-state reaction

The carbon-coated monoclinic Li₃V₂(PO₄)₃ (LVP) cathode materials can be synthesized by a low temperature solid-state reaction route. The influences of different heat treatments on the LVP have been investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM) and electrochemical methods. In the range of 3.0-4.3 V, both LVP/C electrodes present good rate capability and excellent cyclic performance. It is found that the sample (LVP1/C) prepared by the two-step heat treatment with pre-sintering at 350 °C delivers the initial discharge capacity of 99.8 mAh g⁻¹ at 10 C charge-discharge rate and still retains 95.8 mAh g⁻¹ after 300 cycles. For the sample (LVP2/C) synthesized by the one-step heat treatment, 95.9 and 90.0 mAh g⁻¹ are obtained in the 1st and 300th cycles at 10 C rate, respectively. Our results based on the XRD patterns and the SEM images suggest that the good rate capability and cyclic performance may be owing to the pure phases, small particles, large specific surface areas and residual carbon. In the range of 3.0-4.8 V, compared with the LVP2/C, the LVP1/C also exhibits better performance.     

Li3V2(PO4)3 cathode material synthesized by chemical reduction and lithiation method

The monoclinic-type Li₃V₂(PO₄)₃ cathode material was synthesized via calcining amorphous Li₃V₂(PO₄)₃ obtained by chemical reduction and lithiation of V₂O₅ using oxalic acid as reducer and lithium carbonate as lithium source in alcohol solution. The amorphous Li₃V₂(PO₄)₃ precursor was characterized by using TG–DSC and XPS. The results showed that the V⁵⁺ was reduced to V³⁺ by oxalic acid at ambient temperature and pressure. The prepared Li₃V₂(PO₄)₃ was characterized by XRD and SEM. The results indicated the Li₃V₂(PO₄)₃ powder had good crystallinity and mesoporous morphology with an average diameter of about 30 nm. The pure Li₃V₂(PO₄)₃ exhibits a stable discharge capacity of 130.08 mAh g⁻¹ at 0.1 C (14 mA g⁻¹).     

The electrochemical insertion and safety properties of the low-cost Li-ion active material, Li2FeS2

The electrochemical insertion properties and safety characteristics of the alternative Li-ion positive electrode material, lithium iron sulfide (Li₂FeS₂) are presented. The active material is synthesized by a low cost, proprietary solid-state method. In terms of specific energy, the Li₂FeS₂ material offers a significant advantage over conventional lithium-ion positive electrode materials. The fully de-lithiated (charged) Li_2−xFeS₂ phase also demonstrates outstanding thermal stability suggesting that it may represent an excellent choice for safe, large format Li-ion battery applications.     

Electrochemical behaviors of a Li3N modified Li metal electrode in secondary lithium batteries

A lithium conductive Li₃N film is successfully prepared on Li metal surface by the direct reaction between Li and N₂ gas at room temperature. X-ray diffraction (XRD), Auger electron spectroscopy (AES), cyclic voltammetry (CV), scanning electron microscopy (SEM), AC impedance, cathodic polarization and galvanostatic charge/discharge cycling tests are applied to characterize the film. The experimental results show that the Li₃N protective film is tight and dense with high stability in the electrolyte. Its thickness is more than 159.4 nm and much bigger than that of a native SEI film formed on the lithium surface as received. An exchange current as low as 3.244 × 10⁻⁷ A demonstrates the formation of a complete SEI film at the electrode|electrolyte interface with Li₃N modification. The SEI film is very effective in preventing the corrosion of the Li electrode in liquid electrolyte, leading to a decreased Li|electrolyte interface resistance and an average short distance of 3.16 × 10⁻³ cm for Li ion diffusion from electrolyte to Li surface. The Li cycling efficiency depends on N₂ exposing time and is obviously enhanced by the Li₃N (1 h) modification. After cycling, a dense and homogeneous Li layer deposits on the Li₃N (1 h) modified Li surface, instead of a loose and inhomogeneous layer on the Li surface as received.     

Preparation of Li2S-GeSe2-P2S5 electrolytes by a single step ball milling for all-solid-state lithium secondary batteries

Glass-ceramic and glass Li₂S-GeSe₂-P₂S₅ electrolytes were prepared by a single step ball milling (SSBM) process. Various compositions of Li_4−xGe_1−xP_xS_2(1+x)Se_2(1−x) with/without heat treatment (HT) from x = 0.55 to x = 1.00 were systematically investigated. Structural analysis by X-ray diffraction (XRD) showed gradual increase of the lattice constant followed by significant phase change with increasing GeSe₂. HT also affected the crystallinity. Incorporation of GeSe₂ in Li₂S-P₂S₅ kept high conductivity with a maximum value of 1.4 × 10⁻³ S cm⁻¹ at room temperature for x = 0.95 in Li_4−xGe_1−xP_xS_2(1+x)Se_2(1−x) without HT. All-solid-state LiCoO₂/Li cells using Li₂S-GeSe₂-P₂S₅ as solid-state electrolytes (SE) were tested by constant-current constant-voltage (CCCV) charge-discharge cycling at a current density of 50 μA cm⁻² between 2.5 and 4.3 V (vs. Li/Li⁺). In spite of the extremely high conductivity of the SE, LiCoO₂/Li cells showed a large irreversible reaction especially during the first charging cycle. LiCoO₂ with SEs heat-treated at elevated temperature exhibited a capacity over 100 mAh g⁻¹ at the second cycle and consistently improved cycle retention, which is believed to be due to the better interfacial stability.     

In situ X-ray diffraction under H2 of the pseudo-AB2 compounds: YNi3.5Al0.5Mg

The hydrogenation and dehydrogenation behaviours of the YNi_3.5Al_0.5Mg compound were studied by in situ X-ray diffraction under hydrogen pressure and at room temperature. The changes of (i) the lattice parameters, (ii) the crystallite size and (iii) the lattice strain during the sorption process (i.e. along the PC isotherms) were studied. These results indicate that the crystallite size decreases by a factor of 2. The micro deformations increase at first and then tend to almost zero at the end of the sorption cycle. This behaviour is explained in terms of co-existence of the metal (i.e. α$\alpha$ phase) and metal hydride (i.e. β$\beta$ phase) phases. The change in crystallinity is consistent with the hydrogen induced amorphisation process existing in a lot of AB₂ compounds. No anisotropic effects can be highlighted on this pseudo-AB₂ compounds in contrary with what could be observed in AB₅ compounds.     

Catalytic effect of titanium nitride nanopowder on hydrogen desorption properties of NaAlH4 and its stability in NaAlH4

Single crystalline titanium nitride (TiN) nanopowder is synthesized by a mechano-chemical reaction between titanium chloride (TiCl₃) and lithium nitride (Li₃N) by means of high-energy ball milling. The TiN nanopowder has an average particle size of 6 nm and is introduced into sodium alanate (NaAlH₄) as a catalyst. During hydrogen sorption cycles, TiN-catalyzed NaAlH₄ exhibits a greater hydrogen desorption rate and higher hydrogen capacity than TiCl₃-catalyzed NaAlH₄. Contradicting thermodynamic predictions, in situ X-ray diffraction results reveal that TiN nanopowder remains stable and produces no by-products (e.g., Ti-Al compounds) in the reaction with NaAlH₄ during hydrogen desorption. In situ Raman spectroscopy also confirms the stability of TiN nanopowder in NaAlH₄. This implies that the sustained hydrogen sorption kinetics and hydrogen capacity of TiN-catalyzed NaAlH₄ originate from the structural and chemical stability of TiN nanopowder in NaAlH₄ for the given conditions of the hydrogen cycle test.     

Formation of the high lithium ion conducting phase from mechanically milled amorphous Li2S-P2S5 system

The fast ionic conducting structure similar to thio-Lithium Super Ionic Conductor (LISICON) phase is synthesized in the Li₂S-P₂S₅ system. The Li₂S-P₂S₅ glass-ceramics with the composition of xLi₂S·(100−x)P₂S₅ (75 ≤ x ≤ 80) are prepared by the heat-treatment of mechanically milled amorphous sulfide powders. In the binary Li₂S-P₂S₅ system, 78.3Li₂S·21.7P₂S₅ glass ceramic prepared by mechanical milling and subsequent heat-treatment at 260 °C for 3 h shows the highest conductivity of 6.3 × 10⁻⁴ S cm⁻¹ at room temperature and the lowest activation energy for conduction of 30.5 kJ mol⁻¹. The enhancement of conductivity with increasing x up to 78.3 is probably caused by the introduction of interstitial lithium ions at the Li sites which affects the Li ion distribution. The prepared electrolyte exhibits the lithium ion transport number of almost unity and voltage stability of 5 V vs. Li at room temperature.     

Thermal properties of Li4/3Ti5/3O4/LiMn2O4 cell

The thermal properties of Li_4/3Ti_5/3O₄ and Li_1+xMn₂O₄ electrodes were investigated by isothermal micro-calorimetry (IMC). The 150-mAh g⁻¹ capacity of a Li/Li_4/3Ti_5/3O₄ half cell was obtained through the voltage plateau that occurs at 1.55 V during the phase transition from spinel to rock salt. Extra capacity below 1.0 V was attributed to the generation of a new phase. The small and constant entropy change of Li_4/3Ti_5/3O₄ during the spinel/rock-salt phase transition indicated its good thermal stability. Accelerated rate calorimetry confirmed that Li_4/3Ti_5/3O₄ has better thermal characteristics than graphite. The IMC results for a Li/Li_1+xMn₂O₄ half cell indicated less heat variation due to the suppression of the order/disorder change by lithium doping. The heat profiles of the Li_4/3Ti_5/3O₄/Li_1+xMn₂O₄ full cell indicated less heat generation compared with a mesocarbon-microbead graphite/Li_1+xMn₂O₄ cell.     

Characteristics of Li3V2(PO4)3/C powders prepared by ultrasonic spray pyrolysis

Li₃V₂(PO₄)₃ and Li₃V₂(PO₄)₃/C powders are prepared by ultrasonic spray pyrolysis from spray solutions with and without sucrose. The precursor powders have a spherical shape and the crystal structure of V₂O₃ irrespective of the concentration of sucrose in the spray solution. The powders post-treated at 700 °C have the pure crystal structure of the Li₃V₂(PO₄)₃ phase irrespective of the concentration of sucrose in the spray solution. The Li₃V₂(PO₄)₃ powders prepared from the spray solution without sucrose have a non-spherical shape and hard aggregation. However, the Li₃V₂(PO₄)₃/C powders prepared from the spray solution with sucrose have a spherical shape and non-aggregation characteristics. The Li₃V₂(PO₄)₃ powders prepared from the spray solution without sucrose have a low initial discharge capacity of 122 mAh g⁻¹. However, the Li₃V₂(PO₄)₃/C powders prepared from the spray solutions with 0.1, 0.3, and 0.5 M sucrose have initial discharge capacities of 141, 130, and 138 mAh g⁻¹, respectively. After 25 cycles, the discharge capacities of the powders formed from the spray solutions with and without 0.1 M sucrose are 70% and 71% of the initial discharge capacities, respectively.     

Li3V2(PO4)3/C composite as an intercalation-type anode material for lithium-ion batteries

The carbon coated monoclinic Li₃V₂(PO₄)₃ (LVP/C) powder is successfully synthesized by a carbothermal reduction method using crystal sugar as the carbon source. Its structure and physicochemical properties are investigated using X-ray diffraction (XRD), scanning electron microscopy, high-resolution transmission electron microscopy and electrochemical methods. The LVP/C electrode exhibits stable reversible capacities of 203 and 102 mAh g⁻¹ in the potential ranges of 3.0-0.0 V and 3.0-1.0 V versus Li⁺/Li, respectively. It is identified that the insertion/extraction of Li⁺ undergoes a series of two-phase transition processes between 3.0 and 1.6 V and a single phase process between 1.6 and 0.0 V. The ex situ XRD patterns of the electrodes at various lithiated states indicate that the monoclinic structure can still be retained during charge-discharge process and the insertion/deinsertion of lithium ions occur reversibly, which provides an excellent cycling stability with high energy efficiency.     

A fast synthesis of Li3V2(PO4)3 crystals via glass-ceramic processing and their battery performance

A synthesis of Li₃V₂(PO₄)₃ being a potential cathode material for lithium ion batteries was attempted via a glass-ceramic processing. A glass with the composition of 37.5Li₂O-25V₂O₅-37.5P₂O₅ (mol%) was prepared by a melt-quenching method and precursor glass powders were crystallized with/without 10 wt% glucose in N₂ or 7%H₂/Ar atmosphere. It was found that heat treatments with glucose at 700 °C in 7%H₂/Ar can produce well-crystallized Li₃V₂(PO₄)₃ in the short time of 30 min. The battery performance measurements revealed that the precursor glass shows the discharge capacity of 14 mAh g⁻¹ at the rate of 1 μA cm⁻² and the glass-ceramics with Li₃V₂(PO₄)₃ prepared with glucose at 700 °C in 7%H₂/Ar show the capacities of 117-126 mAh g⁻¹ (∼96% of the theoretical capacity) which are independent of heat treatment time. The present study proposes that the glass-ceramic processing is a fast synthesizing route for Li₃V₂(PO₄)₃ crystals.     

In situ fabrication of CDs/g-C3N4 hybrids with enhanced interface connection via calcination of the precursors for photocatalytic H2 evolution

Novel carbon dots (CDs)/graphitic carbon nitride (g-C₃N₄) hybrids were fabricated via an in situ thermal polymerization of the precursors, urea and glucose. This heterojunction catalyst exhibited enhanced photocatalytic H₂ evolution activity under visible-light (λ > 420). A sample of CDs/g-C₃N₄ hybrids, CN/G0.5, which was prepared from 0.5 mg of glucose in 6.0 g of urea (8.3 × 10^?3 wt% glucose), exhibited the best photocatalytic performance for hydrogen production from water under visible light irradiation, which is about 4.55 times of that of the bulk g-C₃N₄ (BCN). The improvement of photocatalytic activity was mainly attributed to the construction of built-in electric field at the interface of CDs and g-C₃N₄, which could improve the separation of photogenerated electron-hole pair. Moreover, the tight connection of CDs with g-C₃N₄ would serve as a well electron transport channel, which could promote the photocatalytic H₂ evolution ability.     

Synthesis of plate-like Li3V2(PO4)3/C as a cathode material for Li-ion batteries

Plate-like Li₃V₂(PO₄)₃/C composite is synthesized via a solution route followed by solid-state reaction. The Li₃V₂(PO₄)₃/C plates are 40-100 nm in thicknesses and 2-10 μm in lengths. TEM images show that a uniform carbon layer with a thickness of 5.3 nm presents on the surfaces of Li₃V₂(PO₄)₃ plates. The apparent Li-ion diffusion coefficient of the plate-like Li₃V₂(PO₄)₃/C is calculated to be 2.7 × 10⁻⁸ cm² s⁻¹. At a charge-discharge rate of 3 C, the plate-like Li₃V₂(PO₄)₃/C exhibits an initial discharge capacity of 125.2 and 133.1 mAh g⁻¹ in the voltage ranges of 3.0-4.3 and 3.0-4.8 V, respectively. After 500 cycles, the electrodes still can deliver a discharge capacity of 111.8 and 97.8 mAh g⁻¹ correspondingly, showing a good cycling stability.     

Synthesis and electrochemical performances of Li3V2(PO4)3/(Ag + C) composite cathode

Li₃V₂(PO₄)₃, Li₃V₂(PO₄)₃/C and Li₃V₂(PO₄)₃/(Ag + C) composites as cathodes for Li ion batteries are synthesized by carbon-thermal reduction (CTR) method and chemical plating reactions. The microstructure and morphology of the compounds are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The Li₃V₂(PO₄)₃/(Ag + C) particles are 0.5-1 μm in diameters. As compared to Li₃V₂(PO₄)₃, Li₃V₂(PO₄)₃/C, the Li₃V₂(PO₄)₃/(Ag + C) composite cathode exhibits high discharge capacity, good cycle performance (140.5 mAh g⁻¹ at 50th cycle at 1 C, 97.3% of initial discharge capacity) and rate behavior (120.5 mAh g⁻¹ for initial discharge at 5 C) for the fully delithiated (3.0-4.8 V) state. Electrochemical impedance spectroscopy (EIS) measurements show that the carbon and silver co-modification decreases the charge transfer resistance of Li₃V₂(PO₄)₃/(Ag + C) cathode, and improves the conductivity and boosts the electrochemical performance of the electrode.     

Synthesis and improved electrochemical performances of porous Li3V2(PO4)3/C spheres as cathode material for lithium-ion batteries

Spherical Li₃V₂(PO₄)₃/C composites are synthesized by a soft chemistry route using hydrazine hydrate as the spheroidizing medium. The electrochemical properties of the materials are investigated by galvanostatic charge-discharge tests, cyclic voltammograms and electrochemical impedance spectrum. The porous Li₃V₂(PO₄)₃/C spheres exhibit better electrochemical performances than the solid ones. The spherical porous Li₃V₂(PO₄)₃/C electrode shows a high discharge capacity of 129.1 and 125.6 mAh g⁻¹ between 3.0 and 4.3 V, and 183.8 and 160.9 mAh g⁻¹ between 3.0 and 4.8 V at 0.2 and 1 C, respectively. Even at a charge-discharge rate of 15 C, this material can still deliver a discharge capacity of 100.5 and 121.5 mAh g⁻¹ in the potential regions of 3.0-4.3 V and 3.0-4.8 V, respectively. The excellent electrochemical performance can be attributed to the porous structure, which can make the lithium ion diffusion and electron transfer more easily across the Li₃V₂(PO₄)₃/electrolyte interfaces, thus resulting in enhanced electrode reaction kinetics and improved electrochemical performance.     

Effects of Fe2P and Li3PO4 additives on the cycling performance of LiFePO4/C composite cathode materials

In this study, a solution method was employed to synthesize LiFePO₄-based powders with Li₃PO₄ and Fe₂P additives. The composition, crystalline structure, and morphology of the synthesized powders were investigated by using ICP-OES, XRD, TEM, and SEM, respectively. The electrochemical properties of the powders were investigated with cyclic voltammetric and capacity retention studies. The capacity retention studies were carried out with LiFePO₄/Li cells and LiFePO₄/MCMB cells comprised LiFePO₄-based materials prepared at various temperatures from a stoichiometric precursor. Among all of the synthesized powders, the samples synthesized at 750 and 775 °C demonstrate the most promising cycling performance with C/10, C/5, C/2, and 1C rates. The sample synthesized at 775 °C shows initial discharge capacity of 155 mAh g⁻¹ at 30 °C with C/10 rate. From the results of the cycling performance of LiFePO₄/MCMB cells, it is found that 800 °C sample exhibited higher polarization growth rate than 700 °C sample, though it shows lower capacity fading rate than 700 °C sample. For Fe₂P containing samples, the diffusion coefficient of Li⁺ ion increases with increasing amount of Fe₂P, however, the sample synthesized at 900 °C shows much lower Li⁺ ion diffusion coefficient due to the hindrance of Fe₂P layer on the surface of LiFePO₄ particles.