A theoretical study of the electronic properties of intercalated graphite

We present the results of electronic structure calculations for first and second stages of lithium intercalated graphite (LiC₆ and LiC₁₂). The various stages of Li intercalated graphite all have hexagonal symmetry, where different carbon layers are stacked with C-atoms directly on top of each other (AIAI…), as opposed to natural graphite where the C-layers are staggered (ABAB…). All our calculations have been performed within the framework of the extended tight binding method with Gaussian type basis sets. From the orbital and total densities of states, we conclude that Li-2s electrons are transferred into carbon π-bands. This results in shifting the Fermi level into the region of high density of states (compared with pure graphite) and, hence, to observed metallic behavior. The calculated density of states for LiC₆ and LiC₁₂ is 0.25 and 0.12/(eV C-atom), respectively. Recall that for pure graphite the value is nearly zero and for copper it is 0.29. We also found it instructive to obtain the electronic structure of LiC₆ and LiC₁₂ based on a rigid band model.     

Structure and phase stability of hydrogenated first-stage alkali- and alkaline-earth metal–graphite intercalation compounds

We report the hydrogenation temperature and pressure dependence of hydrogen absorption capacity in CaC₆. In addition, the structure and phase stability of hydrogenated CaC₆, LiC₆ and KC₈ has been investigated using X-ray diffraction and Raman spectroscopy. It is found that the hydrogen chemisorption in both CaC₆ and LiC₆ leads to either metastable or higher stage intermediate compounds but eventually leaves completely deintercalated graphite by forming stable metal hydrides. In contrast hydrogenation of KC₈ generates a restaged ternary hydride compound. The phase stability of hydrogenated compounds is discussed in the text based on the observed experimental results and correlated to the thermodynamic and kinetic aspects of intercalated metal.     

H NMR study of hydrogen stored in activated carbon powder prepared by mechanical milling

A ¹H NMR study was carried out using hydrogenated activated carbon powder (AC) prepared by mechanical milling in a H₂ atmosphere. Chemical shifts in the hydrogenated milled AC were observed near 0 and 2 ppm. In addition, the peak near 0 ppm was separated into two peaks (α and β) by the deconvolution of the NMR spectra; −0.6 and 0.2 ppm. This indicates that hydrogenated milled AC has three hydrogen components with different molecular mobilities. Measurement of the spin-lattice relaxation time (T₁) revealed that the hydrogen near 0 and 2 ppm consisted of two components (Components 1 and 2) and one component (Component 3), respectively. However, the activation energies (E_a) of each hydrogen component could not be estimated because the plots of inverse temperature (1/T) versus the logarithm of T₁ (ln T₁) were scattered. We assumed that the components near 0 ppm (Component 1 and/or 2) were thermally unstable because the intensity of the chemical shift near 0 ppm decreased as the measurement temperature increased, and this might have an effect on T₁ measurements. The spin-spin relaxation time (T₂) indicated high and low molecular mobility at each chemical shift and several temperatures.     

Microstructure and Adhesion Strength of Ni<Subscript>3</Subscript>Ti Coating Prepared by Mechanical Alloying and HVOF

In the present work we report the development of Ni₃Ti intermetallic compound by high energy ball milling of Ni and Ti powders. The ball milled powders were taken at various intervals (4, 6, 8, 10, and 11 h) to analyze the formation of Ni _x Ti _x intermetallic compounds. The ball milled powders were analyzed using scanning electron microscopy and X-ray diffraction. The layered shaped powder particles of Ni₃Ti phase were formed after 11 h of ball milling, which was confirmed by X-ray peaks. Further High-Velocity Oxy-Fuel (HVOF) process was used to coat Ni₃Ti and Ni₃Ti + (Cr₃C₂ + 20NiCr) on MDN 420 steel. Both the coated materials displayed excellent cohesion with minimal porosity less than 2%. The tensile adhesion strength test was carried out on these coatings to check the bond strength. Out of the two the Ni₃Ti coating showed excellent bond strength of 41.04 MPa compared to that of Ni₃Ti + (Cr₃C₂ + 20NiCr) coating.     

In Situ Production of Fe-TiC Nanocomposite by Mechanical Activation and Heat Treatment of the Fe2O3/TiO2/C Powder

In this study, Fe-TiC nanocomposite was synthesized by carbothermic reduction of activated Fe₂O₃, TiO₂, and graphite powder mixture. The effect of 0, 5, and 20?h of high energy ball milling of mixture on the reduction process was also investigated. Comparing the results of the thermogravimetry analysis of milled and un-milled mixtures clearly showed that the reduction temperature decreased due to the milling process. XRD pattern of 20?h milled powder mixture proved that Fe-TiC nanocomposite was formed after the heat treatment of activated powder at 1100°C for 1?h under vacuum. The microstructure studies of the milled mixture by scanning electron microscope revealed homogenous distribution of TiC particles in the Fe matrix.     

Orientation of nitric acid molecules in graphite nitrate

A new model for HNO₃ intercalation into graphite is proposed. It was supposed that nitric acid forms double parallel layers between the graphit ic network. The new model is characterized by X-ray methods on the 00l line intensity. It is shown that the nitrate molecules are intercalated perpendicular to the graphite planes. The crystallographic stoichiometry gives C_4.3nHNO₃ for the n^th stage.     

In-situ synthesis and characterization of Fe – TiC based cermet produced from enhanced carbothermally reduced ilmenite

In-situ synthesis of Fe-TiC based cermet by carbothermic reduction of South African ilmenite ore has been successfully achieved. This was carried out in an argon containing atmosphere in the temperature range of 850–1350 °C. Powder mixtures of ilmenite concentrate and graphite in molar ratio of 1:4 respectively, were milled for 2 h in a planetary mill (PM 100) to obtain intimate mixtures. The mixtures were then synthesized in a laboratory high temperature furnace at different reduction temperatures. The obtained X-ray diffraction results showed that Fe-TiC has been successfully formed at 1350 °C with 15 min holding time. The sequence of phase evolution showed a stepwise reduction of ilmenite FeTiO₃ initially to form a mixture of Fe and TiO₂, which was successively further reduced to Ti₄O₇, Ti₃O₅, Ti₂O₃ and finally TiC. Three or more major peaks were observed from thermo-analyses (TG, DTG and DTA) corresponding to the initial reduction of FeTiO₃ at 983.3 °C, Ti₄O₇ at 1050 °C, Ti₃O₅ at 1200 °C and finally TiC at 1350 °C. Field Emission SEM coupled with elemental analysis showed a clear phase boundary between the metallic phase (Fe) and the ceramic –TiC in the cermet synthesized at 1350 °C.     

Thermodynamic Reassessment of the Gallium–Lithium Phase Diagram

Thermodynamic optimization of the Ga–Li binary system was carried out using the CALculation of PHAse Diagrams (CALPHAD) method. Ga₁₄Li₃, Ga₇Li₂, Ga₈Li₃, Ga₉Li₅, and Ga₄Li₅ were treated as stoichiometric compounds, while a solution model was used to describe the liquid, ORTHORHOMBIC_CMCA (Ga), and BCC_A2 (Li) phases. The intermetallic compounds GaLi, Ga₂Li₃, and GaLi₂, which have homogeneity ranges, were treated using a two-sublattice model. The results of the calculations based on thermodynamic modeling are in good agreement with phase diagram data and experimental thermodynamic values.     

Cyclic voltammetry studies on 4 V and 5 V plateaus of non-stoichiometric spinel Li1＋xMn2-yO4

1 Introduction As one of the most promising cathode materials for lithium ion batteries, spinel LiMn2O4 has received much attention in recent years. This material has reversible capacities at both 3 V and 4 V plateaus[1]. However, the Li insertion and ex…     

Electrochemical properties of the carbon-coated lithium vanadium oxide anode for lithium ion batteries

Carbon-coated Li_1.1V_0.9O₂ powder was prepared by dissolving pure crystalline Li_1.1V_0.9O₂ powder in an ethanol solution containing 10 wt% sucrose and sintering it under an argon atmosphere. The structures of the bare and carbon-coated Li_1.1V_0.9O₂ powders were analyzed using X-ray diffraction, Raman spectroscopy, scanning electron microscopy, and transmission electron microscopy. These powders were used as anode active materials for lithium ion batteries in order to determine the electrochemical properties via cyclic voltammetry (CV) and constant current methods. CV revealed the carbon-coated Li_1.1V_0.9O₂ anode to have better reversibility during cycling than the bare Li_1.1V_0.9O₂ anode. Carbon-coated Li_1.1V_0.9O₂ also showed a higher specific discharge and charge capacities, as well as lower electrolyte and interfacial resistance properties. The observed specific discharge and charge capacities of the carbon-coated Li_1.1V_0.9O₂ anode were 330 mAh/g and 250 mAh/g, respectively, in the first cycle. In addition, the cyclic efficiency of this cell was 75.8% in the first cycle. After 20 cycles, the specific capacity of the Li_1.1V_0.9O₂ anode was reduced to approximately 50% of its initial capacity, irrespective of the presence of a carbon coating.     

Ni-Cr合金保护气氛钎焊金刚石界面Cr7C3的形核与长大

采用Ni-Cr合金在保护气氛炉中进行钎焊单晶金刚石磨粒试验,使用SEM对Ni-Cr合金钎焊金刚石界面碳化物、断口形貌、钎缝组织进行观察分析,采用XRD对钎焊后金刚石进行物相分析.结果表明,在保护气氛炉中进行钎焊可以实现金刚石的高强度连接,钎焊后金刚石棱角清晰、形貌完好.金刚石的表面生成具有方向性的Cr3C2,Cr3C2的生长方向与金刚石(111)晶面有一定的位向关系,与其所在晶面六边形的边平行,Cr7C3全部在Cr3C2上形核长大,最后形成外层是Cr7C3,内层是具有方向性的Cr3C2.     

X-ray photoelectron spectroscopy studies of SbF5 intercalated graphite

Two samples of graphite intercalated with SbF₅ have been studied by X-ray photoelectron spectroscopy (x.p.s.). In each case two fluorine (1s) peaks were observed, with binding energies 685.0 eV and 688.0 eV. The peak at 685.0 eV is attributed largely to volatile, quasi-liquid SbF₅ present in the interlayer. The peak at 688 eV is assigned to involatile SbF₆^? ions located at fixed sites in the interlayer, the fluorine atoms undergoing strong polarization interaction with the graphite lattice. Changes observed in carbon (1s) and antimony (3d_{$\text{5}\text{2}$}) spectra are consistent with these assignments.     

Effect of the valence states of titanium on the lattice structure and ionic conductivity of Li0.33La0.55TiO3 solid electrolyte

Li_0.33La_0.55TiO₃ solid electrolytes with a pure phase were synthesized by the citric acid-supported sol-gel method and then sintered under controlled redox atmospheres of air, argon and hydrogen. Although of similar morphology and relative density, Li_0.33La_0.55TiO₃ samples sintered under reduction atmosphere such as argon and hydrogen exhibited a tetragonal structure with lattice distortion. The distortion and volume expansion of the crystal lattice was identified as originating from the transformation of the Ti valence state, and this was more clearly observed under sintering of the hydrogen atmosphere. The qualitative analysis of the Ti valence state using an X-ray Photoelectron Spectroscopy experiment was performed and indicated that the largest amount of Ti³⁺(18.6%) was formed in the Li_0.33La_0.55TiO₃ samples sintered under hydrogen atmosphere. The relationship between the lattice distortion and the lithium ion conductivity of the Li_0.33La_0.55TiO₃ sintered under different atmospheres is also discussed on the basis of the lattice distortion.     

Compositional analysis of passivating surface film formed on carbon electrode in organic electrolytic solution using in-situ spectroelectrochemical technique

In-situ spectroelectrochemical technique has been applied to investigate passivating surface film on porous carbon electrode and plasma enhanced chemical vapour deposited (PECVD) carbon film electrode in organic electrolytic solution consisting of ethylene carbonate (EC) and diethyl carbonate (DEC) solvent, and 1 M LiPF₆ and LiAsF₆. Water impurity with the concentration of 0 M, 0.02 M, 0.05 M, and 0.1 M H₂0 was added to 1 M LiPF₆-EC/DEC solution. In-situ Fourier transform infra-red (FTIR) spectra of the surface film on both electrodes with the constituents of ROCO₂Li, Li₂CO₃, and Li_xPF_y suggested that the reduction of EC to ROCO₂Li runs via a one-electron transfer pathway as a result of diffusion of water through the surface film, and then Li2CO3 formation proceeds simultaneously by the chemical reaction of ROCO₂Li with water. From the measured potential dependence of the amount of the salt reduction products, it is suggested that the surface film formed in 1 M LiPF₆EC/DEC solution gives a poorer passivity as compared with that formed in 1 M LiAsF₆-EC/DEC solution, which is due to the considerable interference of LiPE₆ salt reduction with the compact sedimentation of ROCO₂Li on the electrode. In-situ FFIR spectra of the surface film showed that all the peak intensities of the three constituents significantly increase with increasing water content under application of the negative potentials with respect to open circuit potential (OCP). From these experimental results, the dependence of the passivity of the surface film on the carbon electrode on the water concentration of the electrolyte, as well as on the lithium salt type, was discussed in view of the salt and solvent reactivities.     

时效温度对Al-9.0at%Li合金反位原子缺陷的影响

应用微观相场法,计算Al-9.0at%Li合金沉淀相中的反位缺陷Al_(Li)和Li_(Al)随时效温度及时间变化的规律。结果表明,在相同的时效温度下,L12结构的Al_3Li相中反位缺陷Al_(Li)和Li_(Al)随时效时间的增加逐渐减小,且在富Al环境下Al_(Li)反位缺陷的浓度高于Li_(Al),在Al_3Li相中以Al_(Li)为主,同时存在少量的Li_(Al)。当时效温度发生变化时,时效温度越高,在Al_3Li相稳定形核后Li_(Al)、Al_(Li)反位缺陷的浓度越高。在达到平衡浓度前也具有这个特点,但是规律性并不显著。     

Synthesis of Li excess LiFePO4/C using iron chloride extracted from steel scrap pickling

Olivine-type Li rich Li_1+xFePO₄/C composites are synthesized by a solid state reaction process using premilled Li₂CO₃ and pre-synthesized amorphous FePO₄·xH₂O powders. The amorphous FePO₄·xH₂O powders are prepared from an industrial waste liquid (by-product), a FeCl₃ (38%) solution, via a precipitation process. In addition, lithium carbonate is pre-milled using a high energy nano mill to control particle sizes and shape differences for enhancing the reaction activity in the starting materials. The main purpose of this study is to investigate the effect of excess Li on the electrochemical properties of LiFePO₄ cathode materials. The pre-synthesized FePO₄ powders are mixed with pre-milled lithium carbonate and glucose (8 wt%) using a ball-mill process. The structural characteristics of the Li_1+xFePO₄/C composites are examined by XRD and SEM. To investigate the effect of excess Li content on the electrochemical properties in Li_1+xFePO₄/C composites, a Li[LiPF₆ (Ethylene carbonate + Dimethyl carbonate)] Li_1+xFePO₄/C model cell is used. It is demonstrated that the 1% Li rich Li/[Li_1.01FePO₄/C] cell exhibits the best electrochemical performance and delivers an initial discharge capacity of 161 mAhg^?1, which is 25 mAhg^?1 higher than that of the Li/[LiFePO₄/C] cell.     

Electrochemical characteristics of single crystalline nanowires and their driven mechanism

The electrochemical characteristics of single crystalline SnO₂, ZnO and Si nanowires and their driven mechanism are reported as nanostructural anode materials. As intercalation and deintercalation of Li, Si nanowires are converted to amorphous phases of shorter wire shapes caused by the lattice expansion of the single crystalline Si, resulting in the fading of discharge capacity, although the reversible capacity (2500 mAh/g) in the first cycle is very high. However, oxide nanowries (SnO₂ and ZnO) are transformed from a single crystalline structure into a polycrystalline form consisting of nano-sized metallic particles and Li₂O crystals within the wires, which maintain their discharge capacity. The results of this study imply that the large surface area and high electrochemical activity of nanowires and nano-sized polycrystalline particles can provide a method to develop a new class of one-dimensional anode nanostructures in lithium-ion rechargeable batteries.     

Synthesis and physical properties of new metallic compound LixC60(THF)y

Electrocrystallization of C₆₀ in chlorobenzene and lithium tetraphenylborate in THF efficiently produced the new metallic species Li_xC₆₀(THF)_y; the ESR spectra show clearly the existence of a typical broad signal due to the C₆₀ anion radical. The change of the intensity of the ESR signal corresponds to the metal-insulator transition at 150 K, and the ¹³C and ⁷Li solid-state NMR spectra are also reported.     

Effects of Sn on microstructure of as-cast and as-extruded Mg–9Li alloys

The effects of Sn addition on the microstructure of as-cast and as-extruded Mg–9Li alloys were investigated. The results show that α-Mg, β-Li, Li₂MgSn, and Mg₂Sn are primary phases in the microstructures of the as-cast and as-extruded Mg–9Li–xSn (x=0, 5; in mass fraction, %) alloys. Li₂MgSn phase evolves from continuously net-like structure in the as-cast state to fine granular in the as-extruded state. After the extrusion, Mg–9Li–5Sn alloy has finer microstructures. Li₂MgSn or Mg₂Sn compound can act as the heterogeneous nucleation sites for dynamic recrystallization during the extrusion due to the crystallography matching relationship. Extrusion deformation leads to dynamic recrystallization, which results in the grain refinement and uniform distribution. The as-extruded Mg–9Li–5Sn alloy possesses the lowest grain size of 45.9 μm.     

Electrochemical behavior of Li+, Mg2+, Na+ and K+ in LiFePO4/ FePO4 structures

Besides Li⁺ and Mg²⁺, the electrochemical behavior of Na⁺ and K⁺ in LiFePO₄/ FePO₄ structures was studied since they naturally coexist with Li⁺ and Mg²⁺ in brine. The cyclic voltammogram (CV) results indicated that Na⁺ exhibits some reversibility in LiFePO₄/FePO₄ structures. Its reduction peak appears at ?0.511 V, more negative than that of Li⁺ (?0.197 V), meaning that a relatively positive potential is beneficial for decreasing Na⁺ insertion. The reduction peak of K⁺ could not be found clearly, indicating that K⁺ is difficult to insert into the FePO₄ structure. Furthermore, technical experiments using real brine with a super high Mg/Li ratio (493) at a cell voltage of 0.7V showed that the final extracted capacity of Li⁺, Mg²⁺ and Na⁺ that can be attained in 1 g LiFePO₄ is 24.1 mg, 7.32 mg and 4.61 mg, respectively. The Mg/Li ratio can be reduced to 0.30 from 493, and the Na/Li ratio to 0.19 from 16.7, which proves that, even in super high Mg/Li ratio brine, if a cell voltage is appropriately controlled, it is possible to separate Li⁺ and other impurities effectively.