Highly ordered staging structural interface between LiFePO4 and FePO4

A highly ordered interface between LiFePO(4) phase and FePO(4) phase with staging structure along the a axis and perpendicular to the b axis direction has been observed for the first time, in a partially chemically delithiated Li(0.90)Nb(0.02)FePO(4) by advanced aberration-corrected annular-bright-field (ABF) scanning transmission electron microscopy (STEM).     

Vibrational spectroscopic investigation of structurally-related LiFePO4, NaFePO4, and FePO4 compounds

Vibrational spectroscopy was utilized to investigate the local structure of LiFePO(4), NaFePO(4), and FePO(4). The factor group splitting of the intramolecular PO(4)(3-) vibrations is between 10 and 20 cm(-1) less for NaFePO(4) than for LiFePO(4). This is because Li(+) ions have a higher charge density than Na(+) ions and can form stronger coordinative bonds with the PO(4)(3-) anions. Thus, the internal modes are more perturbed in LiFePO(4) and exhibit larger factor group splitting effects. The similarity of the factor group multiplets for both LiFePO(4) and NaFePO(4), particularly the PO(4)(3-) bending modes, strongly suggests that the 506 and 470 cm(-1) bands of LiFePO(4) consist almost entirely of lithium translatory motion. There are marked differences between the vibrational spectrum of FePO(4) and those of LiMPO(4) (M=Mn, Fe, Co, or Ni) or NaFePO(4). The monovalent cations interact with the oxygen atoms of the phosphate groups, affecting the frequencies and intensities of the intramolecular PO(4)(3-) modes, in a manner that is absent in FePO(4).     

The magnetic properties of the high pressure phase of ferric phosphate,FePO4-II

FePO₄-II has been prepared by treating berlinite-like FePO₄-I at 850°C and 4 GPa. The high pressure phase is isostructural with the orthorhombic, divalent sulfates MSO₄ (M = Mn, Fe, Co, and Ni) and thus contains chains of 6-coordinate Fe³⁺ ions. Susceptibility and Mössbauer measurements indicate that FePO₄-II is antiferromagnetically ordered at 4.2 K. Weak interchain coupling leads to the observation of relaxation phenomena in the Mössbauer spectra between 43 K and the magnetic ordering temperature of ca. 60 K. A model which can account for these observations is proposed for the magnetic structure; it consists of weakly ferromagnetic [001] chains, coupled together antiferromagnetically.     

Noteworthy electroanalytical features of the stage 4 to stage 3 phase transition in lithiated graphite

A careful study of electrochemical lithiation of a composite graphite electrode at elevated temperatures was performed using a potentiostatic intermittent titration technique. Special emphasis was placed on the stage 4 to stage 3 phase transition, occurring in the range of x (in Li_xC₆) between 0.12 and 0.22 (this corresponds to the partial dimensionless intercalation level ranging from 0 to 1). An abrupt increase in the Li chemical diffusion coefficient, D_chem, when approaching the relative coverage level =1/3, and a specific non-Arrhenius shape of the plot of D_chem vs. the inverse of the absolute temperature were observed. These two features of the phase transition seem to be in agreement with a recent thermodynamic model for adatoms diffusing on a surface with two considerably different barrier energy sites.Presented at the 3rd International Meeting on Advanced Batteries and Accumulators, 16–20 June 2002, Brno, Czech Republic     

激光促进乙烯在磷酸铁上的表面反应

激光促进表面反应;丁二烯;激光促进乙烯在磷酸铁上的表面反应     

溶胶凝法制备LiFePO4正极材料

本文介绍了溶胶凝胶法制备LiFePO4正极材料的基本原理及近几年这一领域的研究进展。对碳包覆活性物质、掺杂和多形态纳米化制备技术三种改性方法以及它们对LiFePO4正极材料性能的提高进行了总结。取得的成效主要有，容量得到提高、颗粒尺寸和碳含量有所减小，热处理过程所需时间大大缩短。文中进一步指出目前存在的若干问题，包括对制备过程的深入认识，产品成本以及环境污染。最后对其商业化的可能性进行了讨论。     

Communication: probable scenario of the liquid-liquid phase transition of SnI4

We have shown from in situ synchrotron x-ray diffraction measurements that there are two thermodynamically stable liquid forms of SnI(4), depending on the pressure. Based on the liquid-liquid critical point scenario, our recent measurements suggest that the second critical point, if it exists, may be located in a region close to the point at which the melting curve of the crystalline phase abruptly breaks. This region is, unlike that of water, experimentally accessible with relative ease.     

High-pressure phase transition in LiBH4

The high-pressure phase transition from ambient pressure α-LiBH₄ to high-pressure β-LiBH₄ was observed by Raman spectroscopy and X-ray diffraction between 0.8 and 1.1 GPa. The phase boundary between these two phases was mapped over a large range of temperatures using thermal conductivity studies and differential thermal analysis. The structure of the high-pressure phase could not be identified due to small number of experimentally observed reflections, but it was shown that it is different from previously reported theoretical predictions.     

Thermal conductivity of LiKSO4 near its high-temperature phase transition

The crystallization processes of Ge _x S_1–x glasses were studied by means of heat flux DSC. It was shown that germanium disulphide crystallizes atx1/3. On the other hand, a higher germanium content (x>1/3) led to the crystallization of both GeS₂ and GeS. Kinetic analysis of the processes of crystallization of the studied glasses was performed using the Arrhenius rate constant and the esták-Berggren kinetic model. The kinetic parameters were calculated through non-linear regression of the experimental DSC curves.

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Zusammenfassung Die Kristallisationsprozesse von Gläsern Ge_xS₁–x wurden durch Wärmefluss-DSC untersucht. Beix0,33 kristallisiert GeS₂, während bei höherem Germaniumgehalt (x>0,333) Kristallisation von GeS₂ und GeS eintritt.Für die kinetische Analyse des Kristallisationsprozessen wurden Arrhenius-Geschwindigkeitskonstanten und das esták-Berggren-Modell verwendet. Die kinetischen Parameter wurden durch nichtlineare Regression der experimentellen DSC-Kurven berechnet.

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GexS1–x. , 1/3 , (>1/3) — , . . . .

     

Moisture driven aging mechanism of LiFePO4 subjected to air exposure

The impact of air exposure on LiFePO₄–C nanocomposites has been investigated at moderate temperature. We show here that the storage in 120 °C hot air for 30 days leads not only to the material delithiation but also to the formation of an amorphous ferric phosphate side-phase, accounting for 38% of the total iron. The formed phase is found to be partially hydrated, suggesting a water-driven aging mechanism and a proposed hypothetic formula: Li_xFePO₄(OH)_x. The side-phase displays new electrochemical activity but poor cyclability and the overall battery performance is thus deteriorated. The regeneration of pristine structure, together with the performance recovery can be achieved by a simple thermal treatment under inert atmosphere.     

Raman and X-ray investigations of ferroelectric phase transition in NH4HSO4

Temperature-dependent Raman spectroscopy and X-ray diffraction studies have been carried out on NH(4)HSO(4) single crystals in the temperature range 77-298 K. Two structural transitions driven by the molecular ordering and change in crystal symmetries are observed below 263 and 143 K. These phase transitions are marked by the anomalies in the temperature dependence of wavenumber and fwhm of several internal vibrational modes. The Raman spectra and X-ray data enable us to understand the nature of the molecular ordering resulting in the ferroelectric phase below 263 K, sandwiched between two nonferroelectric phases. The crystal structure of the ferroelectric phase is determined correctly as Pc, which has been earlier solved in Ba symmetry. The temperature dependent Raman and X-ray results suggest that the disorder to order transition leading to lower symmetry below 263 K is driven by the change in HSO(4)(-) ions and that below 143 K is driven by the change in both HSO(4)(-) and NH(4)(+) ions.     

Surface modification of FePO4 particles with conductive layer of polypyrrole

Polypyrrole–FePO₄ powder was synthesized by an oxidative polymerization of pyrrole monomer on the surface of FePO₄ powder. The polymerization reaction was initiated using hydrogen peroxide in an acidified solution and catalysed with Fe³⁺. The samples were investigated by light microscopy (LM), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDX). These methods confirmed the presence of polypyrrole on FePO₄ particles and its homogeneous distribution in the composite material. To determine the PPy content in the PPy–FePO₄ composites a thermogravimetric analysis was used. Cyclic voltammetry curves (CV) were measured and compared in a non-aqueous lithium salt solution for electrodes consisting of pellets made from pure FePO4 and FePO₄/PPy. Electrochemical impedance spectroscopy (EIS) showed that coating of PPy significantly decreases the charge transfer resistance of PPy–FePO₄ electrodes.     

Synthesis and electrochemical characterizations of nano-crystalline LiFePO4 and Mg-doped LiFePO4 cathode materials for rechargeable lithium-ion batteries

Nano-crystalline LiFePO₄ and LiMg_0.05Fe_0.95PO₄ cathode materials were synthesized by sol–gel method in argon atmosphere using succinic acid as a chelating agent. Physico-chemical characterizations were done by thermogravimetric and differential thermal analysis, X-ray diffraction, scanning electron microscopy, transmittance electron microscopy, and Raman spectroscopy. Electrochemical behavior of the cathode materials were analyzed using cyclic voltammetry, and galvanostatic charge/discharge cycling studies were employed to characterize the reaction of lithium-ion insertion into and extraction from virginal and magnesium-doped LiFePO₄, in the voltage range 2.5 to 4.5 V (Vs Li/Li⁺) using 1 M LiPF₆ with 1:1 ratio of ethylene carbonate and dimethyl carbonate as electrolytes. LiMg_0.05Fe_0.95PO₄ exhibits initial charge and discharge capacities of 159 and 141 mAh/g at 0.2 C rate respectively, as compared to 121 and 107 mAh/g of pristine LiFePO₄. Furthermore, LiMg_0.05Fe_0.95PO₄ has retained more than 89% of the capacity even after 60 cycles. Hence, LiMg_0.05Fe_0.95PO₄ is a promising cathode material for rechargeable lithium-ion batteries.     

FTIR研究LiFePO4的充放电过程

随着便携式电子设备的日益普及,人们对支撑这些设备运行的后备电源提出了越来越高的要求.锂离子电池与传统的铅酸和镍镉电池相比具有更大的电动势、更大的比能量(120～150 Wh/kg,是常用的Ni-Cd电池的2～3倍)以及较好的充放循环性能,因此成为目前使用较多的高性能便携能源设备.作为锂离子电池的重要组成部分,正极材料一直是人们重点研究的一个内容,目前应用较广的是LiCoO2,它具有放电电压高、放电平稳、高倍率放电性能好、比能量高、循环性好和生产工艺简单等优点,但由于Co的毒性大、储量低导致这种材料不环保、价格高,并且由于Co4+的高氧化性使LiCoO2只能获得理论值一半的容量,并存在一定的过充电安全隐患,因此人们一直在寻找更好的正极材料.1997年Padhi等人[1]首次报道了具有橄榄石结构的LiFePO4可以作为锂离子电池正极材料,这种材料具有较平坦的3.4 V电压平台、较高的比容量(大于160 mAh/g)、所含元素储量丰富、绿色环保、易于制备和安全性好等优点,被认为是有望替代LiCoO2的正极材料,成为近年来这一研究领域的热点.为了了解LiFePO4的电化学反应机理,Padhi[1]和Takahashi[2]等人用XRD研究了LiFePO4化学脱锂和电化学脱锂后的结构变化,表明Li+的脱嵌过程中LiFePO4和PO4两相共存.Burba等人[3]也使用FTIR和Raman光谱研究了LiFePO4化学脱锂后的结构变化,表明分子光谱是研究LiFePO4结构变化的很好手段,为了更深入理解LiFePO4电化学反应过程中的变化, 本文使用FTIR对LiFePO4在充放电过程中不同充放电阶段的结构变化进行了研究.     

溶剂热合成磷酸亚铁锂的研究

以溶剂热方法首次合成了橄榄石相的磷酸亚铁锂,并以水热法为参照.使用XRD,TEM和FT-IR等手段进行表征.溶剂热法制备的样品呈球形或多面体状.而水热法制备的颗粒则由纤维状成长为菱形.两种方法产物的形状差异可能是由于溶剂热反应体系中较高的压力抑制了纤维态晶体的产生.溶剂热法和灼烧都有利于磷酸亚铁锂晶相发育.     

Regeneration and characterization of air-oxidized LiFePO4

Phase-pure LiFePO₄ as a cathode material for lithium ion batteries was prepared in argon, then exposed to hot air and, at last, regenerated with polypyrrole (PPy) as a reductant. Mössbauer spectroscopy and Raman spectroscopy were used to characterize the chemistry of LiFePO₄ before and after regeneration. Electrochemical tests were carried out to evaluate the impacts of hot-air oxidation and regeneration on the properties of LiFePO₄.     

Analysis of phase transition and dehydration processes of nevirapine

Solid-state characterization of crystalline drugs is an important pre-formulation step for the development and design of solid dosage forms, such as pellets and tablets. In this study, phase transition and dehydration processes of nevirapine have been studied by differential scanning calorimetry and thermogravimetry differential thermal analysis to overcome the problems of drug formulation, namely poor solubility and poor content uniformity. Phase solubility studies elucidated the mechanism of enhanced nevirapine solubility.     

Phonons and thermodynamics of unmixed and disordered Li0.6FePO4

The lithium-storage material Li(0.6)FePO(4) was studied by inelastic neutron scattering and differential scanning calorimetry. Li(0.6)FePO(4) undergoes a transformation from a two-phase mixture (heterosite and triphylite) to a disordered solid-solution at 200 degrees C. Phonon densities of states (DOS) obtained from the inelastic neutron scattering were similar for the two-phase sample measured at 180 degrees C and the disordered sample measured at 220 degrees C. The vibrational entropy of transformation is 1.8 +/-0.9 J/(K mol), which is smaller than the configurational entropy difference of approximately 3.1 J/(K mol). The measured enthalpy of the disordering transition was estimated as 2.5 kJ/mol. The phonon data show a small change in lattice dynamics upon disordering.     

A phase transition study for two homologues of the terephthalylidene bis-4-n-alkylanilines

The variation of density with temperature across different phases for two homologues of the terephthalylidene bis-4-n-alkylaniline series (TBAA7 and TBAA9) was determined to locate the phase transitions. The temperature dependence of the thermal expansion coefficient and the estimated pressure dependence of the transition temperatures together with the reported experimental P-T data are discussed.