Disordered carbon nanofibers/LiCoPO<Subscript>4</Subscript> composites as cathode materials for lithium ion batteries

LiCoPO₄-coated disordered carbon nanofibers (CNFs/LiCoPO₄) were obtained by a sol–gel method, using triethyl phosphite or triethyl phosphate as the phosphorous source. The crystal structure of the products was analyzed by X-ray powder diffraction, while morphology was studied using scanning electron microscopy, transmission electron microscopy, Auger electron spectroscopy and X-ray photoelectron spectroscopy. Optimal synthesis conditions for the CNFs/LiCoPO₄ in light of the best electrochemical performance are discussed. The best discharge capacity 105 mAh/g (or ca. 63% of the theoretical capacity) shows the material with 40% CNFs/LiCoPO₄ and addition coating by carbon black. This composition has a best purity of active materials and point coverage of CNFs. The X-ray photoelectron C1s spectra of the CNFs surface without and with sputter erosion show enhancement of C–O bonds at the fiber surface, which does not influence significantly electrochemical behavior of the composite materials.     

Effect of different conductive additives on charge/discharge properties of LiCoPO<Subscript>4</Subscript>/Li batteries

Single-phase LiCoPO₄ nanoparticles were synthesized by solid-state reaction method and subsequent high-energy ball milling. The electrochemical properties of LiCoPO₄/Li batteries were analyzed by ac impedance experiments, cyclic voltammetry (CV), and charge/discharge tests. The structural and morphological performance of LiCoPO₄ nanoparticles was investigated by X-ray diffraction (XRD), scanning electron microscope (SEM), and transmission electron microscope (TEM). The XRD result demonstrated that LiCoPO₄ nanoparticles had an orthorhombic olivine-type structure with a space group of Pmnb. Different conductive additives including acetylene black and carbon black (SP270) were used to fabricate electrodes. The morphologies of the electrodes and different conductive additives were observed by field emission-scanning electron microscopy (FE-SEM). LiCoPO₄/Li battery with acetylene black showed the best electrochemical properties, and exhibited a discharge plateau at around 4.7 V with an initial discharge capacity of 110 mAh g⁻¹ at a discharge current density of 0.05 mA cm⁻² at 25 °C.     

Charge distribution and mobility of lithium ions in Li<Subscript>2</Subscript>TiO<Subscript>3</Subscript> from <Superscript>6,7</Superscript>Li NMR data

A comparative analysis of ^6,7Li NMR spectra is performed for the samples of monoclinic lithium titanate obtained at different synthesis temperatures. In the ⁷Li NMR spectra three lines are found, which differ in quadrupole splitting frequencies v _Q and according to ab initio EFG calculations are assigned to three crystallographic sites of lithium: Li1 (v _Q ～ 27 kHz); Li2 (v _Q ～ 59 kHz); Li3 (v _Q ～ 6 kHz). The dynamics of lithium ions is studied in a wide temperature range from 300 K to 900 K. It is found that the narrowing of ⁷Li NMR spectra as a result of thermally activated diffusion of lithium ions in the low-temperature Li₂TiO₃ sample is observed at a higher temperature in comparison with a sample of high-temperature lithium titanate. Based on the analysis of ⁶Li NMR spectra it is assumed that there is mixed occupancy of lithium and titanium sites in the corresponding layers of the crystal structure of low-temperature lithium titanate, which hinders lithium ion transfer over regular crystallographic sites.     

Impact of glucose on the electrochemical performance of nano-LiCoPO4 cathode for Li-ion batteries

LiCoPO₄ nanoparticles were synthesized by standard and glucose-assisted sol–gel methods for use as cathodes in lithium-ion batteries. The effect of glucose on the characteristics of the formed LiCoPO₄ nanoparticles was investigated by TGA, XRD, and FESEM. The TGA results indicated gradual decomposition of glucose in the temperature range 400–700 °C. The XRD results showed olivine phases in addition to small traces of Co₃O₄ for samples calcined at 400 °C while pure olivine phases were confirmed for the 700 °C calcined samples. The addition of glucose strongly suggests promotion of LiCoPO₄ crystallization, as revealed by FESEM studies. The electrochemical measurements pertaining to LiCoPO₄ samples calcined at 400 °C suggested an enhancement of initial discharge capacity from 103.3 to 144.6 mAh/g for the standard and glucose-based electrodes, respectively. Further, the effects of conductive additive and excess lithium on the electrochemical performance of LiCoPO₄ have also been investigated.     

The influence of Li sources on physical and electrochemical properties of LiNi<Subscript>0.5</Subscript>Mn<Subscript>1.5</Subscript>O<Subscript>4</Subscript> cathode materials for lithium-ion batteries

LiNi_0.5Mn_1.5O₄ cathode materials were successfully prepared by sol–gel method with two different Li sources. The effect of both lithium acetate and lithium hydroxide on physical and electrochemical performances of LiNi_0.5Mn_1.5O₄ was investigated by scanning electron microscopy, Fourier transform infrared, X-ray diffraction, and electrochemical method. The structure of both samples is confirmed as typical cubic spinel with Fd3m space group, whichever lithium salt is adopted. The grain size of LiNi_0.5Mn_1.5O₄ powder and its electrochemical behaviors are strongly affected by Li sources. For the samples prepared with lithium acetate, more spinel nucleation should form during the precalcination process, which was stimulated by the heat released from the combustion of extra organic acetate group. Therefore, the particle size of the obtained powder presents smaller average and wider distribution, which facilitates the initial discharge capacity and deteriorates the cycling performance. More seriously, there exists cation replacement of Li sites by transition metal elements, which causes channel block for Li ion transference and deteriorates the rate capability. The compound obtained with lithium hydroxide exhibits better electrochemical responses in terms of both cycling and rate properties due to higher crystallinity, moderate particle size, narrow size distribution and lower transition cation substitute content.     

Investigation on LiCoPO<Subscript>4</Subscript> powders as cathode materials annealed under different atmospheres

An uncomplicated Pechini-assisted sol–gel process in aqueous solutions is used for the synthesis of Li–Co phosphate powders as cathode materials. The powders are annealed under different conditions in flowing nitrogen and in flowing air. The structural, morphological, and electrochemical properties are strongly dependent upon the annealing conditions. After the treatment in air, the X-ray diffraction (XRD) patterns reveal the presence of LiCoPO₄ as a single phase. The morphology of the powders consists of a homogeneous and good interconnected blend of grains with different sizes; the cyclic voltammetry (CV) curves show a very good reversibility with very close values of the mean peak maxima in the cathodic region. The electrochemical measurements deliver a discharge specific capacity of 37 mAhg⁻¹ at a discharge rate of C/25 at room temperature. After annealing in nitrogen, the XRD analysis detects the formation of Li₄P₂O₇ and to Co₂P as secondary phases; the morphological investigation indicated that the LiCoPO₄ particles took shape of prisms with an average size of 2 μm. The CV curves are associated with a large polarization and poor irreversibility. The electrochemical measurements deliver a discharge specific capacity of 42 mAh g⁻¹ at a discharge rate of C/25 at room temperature and lower capacity fade (approx. 35%).     

The synthesis,structure, and electrochemical properties of Li<Subscript>2</Subscript>FeSiO<Subscript>4</Subscript>-based lithium-accumulating electrode material

Different approaches to synthesis of Li₂FeSiO₄-based electrode materials for lithium intercalation, using low-cost and abundant Li-, Si-, and Fe-containing parent substances, are discussed. XRD, SEM, and a laser-diffraction analyzer of particle size were used for structure and morphology characterization of the composite electrode materials. Li₂FeSiO₄ was shown to be the main lithium-accumulating crystalline phase; minor LiFeO₂ and Li₂SiO₃ admixtures are also present. The material microparticles’ average size was shown to vary from tenths of micrometer to 1 μm. Larger objects sized ca. 2–4 μm are the microparticles’ agglomerates. The material electrochemical properties were studied by dc chronopotentiometry (galvanostatic charging–discharging) and cyclic voltammetry with potential linear sweeping. The initial reversible cycled capacity of the best samples is 170 mA h/g. The anodic and cathodic processes manifest obvious hysteresis caused by the presence of several different lithium ion energy states in the material; the transition between the states is kinetically hindered. The dependences of the specific capacity and its stability under cycling on the current load and the conductive carbon component content in the composite were elucidated.     

Structure and lithium mobility of Li<Subscript>4</Subscript>Pt<Subscript>3</Subscript>Si

The lithium-rich silicide Li₄Pt₃Si was synthesised from the elements by high-temperature synthesis in a sealed niobium ampoule. Its structure was refined on the basis of single-crystal X-ray diffraction data: R32, a = 693.7(2), c = 1627.1(4) pm, wR2 = 0.0762, 525 F² values and 21 variables. The striking structural motifs of the Li₄Pt₃Si structure are silicon atoms with a slightly distorted trigonal prismatic platinum coordination with short Si–Pt distances (238–246 pm). Always two trigonal prisms are condensed via a common Pt₃ triangle, and these double units built up a three-dimensional network by condensation via common corners. The channels left by this prismatic network are filled by two crystallographically independent lithium sites in a 3:1 ratio. The single crystal X-ray data were fully confirmed by neutron powder diffraction and ⁷Li magic-angle spinning (MAS)–nuclear magnetic resonance (NMR) results. The two distinct lithium sites are well differentiated by their ⁷Li isotropic chemical shift and nuclear electric quadrupolar interaction parameters. MAS-NMR spectra reveal signal coalescence effects above 300 K, indicating chemical exchange between the lithium sites on the millisecond timescale. The spectra can be simulated with a simple two-site exchange model. From the resulting temperature-dependent correlation times, an activation energy of 50 kJ/mol is extracted.     

Olivine-type nanoparticle for hybrid supercapacitors

LiCoPO₄ nanoparticles were precipitated from polyethylene glycol solution of lithium acetate, cobalt acetate, and ammonium dihydrogen phosphate by refluxing at 250 °C for 35 h. The resultant powder samples were heated at 800 °C for different time periods of 2 and 4 h to study the effect of annealing time on the growth of samples. The X-ray diffraction pattern of the obtained samples exhibited olivine phase. The scanning electron microscopic images of dried powder sample and samples heated at 800 °C for 2 and 4 h showed a homogenous orthorhombic morphology with a particle size of few nanometers range. For the first time, orthorhombic olivine was introduced as positive electrode for a hybrid electrochemical supercapacitor cell with carbon nanofoam as negative electrode in 1 M LiClO₄ in ethylene carbonate and propylene carbonate (1:1 in volume) solution. A sloping voltage profile of 2 to 0 V is observed for all the three hybrid cells. From the impedance results, we inferred that LiCoPO₄ nanoparticles synthesized by polyol process offers less resistance than lithium titanium oxide. According to the results of electrochemical testing for the first time, maximum power density of 192 W/kg at 11 Wh/kg energy density was obtained for LiCoPO₄ nanoparticles annealed at 800 °C for 2 h. The dried sample and the sample heated at 800 °C for 2 and 4 h exhibited high capacitances of 5, 19, and 4 F/g, respectively, with an excellent rate capability over 1,000 cycles.     

CAM sol–gel synthesized LiMPO<Subscript>4</Subscript> (M=Co,Ni) cathodes for rechargeable lithium batteries

The emerging category cathode candidates such as LiCoPO₄ and LiNiPO₄ were synthesized at 800 °C using Citric acid assisted modified sol–gel (CAM sol–gel) method and examined for possible lithium intercalation behavior. Compound formation temperature is confirmed from thermogravimetry and differential thermal analysis (TG/DTA). Powder X-ray diffraction (PXRD) pattern evidenced the absence of undesirable peaks and confirmed the formation of phase pure LiMPO₄ (M=Co, Ni) compounds with an orthorhombic structure and finer crystallite size. Presence of nanosized particles as observed from TEM image of LiCoPO₄ and the presence of preferred local cation environment as understood from FT–IR studies are the added advantages of CAM sol–gel synthesis. Further, Cyclic voltametry (CV) and Impedance spectroscopy (EIS) studies performed on the synthesized LiCoPO₄ and LiNiPO₄ cathodes revealed excellent reversibility and structural stability of CAM sol–gel synthesized cathodes, especially upon storage as well as during cycling.     

Impedance studies on the 5-V cathode material,LiCoPO<Subscript>4</Subscript>

Olivine-structured LiCoPO₄ is synthesized by a Pechini-type polymer precursor method. The structure and the morphology of the compounds are studied by the Rietveld-refined X-ray diffraction, scanning electron microscopy, Brunauer, Emmett, and Teller surface area technique, infrared spectroscopy, and Raman spectroscopy techniques, respectively. The ionic conductivity (σ ionic), dielectric, and electric modulus properties of LiCoPO₄ are investigated on sintered pellets by impedance spectroscopy in the temperature range, 27–50 °C. The σ (ionic) values at 27 and 50 °C are 8.8 × 10⁻⁸ and 49 × 10⁻⁸ S cm⁻¹, respectively with an energy of activation (E _a) = 0.43 eV. The electric modulus studies suggest the presence of non-Debye type of relaxation. Preliminary charge–discharge cycling data are presented.     

Fast synthesis of core-shell LiCoPO4/C nanocomposite via microwave heating and its electrochemical Li intercalation performances

A very simple and rapid method for synthesizing LiCoPO₄/C nanocomposite has been developed via microwave heating. X-ray diffraction confirmed that nanosized olivine LiCoPO₄ was successfully synthesized. Scanning electron microscopy and transmission electron microscopy verified that LiCoPO₄ displays small powders with an average size of ～150 nm and is also coated with uniform amorphous carbon film of ～10 nm in thickness. Compared with pure LiCoPO₄, LiCoPO₄/C composite presented enhanced electrochemical Li-ion intercalation performances. Cyclic voltammetric and electrical tests disclosed that the Li-ion diffusion, the reversibility of lithium extraction/insertion and electrical conductivity were significantly improved in LiCoPO₄/C composite.     

Preparation of LiCoPO<Subscript>4</Subscript> powders and films via sol–gel

Lithium intercalation materials are of special interest as cathodes in rechargeable batteries. An uncomplicated sol–gel process has been used for the synthesis of Li–Co phosphates powders and, for the first time, of LiCoPO₄ films. The powders were prepared from aqueous solutions, containing Li, Co and phosphate precursors to which acid citric and ethylene glycol was added, during the drying process at 75 °C. The X-ray diffraction patterns of the prepared powders confirmed the presence of LiCoPO₄ with an olivine-like structure as main phase. The morphological investigations of the powder showed a platelet-like structure with an average grain size of 0.75 μm. The films of LiCoPO₄ were deposited onto ITO glass substrates with the combination of the dip-coating process under the same conditions. Finally, the films were annealed in inert atmosphere at 300 °C. The morphological investigations reveal a smooth and homogeneous surface of the prepared Li–Co phosphate films. The preliminary electrical investigation on the prepared LiCoPO₄ films showed lithium ions electrochemical activity in the range 3.0–4.5 V.     

Influence of sol–gel derived lithium cobalt phosphate in alkaline rechargeable battery

A lithium cobalt phosphate (LiCoPO₄) cathode was synthesised by citric acid assisted sol?Cgel method and its electrochemical behaviour in alkaline secondary battery (using novel lithium hydroxide as the electrolyte) is reported. The sol?Cgel method using metal acetate precursors with citric acid as a chelating agent influenced the particle size and the homogeneity while yielding a single phase LiCoPO₄ at a considerably lower temperature and shortened heating time, compared to that of the conventional solid state reaction. The cyclic voltammogram of LiCoPO₄ showed a reversible redox process implying that de-intercalation and intercalation of lithium can occur in aqueous electrolyte. This was supported by X-ray diffraction (XRD) and Infra-red (IR) studies. The charge?Cdischarge performance of the Zn/LiCoPO₄ battery showed good capacity retention (after 25 cycles it delivered 90?% of its initial capacity). This enhanced capacity retention was attributed to the synergistic effect of particle homogeneity, reduced Li⁺ diffusion path and stability of the non-reactive aqueous electrolyte between the electrode and the electrolyte interface.     

Analysis of the temperature dependence of the high-frequency EMR spectra of Mn ions in the lithium-ion battery material LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript>

This paper deals with the analysis of the temperature dependence of high-frequency EMR (HF-EMR) spectra due to Mn³⁺ and Mn⁴⁺ ions in the lithium manganese spinel LiMn₂O₄. A range of powder samples obtained by the sol-gel method with calcinations in several temperature ranges were prepared for this study. Based on the initial characterization carried out by a number of techniques, the physicochemical and structural properties of the samples were earlier determined. Independently, temperature magnetization and HF-EMR measurements were carried out. The EMR spectra vary strongly between samples, indicating possible structural or chemical changes. Quantitative analysis of the temperature dependence of the HF-EMR spectra due to Mn³⁺ and Mn⁴⁺ ions in LiMn₂O₄ is presented in this paper. The spectral analysis concerns the line shape, linewidth, intensity and g-factors. Fittings using the Lorentzian spectral shape and, to a certain extent, the Gaussian spectral shape have been carried out in order to parameterize the temperature dependence of the HF-EMR spectra. This parameterization of the HF-EMR experimental data enables a deeper characterization of the samples. Subsequently, a better insight into the role of the Mn³⁺ and Mn⁴⁺ ions in accounting for the characteristics most suitable for application of LiMn₂O₄ as a cathode material may be gained.     

Synthesis and characterization of Pb<Subscript>1–x</Subscript>La<Subscript>x</Subscript>TiO<Subscript>3</Subscript> nanocrystalline powders

Pb_1–xLa_xTiO₃ (PLT) nanocrystalline powders were obtained by polymeric precursor method. The samples were analyzed by differential scanning calorimetry (DSC) and thermogravimetry (TG) techniques to characterize properly the distinct thermal events occurring during synthesis. The X-ray diffraction patterns show a tetragonal structure for the samples with x=0.10 and 0.15. An increase of the lanthanum concentration to x=0.20 led to a highly symmetric structure, cubic on average. The powders obtained at the end of the synthesis had an average particle size of 30 to 70 nm.     

Production of an electrode material modified by a lithium-conducting solid electrolyte

The process for producing the electrode material LiCoPO₄ modified by the lithium-conducting solid electrolyte Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP) was studied. To create a composite consisting of an electrochemically active substance and an electrically conductive additive distributed uniformly between LiCoPO₄ particles, a peroxide solution of a LATP precursor was used. After annealing at 700°C, the two-phase composite LiCoPO₄/LATP was obtained, the conductivity of which was two orders of magnitude higher than that of binary lithium cobalt phosphate at room temperature.     

Electrochemical lithium insertion in β-MoO<Subscript>3</Subscript>: novel Li<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>MoO<Subscript>3</Subscript> bronzes

New lithium molybdenum bronzes have been synthesized by electrochemical lithium insertion. Through electrochemical spectroscopy techniques we have detected that lithium insertion proceeds at least in a two-step reduction process. The maximum amount of lithium inserted in β-MoO₃ leads to a high specific capacity of the cell of 370 Ah kg^–1. However, this capacity was lost after the first charge-discharge cycle, resulting in a total loss of 25%, due to structural transformations. The structural study of the insertion process showed that each step of the process can be associated with the formation of different single phases of variable composition, Li _x MoO₃. Electronic Publication     

Two mechanisms of thermal expansion in perovskite SrCo<Subscript>0.6</Subscript>Fe<Subscript>0.2</Subscript>Nb<Subscript>0.2</Subscript>O<Subscript>3-z</Subscript>

Perovskite SrCo_0.6Fe_0.2Nb_0.2O_3-z attracts attention as a promising material with high oxygen conductivity. The sample was investigated by means of high-temperature X-ray powder diffraction and thermogravimetry. Phase transition was detected near 400 °C and accompanied with significant mass loss. The phase transition affects oxygen mobility, important for the synthesis of oxygen permeable membranes. The unit cell parameters are proved to change with temperature after two effects (1) reversible conventional thermal expansion and (2) irreversible contraction-expansion due to the changes in the oxygen content. In situ high-temperature X-ray diffraction experiments allowed us to separate the contributions and to measure them as a function of temperature.     

Mass spectrometric investigation of vaporization in the Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> system

The thermodynamics of vaporization in the Bi₂O₃-Fe₂O₃ quasibinary system was studied by high-temperature mass-spectrometry. The partial pressures of the constituents of a saturated vapor over the system at 1100 K were determined. Based on the experimental data, the following parameters were calculated: the activities of the components of the Bi₂O₃-Fe₂O₃ system condensed phase, the standard enthalpies of some heterogeneous reactions, and standard enthalpies of formation and enthalpies of formation for crystalline BiFeO₃ and Bi₂Fe₄O₉ from individual oxides. An optimal temperature for the solid-phase synthesis of bismuth ferrites from simple oxides is recommended.