Influence of the synthesis method on the structural and electrochemical properties of LiCo1−yNiyO2 oxides

To optimize the LiCo_1−yNi_yO₂ cathode materials, we have systematically studied the influence of synthesis parameters on their electrochemical behavior using powders synthesized by different wet-chemistry methods (combustion and sol-gel). It is found that the thermal decomposition behavior plays an important role on the intrinsic properties of the powders. Products were studied in details. Characterizations include compositional analysis, morphology, crystallographic structure, and local structure for LiCo_1−yNi_yO₂ materials calcined at 800 °C. A comparison of the different methods is given for the synthesis of LiCo_1−yNi_yO₂ cathode materials. It is found that well-structured oxides with uniformly sized ultrafine particles are obtained using the sol-gel method assisted by succinic acid. Paper presented at the 7th Euroconference on Ionics, Calcatoggio, Corsica, France, Oct. 1–7, 2000.     

Iron doped lithium cobalt oxides as lithium intercalating cathode materials

Layered transition metal oxides of the formula LiMO₂ have good lithium insertion properties for which reason LiCoO₂ and LiNiO₂ have been exploited in practical lithium rocking chair batteries. Another member of the LiMO₂ series, LiFeO₂, should be an attractive cathode material considering the cheapness and environment-friendliness of iron compounds. Its rock-salt structure, however, does not allow significant amounts of lithium to be reversibly intercalated in its structure. Synthesis of layered LiFeO₂ and study of its lithium intercalating properties have been of limited success. Therefore, an attempt has been made here to study LiCo_1−yFe_yO₂ solid solutions (0 ≤ y ≤ 0.4) as prospective cathode materials. XRD, FTIR, Atomic absorption spectroscopy, Particle size and Surface area analysis were carried out in this regard towards the physical characterization of the entire series of LiCo_1−yFe_yO₂ compounds. The electrochemical discharge capacity of these materials is explained as a function of the iron content.     

Synthesis, structure characterization and magnetic properties of nanosize LiCo1 − y Ni y O2 prepared by sol–gel citric acid route

Single phase LiCo_1 − y Ni _y O₂ (y = 0.4 and 0.5) with fine particles and high homogeneity was synthesized by “chimie douce” assisted by citric acid as the polymeric agent and investigated as positive electrodes in rechargeable lithium batteries. The long-range and short-range structural properties are investigated with experiments including X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and superconducting quantum interference device magnetometry. The physicochemical properties of the powders (crystallinity, lattice constants, grain size) have been investigated in this composition. The powders adopted the α-NaFeO₂ structure as it appeared from XRD and FTIR results. Magnetic measurements shows signal at low temperature attributed to the magnetic domains in the nanostructure sample from which we estimated that the cation mixing are 3.35 and 4.74% for y = 0.4 and 0.5 in LiCo_1 − y Ni _y O₂, respectively. LiCo_0.5Ni_0.5O₂ cathode yields capacity (135 mAh g⁻¹) compared to LiCo_0.6Ni_0.4O₂ cathode (147 mAh g⁻¹) when discharged to a cutoff voltage of 2.9 V vs. Li/Li⁺. Lower capacity loss and higher discharge efficiency percentage are observed for the cell of LiCo_0.6Ni_0.4O₂ cathode.     

Structural and electrochemical properties of LiCoO2 and LiAlyCo1−yO2 (y=0.1 and 0.2) oxides: A comparative study of electrodes prepared by the citrate precursor route

We present the characterization and electrode behavior of LiCoO₂ and Al-doped LiAl_yCo_1−yO₂ (y=0.1 and 0.2) oxides prepared by wet-chemical method from the citrate precursor route. We study the phase evolution as a function of the aluminum substitution and the modification on the intercalation and deintercalation of Li ions. Characterization methods include XRD, SEM, and FTIR. X-ray diffraction patterns show that samples belong to the LiCoO₂-LiAlO₂ solid solution and have the layered α-NaFeO₂ structure. FT-infrared vibrational spectroscopy indicates the slight modification in the local structure related to the short-range environment of oxygen coordination around the cations in oxide lattices. The frequencies and relative intensities of the bands are sensitive to the covalency of the (Al, Co)O₂ slabs. The overall electrochemical capacity of the LiAl_yCo_1−yO₂ oxides have been reduced due to the sp metal substitution, however, a more stable charge-discharge cycling performances have been observed when electrodes are charged to 4.3 V as compared to the performances of the native oxide. Differences and similarities between LiCoO₂ and Al-substituted oxides are discussed therefrom. Paper presented at the 9th EuroConference on Ionics, Ixia, Rhodes, Greece, Sept. 15–21, 2002.     

Synthesis and characterization of mixed vanadium oxide cathode materials for lithium-ion batteries

A family of mixed vanadium oxides LiCo_yNi_(1−y)VO₄ (x=0.2, 0.5 and 0.8) of potential use as high voltage cathode materials in lithium batteries, has been synthesized and characterized. In general the x-ray diffraction analysis showed that these compounds have an inverse spinel structure where about 85 % of the Ni²⁺ and Co²⁺ ions occupies octahedral sites and the rest tetrahedral sites along with the V⁵⁺ ions. Moreover, the annealing temperature plays a key role in determining the particle size, as demonstrated by scanning electron microscope analysis. Cycling voltammetry tests showed that the lithium insertion-extraction process in the LiCo_yNi_(1−y)VO₄ electrode materials occurs reversibly at around 4.3–4.4 V vs. Li and these results are confirmed by cycling tests. The cycling capacity is modest; however the trend of the cycling curves leads to foresee that an increase in capacity may be obtained by extending the charging process beyond 4.6 V vs. Li, once a stable electrolyte will be available. Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.     

Structural and electrochemical effects of cobalt doping on lithium manganese oxide spinel

Pure LiMn₂O₄ and lithium manganese oxide spinels with partial replacement of manganese by cobalt up to 20 mole%, LiCo_xMn_2−xO₄, were prepared. The effect of extended cycling on the crystal structure was investigated. A capacity decrease with increasing cobalt content was observed in the potential range about 4100 mV vs. Li/Li⁺. Cycling behavior is significantly improved, compared to LiMn₂O₄. LiCo_xMn_2−xO₄ is discharged in a single phase reaction in the upper potential range around 4100 mV vs. Li/Li⁺, whereas pure LiMn₂O₄ shows a two phase behavior. LiMn₂O₄ shows a significant broadening of peaks in plots of differential capacity and change in shape of the voltage profile upon extended cycling. LiCo_xMn_2−xO₄ shows neither broadening nor change. Voltage profiles and plots of the differential capacity differ significantly compared to spinels with lithium substitution, Li_1+xMn_2−xO₄. In contrast to Li_1+xMn_2-xO₄, LiCo_xMn_2-xO₄ is discharged in a two step process in the range of 0 ≤ × ≤ 0,5. Paper presented at the 3rd Euroconference on Solid State Ionics, Teulada, Sardinia, Italy, Sept. 15–22, 1996     

Layered LT-LiNi1−yCoyO2 (0.3≤y≤0.7) cathodes for rechargeable lithium cells synthesized by co-precipitation method

Lithium nickel-cobalt oxides were synthesized at low temperature using a precipitation method involving dissolution of metal acetates and Li(COOCH₃) in oxalic acid (dicarboxylic acid). Pyrolysis of the precipitate at 400–600 °C produced single phase LiNi_1−yCo_yO₂ (0.3≤y≤0.7) with submicron-sized particles. Oxalic acid acted as a fuel, decomposed the homogeneous precipitate of metal complexes at low temperature, and yielded the free impurity LiNi_1−yCo_yO₂ compounds. The physicochemical properties of synthesized products were investigated by structural (XRD, SEM), spectroscopic (FTIR and Raman) and thermal (DTA/TG) analyses. The electrochemical properties of the LiNi_1−yCo_yO₂ cathode materials were evaluated in rechargeable Li cells by employing a non-aqueous organic electrolyte mixture of 1M LiPF₆ in EC+DMC. The cells maintained excellent cyclability at moderate charge-discharge rates. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998.     

Structural properties of LiNi1−yCoyO2 (0≤y≤1) synthesized by wet chemistry via malic acid-assisted technique

We have synthesized LiNi_1−yCo_yO₂ powders by a sol-gel method using malic acid as a chelating agent. The dependence of physico-chemical properties of the powders (crystallinity, lattice constants, grain size) has been investigated by changing the malic acid quantity and the calcination temperature for the different LiNi_1−yCo_yO₂ oxides in the composition range 0 ≤ y ≤1. Structural studies show that a layered single phase was obtained for the y values 0.2 ≤ y ≤ 1.0. The local cationic environment has been studied by Raman and FTIR spectroscopy. Using acid-assisted LiNi_1−yCo_yO₂ powders (with compositions y=0.8 and y=0.6) calcined at 800 °C, Li//LiNi_1−yCo_yO₂ cells were assembled and tested by galvanostatic titration. These cells had an initial capacity of 140 mAh/g in the voltage range 2.8-4.2 V and showed attractive charge-discharge profiles upon cycling. Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Ital, Sept. 12–19, 1999.     

Chemically deintercalated cathode materials for lithium cells

New layered materials in the Li-Ni-Co-O system have been obtained by reaction of (Ni_yCo_1−y)₃O₄ (0 ≤ y ≤ 0.2) and Ni_yCo_1−yO (y ≥ 0.8) with LiOH between 450 and 850°C. Lithium contents increase with synthesis temperature and for Co-rich compositions. Chemical deintercalation of the ceramic oxides was carried out by hydrochloric acid treatment. The rhombohedral layered structure is preserved and the average oxidation state of metal ions increases significantly in the deintercalated products. Pristine and acid delithiated oxides have been studied as intercalation cathode materials in electrochemical test cells using lithium anodes and LiClO₄ (PC) as electrolyte. The cells using delithiated producs showed a better reversibility, an initial voltage in the 3.5–4 V range and could be discharged without previous charge. The voltages were higher for Co-rich compositions. XRD analysis and IR spectroscopy show good reversibility of lithium insertion and extraction.     

Combustion synthesis and luminescent properties of green-emitting phosphor (Sr, Zn) Al2O4: Eu2+, B3+ for white light emitting diodes (WLED)

In this study, the phosphors (Sr_1−x , Zn _x )_0.9(Al_2−y , B _y )O₄ doped 10 mol % Eu²⁺, were prepared by combustion method as the fluorescent material for white light emitting diodes (WLEDs), performing as a light source. The luminescent properties were investigated by changing the combustion temperature, the boron concentration, and the ratio of Sr to Zn. The luminescence, crystallinity and particle morphology were investigated by using a luminescence spectrometer, X-ray diffractometer (XRD) and transmission electron microscopy (TEM), respectively. The highest intensity of Sr_0.9(Al_2−y , B _y )O₄: Eu_0.1²⁺ phosphor was achieved when the combustion temperature was 600° and the concentration of B³⁺ was 8 mol % of the aluminate. A new blue emission was observed when the high Zn concentration (x ⩾ 0.8), and this blue emission disappeared with the Zn concentration became lower than 0.8. The combustion method synthesized phosphor (Sr_0.6, Zn_0.4)_0.9(Al_1.92, B_0.08)O₄: Eu_0.1²⁺ showed 3.3 times improved emission intensity compared with that of the Sr_0.9(Al_1.92, B_0.08)O₄:Eu_0.1²⁺ phosphor under λ _ex = 390 nm.      

Effect of the aluminium doping on the microstructure and morphology of LiNi0.5Co0.5O2 oxides

LiCo_1−xNi_xO₂ materials having the layeredα-NaFeO₂-type structure are promising positive electrodes for advanced lithium-ion batteries. Their electrochemical performance is very much conditioned by the integrity of the lamellar structure that needs to be stable enough to stand many charge/discharge cycles. LiNi_0.5−xAl_xCo_0.5O₂ (0≤x≤0.3) samples have been prepared by a low-temperature route, the sol-gel method assisted by succinic acid as chelating agent. We study in detail their crystallographic structure by XRD, their chemical composition by ICP and their morphology by SEM. Their local cationic environment is also carried out by means of FTIR spectroscopy. Paper presented at the 8th EuroConference on Ionics, Carvoeiro, Algarve, Portugal, Sept. 16 – 22, 2001.     

Effect of composition disorder on the electron paramagnetic resonance spectrum of the thermally populated excited state of Er<Superscript>3+</Superscript> ions in YLuAG mixed garnets

The results of investigation of electron paramagnetic resonance of Er³⁺ ions in the thermally populated first excited state in (Y_{1 − x} Lu _x )₃Al₅O₁₂ (YLuAG) mixed yttrium-lutetium garnet single crystals (0 ≤ x ≤ 1) are considered. In composition-disordered YLuAG, a number of new (as compared to Y₃Al₅O₁₂ (YAG)) Er³⁺ paramagnetic centers are detected; these centers appear due to a change in the crystal field symmetry and magnitude upon isomorphic substitution of Lu³⁺ for Y³⁺ in the yttrium sublattice of garnets. The origin of new paramagnetic centers is established and their formation probability is calculated.     

Factors determining the energy yield of luminophors based on single-crystal films of Al2−Y2O3−R2O3 oxides

We consider the factors determining the energy yield of crystallophosphors based on single-crystal films of oxides of the system Al₂−Y₂O₃−R₂O₃ (R is the rare-earth ions) that are used as different types of luminescent converters. Special features of producing the films by the method of liquid-phase epitaxy from Pb−B₂O₃ and Bi₂O₃-based fluxes are analyzed, and the advantages of this method for production of efficient luminophors, which are based on high-melting oxides doped with mercury-like impurities, are shown. It is established that the main factor that bounds the crystallophosphor luminous yield is the presence of donor-acceptor complexes of the Pb²⁺−Pb⁴⁺ and Fe³⁺−Fe²⁺-type that form different channels of dissipation of excitation energy. The means of minimizing the contribution of these ions to the processes of excitation-energy relaxation are discussed. Institute of Applied Physics, I. Franko L'vov State University, 49 General Chuprynka Str., L'vov, 290044, Ukraine. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 6, pp. 819–823, November–December, 1999.     

Microstructure characterization and magnetic behavior of NiAlxFe2−xO4 and Ni1−yMnyAl0.2Fe1.8O4 ferrites

NiAl_xFe_2−xO₄ and Ni_1−yMn_yAl_0.2Fe_1.8O₄ ferrites were prepared by the conventional ceramic method and were characterized by X-ray diffraction, scanning electron microscopy, and magnetic measurements. The single spinel phase was confirmed for all prepared samples. A proper explanation of data is possible if the Al³⁺ ions are assumed to replace Fe³⁺ ions in the A and B sites simultaneously for NiAl_xFe_2−xO₄ ferrites, and if the Mn²⁺ ions are assumed to replace Ni²⁺ ions in the B sites for Ni_1−yMn_yAl_0.2Fe_1.8O₄ ferrites. Microstructural factors play an important role in the magnetic behavior of Ni_1−yMn_yAl_0.2Fe_1.8O₄ ferrites with large Mn²⁺ content.     

Magnetic properties of LiNi<Subscript>0.5</Subscript>Mn<Subscript>0.47</Subscript>Al<Subscript>0.03</Subscript>O<Subscript>2</Subscript> as positive electrode for Li-ion batteries

The layered LiNi_0.5Mn_0.47Al_0.03O₂ was synthesized by wet chemical method and characterized by X-ray diffraction and analysis of magnetic measurements. The powders adopted the α-NaFeO₂ structure. This substitution of Al for Mn promotes the formation of Li(Ni_0.47²⁺Ni_0.03³⁺Mn_0.47⁴⁺Al_0.03³⁺)O₂ structures and induces an increase in the average oxidation state of Ni, thereby leading to the shrinkage of the lattice unit cell. The concentration of antisite defects in which Ni²⁺ occupies the (3a) Li lattice sites in the Wyckoff notation has been estimated from the ferromagnetic Ni²⁺(3a)–Mn⁴⁺(3b) pairing observed below 140 K. The substitution of 3% Al for Mn reduces the amount of antisite defects from 7% to 6.4–6.5%. The analysis of the magnetic properties in the paramagnetic phase in the framework of the Curie–Weiss law agrees well with the combination of Ni²⁺ (S = 1), Ni³⁺ (S = 1/2) and Mn⁴⁺ (S = 3/2) spin-only values. Delithiation has been made by the use of K₂S₂O₈. According to this process, known to be softer than the electrochemical one, the nickel ions in the (3b) sites are converted into Ni⁴⁺ in the high spin configuration, while Ni²⁺(3a)–Mn⁴⁺(3b) ferromagnetic pairs remain, as the Li⁺(3b) ions linked to the Ni²⁺(3a) ions in the antisite defects are not removed. The results show that the antisite defect is surrounded by Mn⁴⁺ ions, implying the nonuniform distribution of the cations in agreement with previous NMR and neutron experiments.     

UV excitable Y2? x ? y Gd y SiO5:Ce x phosphors for cool white light emission

Cool white light was realized in Y_2−x−y Gd _x SiO₅: Ce _y phosphor under near UV excitation, due to the occupation Ce³⁺ in Y³⁺ 1st and 2nd site, synthesized using solid state carbothermal reduction route. SEM with elemental analysis show the existence of Gd in Y₂SiO₅:Ce enhances the particles size in comparison to Y₂SiO₅:Ce phosphors alone. Gd³⁺ (0.00≤x≤0.75) and Ce³⁺ (0.02≤y≤0.10) concentration was optimized to 0.50 and 0.08 in Y₂SiO₅, respectively. The CIE chromaticity color coordinates (0.24, 0.20) are close to cool white light value which could be useful for the fabrication of cool white LED.     

Structure property correlation in lithium borophosphate glasses

To investigate the influence of cation mobility variation due to the mixed glass former effect, 0.45Li₂O-(0.55 − x) P₂O₅−x B₂O₃ glasses (0 ≤; x ≤ 0.55) are studied keeping the molar ratio of Li₂O/(P₂O₅ + B₂O₃) constant. Addition of B₂O₃ into lithium phosphate glasses increases the glass transition temperature (T _g) and number density, decreases the molar volume, and generally renders the glasses more fragile. The glass system has been characterised experimentally by XRD, XPS and impedance studies and studied computationally by constant volume molecular dynamics (MD) simulations and bond valence (BV) method to identify the structural variation with increasing the B₂O₃ content, its consequence for Li⁺ ion mobility, as well as the distribution of bridging and non-bridging oxygen atoms. These studies indicate the increase of P-O-B bonds (up to Y = [B₂O₃]/([B₂O₃] + [P₂O₅]) ≈ 0.5 and B-O-B bonds, as well as the decrease of P-O-P bonds and non-bridging oxygens (NBOs) with rising B₂O₃ content. The system with Y ≈ 0.5 exhibits maximum ionic conductivity, 1.0 × 10⁻⁷ S cm⁻¹, with activation energy 0.63 V. Findings are rationalised by a model of structure evolution with varying B₂O₃ content Y and an empirical model quantifying the effect of the various structural building blocks on the ionic conductivity in this mixed glass former system.     

Study of the local order in LiNi1−yCoyO2 oxides synthesized by soft chemistry

Lattice dynamics of LiNi_1−yCo_yO₂ solid solution are investigated using Raman and FTIR measurements. Evolution of the vibrational spectra, i.e., frequency shift and band broadening of the stretching modes of either (Ni_1−yCo_yO₂)O₆ or LiO₆ octahedra, are studied as a function of the calcination temperature of the LiNi_1−yCo_yO₂ solid solution. Results show that the solid solution exhibits a one-mode behavior with an increase of the (Ni_1−yCo_yO₂)_n sheet covalency upon substitution of cobalt for nickel. The change in the covalency corresponds to an increase of the Madelung constant with decreasing the c/a value. The broadening of the vibrational bands for Ni-rich compounds is the result of the cation mixing in the crystal layers. The partially disordered cation distribution appearing in lithium nickelate materials can also explain the observed broadening of the Raman spectra. Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.     

Novel trivalent cation conducting solids and their application

Among the trivalent ion conductors reported, the highest ion conductivity was realized with the trivalent Al³⁺ ion conducting (Al_xZr_1−x)_4/(4−x)Nb(PO₄)₃ solid electrolyte and the value enters into the region between yttria stabilized zirconia (YSZ) and calcia stabilized zirconia (CSZ) that are well known to be high oxide anion conductors commercialized. The improvement of the ion conductivity and the mechanical strength was simultaneously achieved by adding B₂O₃ during the sintering procedure. The Al³⁺ ion conducting (Al_xZr_1−x)_4/(4−x)Nb(PO₄)₃ solid electrolyte with B₂O₃ treatment was combined with YSZ, and the 0.7La₂O₂SO₄-0.3Li₂SO₄ solid was attached on the (Al_xZr_1−x)_4/(4−x)Nb(PO₄)₃ solid surface as the auxiliary electrode for sulfur dioxide (SO₂) gas sensing. The sensor response was rapid, reproducible and continuous with obeying the Nernst theoretical relationship. Paper presented at the Patras Conference on Solid State Ionics — Transport Properties, Patras, Greece, Sept. 14 – 18, 2004.     

In-plane propagation of millimeter waves in periodic magnetoactive semiconductor structures

Non-transmission bands of electromagnetic waves propagating along the layers in periodic structures are studied in the steady magnetic field perpendicular both to the uniaxis and the direction of propagation. The band control range (36÷75 GHz) inn-InSb/Al₂O₃ structures with the carrier densities 4 10¹³ ≤n ≤ 8 10¹⁴ cm⁻³ in magnetic fieldsB _o ≤ 2 T at temperatures 77 ≤T ≤ 200 K is found to agree with the calculated in the effective medium approximation. Attenuation down to −50 dB within the band is observed. The band lineshape is found to indicate additional effects related to the finite layer thickness and periodicity termination predicted by a more rigorous theory of dispersion.