Variation of discharge capacities with C-rate for LiNi1−yMyO2 (M = Ni,Ga, Al and/or Ti) cathodes synthesized by the combustion method

LiNiO₂, LiNi_0.995Al_0.005O₂, LiNi_0.975Ga_0.025O₂, LiNi_0.990Ti_0.010O₂ and LiNi_0.990Al_0.005Ti_0.005O₂ specimens were synthesized by preheating at 400 °C for 30 min in air and calcination at 750 °C for 36 h in an O₂ stream. The variation of the discharge capacities with C-rate for the synthesized samples was investigated. LiNi_0.990Al_0.005Ti_0.005O₂ has the largest first discharge capacities at the 0.1 and 0.2 C rates. LiNi_0.990Ti_0.010O₂ has the largest first discharge capacity at the 0.5 C rate. In case of LiNiO₂ and LiNi_0.990Ti_0.010O₂, the first discharge capacity decreases slowly as the C-rate increases. LiNiO₂ has the largest discharge capacities at n = 10 (after stabilization of the cycling performance) at the 0.1, 0.2 and 0.5 C rates. This is considered to be related with the largest value of I_0 0 3/I_1 0 4 and the smallest value of R-factor (the least degree of cation mixing) among all the samples. LiNi_0.975Ga_0.025O₂ exhibits the lowest discharge capacity degradation rates at 0.1, 0.2 and 0.5 C rates.     

Evolution of the local structure and electrochemical properties of spinel LiNixMn2−xO4 (0 ≤ x ≤ 0.5)

A series of Ni substituted spinel LiNi_xMn_2−xO₄ (0 ≤ x ≤ 0.5) have been synthesized to study the evolution of the local structure and their electrochemical properties. X-ray diffraction showed a few Ni cations moved to the 8a sites in heavily substituted LiNi_xMn_2−xO₄ (x ≥ 0.3). X-ray photoelectron spectroscopy confirmed Ni²⁺ cations were partially oxidized to Ni³⁺. The local structures of LiNi_xMn_2−xO₄ were studied by analyzing the and A_1g Raman bands. The most compact [Mn(Ni)O₆] octahedron with the highest bond energy of Mn(Ni)O was found for LiNi_0.2Mn_1.8O₄, which showed a Mn(Ni)O average bond length of 1.790 Å, and a force constant of 2.966 N cm⁻¹. Electrolyte decomposition during the electrochemical charging processes increased with Ni substitution. The discharge capacities at the 4.1 and 4.7 V plateaus obeyed the linear relationships with respect to the Ni substitution with the slopes of −1.9 and +1.9, which were smaller than the theoretical values of −2 and +2, respectively. The smaller slopes could be attributed to the electrochemical hysteresis and the presence of Ni³⁺ in the materials.     

Effects of Zn or Ti substitution for Ni on the electrochemical properties of LiNiO2

LiNiO₂ and LiNi_1−yM_yO₂ (M = Zn and Ti, y = 0.005, 0.01, 0.025, 0.05, and 0.1) were synthesized with a solid-state reaction method by calcination at 750 °C for 30 h under oxygen stream after preheating at 450 °C for 5 h in air. LiNi_0.995Zn_0.005O₂ among the Zn-substituted samples and LiNi_0.995Ti_0.005O₂ among the Ti-substituted samples showed the best electrochemical properties. For similar values of y, LiNi_1−yTi_yO₂ had in general better electrochemical properties than LiNi_1−yZn_yO₂. Electrochemical properties seem to be closely related to R-factor but less related to I_0 0 3/I_1 0 4 value. In the FT-IR absorption spectra of LiNiO₂ and LiNi_1−yM_yO₂ (M = Zn and Ti, y = 0.005, 0.01, 0.025, 0.05 and 0.1), Li₂CO₃ was detected even if it is not observed from XRD pattern, with the samples LiNi_1−yZn_yO₂ (y = 0.05 and 0.1) showing Li₂ZnO₂ additionally. The smaller cation mixing of the Ti-substituted samples is considered to lead to their better electrochemical properties than the Zn-substituted samples.     

Electrochemical properties of LiNi<Subscript>1−<Emphasis Type="Italic">y</Emphasis></Subscript>M<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>O<Subscript>2</Subscript> (M=Ni,Ga, Al and/or Ti) cathodes synthesized by the combustion method

LiNiO₂, LiNi_0.995Al_0.005O₂, LiNi_0.975Ga_0.025O₂, LiNi_0.990Ti_0.010O₂ and LiNi_0.990Al_0.005Ti_0.005O₂ were synthesized by preheating at 400 °C for 30 min in air and calcination at 750 °C for 36 h in an O₂ stream with excess lithium amount z = 0.10 in Li_1+z Ni_1−y M _y O₂. For these samples, the discharge capacities and discharge capacity degradation rate are compared. LiNiO₂ has the largest discharge capacity at the 20th cycle (n = 20) and the 50th cycle (n = 50). LiNiO₂ and LiNi_0.995Al_0.005O₂ have relatively good cycling performances and their discharge capacities at n = 50 are 134 and 123 mAh/g, respectively, at 0.1 C rate. The crystallite sizes and strains were calculated by the Williamson–Hall method with XRD patterns and compared for the samples as prepared and after 50 charge–discharge cycles.     

Hydrothermal synthesis of LiNixCo1−xO2 cathode materials

Ultrafine powders of LiCoO₂, nonstoichiometric LiNiO₂ and LiNi_0.9Co_0.1O₂ were prepared under mild hydrothermal conditions. The influence of the molar ratio of Li/Co, Li/Ni and Li/(Ni + Co) was studied. The final products were investigated by XRD, TEM and EDS. To synthesize a stoichiometric LiNiO₂ under mild hydrothermal conditions was found to be a big challenge. Transmission electron microscopies (TEM) revealed the formation of well-crystallized LiCoO₂ and LiNi_0.9Co_0.1O₂ with average size of 100 nm and 10 nm, respectively.     

Preparation and cycling performance of Aland F co-substituted compounds Li4AlxTi5−xFyO12−y

Li₄Al_xTi_5−xF_yO_12−y compounds were prepared by a solid-state reaction method. Phase analyses demonstrated that both Al³⁺ and F⁻ ions entered the structure of spinel-type Li₄Ti₅O₁₂. Charge-discharge cycling results at a constant current density of 0.15 mA cm⁻² between the cut-off voltages of 2.5 and 0.5 V showed that the Al³⁺ and F⁻ substitutions improved the first total discharge capacity of Li₄Ti₅O₁₂. However, Al³⁺ substitution greatly increased the reversible capacity and cycling stability of Li₄Ti₅O₁₂ while F⁻ substitution decreased its reversible capacity and cycling stability slightly. The electrochemical performance of the Al³⁺-F⁻-co-substituted specimen was better than the F⁻-substituted one but worse than the Al³⁺-substituted one.     

Electrochemical characterizations of Fe-substituted LiNiO2 synthesized in air by the combustion method

The phases that appear in the intermediate reaction steps for the formation of lithium nickel oxide were deduced from XRD and DTA analyses. XRD analysis and electrochemical measurements were performed for LiNi_1−yFe_yO₂ (0.000 ≤ y ≤ 0.300) samples calcined in air after preheating in air at 400 °C for 30 min. Rietveld refinement of the LiNi_1−yFe_yO₂ XRD patterns (0.000 < y ≤ 0.100) was carried out from a [Li,Ni]_3b[Li,Ni,Fe]_3a[O₂]_6c starting structure model. The samples of LiNi_1−yFe_yO₂ with y = 0.025 and 0.050 had higher first discharge capacities when compared with LiNiO₂ and exhibited better or similar cycling performance at a 0.1 C rate in the voltage range of 2.7–4.2 V. The LiNi_0.975Fe_0.025O₂ sample had the highest first discharge capacity of 176.5 mAh/g and a discharge capacity of 121.0 mAh/g at n = 100. With the exception of Co-substituted LiNiO₂, such a high first discharge capacity has not been previously reported.     

Comparative study of different crystallographic structure of LiNi0.5Mn1.5O4−δ cathodes with wide operation voltage (2.0-5.0 V)

LiNi_0.5Mn_1.5O_4−δ cathode materials with two different structures ( and P4₃32) were synthesized by ultrasonic spray pyrolysis method. The X-ray diffraction (XRD) data was confirmed that face-centered spinel () transformed into primitive simple cubic (P4₃32) structure by annealing process at 700 °C. In spite of two electrons operated cut-off voltage range between 2.0 and 5.0 V, LiNi_0.5Mn_1.5O₄ with P4₃32 structure has better electrochemical behaviors than the cathode with simple cubic structure. Ex situ XRD study of the electrode revealed that LiNi_0.5Mn_1.5O₄ (P4₃32) has reversible crystal transformation between fully lithiated state (2.0 V) and delithiated state (5.0 V) whereas LiNi_0.5Mn_1.5O_4−δ () showed irreversible phase transformed at two voltage region. The LiNi_0.5Mn_1.5O_4−δ () has voltage drop was occurred after 20th cycled compared without any voltage drop of the P4₃32 structure.     

Molten salt synthesis of LiNi0.5Mn1.5O4 spinel for 5 V class cathode material of Li-ion secondary battery

Well-ordered high crystalline LiNi_0.5Mn_1.5O₄ spinel has been readily synthesized by a molten salt method using a mixture of LiCl and LiOH salts. Synthetic variables on the synthesis of LiNi_0.5Mn_1.5O₄, such as synthetic atmosphere, LiCl salt amount, synthetic temperature, and synthetic time, were intensively investigated. X-ray diffraction (XRD) patterns and scanning electron microscopic (SEM) images showed that LiNi_0.5Mn_1.5O₄ synthesized at 900 and 950 °C have cubic spinel structure () with clear octahedral dimension. LiNi_0.5Mn_1.5O₄ spinel phase began to decompose at around 1000 °C accompanied with structural and morphological degradation. LiNi_0.5Mn_1.5O₄ powders synthesized at 900 °C for 3 h delivered an initial discharge capacity of 139 mAh/g with excellent capacity retention rate more than 99% after 50 cycles.     

Measurements and modeling of high-pressure excess molar enthalpies and isothermal vapor-liquid equilibria of the carbon dioxide + N,N-dimethylformamide system

Mixtures of supercritical CO₂ and N,N-dimethylformamide (DMF) are very often involved in supercritical fluid applications and their thermodynamic properties are required to understand and design these processes. Excess molar enthalpies () for CO₂ + DMF mixtures were measured using an isothermal high-pressure flow calorimeter under conditions of temperature and pressure typically used in supercritical processes: 313.15 and 323.15 K at 9.00, 12.00, 15.00 and 18.00 MPa and 333.15 K at 9.00 and 15.00 MPa. The Peng-Robinson and the Soave-Redlich-Kwong equations of state were used in conjunction with the classical mixing rules to model the literature vapor-liquid equilibrium and critical data and the excess enthalpy data. In most cases, CO₂ + DMF mixtures showed very exothermic mixing and excess molar enthalpies exhibited a minimum in the CO₂-rich region. The lowest value (−4526 J mol⁻¹) was observed for a CO₂ mole fraction value of 0.713 at 9.00 MPa and 333.15 K. On the other hand, at 9.00 MPa and 323.15 and 333.15 K varies linearly with CO₂ mole fraction in the two-phase region where a gaseous and a liquid mixture of fixed composition are in equilibrium. The effects of pressure and temperature on the excess molar enthalpy are large. For a given mole fraction, mixtures become less exothermic as pressure increases or temperature decreases. These excess enthalpy data were analyzed in terms of molecular interactions, phase equilibria, density and critical parameters previously reported for CO₂ + DMF. All throughout this paper, the key concepts and modeling tools originate from the work of van der Waals: the paper is intended as a small piece of recognition of van der Waals overwhelming contributions to thermodynamics.     

Transesterification of palm oil on KyMg1 − xZn1 + xO3 catalyst: Effect of Mg-Zn interaction

The Mg-Zn interaction effect of K_yMg_1 − xZn_1 + xO₃ heterogeneous type catalyst and its performance on transesterification of palm oil have been studied using the response surface methodology and the factorial design of experiments. The catalyst was synthesized using the co-precipitation method and the activity was assessed by transesterification of palm oil into fatty acid methyl esters. The ratio of the Mg/Zn metal interaction, temperature and time of calcination were found to have positive influence on the conversion of palm oil to fatty acid methyl ester (FAME) with the effect of metal to metal ratio and temperature of calcination being more significant. The catalytic activity was found to decrease at higher calcination temperature and the catalyst type K₂Mg_0.34Zn_1.66O₃ with Mg/Zn ratio of 4.81 gave FAME content of 73% at a catalyst loading of 1.404 wt.% of oil with molar ratio of methanol to oil being 6:1 at temperature of 150 °C in 6 h. A regression model was obtained to predict conversions to methyl esters as a function of metal interaction ratio, temperature of calcination and time. The observed activity of the synthesized catalyst was due to its synergetic structure and composition.     

Synthesis and characterization of LiNi1/3Mn1/3Co1/3O2 by wet-chemical method

A series of LiNi_1/3Mn_1/3Co_1/3O₂ samples with α-NaFeO₂ structure belonging to the D_3d⁵ space group were synthesized using tartaric acid as a chelating agent by wet-chemical method. Different acid to metal-ion ratios R have been used to investigate the effect of this parameter on the physical and electrochemical properties. We have characterized the reaction mechanism, the structure, and morphology of the powders by TGA, XRD, SEM and TEM imaging, completed by magnetic measurements, Raman scattering spectroscopy, and complex impedance experiments. We find that the LiNi_1/3Mn_1/3Co_1/3O₂ sintered at 900 °C for 15 h with an acid to metal-ion ratio R = 2 was the optimum condition for this synthesis. For this optimized sample, only 1.3% of nickel-ions occupied the 3b Wyckoff site of the lithium-ions sublattice. The electrochemical performance has been investigated using a coin-type cell containing Li metal as the anode. The electronic performance is correlated to the concentration of the Ni(3b) defects that increase the charge transfer resistance and reduce the lithium diffusion coefficient. The optimized cell delivered an initial discharge capacity of 172 mAh g⁻¹ in the cut-off voltage of 2.8-4.4 V, with a coulombic efficiency of 93.4%.     

Effects of cathode fabrication conditions and cycling on the electrochemical performance of LiNiO2 synthesized by combustion and calcination

LiNiO₂ was synthesized by the combustion method with various excess lithium amount z in Li_1 + zNiO₂ (z = 0.04, 0.08, 0.10, 0.12, and 0.15). The sample with z = 0.10 has the largest first discharge capacity of 195 mAh/g at 0.1 C rate and voltage range 2.7-4.4 V with the weight ratio of active material:acetylene black:binder = 85:10:5. The LiNiO₂ cathodes, in which the excess lithium amount z for the synthesis of LiNiO₂ was 0.10, were fabricated with various weight ratios of active material:acetylene black:binder (85:10:5, 85:12:3, and 90:7:3). The cathode with the ratio of active material:acetylene black:binder 85:10:5 has the best electrochemical properties. The variation, with C-rate, of discharge capacity vs. number of cycles curve for the LiNiO₂ cathode with the weight ratio of active material:acetylene black:binder = 85:10:5 was investigated. At 0.1 C rate, the LiNiO₂ cathode has the largest first discharge capacity, the discharge capacity degradation rate of 0.70 mAh/g/cycle and a discharge capacity at n = 50 of 134 mAh/g.     

Surface enhanced infrared spectroscopy—Au(1 1 1-20 nm)/sulphuric acid—new aspects and challenges

The structure and orientation of water molecules on Au(1 1 1-20 nm) film electrodes in contact with aqueous sulphuric acid solution was studied by surface enhanced infrared reflection-absorption spectroscopy (SEIRAS) employing an attenuated total reflection (ATR) configuration (ATR-SEIRAS) with a vertical spectroelectrochemical cell. The spectrum of interfacial water is strongly dependent on electrode potential, ionic strength and pH. Coadsorption of hydronium ions with weakly hydrogen-bonded water molecules was found at E<E_pzc. At E>E_pzc strong hydrogen-bonding among water molecules and to coadsorbed sulphate species exists. Based on pH-dependent electrochemical, spectroscopic and in situ STM investigations, a new model is suggested to represent the ordered () adlayer at maximum sulphate coverage θ=0.2: a Zundel or hydrated hydronium ion H₅O₂⁺, in which one proton is shared between two water molecules, bridges adjacent sulphate species via hydrogen-bonds along the main diagonal of the () unit cell. This alternating arrangement gives rise to a long-range ordered, 2D network of sulphate and water species interconnected by hydrogen-bonds, and capable to form hydrogen-bonds with second-layer water species. The suggested model is consistent with all experimental observations as well as predictions from quantum-chemical and molecular dynamic simulations.