Electrochemical and thermal studies of Li[NixLi(1/3−2x/3)Mn(2/3−x/3)]O2 (x = 1/12, 1/4, 5/12, and 1/2)

Samples of the layered cathode materials, Li[Ni_xLi_(1/3−2x/3)Mn_(2/3−x/3)]O₂ (x = 1/12, 1/4, 5/12, and 1/2), were synthesized at 900 °C. Electrodes of these samples were charged in Li-ion coin cells to remove lithium. The charged electrode materials were rinsed to remove the electrolyte salt and then added, along with EC/DEC solvent or 1 M LiPF₆ EC/DEC, to stainless steel accelerating rate calorimetry (ARC) sample holders that were then welded closed. The reactivity of the samples with electrolyte was probed at two states of charge. First, for samples charged to near 4.45 V and second, for samples charged to 4.8 V, corresponding to removal of all mobile lithium from the samples and also concomitant release of oxygen in a plateau near 4.5 V. Li[Ni_xLi_(1/3−2x/3)Mn_(2/3−x/3)]O₂ samples with x = 1/4, 5/12 and 1/2 charged to 4.45 V do not react appreciably till 190 °C in EC/DEC. Li[Ni_xLi_(1/3−2x/3)Mn_(2/3−x/3)]O₂ samples charged to 4.8 V versus Li, across the oxygen release plateau, start to significantly react with EC/DEC at about 130 °C. However, their high reactivity is similar to that of Li_0.5CoO₂ (4.2 V) with 1 μm particle size. Therefore, Li[Ni_xLi_(1/3−2x/3)Mn_(2/3−x/3)]O₂ samples showing specific capacity of up to 225 mAh/g may be acceptable for replacing LiCoO₂ (145 mAh/g to 4.2 V) from a safety point of view, if their particle size is increased.     

Improved electrochemical properties of Li1+x(Ni0.3Co0.4Mn0.3)O2−δ (x = 0, 0.03 and 0.06) with lithium excess composition prepared by a spray drying method

Layered Li_1+x(Ni_0.3Co_0.4Mn_0.3)O_2−δ (x = 0, 0.03 and 0.06) materials were synthesized through the different calcination times using the spray-dried precursor with the molar ratio of Li/Me = 1.25 (Me = transition metals). The physical and electrochemical properties of the lithium excess and the stoichiometric materials were examined using XRD, AAS, BET and galvanostatic electrochemical method. As results, the lithium excess Li_1.06(Ni_0.3Co_0.4Mn_0.3)O_2−δ could show better electrochemical properties, such as discharge capacity, capacity retention and C rate ability, than those of the stoichiometric Li_1.00(Ni_0.3Co_0.4Mn_0.3)O_2−δ. In this paper, the effect of excess lithium on the electrochemical properties of Li_1+x(Ni_0.3Co_0.4Mn_0.3)O_2−δ materials will be discussed based on the experimental results of ex situ X-ray diffraction, transmission electron microscopy (TEM) and galvanostatic intermittent titration technique (GITT)     

Phase equilibria and glass formation studies in the (1 − x)TeO2-xCdO (0.05 ≤ x ≤ 0.33 mol) system

Phase equilibria and glass formation studies of the (1 − x)TeO₂-xCdO system (0.05 ≤ x ≤ 0.33 mol) were realized by using differential thermal analysis (DTA), X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. The samples were prepared by applying a conventional melt-quenching technique at 800 °C. The glass formation range of the system was determined as 0.05 ≤ x < 0.15 and the sample containing 10 mol% CdO showed the highest glass stability. Crystallization behavior of the TeO₂-CdO glasses was investigated and formation and/or transformation of different phases were detected for each crystallization reaction. In order to obtain thermal stability of the system, as-cast samples were heat-treated above all crystallization reaction temperatures at 550 °C for 24 h. A binary eutectic: liquid → TeO₂ + CdTe₂O₅ was detected at 638 ± 4 °C. Crystallization behavior of the TeO₂-CdO glasses and microstructural characterization of the TeO₂-CdTe₂O₅ system was realized.     

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.     

Improvement of cycling performance of Li1.1Mn1.9O4 at 60 °C by NiO addition for Li-ion secondary batteries

We report on electrochemical properties of NiO-blended spinel Li_1.1Mn_1.9O₄ at elevated temperature (60 °C). Thus, we employed two kinds of NiO powders, those are, larger particle size (>10 μm) and submicron-sized NiO powders obtained by a ball-milling. These NiO powders were blended to the spinel Li_1.1Mn_1.9O₄ as an additive for fabrication of cathode. The resulting discharge capacity for the larger NiO particle-blended Li_1.1Mn_1.9O₄ had similar electrochemical properties to the bare Li_1.1Mn_1.9O₄. On the other hand, submicron-sized NiO-blended Li_1.1Mn_1.9O₄ brought about slightly increased capacity and excellent capacity retention, maintaining its initial capacity of 99.2% at 25 °C and 94% at 60 °C when Li metal was employed as the anode. In Li-ion cell using graphite as the anode, the capacity retention was of about 80% during cycling at 60 °C, whereas C/Li_1.1Mn_1.9O₄ cell retained around 68% of its initial capacity. Such improved properties would be ascribed to the HF scavenging into the electrolyte by presence of the submicron-sized NiO particles in Li_1.1Mn_1.9O₄ cathode.     

Effects of the nanostructured SiO2 coating on the performance of LiNi0.5Mn1.5O4 cathode materials for high-voltage Li-ion batteries

LiNi_0.5Mn_1.5O₄ spinels coated with various amounts of fumed silica have been synthesized and investigated as cathode materials for high-voltage lithium-ion batteries at the elevated temperature (55 °C). The morphology and structure of the coated LiNi_0.5Mn_1.5O₄ samples were characterized by XRD, TEM and EDX. It was found that the surfaces of the coated LiNi_0.5Mn_1.5O₄ samples are covered with a porous, amorphous, nanostructured SiO₂ layer. The results of electrochemical experiments showed that the SiO₂-coated LiNi_0.5Mn_1.5O₄ samples display obviously improved capacity retention rate, and the improvement effect enhances with the increase of SiO₂ content. The XPS results revealed that the surfaces of the SiO₂-coated LiNi_0.5Mn_1.5O₄ cathode materials have relatively low content of LiF, and this is mainly responsible for their improved electrochemical cycling stability.     

Enhanced electrochemical stability of Al-doped LiMn2O4 synthesized by a polymer-pyrolysis method

LiAl_xMn_2−xO₄ samples (x = 0, 0.02, 0.05, 0.08) were synthesized by a polymer-pyrolysis method. The structure and morphology of the LiAl_xMn_2−xO₄ samples calcined at 800 °C for 6 h were investigated by powder X-ray diffraction and scanning electron microscopy. The results show that all samples have high crystallinity, regular octahedral morphology and uniform particle size of 100-300 nm. The electrochemical performances were tested by galvanostatic charge-discharge and cyclic voltammetry. The results demonstrate that the Al-doped LiMn₂O₄ can be very well cycled at an elevated temperature of 55 °C without severe capacity degradation. In particular, the LiAl_0.08Mn_1.92O₄ sample demonstrates excellent capacity retention of 99.3% after 50 cycles at 55 °C, confirming the greatly enhanced electrochemical stability of LiMn₂O₄ by a small quantity of Al-doping.     

Low-temperature solution combustion synthesis of high performance LiNi0.5Mn1.5O4

High-voltage LiNi_0.5Mn_1.5O₄ spinels were synthesized by a low temperature solution combustion method at 400 °C, 600 °C and 800 °C for 3 h. The phase composition, structural disordering, micro-morphologies and electrochemical properties of the products were investigated by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM) and constant current charge–discharge test. XRD analysis indicated that single phase LiNi_0.5Mn_1.5O₄ powders with disordered Fd-3m structures were obtained by the method at 400 °C, 600 °C and 800 °C. The crystallinity increased with increasing preparation temperatures. XRD and FTIR data indicated that the degree of structural disordering in the product prepared at 800 °C was the largest and in the product prepared at 600 °C was the least. SEM investigation demonstrated that the particle size and the crystal perfection of the products were increased with increasing temperatures. The particles of the product prepared at 600 °C with ~200 nm in size are well developed and homogeneously distributed. Charge/discharge curves and cycling performance tests at different current density indicated that the product prepared at 600 °C had the largest specific capacity and the best cycling performance, due to its high purity, high crystallinity, small particle size as well as moderate amount of Mn³⁺ ions.     

Co-precipitation synthesis of spherical Li1.05M0.05Mn1.9O4 (M = Ni, Mg, Al) spinel and its application for lithium secondary battery cathode

Metal-doped spinel lithium manganese oxides were successfully synthesized via co-precipitation. The synthesized Li_1.05M_0.05Mn_1.9O₄ (M = Ni, Mg, Al) had highly crystalline cubic spinel phase with the space group Fd3m, as confirmed by X-ray diffraction study. From scanning electron microscopic observation, it was found that the prepared materials had spherical morphology with high tap-density. The Li_1.05M_0.05Mn_1.9O₄ (M = Ni, Mg, Al) electrode exhibited improved cycling performance at elevated temperature. Especially, the Li_1.05Al_0.05Mn_1.9O₄ showed capacity retention of 91.5% during 100 cycles at elevated temperature (55 °C). Also the thermal stability of the Li_1.05M_0.05Mn_1.9O₄ (M = Ni, Mg, Al) was also significantly improved.     

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.     

Densification, microstructural evolution and microwave dielectric properties of fluxed sintered 12R-Ba(Ti0.5Mn0.5)O3 ceramics

Doped hexagonal BaTiO₃ (h-BaTiO₃) ceramics have recently been identified as potential candidates for use in microwave dielectric resonators. However, similar to other common microwave ceramics, doped h-BaTiO₃ ceramics require a sintering temperature higher than 1400 °C. In this study, the effects of Bi₂O₃ and Li₂CO₃ on the densification, microstructural evolution and microwave properties of hexagonal 12R-Ba(Ti_0.5Mn_0.5)O₃ ceramics were examined. Results indicate that Bi₂O₃ and Li₂CO₃ are able to effectively reduce the sintering temperature of 12R-Ba(Ti₀₅Mn_0.5)O₃ ceramics through liquid phase sintering while retaining the hexagonal structure and the microwave dielectric properties. The best results were obtained for the 12R-Ba(Ti_0.5Mn_0.5)O₃ with the additions of 5 wt% Bi₂O₃ sintered at 1200 °C (?_r: 36.0, Qf_r: 6779 GHz, and τ_f: 25.3 ppm/°C), and 5 wt% Li₂CO₃ sintered at 1200 °C (?_r: 28.1, Qf_r: 5304 GHz, and τ_f: 35.3 ppm/°C).     

Optimum synthesis of Li2Fe1−xMnxSiO4/C cathode for lithium ion batteries

Li₂Fe_1−xMn_xSi0₄/C cathode materials were synthesized by mechanical activation-solid-state reaction. The effects of Mn-doping content, roasting temperature, soaking time and Li/Si molar ratio on the physical properties and electrochemical performance of the Li₂Fe_1−xMn_xSi0₄/C composites were investigated. The materials were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM), charge-discharge tests and AC impedance measurements. SEM images suggest that the morphology of the Li₂Fe_1−xMn_xSi0₄/C composite is sensitive to the reaction temperature. Samples synthesized at different temperatures have different extent of agglomeration. Being charged-discharged at C/32 between 1.5 and 4.8 V, the Li₂Fe_0.9Mn_0.1Si0₄/C synthesized at the optimum conditions shows good electrochemical performances with an initial discharge capacity of 158.1 mAh g⁻¹ and a capacity retention ratio of 94.3% after 30 cycles. AC impendence investigation shows Li₂Fe_0.9Mn_0.1SiO₄/C have much lower resistance of electrode/electrolyte interface than Li₂FeSiO₄/C.     

Particle size effect of Li[Ni0.5Mn0.5]O2 prepared by co-precipitation

Spherical (Ni_0.5Mn_0.5)(OH)₂ with different secondary particle size (3 μm, 10 μm in diameter) was synthesized by co-precipitation method. Mixture of the prepared hydroxide and lithium hydroxide was calcined at 950 °C for 20 h in air. X-ray diffraction patterns revealed that the prepared material had a typical layered structure with space group. Spherical morphologies with mono-dispersed powders were observed by scanning electron microscopy. It was found that the layered Li[Ni_0.5Mn_0.5]O₂ delivered an initial discharge capacity of 148 mAh g⁻¹ (3.0-4.3 V) though the particle sizes were different. Li[Ni_0.5Mn_0.5]O₂ having smaller particle size (3 μm) showed improved area specific impedance due to the reduced Li⁺ diffusion path, compared with that of Li[Ni_0.5Mn_0.5]O₂ possessing larger particle size (10 μm). Although the Li[Ni_0.5Mn_0.5]O₂ (3 μm) was electrochemically delithiated to Li_0.39[Ni_0.5Mn_0.5]O₂, the resulting exothermic onset temperature was around 295 °C, of which the value is significantly higher than that of highly delithiated Li_1−δCoO₂ (∼180 °C).     

Theoretical approach to cell-impedance-controlled lithium transport through Li1 − δMn2O4 film electrode with partially inactive fractal surface by analyses of potentiostatic current transient and linear sweep voltammogram

Lithium transport through the partially inactive fractal Li_1 − δMn₂O₄ film electrode under the cell-impedance-controlled constraint was theoretically investigated by using the kinetic Monte Carlo method based upon random walk approach. Under the cell-impedance-controlled constraint, all the potentiostatic current transients calculated from the totally active and partially inactive fractal electrodes hardly exhibited the generalised Cottrell behaviour and they were significantly affected in shape by the interfacial charge-transfer kinetics. In the case of the linear sweep voltammogram determined from the totally active and partially inactive fractal electrodes, all the power dependence of the peak current on the scan rate above the characteristic scan rate deviated from the generalised Randles-Sev?ik behaviour. From the analyses of the current transients and the linear sweep voltammograms simulated with various values of the simulation parameters, it was further recognised that the cell-impedance-controlled lithium transport through the partially inactive fractal Li_1 − δMn₂O₄ film electrode strongly deviates from the generalised diffusion-controlled transport behaviour of the electrode with the totally active surface, which is attributed to the impeded interfacial charge-transfer kinetics governed by the surface inhomogeneities including the fractal dimension of the surface and the surface coverage by active sites and by the kinetic parameters including the internal cell resistance.     

Properties of Ni1−xFex (0.1 < x < 0.9) and Invar (x = 0.64) alloys obtained by electrodeposition

Electrodeposition of Ni_1−xFe_x (x = 0.1-0.9) films was carried out from a chloride plating solution containing saccharin as an organic additive at a constant current density (5 mA/cm²) and a controlled pH of 2.5. X-ray diffraction studies revealed the existence of an fcc, or γ phase, in the range of 10-58 wt.% Fe, a mixed fcc/bcc phase in the range of 59-60 wt.% Fe, and a bcc, or α phase in the range of 64-90 wt.% Fe. The saturation magnetization, B_s, of electrodeposited Ni_1−xFe_x alloys at the room temperature was found to increase with the increase of Fe-content and follows the Slater-Pauling curve, but deviates from as-cast bulk NiFe alloys. The coefficient of thermal expansion, CTE, of electrodeposited alloys at room temperature also deviates from as-cast bulk NiFe alloys. Annealing of α-Ni₃₆Fe₆₄ alloy results in a martensitic α → γ phase transformation, which takes place between 300 and 400 °C. It was demonstrated that thermal treatment above 400 °C was necessary to obtain magnetic and mechanical properties similar to those to conventional Invar alloy. Annealing of α-Ni₃₆Fe₆₄ alloy at 700 °C brings about a decrease of B_s from 1.75 to 0.45 T. By controlling the annealing conditions of α → γ martensitic transformation, it is possible to adjust the CTE of Ni₃₆Fe₆₄ alloy over the broad limits from 2.7 to 8.7 × 10⁻⁶/°C.     

Synthesis and tunable thermal expansion properties of Sc2−xYxW3O12 solid solutions

A new series of rare earth solid solutions Sc_2−xY_xW₃O₁₂ was successfully synthesized by the conventional solid-state method. Effects of doping ion yttrium on the crystal structure, morphology and thermal expansion property of as-prepared Sc_2−xY_xW₃O₁₂ ceramics were investigated by X-ray diffraction (XRD), thermogravimetric analysis (TG), field emission scanning electron microscope (FE-SEM) and thermal mechanical analyzer (TMA). Results indicate that the obtained Sc_2−xY_xW₃O₁₂ samples with Y doping of 0≤x≤0.5 are in the form of orthorhombic Sc₂W₃O₁₂-structure and show negative thermal expansion (NTE) from room temperature to 600 °C; while as-synthesized materials with Y doping of 1.5≤x≤2 take hygroscopic Y₂W₃O₁₂·nH₂O-structure at room temperature and exhibit NTE only after losing water molecules. It is suggested that the obvious difference in crystal structure leads to different thermal expansion behaviors in Sc_2−xY_xW₃O₁₂. Thus it is proposed that thermal expansion properties of Sc_2−xY_xW₃O₁₂ can be adjusted by the employment of Y dopant; the obtained Sc_1.5Y_0.5W₃O₁₂ ceramic shows almost zero thermal expansion and its average linear thermal expansion coefficient is −0.00683×10⁻⁶ °C⁻¹ in the 25–250 °C range.     

Nanorod-assembled spinel Li1.05Mn1.95O4 rods with a central tunnel along the rod-axis for high rate capability of rechargeable lithium-ion batteries

Nanorod-assembled spinel Li_1.05Mn_1.95O₄ rods with a central tunnel along the rod-axis were synthesized using highly crystalline β-MnO₂ rods as self-templates. The synthesized spinel Li_1.05Mn_1.95O₄ is an assembly of several single crystal-like nanorods with an average diameter and length of 100 and 400 nm, respectively, which was determined by microstructural Rietveld refinement using the synchrotron powder XRD data. Galvanostatic battery testing showed that central-tunneled and nanorod-assembled Li_1.05Mn_1.95O₄ rods have a high charge storage capacity at high current densities in comparison with those of the spinel rods without a tunnel structure and commercial powders. Moreover, a capacity retention value of ∼81% was observed at the end of 100 cycles at a current of 250 mAh g⁻¹.     

A delithiated LiNi0.65Co0.25Mn0.10O2 electrode material: A structural, magnetic and electrochemical study

A crystalline LiNi_0.65Co_0.25Mn_0.10O₂ electrode material was synthesized by the combustion method at 900 °C for 1 h. Rietveld refinement shows less than 3% of Li/Ni disorder in the structure. Lithium extraction involves only the Ni²⁺/Ni⁴⁺ redox couple while Co³⁺ and Mn⁴⁺ remain electrochemically inactive. No structural transition was detected during cycling in the whole composition range 0 < x < 1.0. Furthermore, the hexagonal cell volume changes by only 3% when all lithium was removed indicating a good mechanical stability of the studied compound. LiNi_0.65Co_0.25Mn_0.10O₂ has a discharge capacity of 150 mAh/g in the voltage range 2.5-4.5 V, but the best electrochemical performance was obtained with an upper cut-off potential of 4.3 V. Magnetic measurements reveal competing antiferromagnetic and ferromagnetic interactions - varying in strength as a function of lithium content - yielding a low temperature magnetically frustrated state. The evolution of the magnetic properties with lithium content confirms the preferential oxidation of Ni ions compared to Co³⁺ and Mn⁴⁺ during the delithiation process.     

Low temperature solid-state synthesis routine and mechanism for Li3V2(PO4)3 using LiF as lithium precursor

Monoclinic lithium vanadium phosphate, Li₃V₂(PO₄)₃, has been successfully synthesized using LiF as lithium source. The one-step reaction with stoichiometric composition and relative lower sintering temperature (700 °C) has been used in our experimental processes. The solid-state reaction mechanism using LiF as lithium precursor has been studied by X-ray diffraction and Fourier transform infrared spectra. The Rietveld refinement results show that in our product sintered at 700 °C no impurity phases of VPO₄, Li₅V(PO₄)₂F₂, or LiVPO₄F can be detected. The solid-state reaction using Li₂CO₃ as Li-precursor has also been carried out for comparison. X-ray diffraction patterns indicate that impurities as Li₃PO₄ can be found in the product using Li₂CO₃ as Li-precursor unless the sintering temperatures are higher than 850 °C. An abrupt particle growth (about 2 μm) has also been observed by scanning electron microscope for the samples sintered at higher temperatures, which can result in a poor cycle performance. The product obtained using LiF as Li-precursor with the uniform flake-like particles and smaller particle size (about 300 nm) exhibits the better performance. At the 50th cycle, the reversible specific capacities for Li₃V₂(PO₄)₃ measured between 3 and 4.8 V at 1C rate are found to approach 147.1 mAh/g (93.8% of initial capacity). The specific capacity of 123.6 mAh/g can even be hold between 3 and 4.8 V at 5C rate.