Electrochemical performance of carbon-coated Li3V2(PO4)3 cathode materials derived from polystyrene-based carbon-thermal reduction synthesis

Li₃V₂(PO₄)₃ cathode materials were synthesized by a simple carbon-thermal reduction method using polystyrene as a carbon source. The residual carbon produced by the pyrolysis of polystyrene produced fine particle sizes and uniform carbon distribution on the Li₃V₂(PO₄)₃ particle surface. By increasing the amount of polystyrene added in the range of 0-16 wt.%, the thickness of the carbon coating increased, and the coating thickness was found to influence the electrochemical performance of the Li₃V₂(PO₄)₃ significantly. Our results indicate that the 6 wt.% polystyrene added Li₃V₂(PO₄)₃ with a 0.5-1 nm thick carbon coating possesses the highest initial discharge capacity of 132.7 mAh g⁻¹ between 3.0 and 4.3 V at 0.1 C. However, at high current densities, the higher polystyrene added Li₃V₂(PO₄)₃/C with a thicker carbon coating shows better performance in terms of the discharge capacity and cycling stability than that with the thin carbon coating. The improved cycling performance at higher current densities is attributed to the relatively small particle size and the suppressed impedance increase because of the thicker carbon coating.     

Crystallographic and electrochemical characteristics of TiZrNiCu quasicrystal ball-milled with La0.9Zr0.1Ni4.5Al0.5 alloy

Icosahedral quasicrystalline Ti₄₅Zr₃₅Ni₁₇Cu₃ alloy was ball-milled with 30 mass% La_0.9Zr_0.1Ni_4.5Al_0.5 alloy (LaNi₅ phase), the effect of the milling time on crystallographic and electrochemical characteristics of the alloy powder was investigated. The amount of amorphous phase increased with increasing milling time from 60 to 360 min, and the LaNi₅ phase cannot be observed when milling time was 240 min or more. The maximum discharge capacity and high-rate dischargeability of milled alloy electrodes were obviously higher than those of the alloy electrode before milling. The cycling capacity retention rate after 40 cycles increased from 52.8% (t = 60 min) to 62.9% (t = 360 min).     

Effects of synthetic route on structure and electrochemical performance of Li3V2(PO4)3/C cathode materials

Three different synthetic routes, including solid-state reaction, sol–gel and hydrothermal methods are successfully used for preparation of Li₃V₂(PO₄)₃/C. Ascorbic acid is used as a reducing agent and/or as a chelating agent. The Li₃V₂(PO₄)₃/C synthesized by hydrothermal method with fine particles exhibits lower impedance and smaller potential difference values between oxidation and reduction peaks than those by solid-state reaction and sol–gel methods. Thus as cathode material for Li-ion batteries, the Li₃V₂(PO₄)₃/C synthesized by hydrothermal method shows higher discharge capacity, better rate capability and cyclic performance. Even at a high charge–discharge rate of 10 C, it still can deliver a discharge capacity of 101.4 mAh g⁻¹ and 106.6 mAh g⁻¹ in the potential range of 3.0–4.3 V and 3.0–4.8 V, respectively. The hydrothermal synthesis has been considered to be a competitive process to prepare Li₃V₂(PO₄)₃/C cathode materials with excellent electrochemical performances.     

Electrochemical investigation of LiNi0.5Mn1.5O4 thin film intercalation electrodes

This work provides kinetic and transport parameters of Li-ion during its extraction/insertion into thin film LiNi_0.5Mn_1.5O₄ free of binder and conductive additive. Thin films of LiNi_0.5Mn_1.5O₄ (0.2 μm thick) were prepared on electronically conductive gold substrate utilizing the electrostatic spray deposition technique. High purity LiNi_0.5Mn_1.5O₄ thin film electrodes were observed with cyclic voltammetry, to exhibit very sharp peaks, high reversibility, and absence of the 4 V signal related to the Mn³⁺/Mn⁴⁺ redox couple. The electrode subjected to 100 CV cycles of charge/discharge delivered a capacity of 155 mAh g⁻¹ on the first cycle and sustained a good cycling behavior while retaining 91% of the initial capacity after 50 cycles. Kinetics and mass-transport of Li-ion extraction at LiNi_0.5Mn_1.5O₄ thin film electrode were investigated by means of electrochemical impedance spectroscopy. The apparent chemical diffusion coefficient (D_app) value determined from EIS measurements changed depending on the electrode potential in the range of 10⁻¹⁰-10⁻¹² cm² s⁻¹. The D_app profile shows two minimums at the potential values close to the peak potentials of the corresponding cyclic voltammogram.     

Effect of AB5 alloy on Ti0.10Zr0.15V0.35Cr0.10Ni0.30 hydrogen storage alloy

The structure and electrochemical characteristics of melted composite Ti_0.10Zr_0.15V_0.35Cr_0.10Ni_0.30 + x% LaNi₄Al_0.4Mn_0.3Co_0.3 (x = 0, 1, 5) hydrogen storage alloys have been investigated systematically. XRD shows that though the main phase of the matrix alloy remains unchanged after LaNi₄Al_0.4Mn_0.3Co_0.3 alloy is added, a new specimen is formed. The amount of the new specimen increases with increasing x. SEM-EDS analysis indicates that the V-based solid solution phase is mainly composed of V, Cr and Ni; C14 Laves phase is mainly composed of Ni, Zr and V; the new specimen containing La is mainly composed of Zr, V and Ni. The electrochemical measurements suggest that the activation performance, the low temperature discharge ability, the high rate discharge ability and the cyclic stability of composite alloy electrodes increase greatly with the growth of x. The HRD is controlled by the charge-transfer reaction of hydrogen and the hydrogen diffusion in the bulk of the alloy under the present experimental conditions.     

Crystal structure and electrochemical performance of Li3V2(PO4)3 synthesized by optimized microwave solid-state synthesis route

To investigate the crystal structure and electrochemical performance of samples synthesized under different microwave solid-state synthesis condition, a series of Li₃V₂(PO₄)₃ samples has been synthesized at five different temperatures for 3-5 min and at 750 °C for various time. The as-synthesized Li₃V₂(PO₄)₃ samples are characterized and studied by ICP-AES analysis, X-ray diffraction (XRD), Rietveld analysis, scanning and transmission electron microcopy (SEM and TEM). At relatively lower temperature (650 °C) and very short reaction time (3 min), pure phase of Li₃V₂(PO₄)₃ could be synthesized in microwave irradiation field. The crystal structure and Li atomic fractional coordinate present a significant deviation upon the change of microwave irradiation temperature and time. Relatively, the diffusion ability of lithium cations and the electrochemical performance are affected. Under the proper reaction temperature and time, the carbon-free samples MW750C5m and MW850C3m show the best specific discharge capacity 126.4 and 132 mAh g⁻¹ at the voltage range of 3.0-4.3 V, near the reversible cycling of two lithium ions per Li₃V₂(PO₄)₃ formula unit (133 mAh g⁻¹). At the voltage range of 3-4.8 V, the sample MW750C5m presents the best initial specific charge capacity of 197 mAh g⁻¹, equivalent to the reversible cycling of three lithium ions per Li₃V₂(PO₄)₃ formula unit (197 mAh g⁻¹). The initial discharge capacity, the samples MW750C5m and MW850C3m present high specific discharge capacity 183.4 and 175.7 mAh g⁻¹, respectively. The relationship among microwave irradiation condition, crystal structure, lithium atomic fractional coordinates and the electrochemical performance have been discussed in detail.     

钴、铝掺杂对Li_(1.1)Ni_(0.4)Mn_(0.4)Co_(0.15)Al_(0.05)O_2结构和电化学性能的影响

采用草酸盐共沉淀法合成Li1.1Ni0.4Mn0.4Co0.15Al0.05O2正极材料,通过XRD分析,计算得出掺杂后晶胞体积收缩,层状结构更加稳定。恒电流充放电测试显示了其良好的充放电性能和循环稳定性。对于不同充电状态的EIS分析,显示其较小的电荷转移阻抗及较好的电荷转移动力学。     

Electrochemical performance of the carbon coated Li3V2(PO4)3 cathode material synthesized by a sol-gel method

A carbon coated Li₃V₂(PO₄)₃ cathode material for lithium ion batteries was synthesized by a sol-gel method using V₂O₅, H₂O₂, NH₄H₂PO₄, LiOH and citric acid as starting materials, and its physicochemical properties were investigated using X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) spectroscopy, scanning electron microscopy (SEM), energy dispersive analysis of X-ray (EDAX), transmission electron microscope (TEM), and electrochemical methods. The sample prepared displays a monoclinic structure with a space group of P2₁/n, and its surface is covered with a rough and porous carbon layer. In the voltage range of 3.0-4.3 V, the Li₃V₂(PO₄)₃ electrode displays a large reversible capacity, good rate capability and excellent cyclic stability at both 25 and 55 °C. The largest reversible capacity of 130 mAh g⁻¹ was obtained at 0.1C and 55 °C, nearly equivalent to the reversible cycling of two lithium ions per Li₃V₂(PO₄)₃ formula unit (133 mAh g⁻¹). It was found that the increase in total carbon content can improve the discharge performance of the Li₃V₂(PO₄)₃ electrode. In the voltage range of 3.0-4.8 V, the extraction and reinsertion of the third lithium ion in the carbon coated Li₃V₂(PO₄)₃ host are almost reversible, exhibiting a reversible capacity of 177 mAh g⁻¹ and good cyclic performance. The reasons for the excellent electrochemical performance of the carbon coated Li₃V₂(PO₄)₃ cathode material were also discussed.     

Investigation of hydrogen transport through Mm(Ni3.6Co0.7Mn0.4Al0.3)1.12 and Zr0.65Ti0.35Ni1.2V0.4Mn0.4 hydride electrodes by analysis of anodic current transient

Hydrogen transport through such metal-hydride electrodes as Mm(Ni_3.6Co_0.7Mn_0.4Al_0.3)_1.12 and Zr_0.65Ti_0.35Ni_1.2V_0.4Mn_0.4 was investigated in 6 M KOH solution by using potentiostatic current transient technique. From the shape of the anodic current transient and the dependence of the initial current density on the discharging potential, the boundary conditions at the electrode surface were established during hydrogen extraction from the as-annealed and as-surface-treated electrodes. Especially, it was experimentally confirmed that the diffusion-limited boundary condition is no longer valid at the electrode surface during hydrogen transport in case hydrogen diffusion is coupled with either the interfacial charge transfer reaction or the hydrogen transfer reaction between adsorbed state on the electrode surface and absorbed state at the electrode sub-surface. From the transition behaviour of the boundary condition, it was further recognised that the boundary condition at the electrode surface during hydrogen transport is not fixed at the specific electrode/electrolyte system by itself, but it is rather simultaneously determined even at any electrode/electrolyte system by the potential step and the nature of the electrode surface, depending upon e.g. the presence or absence of the surface oxide scales.     

Layered monodiphosphate Li9V3(P2O7)3(PO4)2: A novel cathode material for lithium-ion batteries

Single phase Li₉V₃(P₂O₇)₃(PO₄)₂ is synthesized at 750 °C via solid-state reaction method for the first time. The Rietveld refinement results show that the trigonal system (space group: ) with the lattice parameters a = 0.9724 nm, c = 1.3596 nm are obtained. Its intrinsic electrical conductivity of 1.43 × 10⁻⁸ S cm⁻¹ is higher than that of LiFePO₄ and as the same order of Li₃V₂(PO₃)₄. The electrochemical measurement results show that there are two plateaus (3.77 V and 4.51 V) and three plateaus (3.77 V, 4.51 V and 4.75 V) in the potential ranges of 2.0–4.6 V and 2.0–4.8 V, respectively. In the range of 2.0–4.6 V, two discharge plateaus (4.46 V and 3.74 V) can be observed and 110 mAh g⁻¹ of discharge capacity is achieved. The Rietveld refinement result of the X-ray diffraction (XRD) data at the end of discharge after the first cycle suggests that the structural reversibility can be retained during electrochemical reactions in Li₉V₃(P₂O₇)₃(PO₄)_2. In the range of 2.0–4.8 V, almost six lithium ions are extracted and the trigonal structure is still recovered after 30 cycles. Therefore, this novel layered vanadium monodiphosphate offers a promising candidate as cathode material for lithium-ion batteries.     

Electrochemical behavior of electrodes comprising micro- and nano-sized particles of LiNi0.5Mn1.5O4: A comparative study

A comparative analysis of the electrochemical behavior and Li⁺ transport characteristics of thin-layer LiNi_0.5Mn_1.5O₄ intercalation electrodes comprised of micro- or nano-sized particles in standard Li salt solutions, was carried out and is reported herein. These electrodes were free of any conductive additives and polymeric binder in order to avoid their complex impact on the electrochemical response of the active mass. It was clearly established that the electrodes prepared from nanoparticles of the active mass show faster kinetics and a more reversible electrochemical behavior compared to the electrodes comprising microparticles.

The response of the nanoparticles to electrochemical techniques such as linear sweep voltammetry and potentiostatic intermittent titration (PITT) is characterized by high resolution. Thus D versus E could be calculated very precisely. It was encouraging to realize the good performance of the electrodes comprising nano-LiMn_1.5Ni_0.5O₄, in spite of their high surface area and their high operating voltage.     

Structure and electrochemical characteristics of melted composite Ti0.10Zr0.15V0.35Cr0.10Ni0.30-LaNi5 hydrogen storage alloys

The structure and electrochemical characteristics of melted composite Ti_0.10Zr_0.15V_0.35Cr_0.10Ni_0.30 + x% LaNi₅ (x = 0, 1, 5 and 10) hydrogen storage alloys have been investigated systematically. XRD shows that the matrix phase structure of V-based solid solution phase with a BCC structure and C14 Laves phase with hexagonal structure is not changed after adding LaNi₅ alloy. However, the amount of the secondary phase increases with increasing LaNi₅ content. Field emission scanning electron microscopy-energy dispersive spectroscopy (FESEM-EDS) shows that the C14 Laves phase contains more Zr and the white lard phase has a composition close to (Zr, Ti)(V, Cr, Ni, La)₂. The electrochemical measurements show that the hysteresis effect decreases dramatically with increasing x. The activation performance, the low temperature dischargeability, high rate dischargeability and cyclic stability of composite alloy electrodes increase greatly with increasing x. The maximum discharge capacity first increases as x increases from 0 to 5 and then decreases when x increases further from 5 to 10. The improvement of the electrochemical characteristics caused by adding LaNi₅ seems to be related to formation of the secondary phase.     

The role of structural and electronic alterations on the lithium diffusion in LixCo0.5Ni0.5O2

The mechanisms for lithium diffusion in Li_xCo_0.5Ni_0.5O₂ were investigated using the galvanostatic intermittent titration technique (GITT). Membrane electrodes prepared with poly(vinylidene fluoride) and carbon black were employed in this study. The measured Brunauer-Emmett-Teller (BET) area of the Li_xCo_0.5Ni_0.5O₂ powder was combined with the GITT data to obtain the lithium chemical diffusion coefficient (), the lithium self-diffusion coefficient (D_Li⁺) and the thermodynamic factor (Φ) as a function of Li concentration (x). All three parameters vary non-monotonically with x. A minimum in and D_Li⁺ at x=0.5, along with structural changes, suggests the formation of a lithium superlattice at that concentration. The behavior of is complex but for x<0.34 it eventually undergoes a continuous decrease due to the metallic character of Li_xCo_0.5Ni_0.5O₂. We attribute the limitation of the specific reversible capacity of Li_xCo_0.5Ni_0.5O₂ to this decrease in and to elevated electrode voltages. Li transport in Li_xCo_0.5Ni_0.5O₂ is analyzed taking the variations in the cell parameters and the oxidation states of the Ni, Co and O ions into account.     

Electrochemical impedance characterization of FeSn2 electrodes for Li-ion batteries

This work reports the electrochemical characterization of a micro-scale FeSn₂ electrode in a lithium battery. The electrode is proposed as anode material for advanced lithium ion batteries due to its characteristics of high capacity (500 mAh g⁻¹) and low working voltage (0.6 V vs. Li). The electrochemical alloying process is studied by cyclic voltammetry and galvanostatic cycling while the interfacial properties are investigated by electrochemical impedance spectroscopy. The impedance measurements in combination with the galvanostatic cycling tests reveal relatively low overall impedance values and good electrochemical performance for the electrode, both in terms of delivered capacity and cycling stability, even at the higher C-rate regimes.     

A novel layered Li [Li0.12NizMg0.32−zMn0.56]O2 cathode material for lithium-ion batteries

Layered Li[Li_0.12Ni_zMg_0.32−zMn_0.56]O₂ oxide cathodes containing lithium atoms in the transition metal layers were synthesized and characterized using X-ray diffraction (XRD), galvanostatic cycling, and differential scanning calorimetry (DSC). The Li[Li_0.12Ni_zMg_0.32−zMn_0.56]O₂ cathodes deliver a specific discharge capacity of about 190 mAh/g at room temperature and 236 mAh/g at 55 °C when cycled between 2.7 and 4.6 V versus Li/Li⁺. Excellent capacity retention and smooth potential profiles at room and elevated temperatures over extended cycles suggest that this material does not convert into a spinel structure.     

FexCo0.5−xNi0.5-SDC anodes for low-temperature solid oxide fuel cells

Trimetal alloys, Fe_xCo_0.5−xNi_0.5 (x = 0.1, 0.2, 0.25, 0.3, 0.4), were studied as anodes for low-temperature solid oxide fuel cells (LT-SOFCs) based on GDC (Ce_0.9Gd_0.1O_1.95) electrolytes. The alloys were formed by in situ reduction of Fe_xCo_0.5−xNi_0.5O_y composites, which were synthesized using a glycine-nitrate technique. Symmetrical cells consisted of Fe_xCo_0.5−xNi_0.5-SDC electrodes and GDC electrolytes, and single cells consisted of Fe_xCo_0.5−xNi_0.5-SDC (Ce_0.8Sm_0.2O_1.9) anodes, GDC electrolytes, and SSC (Sm_0.5Sr_0.5CoO₃)-SDC cathodes were prepared using a co-pressing and co-firing process. Interfacial polarization resistances and I-V curves of these cells were measured at temperature from 450 to 600 °C. With Fe_0.25Co_0.25Ni_0.5-SDC as anodes, the cells showed the lowest interfacial resistance and highest power density. For example, at 600 °C, the resistance was about 0.11 Ω cm² and power density was about 750 mW cm⁻² when humidified (3% H₂O) hydrogen was used as fuel and stationary air as oxidant. Further, the cell performance was improved when the molar ratio of Fe:Co:Ni approached 1:1:2, i.e. Fe_0.25Co_0.25Ni_0.5. In addition, higher power density and lower interfacial resistance were obtained for cells with the Fe_0.25Co_0.25Ni_0.5-SDC anodes comparing to that with Ni-SDC anodes, which have been usually used for LT-SOFCs. The promising performance of Fe_xCo_0.5−xNi_0.5 as anodes suggests that trimetallic anodes are worth considering for SOFCs that operate at low-temperature.     

Electrochemical hydrogen storage in (Ti1−xVx)2Ni (x = 0.05-0.3) alloys comprising icosahedral quasicrystalline phase

For (Ti_1−xV_x)₂Ni (x = 0.05, 0.1, 0.15, 0.2 and 0.3) ribbons, synthesized by arc-melting and subsequent melt-spinning techniques, an icosahedral quasicrystalline phase was present, either in the amorphous matrix or together with the stable Ti₂Ni-type phase. With increasing x values, the maximum discharge capacity of the alloy electrodes increased until reached 271.3 mAh/g when x = 0.3. The cycling capacity retention rates for these electrodes were approximately 80% after a preliminary test of 30 consecutive cycles of charging and discharging. Ti_1.7V_0.3Ni alloy electrode displayed the best high-rate discharge ability of 82.7% at the discharge current density of 240 mA/g.     

Structure and electrochemical performances of Mg2Ni1−xMnx (x = 0-0.4) electrode alloys prepared by melt spinning

The melt-spinning technique is applied to the preparation of the nanocrystalline and amorphous Mg₂Ni-type alloys with nominal compositions of Mg₂Ni_1−xMn_x (x = 0, 0.1, 0.2, 0.3, 0.4). The as-spun alloy ribbons possessing a continuous length, a thickness of about 30 μm and a width of about 25 mm were prepared. The structures of the as-spun alloy ribbons are characterized by XRD and TEM. The electrochemical performances of the as-spun alloy ribbons are measured by an automatic galvanostatic system. The results show that no amorphous structure is detected in the as-spun Mg₂Ni alloy, whereas the as-spun Mg₂Ni_0.6Mn_0.4 alloy displays a nanocrystalline and amorphous structure, confirming that the substitution of Mn for Ni notably intensifies the amorphous forming ability of the Mg₂Ni-type alloy. The amorphization degree of the as-spun alloys containing Mn increases with increasing spinning rate. The melt spinning also significantly enhances the electrochemical performances such as the discharge capacity and the electrochemical cycle stability of the Mn-containing alloys. Furthermore, the high rate dischargeability (HRD) of the (x ≤ 0.1) alloys increases with an increase in the spinning rate, while for the (x ≥ 0.2) alloys, the HRD exhibits a maximum value at a particular spinning rate, and it varies with the change in Mn contents of the alloys.     

Effects of mechanical grinding on initial activation and rate capability of Zr–Ti based Laves phase alloy electrode

Surface modification of Zr_0.9Ti_0.1Ni_1.1Co_0.1Mn_0.5V_0.2Cr_0.1 alloy was accomplished by mechanical grinding (MG). The decrepitation of alloy particles gave rise to a new surface. The effect of MG was systematically studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), pressure-composition (PC) isotherms and electrochemical impedance spectroscopy (EIS). Initial activation and rate capability of a negative electrode made from this alloy were significantly improved by MG treatment at 300 rpm for 0.5–1 h, but prolonged MG treatment (10 h) reduced the nominal discharge capacity from 350 mAh g⁻¹ to 180 mAh g⁻¹. Under these conditions the alloy particles disintegrate and become nanocrystalline, which reduces the discharge capacity owing to the change in the stereology of tetrahedral interstices available for hydrogen storage. The data based on PC isotherms and hydriding kinetics indicates that the equilibrium hydrogen pressure increases and the hydriding rate and storage capacity are significantly reduced by prolonged MG treatment. EIS data reveal that the improved rate capability can be ascribed to an enhanced charge-transfer reaction which is the rate-determining step in the hydriding and dehydriding reactions.     

Electrochemical performance of Li3V2(PO4)3/C cathode material using a novel carbon source

Polyethylene glycol (PEG, mean molecular weight of 10,000) has been used to prepare a Li₃V₂(PO₄)₃/C cathode material by a simple solid-state reaction. The Raman spectra shows that the coating carbon has a good structure with a low I_D/I_G ratio. The images of SEM and TEM show that the carbon is dispersed between the Li₃V₂(PO₄)₃ particles, which improves the electrical contact between the corresponding particles. The electronic conductivity of Li₃V₂(PO₄)₃/C composite is 7.0 × 10⁻¹ S/cm, increased by seven orders of magnitude compared with the pristine Li₃V₂(PO₄)₃ (2.3 × 10⁻⁸ S/cm). At a low discharge rate of 0.28C, the sample presents a high discharge capacity of 131.2 mAh/g, almost achieving the theoretical capacity (132 mAh/g) for the reversible cycling of two lithium. After 500 cycles, the discharge capacity is 123.9 mAh/g with only 5.6% fading of the initial specific capacity. The Li₃V₂(PO₄)₃/C material also exhibits an excellent rate capability with high discharge capacities of 115.2 mAh/g at 1C and 106.4 mAh/g at 5C.