Properties of mechanically alloyed Mg–Ni–Ti ternary hydrogen storage alloys for Ni-MH batteries

MgNiTi_x, Mg_1−xTi_xNi and MgNi_1−xTi_x (with x varying from 0 to 0.5) alloys have been prepared by high energy ball milling and tested as hydrogen storage electrodes. The initial discharge capacities of the Mg–Ni–Ti ternary alloys are inferior to the MgNi electrode capacity. However, an exception is observed with MgNi_0.95Ti_0.05, which has an initial discharge capacity of 575 mAh/g compared to 522 mAh/g for the MgNi electrode. The Mg–Ni-Ti ternary alloys show improved cycle life compared to Mg–Ni binary alloys with the same Mg/Ni atomic ratio. The best cycle life is observed with Mg_0.5Ti_0.5Ni electrode which retains 75% of initial capacity after 10 cycles in comparison to 39% for MgNi electrodes, in addition to improved high-rate dischargeability (HRD). According to the XPS analysis, the cycle life improvement of the Mg_0.5Ti_0.5Ni electrode can be related to the formation of TiO₂ which limits Mg(OH)₂ formation. The anodic polarization curve of Mg_0.5Ti_0.5Ni electrode shows that the current related to the active/passive transition is much less important and that the passive region is more extended than for the MgNi electrode but the corrosion of the electrode is still significant. This suggests that the cycle life improvement would be also associated with a decrease of the particle pulverization upon cycling.     

Electrochemical properties of amorphous and icosahedral quasicrystalline Ti45Zr35Ni17Cu3 powders

Ti₄₅Zr₃₅Ni₁₇Cu₃ amorphous and single icosahedral quasicrystalline powders were synthesized by mechanical alloying and subsequent annealing at 855 K. Microstructure and electrochemical properties of two alloy electrodes were characterized. When the temperature was enhanced from 303 to 343 K, the maximum discharge capacities increased from 86 to 329 mAh g⁻¹ and 76 to 312 mAh g⁻¹ for the amorphous and quasicrystalline alloy electrodes, respectively. Discharge capacities of two electrodes decrease distinctly with increasing cycle number. The I-phase is stable during charge/discharge cycles, and the main factors for its discharge capacity loss are the increase of the charge-transfer resistance and the pulverization of alloy particles. Besides the factors mentioned above, the formation of TiH₂ and ZrH₂ hydrides is another primary reason for the discharge capacity loss of the amorphous alloy electrode.     

The effects of partial substitution of Cr for Ni on the electrochemical properties of Mg1.75Al0.25Ni1−xCrx (0 ≤ x ≤ 0.3) electrode alloys

In this paper, the substitution of different amounts of Cr for Ni in the hydrogen storage electrode alloy of Mg_1.75Al_0.25Ni has been carried out to form quaternary Mg_1.75Al_0.25Ni_1−xCr_x (0 ≤ x ≤ 0.3) alloys by means of solid diffusion method (DM). The XRD profiles exhibited that the quaternary alloys still kept the same main phase of Mg₃AlNi₂ (S.G. Fd3m) as that of ternary Mg_1.75Al_0.25Ni alloy. The electrochemical studies found that Cr substituted quaternary alloy reached its maximum discharge capacity (165 mAh g⁻¹) after 2 cycles, which was larger than that of the Mg_1.75Al_0.25Ni alloy (154 mAh g⁻¹). Among these quaternary alloys, the Mg_1.75Al_0.25Ni_0.9Cr_0.1 electrode alloy was found possessing the highest cycling capacity retention rate. Cyclic voltammetry (CV) results and anodic polarization curves demonstrated that appropriate content (x lower than 0.1) of Cr effectively improved the reaction activity of electrode and inhibited the cycling capacity degradation to some degree. Electrochemical impedance spectroscopy (EIS) analyses indicated that the increase of Cr content would raise the polarization resistance R_p on the particle surface of these quaternary alloys.     

Effects of Pd substitution on the electrochemical properties of Mg0.9−xTi0.1PdxNi (x = 0.04–0.1) hydrogen storage alloys

Amorphous Mg_0.9−xTi_0.1Pd_xNi (x = 0.04–0.1) hydrogen storage alloys were prepared by mechanical alloying (MA). The effects of Pd substitution on the electrochemical properties of the Mg_0.9−xTi_0.1Pd_xNi (x = 0.04–0.1) electrode alloys were studied by cyclic charge–discharge, linear polarization, anodic polarization, electrochemical impedance spectroscopy (EIS), and hydrogen diffusion coefficient experiments. It was found that the cyclic capacity retention rate C₅₀/C₁ of the quaternary alloys was greatly improved due to the substitution of Pd for Mg. Mg_0.8Ti_0.1Pd_0.1Ni electrode alloy retained the discharge capacity above 200 mAh g⁻¹ even after 80 charge–discharge cycles, possessing the longest cycle life in the studied quaternary alloys. The improvement of cycle life was ascribed to the formation of passive film on the surface of these electrode alloys. X-ray photoelectron spectroscopy (XPS) analysis proved that the passive film was composed of Mg(OH)₂, TiO₂, NiO, and PdO, which synergistically protected the alloy from further oxidation. The Auger Electron Spectroscopy (AES) study revealed that the thickness of passive film increased with augmentation of the Pd content. The electrochemical impedance study of electrode alloys after different cycles demonstrated that the passive film became thicker during cycles and its thickness also increased with Pd content augmentation. It was also found that the augmentation of Pd content resulted in the decrease of exchange current density I₀ and the increase of the charge-transfer resistance R_ct. With increasing the Pd amount in the Mg_0.9−xTi_0.1Pd_xNi (x = 0.04–0.1) electrode alloys, hydrogen diffusion coefficient D was gradually enhanced at first. Then, it decreased with augmentation of cycle due to the growth of passive film on the surface of the alloys.     

A study of calcium zincate as negative electrode materials for secondary batteries

The properties of calcium zincate as negative electrode materials for secondary batteries were examined by powder microelectrode, cyclic voltammetry, charge–discharge cycle measurements and X-ray diffraction (XRD) analysis. The results show that the cycleability of calcium zincate is obviously superior to that of ZnO and that of the mixture of ZnO and Ca(OH)₂ (the molar ratio of Zn:Ca=2:1). Calcium zincate forms zinc metal during the charging and exhibits an initial discharge capacity 230 mAh g⁻¹. With the discharge cut-off voltage of 1.0 V, the discharge capacity of the experimental Zn/NiOOH cell does not decay much during 500 cycles, exhibiting good prospect for practical use.     

Electrochemical characteristics of needle coke refined by molten caustic leaching as an anode material for a lithium-ion battery

Needle coke, the remaining material after refining petroleum, is used as an anode of a lithium-ion secondary battery. Sulfur is separated from the needle coke to below 0.1 wt.% using the molten caustic leaching (MCL) method developed at the Korea Institute of Energy Research. The needle coke with high-purity is carbonized at various temperatures, namely 0, 500, 700 and 900 °C. The coke treated at 700 °C gives a first and second discharge capacity of more than 560 and 460 mAh g⁻¹, respectively, between 0 and 2.0 V. By contrast, the first and second discharge capacity of untreated coke is over 420 and 340 mAh g⁻¹, respectively, between 0.05 and 2.0 V.The first discharge capacity of 560 mAh g⁻¹ is beyond the theoretical maximum capacity of 372 mAh g⁻¹ for LiC₆. Though the cycle efficiency is not consistent, the needle coke heat-treated at 700 °C persistently maintains an efficiency of over 90% until the 50th cycle, except on the first cycle. This study demonstrates that the needle coke with high-purity could be a good candidate for an anode material in fabricating high-capacity lithium-ion secondary batteries.     

Synthesis of olivine-type LiFePO4 by emulsion-drying method

Olivine-type, orthorhombic, LiFePO₄ powders with small particle size have been successfully synthesized by the emulsion-drying method. The electronic and crystal structure is analyzed by X-ray absorption spectroscopy (XAS) and X-ray diffraction Rietveld refinement. The powder calcined at 750 °C shows the highest discharge capacity of 125 mAh g⁻¹ with excellent cycle stability. The discharge capacity of this powder increases to 154 mAh g⁻¹ on increasing the addition of carbon black as a conductive agent up to 40 wt.%. In a rate capability test, the discharge capacity is completely recovered and retained up to the 700th cycle.     

Novel organic–inorganic poly (3,4-ethylenedioxythiophene) based nanohybrid materials for rechargeable lithium batteries and supercapacitors

Synthesis and characterization of poly (3,4-ethylenedioxythiophene) (PEDOT) interleaved between the layers of crystalline oxides of V and Mo is discussed with special emphasis on their application potential as electrodes for rechargeable Li batteries and supercapacitors. The expansion of the interlayer spacing of crystalline oxides (for example, V₂O₅ causes expansion from 0.43 to 1.41 nm) is consistent with a random layer stacking structure. These hybrid nanocomposites when coupled with a large-area Li foil electrode in 1 M LiClO₄ in a mixture of ethylene and dimethylcarbonate (1:1, v/v), give enhanced discharge capacity compared to pristine oxides. For example a discharge capacity of ∼350 mAh g⁻¹, in the potential range 4.2–2.1 V (versus Li⁺/Li) is obtained for PEDOT–V₂O₅ hybrid which is significantly large compared to that for simple Li-intercalated V₂O_5. The improvement of electrochemical performance compared with that of pristine oxides is attributed to higher electric conductivity, enhanced bi-dimensionality and increased structural disorder. Although these conducting polymer-oxide hybrids delivered more than 300 mAh g⁻¹ in the potential range 1.3–4.3 V, their cycle life needs further improvements to realize their commercial potential. Similarly, the double layer capacitance of MoO₃ increases from ∼40 mF g⁻¹ to ∼300 F g⁻¹ after PEDOT incorporation in the interlayer gap of MoO₃ under similar experimental conditions and the nanocomposite displays intriguing effects with respect to electrochemical Li⁺ insertion. The PEDOT–MoO₃ nanocomposite appears to be a promising electrode material for non-aqueous type supercapacitors.     

Multilayered Sn–Zn–Cu alloy thin-film as negative electrodes for advanced lithium-ion batteries

Sn-based alloy compounds have been considered as possible alternatives for carbon in lithium-ion batteries and attract great attentions because of their large electrochemical capacity compared with that of carbon. In this work, a multilayered Sn–Zn/Zn/Cu alloy thin-film electrode has been prepared by electroplating method. The structure and performance of the electrode before and after heat treatment have been investigated. It is found that Cu₆Sn₅ phase and multilayered structure in electrode are formed after heat treatment. This optimized structure of the heat-treated electrode results in enhanced cycle life. The capacity of the electrode is over 320 mA h g⁻¹ after 100 cycles; though it is 83 mA h g⁻¹ after 20 cycles for as-plated electrode. The Sn–Cu and Zn–Cu alloy formed a network in the electrode is considered to strengthen the electrode and reduce the effect of volume expansion and phase transition during cycling. Experimental results also reveal that lower cut-off potential (0.05 V) for charging and higher one (1.2 V) for discharging result in long cycle life and high discharge capacity, respectively. The reason of capacity decay of the heat-treatment electrode during cycling has also been investigated. All these results show that the electroplated Sn–Zn-based alloy film on Cu foil would be a promising negative material with high capacity and low cost for Li secondary batteries.     

Novel architecture of composite electrode for optimization of lithium battery performance

We show that the polymeric binder of the composite electrode may have an important role on the lithium trivanadate Li_1.2V₃O₈ electrode performance. We describe a new tailored polymeric binder combination with controlled polymer–filler (carbon black) interactions that allows the preparation of new and more efficient electrode architecture. Using this polymeric binder, composite electrodes based on Li_1.2V₃O₈ display a room temperature cycling capacity of 280 mAh g⁻¹ (C/5 rate, 3.3–2 V) instead of 150 mAh g⁻¹ using a standard-type (poly(vinylidene fluoride)–hexafluoropropylene (PVdF–HFP) binder) composite electrode. We have coupled scanning electron microscopy (SEM) observations, galvanostatic cycling and electrochemical impedance spectroscopy in order to define and understand the impact of the microstructure of the composite electrode on its electrochemical performance. Derived from these studies, the main key factors that provide efficient charge carrier collection within the composite electrode complex medium are discussed.     

Improvement of electrochemical properties of LiNi0.5Mn1.5O4 spinel

LiNi_0.5Mn_1.5O₄ material with a spinel structure is prepared by a sol–gel method. The material is initially fired at 850 °C and then subjected to a post-reaction annealing at 600 °C in order to minimize the nickel deficiency. The elevated firing temperature produces materials with a small surface-area which is beneficial for good capacity retention. Indeed, the spinel LiNi_0.5Mn_1.5O₄ not only shows a good cycle performance, but exhibits an excellent discharge capacity, i.e. 114 mAh g⁻¹ at 4.66 V plateau and 127 mAh g⁻¹ in total. Cyclic voltammetry and ac impedance spectroscopy are employed to characterize the reactions of lithium insertion and extraction in the LiNi_0.5Mn_1.5O₄ electrode. Excellent electrochemical performance and low material cost make this compound an attractive cathode for advanced lithium batteries.     

Cracking causing cyclic instability of LiFePO4 cathode material

The capacity of pure LiFePO₄ faded gradually from initial 149 mAh g⁻¹–117 mAh g⁻¹ under current density of 30 mA g⁻¹ at room temperature after 60 cycles. Some obvious cracks are observed in LiFePO₄ particles after cycling. The formation of cracks would lead to poor electric contact and capacity fading. A possible mechanism is proposed for the appearance of the cracks.     

Synthesis and characterization of spinel Li4Ti5O12 anode material by oxalic acid-assisted sol–gel method

Anode material Li₄Ti₅O₁₂ for lithium-ion batteries has been prepared by a novel sol–gel method with oxalic acid as a chelating agent and Li₂CO₃ and tetrabutyl titanate [Ti(OC₄H₉)₄] as starting materials. Various initial conditions were studied in order to find the optimal conditions for the synthesis of Li₄Ti₅O₁₂. Oxalic acid used in this method functioned as a fuel, decomposed the metal complexes at low temperature and yielded the free impurity Li₄Ti₅O₁₂ compounds. Thermal analyses (TG–DTA) and XRD data show that powders grown with a spinel structure (Fd3m space group) have been obtained at 800 °C for 16 h. SEM analyses indicated that the prepared Li₄Ti₅O₁₂ powders had a uniform cubic morphology with average particle size of 200 nm. The influence of synthesis conditions on the electrochemical properties was investigated and discussed. The discharge capacity of Li₄Ti₅O₁₂ synthesized with an oxalic acid to titanium ratio R = 1.0 was 171 mAh g⁻¹ in the first cycle and 150 mAh g⁻¹ after 35 cycles under an optimal synthesis condition at 800 °C for 20 h. The very flat discharge and charge curves indicated that the electrochemical reaction based on Ti⁴⁺/Ti³⁺redox couple was a typical two-phase reaction.     

Effect of poly(acrylic acid) on adhesion strength and electrochemical performance of natural graphite negative electrode for lithium-ion batteries

High-capacity natural graphite negative electrodes for use in prismatic lithium-ion batteries are fabricated from an aqueous suspension precursor. The effects of poly(acrylic acid) (PAA) on suspension stability and the resulting mechanical properties of the electrodes are investigated. Precursor suspensions consisting of graphite particles, sodium carboxymethyl cellulose (CMC), emulsified styrene-butadiene (SB) copolymer latex and PAA are prepared in an aqueous medium and tape-cast on to a copper foil. The addition of PAA enhances the stability of the suspension at low shear rates without compromising the solvent-thickening effect of CMC. Peel test results showed that the adhesion strength of the graphite electrode on the copper substrate is significantly improved by PAA. Graphite negative electrodes fabricated using PAA are characterized by gravimetric and volumetric energy densities of more than 340 mAh g⁻¹ and 560 mAh cm⁻³, respectively. The PAA formulation also leads to improved cycle life, with a discharge capacity exceeding 90% of initial capacity after 500 cycles.     

Properties of containing Sn nanoparticles activated carbon fiber for a negative electrode in lithium batteries

Activated carbon fiber (ACF) containing Sn nanoparticles were prepared by impregnation and were investigated as a negative electrode material in lithium batteries. The tin particle size was controlled by selecting an ACF with an adequate surface structure. This Sn/ACF composite cycled versus Li metal showed a first discharge capacity as high as 200 mAh g⁻¹ compared to the pristine ACF which showed only 87 mAh g⁻¹. Excellent cyclability with these composites was obtained with ACF BET SSA as large as 2000 m² g⁻¹ and 30 wt.% Sn.     

Electrochemical properties of the Li-ion polymer batteries with P(VdF-co-HFP)-based gel polymer electrolyte

Gel polymer electrolytes consisting of 25 wt.% P(VdF-co-HFP), 65 wt.% ethylene carbonate + propylene carbonate and 10 wt.% LiN(CF₃SO₂)₂ are prepared using by a solvent-casting technique. The electrodes are for use in lithium-ion polymer batteries. The electrochemical characteristics of the gel polymer electrolytes are evaluated by means of ac impedance and cyclic voltammetry. The charge–discharge performance of lithium polymer and lithium-ion polymer batteries is examined. A LiCoO₂ | gel polymer electrolyte (GPE) | mesocarbon microbeads (MCMB) cell delivers a discharge capacity of 146.8 and 144.5 mAh g⁻¹ on the first and the 20th cycle, respectively. The specific discharge capacity is greater than 140 mAh g⁻¹ for up to 20 cycle at all the current densities examined.     

Tin–graphite materials prepared by reduction of SnCl4 in organic medium: Synthesis,characterization and electrochemical lithiation

Tin–graphite materials were prepared by chemical reduction of SnCl₄ by t-BuONa-activated NaH. TEM imaging showed that the crude material is composed of an amorphous organic matrix containing tin present either as nanosized particles deposited on the graphite surface or as free aggregates. Subsequent washings with ethanol and water allow removal of side products as well as most part of the organic matrix. Electrochemical insertion of lithium occurred in graphite and in tin. The initial reversible massic capacity of 630 mAh g⁻¹ decayed to a stable value of 415 mAh g⁻¹ after 12 cycles. This capacity value was lower than the expected maximum one of 650 mAh g⁻¹ corresponding to a Sn/12C molar composition and assuming the formation of LiC₆ and Li₂₂Sn₅. Even if this massic capacity is not much improved by comparison with that of graphite, it must be pointed out that the volume capacity of this graphite/Sn material is much larger (2137 mAh cm⁻³) than that corresponding to graphite (837 mAh cm⁻³). It was hypothesized that the part of tin bound to graphite could be responsible for the stable reversible capacity. To the contrary, graphite unsupported tin aggregates would contribute to the observed gradual decline in the storage capacity. Therefore, the improvement in cycleability, compared to that of massive metals, could be attributed both to the nanoscale dimension of the metal particles and to interactions between graphite and metal the nature of which remaining to be precised.     

Preparation and characterization of high-density spherical Li4Ti5O12 anode material for lithium secondary batteries

Li₄Ti₅O₁₂ is a very promising anode material for lithium secondary batteries. A novel technique has been developed to prepare Li₄Ti₅O₁₂. The spherical precursor is prepared via an “inner gel” method by TiCl₄ as the raw material. Spherical Li₄Ti₅O₁₂ powders are synthesized by sintering the mixture of spherical precursor and Li₂CO₃. The investigation of XRD, SEM and the determination of the electrochemical properties show that the Li₄Ti₅O₁₂ powders prepared by this method are spherical, and have high tap-density and excellent electrochemical performance. It is tested that the tap-density of the product is as high as 1.64 g cm⁻³, which is remarkably higher than the non spherical Li₄Ti₅O₁₂. Between 1.0 and 3.0 V versus Li, a reversible capacity is as high as 161 mAh g⁻¹ at a current density of 0.08 mA cm⁻².     

Electrode properties of melt-spun Mg–Ni–Nd amorphous alloys

Amorphous and nanocrystalline Mg-based alloys (Mg₆₀Ni₂₅)_100−xNd_x (x = 2, 5, 10, 15) were prepared by rapid solidification. The microstructure of the as-quenched ribbons was characterized by TEM, X-ray and electron diffraction. The electrode properties of these alloys were measured. The experimental results showed that the discharge capacities increased with increasing Nd atomic content and the optimum Nd content is between in 10–15 mol%. The highest discharge capacity reached more than 580 mAh g⁻¹ at the discharge current densities of 50 mA g⁻¹ for (Mg₆₀Ni₂₅)₉₀Nd₁₀ samples. The nanocrystalline (Mg₆₀Ni₂₅)₉₈Nd₂ alloy showed the lowest discharge capacity compared to the other Mg–Ni–Nd amorphous alloys. The alloy (Mg₆₀Ni₂₅)₉₀Nd₁₀ displays the broadest discharge potential flat from 1.3 to 1.0 V. It was conformed that the increase of discharge capacities is not only a function of the sample composition but strongly influenced by the amorphous phase proportion in the alloys.     

Carbon anode for dry-polymer electrolyte lithium batteries

A spherical carbon material of meso-carbon microbead (MCMB) was examined as an anode in a polyethylene oxide (PEO) based polymer electrolyte lithium battery. The electrochemical performance of the carbon electrode with the polymer electrolyte depended on the electrode thickness and the particle size of MCMB. The 30 μm-thick electrode of MCMB with the particle size of 20–30 μm showed a reversible capacity comparable with that in a liquid electrolyte, but the 100 μm-thick electrode showed a half of the 30 μm-thick electrode. The smaller particle size of 5–8 μm exhibited a high irreversible capacity at the first charge–discharge cycle. The reaction heat between MCMB and the polymer electrolyte was 0.5 J mAh^?1, which was much lower compared to those between lithium metal and the polymer electrolyte, 1.2 J mAh^?1, and MCMB and conventional liquid electrolyte, 4.3 J mAh^?1.