Hye Ryoung Park

Capacity fading of spinel phase LiMn2O4 with cycling

Abstract The capacity fading with cycling between 3.5 V and 4.3 V of the spinel phase LiMn2O4 reveals the decrease in the lengths of two voltage plateaus at 4.15 V and 4.0 V in the V vs. x curves, corresponding to the two-phase reaction and the one-phase reaction, respectively. These reactions are also shown by the oxidation and reduction peaks in the cyclic voltammogram. As the number of cycle increases, the regions of the two-phase reaction and the one-phase reaction in the V vs. x curves decrease simultaneously. The lattice destruction induced by strain causes the capacity fading of LiMn2O4 with cycling. The expansion and contraction of spinel phase LiMn2O4 due to the intercalation and deintercalation make the unit cell strained and distorted. With cycling, the interstitial sites and thus the spinel structure will be destroyed. This decreases the fraction of the spinel phase, leading to the capacity fading of LiMn2O4 with cycling. Vacancy of interstitial sites by the deintercalation of Li ions and the attraction of oxygen with the remaining Li ions make shorter the lattice parameters of the cubic structure phases as the deintercalation proceeds.

Cycling performance of LiNi1−yMyO2 (M=Ni, Ga, Al and/or Ti) synthesized by wet milling and solid-state method

The LiNi1−yMyO2 specimens with compositions of LiNiO2, LiNi0.975Ga0.025O2, LiNi0.975Al0.025O2, LiNi0.995Ti0.005O2, and LiNi0.990Al0.005Ti0.005O2 were synthesized by wet milling and a solid-state reaction method. Among all the specimens, LiNi0.990Al0.005Ti0.005O2 has the largest first discharge capacity of 196.3 mAh/g at a rate of 0.1 C. At n=50, LiNiO2 has the largest discharge capacity of 126.7 mAh/g. LiNiO2 has the best cycling performance, its degradation rate of discharge capacity being 0.73 mAh/g/cycle. LiNi0.975Al0.025O2 shows the lowest decrease rate of the first discharge capacity with C rate. An equation describing the variation of the discharge capacity with the number of charge-discharge cycles, n, is obtained. The Williamson-Hall method is applied to calculate the crystallite size and the strain of the samples before and after charge-discharge cycling.

Variations in the electrochemical properties of metallic elements-substituted LiNiO2 cathodes with preparation and cathode fabrication conditions

Variations in the electrochemical properties of LiNiO2 with preparation and cathode fabrication conditions were studied. The LiNiO2 cathode fabricated with a weight ratio of LiNiO2: acetylene black: binder = 85:10:5, after wet Spex milling for 60 min and drying in a shaking incubator showed the best cycling performance, with a discharge capacity degradation rate of 1.06 mAh/g/cycle between the first cycle and the 20th cycle and a discharge capacity at n = 20 of 143.5 mAh/g at 0.1 C rate. LiNi1−yMyO2 cathodes with active material compositions of LiNiO2, LiNi0.975Ga0.025O2, LiNi0.975Al0.025O2, LiNi0.995Ti0.005O2, and LiNi0.990Al0.005Ti0.005O2 were fabricated under these conditions. Among all the samples, LiNi0.990Al0.005Ti0.005O2 had the highest first discharge capacities at 0.1 C rate (196.3 mAh/g) and 0.5 C rate (147.3 mAh/g). This sample had the smallest R-factor value, indicating that it had the lowest degree of cation mixing. At 0.1 C rate LiNiO2 had the best cycling performance. At 0.5 C rate LiNi0.975Al0.025O2 had the best cycling performance and its discharge capacity degradation rate was 1.28 mAh/g/cycle between the first cycle and the 20th cycle.

Hydrogen storage properties of a Ni, Fe and Ti-added Mg-based alloy

Mg-5wt%Ni-2.5wt%Fe-2.5wt%Ti (referred to as Mg-5Ni-2.5Fe-2.5Ti) hydrogen storage material was prepared by reactive mechanical grinding, after which the hydrogen absorption and desorption kinetics were investigated using a Sievert-type volumetric apparatus. A nanocrystalline Mg-5Ni-2.5Fe-2.5Ti sample was prepared by reactive mechanical grinding and hydriding-dehydriding cycling. Analysis by the Williamson-Hall method from an XRD pattern of this sample after 10 hydriding-dehydriding cycles showed that the crystallite size of Mg was 37.0 nm and that its strain was 0.0407%. The activation of Mg-5Ni-2.5Fe-2.5Ti was completed after three hydriding-dehydriding cycles. The prepared Mg-5Ni-2.5Fe-2.5Ti sample had an effective hydrogen-storage capacity near 5 wt% H. The activated Mg-5Ni-2.5Fe-2.5Ti sample absorbed 4.37 and 4.90 wt% H for 5 and 60 min, respectively, at 593K under 12 bar H2, and desorbed 1.69, 3.81, and 4.85 wt% H for 5, 10 and 60 min, respectively, at 593K under 1.0 bar H2.

Characterization of a magnesium-based alloy after hydriding-dehydriding cycling (n=1–150)

The cycling performance of Mg-15 wt% Ni-5 wt% Fe2O3 alloy (named Mg-15Ni-5Fe2O3) was investigated by measuring the absorbed hydrogen quantity as a function of the number of cycles and by examining the variations in the phases and microstructures with cycling. The sample was hydriding-dehydriding cycled 150 times. The absorbed hydrogen quantity decreased as the number of cycles increased from the second to the 150th cycle. The Ha value varied almost linearly with the number of cycles. The maintainability of the absorbed hydrogen quantity was 73.8%, and the degradation rate was 0.007 wt%/cycle for the hydriding reaction time of 60 min. After the 9th hydriding-dehydriding cycle, Mg, Mg2Ni, MgO, and Fe were observed. After 150 cycles, the quantity of the MgO increased. The phases were analyzed using MDI JADE 6.5, a software system designed for XRD powder pattern processing, from the XRD pattern of the Mg-15Ni-5Fe2O3 alloy after the 9th hydriding-dehydriding cycle. The crystallite size and strain of the Mg were then estimated using the Williamson-Hall technique.

Improvement of hydrogen-storage properties of MgH2 by addition of Ni and Ti via reactive mechanical grinding and a rate-controlling step in its dehydriding reaction

In a shift from prior work, MgH2, instead of Mg, was used as a starting material in this work. A sample with a composition of 86 wt% MgH2-10 wt% Ni-4 wt% Ti was prepared by reactive mechanical grinding. Activation of the sample was completed after the first hydriding cycle. The effects of reactive mechanical grinding of Mg with Ni and Ti were discussed. The formation of Mg2Ni increased the hydriding and dehydriding rates of the sample. The addition of Ti increased the hydriding rate and greatly increased the dehydriding rate of the sample. The titanium hydride, TiH1.924, was formed during reactive mechanical grinding. This titanium hydride, which is brittle, is thought to help the mixture pulverized by being pulverized during reactive mechanical grinding and further to prevent agglomeration of the magnesium by staying as a hydride among Mg particles. A rate-controlling step for the dehydriding reaction of the hydrided MgH2-10Ni-4Ti was analyzed by using a spherical moving boundary model on an assumption that particles have a spherical shape with a uniform diameter.

In situ preparation of powder and the sorption behaviors of molecularly imprinted polymers through the complexation between polymer ion of methyl methacrylate/acrylic acid and Ca++ ion.

Molecularly imprinted polymer (MIP) powders were prepared using a simple complexation strategy between the polymer carboxylate groups and template molecule followed by metal cation cross-linking of residual polymer carboxylates. Polymer powders were formed in situ by templating carboxylic acid containing polymers with 4-ethylaniline (4-EA), followed by addition of an aqueous CaCl2 solution. The solution remained homogeneous. The powders were prepared by precipitation by slowly adding a non-solvent, H2O, to the mixture. The resulting particles were very porous with uptake capacity that approached the theoretical value. We suggest two types of complexes are formed between the template, 4-EA, and polymer. The isolated entry type forms well defined cavities for the template with high specific selectivity, while the adjacent entry type forms wider binding sites without specific sorption for isomeric molecules. To evaluate conditions for forming materials with high affinity and selectivity, three MIPs were prepared containing 0.5, 1.0, and 1.5 equivalents of template to the base polymer. The MIP containing 0.5 eq showed higher specific selectivity to 4-EA, but the MIP containing 1.5 eq had noticeably lower selectivity. The lower selectivity is attributed to poorly formed binding sites with little selective sorption to any isomer when the higher ratio of template was used. However at the lower ratio of template the isolated entry is preferably formed to produce well defined binding cavities with higher selectivity to template.

Verification of the rate-controlling steps in the hydriding reaction of Mg2Ni

The rate-controlling steps in the hydriding reaction of Mg2Ni are verified by comparing the incubation periods in the hydriding reaction of the Mg2Ni alloy and a mechanically-alloyed mixture of 2Mg and Ni, by using gas mixtures of hydrogen and argon, and by varying the sample weight in the hydriding reaction.

Hydriding and dehydriding rates of Mg, Mg-10TaF5, and Mg-10NbF5 prepared via reactive mechanical grinding

In this work, TaF5 and NbF5 were chosen as additives to enhance the hydriding and dehydriding rates of Mg. Mg, Mg-10TaF5, and Mg-10NbF5 samples were prepared by reactive mechanical grinding. The hydriding and dehydriding properties of the samples were then examined. Mg-10TaF5 had the largest amount of hydrogen absorbed for 30 min and the highest initial dehydriding rate after incubation period, followed in order by Mg-10NbF5, and Mg. At 593 K under 12 bar H2 at the first cycle, Mg-10TaF5 absorbed 3.63 wt% H for 5 min and 4.53 wt% H for 30 min. At 593 K under 1.0 bar H2 at the first cycle, Mg-10TaF5 desorbed 0 wt% H for 2.5 min, 0.59 wt% H for 5 min, 3.42 wt% H for 30 min, and 4.24 wt% H for 60 min. The reactive mechanical grinding of Mg with TaF5 or NbF5 is believed to have facilitated the nucleation and to have decreased the diffusion distances of hydrogen atoms. These two effects are believed to have increased the hydriding and dehydriding rates of Mg. The MgF2 and Ta2H formed in Mg-10TaF5, and the MgF2, NbH2, and NbF3 formed in Mg-10NbF5 are considered to have enhanced both of these effects.

Phase transformations and hydrogen-storage characteristics of Mg-transition metal-oxide alloys

Samples with the compositions of 76.5 wt%Mg-23.5 wt%Ni (Mg-Ni), 71.5 wt%Mg-23.5 wt%Ni-5 wt% Fe2O3 (Mg-Ni-Fe2O3) and 71.5 wt%Mg-23.5 wt%Ni-5 wt% Fe2O3 (spray conversion) (Mg-Ni-scFe2O3), 71.5 wt%Mg-23.5 wt%Ni-5 wt% Fe (Mg-Ni-Fe) and 80 wt%Mg-13.33 wt%Ni-6.67 wt%Fe (Mg-13Ni-7Fe) were prepared by reactive mechanical grinding. Mg-13Ni-7Fe has the highest hydriding and dehydriding rates. After hydriding-dehydriding cycling, all the samples contain the Mg2Ni phase. The samples with Fe2O3 and Fe2O3(spray conversion) as starting materials contain the Mg(OH)2 phase after hydriding-dehydriding cycling as well as after reactive mechanical grinding. Mg-Ni-Fe and Mg-13Ni-7Fe contain the MgH2 phase after reactive mechanical grinding. Phases, space groups, cell parameters, contents and crystallite sizes were analyzed by Full Pattern Matching Refinement program, one of the Rietveld analysis programs, from the XRD powder patterns of Mg-Ni-scFe2O3 after reactive mechanical grinding and after hydriding-dehydriding cycling. The MgH2 phase formed in the Mg-Ni-Fe and Mg-13Ni-7Fe mixtures after reactive mechanical grinding is considered to help the pulverization of the materials during reactive mechanical grinding, leading to the high hydriding and dehydriding rates of these mixtures.