IkHyun Kwon

Synthesis by sol–gel method and electrochemical properties of LiNi1−yAlyO2 cathode materials for lithium secondary battery

Abstract Single-phase LiNi 1− y Al y O 2 ( y =0, 0.05, 0.1, 0.2, 0.25 and 0.3) powders with α-NaFeO 2 structure are synthesized by sol–gel method using citric acid as a chelating agent, and LiNO 3 , Ni(NO 3 ) 2 ·6H 2 O and Al(NO 3 ) 3 ·9H 2 O as starting materials. Calcination is carried out at 800 °C for 13 h in oxygen stream after preheating at 600 °c for 5 h in air. The TGA and DTA curves show that the addition of Al decreases the weight loss rate and increases the temperature for the combustion of the residual organic matter and the formation of the LiNiO 2 phase from the decomposition of Li 2 CO 3 and NiO phases. The LiNiO 2 particles are roughly in the form of smoothly edged polyhedra. As y increases, the particles become smaller. The LiNiO 2 electrode has the largest first discharge capacity 168 mA h/g, and the discharge capacity 155 mA h/g at the 20th cycle. The shifting of the voltage range for intercalation and deintercalation to the higher voltage side leads to the decrease in the first discharge capacity of LiNi 1− y Al y O 2 in the fixed voltage range 3.0–4.2 V as y increases. The larger Δ x and the occurrence of phase transitions at more times for y =0 are considered to bring about larger capacity fading with cycling. The smaller Δ x and the occurrence of phase transitions at fewer times lead probably to the smaller capacity fading with cycling. Among the LiNi 1− y Al y O 2 ( y =0, 0.05, 0.1, 0.2, 0.25 and 0.3) samples, LiNi 0.95 Al 0.05 O 2 has the best electrochemical properties with a relatively large first discharge capacity 146 mA h/g and an excellent cycling performance.

Development of AB2-type Zr–Ti–Mn–V–Ni–Fe hydride electrodes for Ni–MH secondary batteries

Abstract A series of multicomponent Zr 0.5 Ti 0.5 Mn 0.4 V 0.6 Ni 1− x Fe x ( x =0.00, 0.15, 0.30, 0.45 and 0.60) alloys are prepared and their crystal structure and P – C – T curves are examined. The electrochemical properties of these alloys such as discharge capacity, cycling performance and rate capability are also investigated. Zr 0.5 Ti 0.5 Mn 0.4 V 0.6 Ni 1− x Fe x alloys ( x =0.00, 0.15, 0.30, 0.45 and 0.60) have the C14 hexagonal Laves phase structure. Their hydrogen storage capacities do not show significant differences. The discharge capacity just after activation decreases with the increase in the amount of the substituted Fe but the cycling performance is improved. The discharge capacity after activation of the alloy with x =0.00 is about 240 mAh/g at the current density 60 mA/g. Zr 0.5 Ti 0.5 Mn 0.4 V 0.6 Ni 0.85 Fe 0.15 is the best composition with a relatively large discharge capacity and a good cycling performance. The increase in the discharge capacity of Zr 0.5 Ti 0.5 Mn 0.4 V 0.6 Ni 0.85 Fe 0.15 with the increase in the current density (from 60 to 125 mA/g) is considered to result from the self-discharge property of the electrode. During activation Ni-rich regions form on the electrode surface, which may act as active sites for the electrochemical reaction. The formation of more minute cracks in the large particles of the alloys with higher Fe content is considered to result from the more severe destruction of the crystal structure due to more dissolution of zirconium and iron into the solution.

Electrochemical properties of LiCoyMn2−yO4 synthesized by the combustion method for lithium secondary battery

Abstract LiCo y Mn 2− y O 4 ( y =0.00, 0.04 and 0.08) were synthesized by the combustion method and the electrochemical properties are examined. The sample for y =0.00 has the largest first discharge capacity (125.3 mA h/g). The cycling performance improves as y increases. The sample with y =0.08 has the first discharge capacity 118.8 mA h/g and the discharge capacity at the 100th cycle 95.0 mA h/g. The decrease in the length of the higher-voltage plateau (around 4.07 V) with cycling has a strong influence on the decrease in the cycling performance. The better crystallinity, the larger lattice parameter, and the finer and more homogeneous particles of LiCo y Mn 2− y O 4 by the combustion method, compared with those for LiMn 2 O 4 by the solid-state method, are considered to lead to the larger discharge capacities.

Improvement in the electrochemical properties of ZrMn2 hydrides by substitution of elements

The hydrogen-storage properties and the electrochemical properties are investigated for the alloys ZrMn2Nix, ZrMnNi1+x, Zr0.5Ti0.5Mn0.4V0.6Ni1−xFex and Zr0.5Ti0.5Mn0.4V0.6Ni0.85M0.15. The C14 Laves phase forms in all the alloys ZrMn2Nix (x=0.0, 0.3, 0.6, 0.9 and 1.2). Among the alloys ZrMn2Nix, ZrMn2Ni0.6 has the largest discharge capacity (29 mAh/g) and a relatively good cycling performance, and shows a relatively easy activation. The C14 Laves phase also forms in all the alloys ZrMnNi1+x (x=0.0, 0.1, 0.2, 0.3 and 0.4). Among the alloys ZrMnNi1+x, ZrMnNi1.0 has the largest discharge capacity (42 mAh/g) and a relatively good cycling performance, and shows the easiest activation. Zr0.5Ti0.5Mn0.4V0.6Ni1−xFex (x=0.00, 0.15, 0.30, 0.45 and 0.60) has the C14 Laves phase hexagonal structure. Their hydrogen storage capacities do not show significant differences. The discharge capacity just after activation decreases with an increase in the amount of the substituted Fe but the cycling performance is improved. The discharge capacity after activation of the alloy with x=0.00 is about 240 mAh/g at the current density 60 mA/g. Zr0.5Ti0.5Mn0.4V0.6Ni0.85Fe0.15 is the best composition with a relatively large discharge capacity and a good cycling performance. The increase in the discharge capacity of Zr0.5Ti0.5Mn0.4V0.6Ni0.85Fe0.15 with the increase in the current density (from 60 mA/g to 125 mA/g) is considered to result from the self-discharge property of the electrode. Zr0.5Ti0.5Mn0.4V0.6Ni0.85M0.15 (M=Fe, Co, Cu, Mo and Al) alloys also have the C14 Laves phase hexagonal structure. The alloys with M=Co and Fe have relatively larger hydrogen storage capacities. The discharge capacities just after activation are relatively large in the case of the alloys with M=Co and Fe. The Zr0.5Ti0.5Mn0.4V0.6Ni0.85Co0.15 alloy is best with a relatively large discharge capacity (257 mAh/g at the current density 250 mA/g for the 12th cycle) and a good cycling performance. During activation form Ni-rich and Fe-rich regions on the surface of the Zr0.5Ti0.5Mn0.4V0.6 Ni0.85Fe0.15 alloy. They may act as the active sites for the electrochemical reaction. With the increase in the number of charge-discharge cycles for the Zr0.5Ti0.5Mn0.4V0.6Ni0.85Fe0.15 alloy, the quantities of the Zr and Fe dissolved in the electrolyte solution increase.

Influences on the H2-sorption properties of Mg of Co (with various sizes) and CoO addition by reactive grinding and their thermodynamic stabilities

We attempted to improve the H2-sorption properties of Mg by mechanical grinding under H2 (reactive grinding) with Co (with various particle sizes) and with CoO. The thermodynamic stabilities of the added Co and CoO were also investigated. CoO addition has the best influence and addition of smaller particles of Co (0.5–1.5 μm) has a better effect than the addition of larger particles of Co on the H2-sorption properties of Mg. The activated Mg+10 wt.% CoO sample has about 5.54 wt% hydrogen-storage capacity at 598 K and the highest hydriding rate, showing an Ha value of 2.39 wt.% after 60 min at 598 K, 11.2 bar H2. The order of the hydriding rates after activation is the same as that of the specific surface areas of the samples. The reactive grinding of Mg with Co or CoO and hydriding-dehydriding cycling increase the H2-sorption rates by facilitating nucleation of magnesium hydride or α solid solution of Mg and H (by creating defects on the surface of the Mg particles and by the additive), and by making cracks on the surface of Mg particles and reducing the particle size of Mg, thus shortening the diffusion distances of hydrogen atoms. The cobalt oxide is stable even after 14 hydriding cycles at 598 K under 11.2 bar H2. Discharge capacities are measured for the sampple Mg+10 wt.%CoO and Mg+10wt.%Co (0.5−1.5 μm) with good hydrogen-storage properties.

Electrochemical properties of ZrMnNi1+x hydrogen-storage alloys

Abstract In order to improve the performance of the AB2-type hydrogen-storage alloy for nickel–metal hydride Ni–MH secondary battery, AB2-type ZrMnNi1+x hydrogen-storage alloys were prepared and their electrochemical properties were investigated. The C14 Laves phase formed in all the alloys ZrMnNi1+x (x=0.0–0.4). The lattice parameters a and c, and the volumes of the unit cell for the alloys tend to decrease as the value of x increases. Among these alloys ZrMnNi1.0 was activated after three charge–discharge cycles and had the largest discharge capacity of 42 mAh/g . For all the alloys Zr was dissolved most, on the whole, into the 6 M KOH solution. For the ZrMnNi1.0 alloy Mn was dissolved much more into the 6 M KOH solution, and Ni was dissolved more, as compared with the other alloys. These may be related with the largest discharge capacity of the ZrMnNi1.0 alloy. The absorption and desorption of hydrogen in extended ranges y for ZrMnNi1.0Hy may lead to the greater dissolution of Zr, Mn and Ni. Among the alloys ZrMnNi1+x (x=0.0–0.4), ZrMnNi1.0 had the largest discharge capacity and a relatively good cycling performance, and it showed the easiest activation. The added or substituted Ni acted as the catalyst for the electrochemical reaction.

Discharge Capacity Fading of LiCo y Mn 2-y O₄ with Cycling

LiCo y Mn 2-y O₄ samples were synthesized by calcining a mixture of LiOH·H₂O, MnO₂(CMD) and CoCO₃ calcining at 400℃ for 10 h and then calcining twice at 750℃ for 24 h in air with intermediate grinding. All the synthesized samples exhibited XRD patterns for the cubic spinel phase with a space group Fd3m. The electrochemical cells were charged and discharged for 30 cycles at a current density 600 μA/㎠ between 3.5 and 4.3 V. As the value of y increases, the size of particles becomes more homogeneous. The first discharge capacity decreases as the value of y increases, its value for y=0.00 being 92.8 mAh/g. The LiMn₂O₄ exhibits much better cycling performance than that reported earlier. The cycling performance increases as the value of y increases. The efficiency of discharge capacity is 98.9% for y=0.30. The larger lattice parameter for the smaller value of y is related to the larger discharge capacity. The more quantity of the intercalated and the deintercalated Li in the sample with the larger discharge capacity brings about the larger capacity fading rate.