Shin Fujitani

Study of LiFePO4 by Cyclic Voltammetry

A systematic study of LiFePO 4 with cyclic voltammetry (CV) was conducted using thin electrodes with a loading of 4 mg/cm 2 . Peak current of the CV profile was proportional to the square root of scan rate under 0.2 mV/s. Results were analyzed using a reversible reaction model with a resistive behavior. This resistance was consistent with other resistances obtained from electrochemical impedance spectroscopy and charge-discharge curves. Apparent Li diffusion constants of 2.2 ×10 -14 and 1.4 X 10 -14 cm 2 /s were obtained at 25°C for charging and discharging LiFePO 4 electrodes in 1 M LiPF 6 ethylene carbonate/diethyl carbonate=3:7 by volume, respectively. Activation energies of the apparent diffusion constants and electrode resistance are about 0.4 eV. These parameters are good indicators for assessing the effectiveness of material modifications such as surface coating and doping.

Advanced Structures in Electrodeposited Tin Base Negative Electrodes for Lithium Secondary Batteries

Tin anodes deposited electrochemically on a copper foil current collector are studied to develop a next-generation lithium-ion battery with higher energy density. Better cycle performance through ten initial cycles under full charge and discharge conditions was attained by annealing tin electrodeposited on a rough surface copper foil. The annealing process was found to change the main active material from Sn to Cu 6 Sn 5 with some minor compounds. Furthermore, a microcolumnar structure of the active material portion was found to he self-organized in accordance with the surface profile of the foil during the first charge-discharge cycle. Advantages of these structural features are discussed in terms of the initial charge and discharge performance. including specific capacity and coulombic efficiency measured by using a three-electrode cell.

Microstructures and hydrogen absorption/desorption properties of LaNi alloys in the composition range of La77.8 ∼ 83.2 at.%Ni

Abstract Microstructure and hydrogen absorption/desorption properties of La Ni alloys have been investigated as a function of alloy composition in the range of La 77.8 ∼ 83.2 at.%Ni, which corresponds to compositions between two intermetallic phases, La 2 Ni 7 and LaNi 5 . The intermetallic phase, La 5 Ni 19 of the Ce 5 Co 19 -type is found for the first time to exist as an equilibrium phase at a composition between La 2 Ni 7 and LaNi 5 . This phase is stable at high temperatures around 1000°C but decomposes into La 2 Ni 7 and LaNi 5 below 900°C. Hydrogen absorption/desorption properties described in terms of pressure-composition isotherms decline with decreasing Ni content (i.e. with increasing volume fraction of intermetallic phases other than LaNi 5 ). In particular, the plateau at the equilibrium pressure corresponding to the hydrogen absorption in the LaNi 5 phase is narrowed with decreasing Ni content and additional plateaus with higher equilibrium pressures come into existence. The degradation becomes more pronounced in the presence of La 2 Ni 7 than La 5 Ni 19 . This can be understood in terms of the ratio of the number of LaNi 2 (Laves) unit layers to that of LaNi 5 unit layers in the unit cell of the two intermetallic phases.

Effect of Electrode Parameters on LiFePO4 Cathodes

LiFePO 4 electrodes with thicknesses from 15 to 120 μm were coated on Al current collectors. The electrochemical characteristics of these electrodes depend strongly on film thickness, with the largest rate capability for the thinnest film-a 15-μm electrode can be discharged at a current rate of 25 C and still give a capacity of 70 mAh/g. This shows great promise for high-power applications such as hybrid electrical vehicles. Increasing the amount of carbon in the electrode, decreasing the packing density, or using an electrolyte with lower viscosity and higher ionic conductivity improved the rate performance. This suggests that the thickness effect is caused by a larger electrode resistance and a slower Li-ion conduction through the electrolyte for thicker films. Electrode thickness in turn affects the energy density of a battery, because the percentage of inactive materials increases with decreasing film thickness. An energy density prospect for a 18650-type battery with these LiFePO 4 electrodes gives a maximum capacity of 1050 mAh at 1-C rate for a 60-μm electrode. This corresponds to a volumetric and gravimetric energy density of 214 Wh/L and 96.5 Wh/kg, respectively. The effective Li diffusivity in the active material is estimated to be of the order of 10 -13 cm 2 /s.

Homogenizing behaviour in a hydrogen-absorbing LaNi4.55Al0.45 alloy through annealing and rapid quenching

Abstract Homogenizing behaviour in a hydrogen-absorbing alloy with a composition of LaNi 4.55 Al 0.45 through annealing and rapid quenching was investigated to improve its equilibrium characteristics with hydrogen. Annealing has an effect on homogenizing the Al distribution in the dominant phase of CaCu 5 structure and decreasing the plateau slope in the P - C isotherms. The rapidly quenched alloy exhibited a flatter but narrower plateau region than the induction-melted and annealed alloy, which became wider after short-time annealing for 7.2 ks at 1323 K. The effect of the annealing in the rapidly quenched alloy was attributed to a decrease in lattice defects introduced by the rapid quenching process.

Impurities in LiFePO4 and Their Influence on Material Characteristics

Li 3 PO 4 impurity was found in hydrothermally grown LiFePO 4 samples made with excess Li. The impurity dissociates in water, leading to an apparent alkaline nature of the LiFePO 4 sample. The impurity can be removed by washing the sample in a neutral buffer solution, after which an increase in specific capacity of the LiFePO 4 sample was observed. Li 3 PO 4 , as an inactive component within the active material, reduces the energy density of LiFePO 4 . Additional experiments were performed to study the reactivity of LiFePO 4 in different environments. It is found that LiFePO 4 decomposes in a strong alkaline solution and the decomposition can be slowed down by carbon coating the material. After prolonged storage in water, the specific capacity of carbon-coated LiFePO 4 is reduced and Fe dissolution is observed. Material degradation is thought to be due to interactions among LiFePO 4 , impurities, and water.

Effect of additives in zinc alloy powder on suppressing hydrogen evolution

Abstract The addition of 0.025 wt.% bismuth and 0.025 wt.% lead to zinc particles modified with 0.10 wt.% indium by a dry-coating process, or a conventional wet-coating process is examined to clarify the effect on suppressing hydrogen gas evolution due to the self-discharging reactions of zinc in alkaline manganese batteries. The dry-coating process of indium metal modifies the zinc alloy powder, such that oxidation of the powder is less, and hydrogen-gas evolution is suppressed more effectively than in the case of the conventional wet-coating process. In the dry-coating process, the bismuth diffuses into the surface to be alloyed with the indium. As a result, zinc alloy powder containing 0.025 wt.% bismuth modified with 0.10 wt.% indium (Zn–In–Bi) by the dry-coating process suppresses hydrogen gas evolution on a competitive level with zinc powder containing 0.15 wt.% mercury (Zn–Hg).

Study on Sn–Co Alloy Anodes for Lithium Secondary Batteries I. Amorphous System

An amorphous Sn-Co alloy film electro-codeposited on rough Cu foil was found in our previous works to self-organize a microisland structure, which is a crucial factor to improve Sn alloy anodes for cyclability. This work focused on the electrochemical properties, film morphology, and phase structure, and the behavior of Co atoms was analyzed by extended X-ray absorption fine structure and magnetic susceptibility measurements. Cycle performance up to 20 cycles, in which the capacity anomalously fluctuated, is discussed based on these results, where the capacity change is explained by the morphological and phase structural changes. The stabilization process of the phase is clarified in connection with formation of ferromagnetic face-centered cubic Co particles.

Development of Lithium-Ion Batteries with a LiCoO2 Cathode Toward High Capacity by Elevating Charging Potential

Elevating the charging voltage of lithium-ion batteries with a LiCoO 2 cathode is investigated to develop them toward high capacity and energy density. Three countermeasures are found to be essential to overcome side reactions with subsequent cycle degradations caused by higher cathode potential: (i) limiting the charging cut-off potential below 4.5 V vs Li/Li + for the LiCoO 2 cathode, (ii) modification of LiCoO 4 particles with Zr element, and (iii) controlling the ratio of ethylene carbonate in electrolyte, which is found to be a major cause of cycle degradation in an elevated charging potential condition. It is suggested that ethylene carbonate is oxidized and dissolves cobalt on the surface of the LiCoO 2 cathode, degrading cycle performance especially at high potentials. Raising the charging voltage up to 4.4 from 4.2 V for a 650 mAh class test cell demonstrates 10% higher cell capacity with 20% higher LiCoO 2 capacity of 190 mAh g -1 and practical cycle performance up to 500 cycles.

Relation between equilibrium hydrogen pressure and lattice parameters in pseudobinary ZrMn alloy systems

Abstract The influence of transition elements (M ≡ V, Fe, Co, Ni) in ZrMn2 − xMx alloy systems on the equilibrium hydrogen pressure and lattice parameters was studied in order to develop metal hydride materials for use in heat storage and heat transportation systems operating at temperatures between 100 and 250 °C, for which there is great demand in industrial heat processes. The equilibrium characteristics and lattice parameters of as-cast pseudobinary alloys ZrMn2 − xMx (M ≡ V, Fe, Co, Ni; 0 ⩽ x ⩽ 0.6) were evaluated by measuring pressure-composition isotherms at 200 °C and by analysing X-ray diffraction patterns using Rietvelds method. The crystal structures of all the pseudobinary alloys were the C14-type Laves phase. The decrease of the unit cell volume increased the equilibrium hydrogen pressure in the alloys modified by vanadium, iron or cobalt, but the alloys modified by nickel showed an opposite tendency. These results were interpreted in terms of Miedemas rule. Consequently, cobalt and vanadium were found to be the most effective elements for control of the equilibrium hydrogen pressure and therefore for extension of the available temperature range of pseudobinary ZrMn alloy systems of the C14-type Laves phase.