Yoshiyasu Saito

Synthesis of Triaxial LiFePO4 Nanowire with a VGCF Core Column and a Carbon Shell through the Electrospinning Method

A triaxial LiFePO4 nanowire with a multi wall carbon nanotube (VGCF:Vapor-grown carbon fiber) core column and an outer shell of amorphous carbon was successfully synthesized through the electrospinning method. The carbon nanotube core oriented in the direction of the wire played an important role in the conduction of electrons during the charge-discharge process, whereas the outer amorphous carbon shell suppressed the oxidation of Fe2+. An electrode with uniformly dispersed carbon and active materials was easily fabricated via a single process by heating after the electrospinning method is applied. Mossbauer spectroscopy for the nanowire showed a broadening of the line width, indicating a disordered coordination environment of the Fe ion near the surface. The electrospinning method was proven to be suitable for the fabrication of a triaxial nanostructure.

Thermal studies of a lithium-ion battery

In order to characterize the heat generation behavior of a lithium-ion secondary battery during discharge, calorimetry of the battery was carried out. The temperature dependency of the heat generation was studied between 283 and 333 K. An exothermic peak and an endothermic peak were observed around 4 V, except at 333 K. These peaks are caused by a phase transition of the positive electrode material, LixCoO2, between the hexagonal and monoclinic structures, which is observed at around x = 0.5. The heat of discharge consists of mainly two factors, the battery reaction and the electrochemical polarization. In the temperature range between 283 to 333 K, if the battery is discharged at 50 mA, the least heat generation is observed at about 300 K owing to the temperature dependence of both factors.

Simulation study of electrical dynamic characteristics of lithium-ion battery

The electrical dynamic characteristics of a lithium-ion battery have been simulated by an equivalent circuit, which is derived from the measured impedance. The transient voltage response to the various kinds of applied current waves such as single pulse, single rectangular, triangle, and sawtooth waves is experimentally examined and calculated by using the numerical Laplace transform with the equivalent circuit. The experimental and calculated results are compared and discussed, focusing on the range of current where the linear relationship is valid. Changing the time range, the state of charge (SOC) and the battery temperature as parameters, their influence on the linear range of the applied current has been investigated.

Comparative study of thermal behaviors of various lithium-ion cells

Heat generation behaviors during charge and discharge are examined by calorimetric measurement for several lithium-ion cells that are commercially available. Heat flow from the cell shows interesting variation reflecting the characteristics of the electrode reactions. Heat generation of the cell with graphitic carbon anode draws extremely complicated curve caused by variation of stage structure of lithium intercalated graphite. The cell with hard carbon anode dissipates excess heat resulting in a voltage hysteresis between charge and discharge. In the LiCoO2 cathode, thermal behaviors due to the crystal phase transition are clearly observed.

Entropy change in lithium ion cells on charge and discharge

Open circuit voltage (OCV) was measured as a function of temperature and state of charge (SOC) for six kinds of lithium ion cells. The following cells were used: four kinds of commercial cell using a LiCoO2 cathode and a graphite or hard carbon anode; a trial manufacture cell using a Li–Ni–Co complex oxide cathode and a graphite-coke hybrid carbon anode; and a trial manufacture cell using a LiMn2O4 cathode and a graphite anode. The entropy change in the cell reaction was determined by calculating the derivative of the OCV with temperature. Results were compared and discussed to determine the influence of the phase transition in the electrode materials due to cell reaction. It was clarified that the entropy change in cells using a LiCoO2 cathode is negative except for the part of the SOC region where LixCoO2 phase transition occurred. An endothermic reaction then occurs during discharge and an exothermic reaction during charge. In cells using LiCoO2 cathodes, there was a fluctuation in the entropy change originating from the LixCoO2 phase transition in the SOC range between 70% and 90%. This fluctuation was influenced by temperature and by additives or excess lithium in the cathode material. The entropy change in both cells using a Li–Ni–Co complex oxide cathode or a LiMn2O4 cathode was comparatively small.

Thermal behaviors of lithium-ion cells during overcharge

Abstract The heat generation behavior in the overcharge has been investigated experimentally for various lithium-ion cells. Generated heat flow was examined by calorimetric measurement for different charging rates as a parameter and discussed. It is clarified that the heat generation during the overcharge is almost proportional to charging current. It is considered that if the cooling system can remove the equivalent heat to electrical input, it is possible to prevent the thermal-runaway which originates from the overcharge.

Impedance spectroscopy by voltage-step chronoamperometry using the Laplace transform method in a lithium-ion battery

Impedance spectroscopy by voltage-step chronoamperometry (VSCA) using the Laplace transform method is proposed. With this method an impedance Bode diagram for a very wide range of frequencies can be obtained by only one measurement of the current response to an applied voltage step change. Further, by applying staircase voltage-step chronoamperometry (SVSCA), each impedance diagram at each voltage step can be obtained, and it is easy to examine the change in the impedance of batteries due to discharging or charging. The VSCA method has been applied to a lithium-ion battery. It has been demonstrated that an impedance diagram for a wide range of frequencies between 1 mHz and several kilohertz can be obtained by only one VSCA and that the results obtained by this method agree with the results from applied sine-wave measurements in a battery at equilibrium. The impedance change at every 50 mV between 2.8 and 4.2 V due to discharging or charging is measured by applying SVSCA in a lithium-ion battery, and the results are discussed. The battery impedance is realized to increase due to discharging, and the impedance may be affected by the previous treatment applied to the battery.

Characterization of reaction in lithium-ion cells by calorimetry and staircase voltage step coulometry

In order to characterize the reaction mechanism of lithium-ion cells during charge and discharge, two experimental methods, calorimetry and staircase voltage step coulometry (SVSC), are examined. As a result of calorimetry during charge and discharge, the influence of previous treatment applied to the cell is observed in the heat generation behavior. SVSC gives kinetic information of the rate-determining step in the cell reaction. It is found that there is a slow-rate reaction besides the main cell reaction during charge and discharge. It is suggested that the irreversibility of the slow-rate reaction causes the voltage hysteresis between charge and discharge. The cell reaction mechanism is discussed, mainly focusing the reaction at the hard carbon anode used in the test cell.