Shin-ichi Tobishima

Reaction behavior of LiFePO4 as a cathode material for rechargeable lithium batteries

Abstract We investigated the effect of cell temperature on the electrochemical reaction behavior of LiFePO4. We evaluated its electrochemical characteristics by using cyclic voltammetry and electrochemical impedance spectroscopy (EIS). Both the electron transfer activity and the lithium ion diffusion rate in the LiFePO4 electrode increased as we increased the operating temperature. The apparent activation energy for the lithium diffusion was more than twice that for charge transfer. This result supports the idea that the large discharge capacity of LiFePO4 at elevated temperatures is mainly caused by the high lithium diffusion rate in the LiFePO4 particles, which makes it possible to utilize more material near the center of the particles. We also evaluated the thermal stability of LiFePO4 by differential scanning calorimetry (DSC). Charged LiFePO4 in contact with an electrolyte solution exhibited no obvious endothermic or exothermic reaction below 340 °C. This means that LiFePO4 has higher thermal stability than such other cathode materials as LiCoO2, LiNiO2 and LiMn2O4.

Characterization of LiFePO4 as the cathode material for rechargeable lithium batteries

Abstract We investigated the electrochemical characteristics of LiFePO 4 when used as the cathode material for rechargeable lithium batteries. We also studied the change in the crystal structure of the material during discharge. The material prepared at a relatively low temperature of 675°C showed a higher charge and discharge capacity than that prepared at higher temperatures. This was because material prepared at a low temperature has a relatively small particle size and a rough surface morphology. We also found that the crystal structure of electrochemically delithiated LiFePO 4 was similar to a heterosite structure and the sample reverted to its original triphylite structure after electrochemical lithiation.

A consideration of lithium cell safety

The safety characteristics of commercial lithium ion cells are examined in relation to their use as batteries for cellular phones. This report describes a theoretical approach to an understanding of cell safety, example results of safety tests that we performed on lithium ion cells, and also presents our views regarding cell safety.

Lithium ion cell safety

The safety characteristics of recent commercial lithium ion cells are examined in relation to their use for cellular phones. These are prismatic cells with an aluminum cell housing (can) and a 500–600 mA h capacity. They have one of two types of 4-V class cathodes, lithium cobalt oxide (LiCoO2) or lithium manganese oxide (LiMn2O4). This report provides results of the safety tests that we performed on lithium ion cells and outlines our views regarding their safety.

Study of Li3 − xMxN (M: Co, Ni or Cu) system for use as anode material in lithium rechargeable cells

We have investigated the Li3 − xMxN (M: Co, Ni, or Cu, x = 0.1−0.6) system as an anode material for lithium rechargeable cells. Li metalLi3 − xMxN cells were prepared and the anode properties evaluated galvanostatically. Li2.6Co0.4N exhibits a high specific capacity of 760 mA h/g in the 0.0–1.4 V range. This value is more than twice the theoretical capacity of C6Li (372 mA h/g). The capacity of the Li3 − xCoxN system depends on the amount of lithium ions removed during the first extraction. These systems are expected to increase substantially the energy density of lithium rechargeable cells.

Lithium cycling efficiency and conductivity for high dielectric solvent/low viscosity solvent mixed systems

Lithium cycling efficiency on a lithium substrate (Li-on-Li cycling) and conductivity for various mixed solvent systems of high dielectric solvent (HDS) and low viscosity solvent (LVS) were examined for secondary lithium batteries. For the HDS, sulfolane, dimethylsulfoxide, γ-lactones, propylene carbonate (PC) and ethylene carbonate (EC) were used. For the LVS, tetrahydrofuran (THF), 2-methyl-THF, 1,2-dialkoxyethanes and 1,3-dioxolane (DOL) were used. For the solute, LiAsF6, LiBF4, LiCF3CO3 and LiClO4 were used. Lithium cycling efficiencies newly measured on a Li substrate (Ea) for EC/LVS or PC/LVS were ca 5% or 15% higher than those previously obtained by simple cycling of Li on a Pt substrate, while the order of Li cycling efficiencies to LVS change is similar in both cases, except for EC/DOL or PC/DOL. The reasons seem to be that the Li-on-Li cycling minimizes the influence of electrochemical Li/Pt alloying and partial solvent oxidation during the cycle on Li cycling efficiency. The Ea values in HDS/LVS mixed systems incorporating LiAsF6 or LiClO4 tended to increase with a decrease in the reactivity to Li, of not only LVS but also HDS. EC/THF systems incorporating LiAsF6 or LiClO4 showed high Ea values of ca 95% even by Li-on-Li cycling, the value being higher than those (ca 92%) for LiBF4 or LiCF3SO3 systems. In addition, for all the HDS/LVS mixed systems examined in this work, conductivities were higher than those for HDS or LVS single solvent systems. In regard to both conductivity and Li cycling efficiency, HDS/LVS mixed systems are considered to be effective in various lithium battery applications.

Lithium electrode cycleability and morphology dependence on current density

Abstract It is known that the cycleability of lithium secondary batteries depends on both charge and discharge current densities. This paper investigates the cycle life and lithium morphology dependence on charge and discharge current densities. The amount of needle-like lithium increases with decreases in discharge current density. The morphology of the needle-like lithium leads to the formation of ‘lead lithium’ which plays no role in charge/discharge cycles. Localized deposition and dissolution may be reason for ‘dead lithium’ formation. The decrease in cycle life with the increase in charge current density is also explained by this mechanism. Furthermore, high rate discharge leads to the recombination of isolated lithium which results in cycle life increase.

Anode performance of a new layered nitride Li3-xCoxN (x = 0.2-0.6)

Abstract The Li 3 − x Co x N ( x = 0.2–0.6) system was investigated for use as an anode material for lithium rechargeable cells. The anode performance of this system was evaluated by using Li metal/Li 3 − x Co x N cells. Li 2.6 Co 0.4 N in this system exhibits the highest specific capacity of 760 mAh/g in the 0–1.4 V range, and shows the best cycle performance. The capacity of the first extraction was 2 − x Li/mol, and depended on the amount ( x ) of substituted Co. In addition, the structure changed gradually from the crystalline phase with hexagonal symmetry to the amorphous phase as lithium was removed during the first extraction. The cycle performance of a lithium-ion cell was evaluated by using the Li 1.6 Co 0.4 N/LiNiO 2 cell. This cell showed good cycleability of more than 240 cycles. This system is thus very promising for application to lithium-ion cells.

Mixed solvent electrolyte for high voltage lithium metal secondary cells

We have examined various solvents and solutes as electrolytes for high voltage lithium metal secondary cells in terms of the oxidation potential, specific conductivity and cycling performance of Li/LiMn1.9Co0.1O4 cells. We selected propylene carbonate (PC), ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), 1,2-dimethoxyethane (DME) and 1,2-diethoxyethane (DEE) as the solvents, and LiClO4, LiPF6, LiAsF4 and LiBF4 as the solutes. Of the mixed solvent electrolytes we used, 1.0 mol dm−3 LiPF6 in a mixture of EC and DMC with a ratio of 1:1, provided a high figure of merit (FOM) of 43.5 for lithium cycling efficiency with Li/LiMn1.9Co0.1O4 cells as well as a high potential of over 5 V vs. Li/Li+ and a high specific conductivity of 3.1 mS cm−1 at −20°C and 18.7 mS cm−1 at 60°C.

Ethylene carbonate-propylene carbonate mixed electrolytes for lithium batteries

Abstract Electrolytic characteristics of propylene carbonate (PC)ethylene carbonate (EC) mixed electrolytes were studied, compared with those in PC electrolytes. Conductivity and Li charge—discharge efficiency values increased with EC contents increasing. For example, 1 M LiClO4ECPC (EC mixing molar ratio; [EC]/[PC] = 4) showed the conductivity of 8.5 ohm−1 cm−1, which value was 40% higher than that in PC. Also, 1 M LiClO4ECPC([EC]/[PC] = 5) showed the Li charge—discharge efficiency of 90.5% at 0.5 mA cm−2, 0.6 C cm−2, which value was ca. 25% higher than that in PC. ECPC mixed electrolytes were considered to be practically available for ambient lithium batteries in regard to the high Li+ ion conductivity and also high Li charge—discharge efficiency.