Hiroyuki Katsukawa

Degradation Mechanism and Life Prediction of Lithium-Ion Batteries

Lithium-ion cells with 5-Ah capacity were fabricated using a spinel Li 1.1 (Ni 0.025 Ti 0.025 Mg 0.02 )Mn 1.83 O 4 as a cathode active material, graphitized carbon as an anode active material, and 1 M LiPF 6 /ethylene carbonate + diethyl carbonate + dimethyl carbonate as an electrolyte. In order to improve the calendar life of the cell, we investigated the degradation mechanism by measuring the thickness of the solid electrolyte interphase (SEI) on anode active material. The SEI thickness was measured by focused ion beam, scanning electron microscope, and X-ray photoelectron spectroscopy. The thickness of the SEI was initially 0.04 μm, and after storage for 392 days at 25 and 40°C, the thickness was 0.15 and 0.45 μm, respectively. The capacity decreased with increase in the thickness of SEI, because Li in the cell is consumed by forming SEI. The amount of Li consumption was estimated theoretically assuming that SEI is formed by a reaction between intercalated Li and the electrolyte in SEI on the negative carbon surface, and a diffusion of the electrolyte in the SEI is the rate-determining step of the reaction. The theoretical equation showed a good agreement with experimental capacity fade at 25, 40, and 60°C for the storing days up to 380 days. A voltage decrease of the cell after 1-s at 20 A of discharge current was measured to estimate roughly the increase of the cell internal resistance during storage. The increase of SEI resistance was estimated by the theoretical equation and compared with the experimental voltage drop data after 1-s discharge. However, the theoretical data was not in a good agreement with the experimental data. The reason is that the charge-transfer resistance on the anode also increases during storage. Another reason is the resistance change of the cathode during the storage.

Safety Performance of Large and High-Power Lithium-Ion Batteries with Manganese Spinel and Meso Carbon Fiber

Three different-sized lithium-ion cells for hybrid electric vehicles nominal capacity 5, 8, and 17 Ah, were fabricated using lithium-rich mangnese spinel Li 1.1 Mn 1.9 O 4 (LM) as a positive active material and meso carbon fiber (MCF) as a negative active material. Overcharge tests at about 5, 16, 100 and 200 A current were carried out to estimate a safety level of LM/MCF cells. As a result of the overcharge test, all cells were not fired and did not burst and themal runaway of the 5 and 17 Ah cells did not occur at 1C rate. LM/MCF cells have great ability for safety under high-rate overcharge. However, thermal runaway occurred at 12C-40C rate overcharge. Thermal runaway depends strongly on the C rate at overcharge.

Influence of electrode active materials on safety performance of large and high-power Li-Ion batteries for hybrid electric vehicles

Large lithium-ion cells with more than 1200 W/kg power were fabricated using Li 1.1 (Ni 0.025 Ti 0.025 )Mn 1.85 O 4 (LM) and LiNi 0.78 Co 0.19 Al 0.03 M x O 2 (M: Na, Sr, Ba, x = 0.0005-0.1) (LN) as a positive active material, and graphitized carbon (KS25) and meso carbon fiber (MCF) as a negative active material. Overcharge tests at 100 A were carried out to estimate a safety level. From the overcharge test, the safety level of the cells are arranged Cell C (LN/MCF) > Cell A(LM/KS25) > Cell B(LN/KS25). Thermal stability of the active materials are arranged LM > LN as positive, and MCF > KS25 as negative from differential thermal analysis of each material with electrolyte. The safety level of the cells can be explained by the thermal stability of the each active material.