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Safety and High Discharge Rate of 5C Characteristics of 100-Ah-Class Lithium Ion Cells for Backup and Cycle Use Application
Recently, MW-class battery systems using large lithium ion batteries have been developed and applied for load-leveling [1]. We previously reported on the characteristics and lifetime of 200-Ah-class lithium ion cells, which exhibit improved safety due to the use phosphazene flame retardants [2]. However, these cells exhibited a large amount of capacity loss at high current density because of high internal resistance. For this study, we developed 100-Ah-class lithium ion cells with improved high-rate discharge characteristics.
Sample battery
A mixed solvent of ethylene carbonate with dimethyl carbonate containing a phosphazene flame retardant was used as the electrolyte. The lithium manganese oxide spinel, in which some of the manganese was replaced with magnesium, was used as the cathode active material, and carbon-coated graphite was used as the anode active material [3]. We optimized the electrode configuration to improve the high discharge rate characteristics.
Experimental and results
The discharge rate characteristics of a 100-Ah-class cell showed that more than 95% of nominal capacity was obtained at a rate of 5 C, as shown in Figure 1. Figure 2 shows the float life of 5C at 25°C. The cell was float charged at 4.1 V. In the figure, we can see that cell lifetime of about 7 years can be expected. Figure 3 shows the cycle life of 5 C at 25°C. The cell was charged to 4.1 V at 0.2 C. Noticeable capacity decay was observed during the first 100 cycles because of cell temperature increasing due to discharge. However, capacity decay was moderate after 100 cycles.
Safety evaluation of the 100-Ah-class cells was carried out in accordance with JIS C 8715-2. There were no abnormal events. Original safety tests, such as nail penetration by using a ceramic nail and overcharge until thermal runaway, were also carried out. The cells were vented with smoke during the ceramic nail penetration test; however, there was no fire due to the effect of the flame retardant and the surface temperature of cell being less than 300°C. Short circuit was observed during the overcharge test at 161%; however, no smoke and or fire were observed. The maximum temperature of the cell surfaces was about 100°C.
We constructed a 50-kW system using 100-Ah Li ion cells for a UPS system. One hundred thirty-seven cells were connected in series in which each cell was controlled between -20 and +20 mV mean cell voltage during float charging by using a cell control unit. This system can also be used temporarily for load-leveling by connecting them in parallel.
References
[1] N. Shinoda et. Al., Mitsubishi Heavy Industries Technical Review, 50 (3), 36 (2013).
[2] T. Tsujikawa, K. Yabuta, M. Arakawa and K. Hayashi, J. Power Sources, 244, 11 (2013).
[3] T. Tsujikawa, K. Yabuta, T. Matsushita, T. Matsushima, K. Hayashi, M. Arakawa, J. Power Sources, 189, 429 (2009).