Lithium-ion batteries based on intercalation compound electrodes exhibit poor performance at temperatures below 0°C. Insufficient ionic conductivity and freezing of the electrolyte are generally seen as the main reasons for the poor performance of rechargeable batteries at low temperatures. Although several attempts have been made to solve this problem, those attempts involved additional materials that added extra weight, which is detrimental for portable devices.
Now a team from Fudan University (Shanghai, China; www.fudan.edu.cn) has developed an ethyl acetate-based electrolyte that exhibits sufficient ionic conductivity at –70°C. Ethyl acetate was a promising choice due to its low freezing point of –84°C, but the low-temperature performance of ethyl acetate electrolyte had never been investigated previously.
For the electrodes, the team used two organic polymers: a polytriphenylamine (PTPAn) cathode; and a 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTCDA)-derived polymide (PNTCDA) anode. Unlike the electrodes used in Li-ion batteries, these organic compounds do not rely on intercalation — the process of integrating ions into the electrodes’ molecular matrix.
The team noted that their battery offers several other advantages. “Compared to the transition-metal-containing electrode materials in conventional Li-ion batteries, organic materials are abundant, inexpensive and environmentally friendly,” says team member Yongyao Xia. He estimates the price of the team’s electrode materials at about one third that of the electrodes in a commercial Li-ion battery.
More work will be required before the new battery can be commercialized, however. The specific energy (energy per unit mass) of the new battery is still low compared with commercial Li-ion batteries, and the assembly process needs to be further improved.