Cold temperature negatively impacts the performance of rechargeable lithium-ion batteries (LIBs) and limits their range of applications. Presently, most LIBs can only operate at about 12% of their optimal level when the temperature drops down to minus 40 degrees Celsius. However, major scientific publications recently reported that a team of chemists in China have made a breakthrough that allows LIBs to operate at 70% of their optimal level in minus 70 degrees Celsius. Additional research and development efforts that build on the findings by the Chinese chemists may one day turn LIBs into a viable energy storage technology for extremely frigid environments such as the outer space.
With significant advantages in terms of performance and scalability, LIBs are being widely used not only in consumer electronics but also in other newly emerged applications such as electric vehicles. However, a noticeable drawback of this technology is the weakening of performance when the environment temperature drops to zero degree Celsius or under. Typically, LIBs can only operate at 50% of their optimal level (or room temperature level) when the temperature falls to minus 20 degrees Celsius. At minus 40 degrees Celsius, they may operate at just 12% of their optimal level. This shortcoming is especially problematic in high-latitude countries such as Russia and Canada, where temperatures can sink below minus 50 degrees Celsius.
The electrolyte in conventional LIBs is ester-based, and its conductivity is temperature dependent. As the surrounding environment becomes colder, the ester-based electrolyte also becomes slower at moving ions. This in turn also impede the electrochemical reactions that take place at the interface of the electrolyte and the electrode, causing the battery to struggle to function.
Solving the temperature dependence problem has long been a challenge for developers of LIB technologies, but now a breakthrough appears to have been found. A research team at China’s Fudan University led by physical chemistry professors Yongyao Xia and Yonggang Wang announced that they have developed a rechargeable LIB that can work in extremely cold environments such as Antarctica or the outer space. The findings of the Fudan researchers were first published in the scientific journal Joule on 28 February 2018.
The solution devised by the researchers replaces the standard ester-based electrolyte with ethyl acetate, which is also an ester but has a freezing point of minus 84 degrees Celsius. At the same time, the researchers replaces traditional metal-based electrodes with organic polymers – polytriphenylamine for cathode and 1,4,5,8-naphthalenetetracarboxylic dianhydride-derived polyimide for anode.
Lab results show that the LIB solution created by the Fudan team can maintain operational stability even when the temperature falls to minus 40 degrees Celsius. When the temperature drops further to minus 55 and minus 70 degrees Celsius, the new battery technology can still operate at 90% and 70%, respectively, of the normal level within the room temperature range.
Organic polymers can serve as better electrodes than metals can because they do not require intercalation, a process that is specific to metal-based electrodes. Intercalation involves the insertion of lithium ions into the structure of the metal that acts as the electrode. This can proceed at a snail’s pace in sub-zero temperatures. Polymers, by contrast, absorb lithium ions faster and are relatively unaffected by low temperatures.
Previously, battery developers tried to address the temperature dependence issue by introducing special additives to the composition of the battery or by maintaining the ideal operational temperatures with an external heating system. These fixes can significantly increase both the production cost and the weight of the battery.
Compared with transition metals, organic polymers are plentiful and economical materials for electrodes. Furthermore, they can also be environmentally friendly. The Fudan team estimates that the cost of the electrode materials used in their solution is about one third of the cost of the typical electrode materials for LIBs.
Although the LIB technology developed by the researchers at Fudan University performs impressively in low temperatures, it is not yet ready for commercialization because it has low specific energy (i.e. energy per mass unit). The technology can be further refined for deployment in special fields. For example, LIBs that use ethyl acetate as electrolyte and organic polymers as electrodes will have lesser risk of going dead in the middle of a space mission.
(This article is an English translation of news content provided by EnergyTrend’s media partner TechNews. Photo at the top courtesy of the US National Aeronautics and Space Administration, or NASA.)