Although lithium-ion batteries only convert less than 10% of their energy into heat during operation, if this heat is not effectively controlled, it will accelerate battery aging and even cause thermal runaway and fire in extreme cases. In sharp contrast, the "inefficient" electrochemical system of humans generates enough heat to boil hundreds of cups of tea every day, but it can still maintain a stable body temperature. The key lies in the skin and its sweating heat dissipation mechanism.Inspired by this, a research team from City University of Hong Kong recently developed a "skin-like adaptive nanocomposite cooling film" that allows batteries to "sweat and cool down" like mammalian skin.
For many years, almost all lithium battery systems from mobile phones to electric vehicles have been equipped with thermal management systems, including fans, heat sinks, liquid cooling circuits and phase change materials, to maintain the cell temperature within a safe range. Although these solutions are mature and effective, they often have complex structures, take up space, and require additional power consumption. The research team believes that nature has already provided an efficient and elegant solution - mammalian skin achieves extremely efficient body temperature regulation through "sweating + evaporation". If this mechanism can be engineered and transplanted to the battery, it will be expected to simultaneously improve performance, safety and system simplicity.
According to reports, this new cooling film covers the surface of the battery like a "skin". It is composed of functional materials such as lithium chloride (LiCl), graphene oxide (GO), activated carbon fiber (ACF), and is encapsulated in a porous polytetrafluoroethylene (PTFE) membrane and supported by a copper frame. Each component has a clear division of labor: LiCl is a highly hygroscopic salt that can absorb and store moisture from the air when the temperature is low; graphene oxide forms an efficient thermal conductive network, which quickly spreads the heat generated by the battery within the membrane; the porous structure of the activated carbon fiber significantly increases the evaporation area; the copper frame helps distribute heat evenly and avoids local saturation; the PTFE outer membrane prevents solution leakage while allowing water vapor to pass freely.
As the battery heats up, the moisture stored in the membrane absorbs heat and evaporates rapidly, taking the heat away from the battery surface, a process known as "desorption cooling." When the battery cools down, the membrane will spontaneously reabsorb water from the surrounding air, restoring its "moisture inventory" and preparing for the next round of work. The research team pointed out that this adaptive moisture absorption and release characteristic allows the cooling film to automatically adjust its own state under different working conditions and achieve continuous circulation without the need for an external control system.
Experimental data showed that the adaptive cooling film achieved an average cooling power of 802.5 W·m⁻² in proof-of-concept testing and lowered the temperature by 34.3 degrees Celsius (approximately 61.7 degrees Fahrenheit) at a high heat flux density of 2.7 kW·m⁻². When conducting high-rate charge and discharge tests on a commercial lithium-ion battery with a nominal 3.7 V/12 Ah, the battery cycle life using this cooling film was extended from 118 times to 233 times, which is almost doubled. The researchers pointed out that under strong thermal load conditions that are close to the real working conditions of high-performance batteries, the material can still achieve a cooling of more than 30 degrees Celsius, which is enough to significantly suppress performance degradation and safety risks.
In addition to its cooling capabilities, the nanocomposite film also has excellent flame retardant properties, effectively preventing the spread of thermal runaway under conditions that would normally trigger combustion. In tests, the membrane maintained stable thermal management performance after more than 1,000 hours of harsh cycle use, showing good durability and repeatability. More importantly, the entire system is passively cooled and does not require any additional power supply: the LiCl in the film will automatically reabsorb moisture from the air when the battery temperature drops, "charging" the next heat dissipation.
"Our goal is to develop a passive, compact, low-cost and practical thermal management solution that provides strong cooling capabilities without external energy input, while taking into account reliability and safety during actual battery operation." said project leader Dr. Sui Zengguang. Due to its simple structure and compact size, this cooling film is highly scalable in design and can be enlarged or reduced in size according to needs. It is expected to be applied in everything from portable electronic devices to large electric vehicle battery packs.
However, the research team also reminds that this technology is currently more suitable for scenarios where the heat load changes intermittently or periodically. Under continuous high heat flow density conditions, the cooling capacity will be limited because the material needs time to cool down and reabsorb moisture. In other words, this is a passive cooling solution that's well-suited for "intermittent high-power work," rather than a one-size-fits-all answer for sustained extreme heat environments.
Although it is still in a relatively early stage and requires further research and development and verification before full industrialization, researchers are quite optimistic about its prospects. They believe that this technology is very attractive for any scenario that is lightweight, compact, requires no additional power supply but requires "meaningful cooling capacity", especially in fields that are extremely sensitive to weight and packaging constraints, such as humanoid robots and drones. The relevant research paper has been published in the journal "ACS Nano", and more detailed technical details are also disclosed in the article.