The scientists from the Universities of Córdoba and Extremadura in Spain, as well as the Center for Research and Development of Advanced Materials in the province of Jujuy, Argentina, have found an unexpected use for discarded medical masks left after the COVID-19 pandemic. They proposed converting them into porous carbon for lithium-sulfur batteries, which is one of the most promising types of energy storage for the future. The batteries are theoretically capable of storing several times more energy than modern lithium-ion batteries, and the sulfur used in them is significantly cheaper and more environmentally friendly than cobalt and other scarce metals. However, two major drawbacks are currently hindering a widespread adoption of this technology: sulfur is a poor electrical conductor, and the polysulfides forming during battery operation dissolve in the electrolyte and gradually remove the active material from the reaction, reducing the battery capacity and lifespan.
To solve this problem, the researchers used the activated carbon derived from the recycled face masks as a kind of porous “cage” for the sulfur. The entire process consisting of just two steps did not require the use of complex chemical activators. First, the masks were ground up and treated with concentrated sulfuric acid, which converted the polypropylene base into a carbonaceous residue. Then the material was heated to 800°C in a stream of carbon dioxide or water vapor, which resulted in a highly developed pore system of various sizes formed within the carbon.
The most efficient material was activated with water vapor. Its specific surface area reached 633 m² per gram, compared to 525 m² per gram for the sample treated with carbon dioxide. Moreover, it contained not only micropores but also larger mesopores. Thanks to this structure, the material simultaneously retained sulfur within the electrode and facilitated the movement of electrolyte and lithium ions, which the battery performance depends on.
When the resulting porous carbon was mixed with sulfur and heated, the molten sulfur filled the pores of the material. The resulting composite was used for manufacturing cathodes for lithium-sulfur batteries. Tests showed that at a relatively low loading (1.3 mg of sulfur per square centimeter), the electrodes delivered a capacity of about 1,000 mA·h per gram and retained more than 850 mA·h per gram even after 400 charge-discharge cycles. These figures exceed the ones of many similar biomass-derived materials.
Particular interest was the performance of electrodes under high sulfur loadings, up to 8 mg per square centimeter, which is considered one of the key requirements for practical application of lithium-sulfur batteries. The capacity per unit area reached 5 mA·h/cm², which is comparable to the performance required for the advanced batteries for electric vehicles and energy storage systems.
To understand the reasons for high efficiency of the new material, the scientists made additional tests. In particular, they examined the ability of carbon to trap polysulfides – the compounds responsible for degradation of lithium-sulfur batteries. During the experiment, the steam activated material almost completely removed the polysulfides from the solution within eight hours. The solution with the carbon dioxide activated sample retained its yellow color retained during this period of time, indicating less effective binding of the polysulfides. In other words, the porous structure better retained the reaction products inside the electrode and prevented their migration through electrolyte. Exactly this process, known as the shuttling effect, is considered one of the primary causes of capacity loss in lithium-sulfur batteries.
Additional electrochemical measurements show that lithium ions move rapidly within a porous structure, which promotes more complete utilization of sulfur and reduces energy losses during a battery operation.
Separately, the researchers assessed the new battery’s self-discharge rate: a gradual loss of charge during storage without use. To do this, they left the cells idle for a week after which they restarted charging and discharging. The experiment lasted for over seven months. Even after a prolonged storage, the cells retained high capacity and demonstrated stable performance, indicating that the active sulfur is effectively retained within the porous carbon structure.
Overall, the work of the Spanish and Argentine researchers has shown that medical waste can be used not only for recycling but also for creating highly efficient materials. If they succeed in scaling up the technology, millions of discarded medical masks would make their way from an environmental problem to a source of valuable raw materials for the next-generation batteries.
