Lithium (Li) is crucial for rechargeable batteries and thus sustainable electric power. However, the finite supply of ore-based Li cannot keep pace with the rapidly growing energy demand. Harvesting Li from brines has recently attracted much attention due to their abundant Li reserves. Solar evaporation — leveraging solar energy to concentrate Li through a series of evaporation and precipitation steps — has been considered a cost-effective extraction method. However, the accumulation of salts and other impurities on the surface of solar evaporators can block the active sites needed for water evaporation and decrease the operational life of evaporators. Now, Beidou Xi, Guandao Gao, Chao Wang, and colleagues in China present a floating evaporator with spherical microparticles whose surfaces have a self-cleaning ability to efficiently separate Li, water, and other salts.

In their evaporator, lithium titanium oxide nanoparticles — known for their high Li-ion adsorption capacity — are uniformly distributed inside nitrogen-doped mesoporous carbon nanospheres, which are then anchored onto lightweight spherical polystyrene microbeads. Due to the high solar–thermal conversion of the carbon nanospheres, the evaporator exploits localized high-temperature spots to selectively extract Li ions from the brines. As water evaporates, leftover salts form in the evaporator. In contrast to conventional evaporators, however, Xi and colleagues demonstrate that the salts generate a torque on the spherical particles of the evaporator, causing them to rotate and subsequently leading to the slipping and removal of the salts. The self-cleaning allows the evaporators to perform continuous water evaporation and Li extraction.