Photothermal lithium extraction assists carbon neutrality

Lithium, as an indispensable element in lithium-ion batteries (LIBs), is critical for the ongoing renewable energy transition. Primary lithium sources include brine, ore, and battery recycling. Ore extraction is energy/capital-intensive and polluting, while battery recycling is still early. Brine extraction (mainly salt lakes) dominates the lithium supply.1 Sustainable methods can decarbonize extraction and support LIB manufacturing. Essentially, brine-based extraction is the separation of lithium ions (Li+) from other competing ions, such as sodium ions (Na+), potassium ions (K+), magnesium ions (Mg2+), and calcium ions (Ca2+). The chemical similarity between Li+ and Mg2+ particularly complicates this separation, thus requiring a decreased mass ratio of Mg2+ to Li+ (MLR) as the primary objective in this process. Direct lithium extraction methods (e.g., adsorption, membrane separation, and electrochemical) boost extraction yet face significant barriers—slow kinetics, poor selectivity, and cyclic instability.2 Despite its dominance, evaporitic lithium concentration suffers from slow speed and high water loss.3 Photothermal evaporation emerged as a promising method to boost brine concentration and direct lithium extraction through tailored structures, yet risks pore blockage and surface salt deposition. This impairs both hydraulic conductivity and solar absorption and poses difficulty in harvesting.

Recently, Song et al. published an article in Science that explores a new method to enhance the capacity and purity of lithium extraction by optimizing the photothermal system.4 Inspired by the extraction-storage-release (ESR) mechanisms of halophytes, they developed a solar transpiration-powered lithium ESR device aimed at more environmentally friendly lithium production (Figure 1A). This device utilizes aluminum nanoparticle-modified anodic aluminum oxide (AAO) as a light absorber, featuring a highly porous matrix with an average pore size of 100 nm and a theoretical transmembrane pressure of 18.5 bar (at θ = 50°). Such high evaporation pressure meets the operational requirements for nanofiltration membranes (5–20 bar). Under pressure, lithium-containing brine is filtered at the membrane surface, allowing Li+ to pass through the membrane pores into a lithium storage layer based on a ceramic frit, while Mg2+ is intercepted. The storage layer has a porosity of approximately 45%, providing ample capacity for Li+ (Figure 1B). Through water circulation, the extracted lithium can be easily collected. The authors investigated lithium permeability and selectivity in different types of brines and conducted cycling experiments with brine at 1.0 g L−1 salinity and MLR of 80, using 12 h of 1.0 kW m−2 light and 12 h of darkness to assess long-term performance. Regeneration operations of 0.5 h were performed at 96, 144, 240, and 528 h, with a final Li+ selectivity (SLi) of 168, thereby validating the primary scalability and practicality of the device.