Identifying critical features of iron phosphate particle for lithium preference

One-dimensional (1D) olivine iron phosphate (FePO₄) is widely proposed for selective electrochemical lithium (Li) extraction from dilute water sources, however, significant variations in Li selectivity were observed for particles with different physical attributes (e.g., sizes, morphologies, dominant facets, etc.). The fundamental understanding of particle feature-dependent host response upon lithium and sodium (Na) ions co-intercalation is still lacking, which impedes the design of extraction system and the improvement of Li selectivity. Here, we investigated a series of FePO₄ particles with various features and revealed the importance of harnessing the kinetic and chemo-mechanical barrier difference between lithiation and sodiation to promote Li selectivity. The thermodynamic preference of FePO₄ provides the baseline of Li preference while the particle features are critical to induce different kinetic pathways and barrier energies, which results in different Li to Na selectivity, ranging from 6.2 × 10² to 2.3 × 10⁴. Importantly, we categorized the FePO₄ particles into two groups by size based on their distinctly paired phase evolution behaviors upon lithiation and sodiation, and generated quantitative correlation maps among Li preference, particle features, and electrochemical properties. The correlations indicate the existence of an optimal size range ([010] length 155-420 nm) for achieving both high Li selectivity and structural reversibility. By choosing FePO₄ particles with different features, we demonstrated fast (636 mA/g) Li extraction from a high Li source (1: 100 Li to Na) with (96.6±0.2)% purity, and high selectivity (2.3 × 10⁴) for Li extraction from a low Li source (1: 1000 Li to Na) with (95.8±0.3)% purity in a single step.