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Abstract One-dimensional (1D) olivine iron phosphate (FePO₄) is widely proposed for electrochemical lithium (Li) extraction from dilute water sources, however, significant variations in Li selectivity were observed for particles with different physical attributes. Understanding how particle features influence Li and sodium (Na) co-intercalation is crucial for system design and enhancing Li selectivity. Here, we investigate a series of FePO₄particles with various features and revealed the importance of harnessing kinetic and chemo-mechanical barrier difference between lithiation and sodiation to promote selectivity. The thermodynamic preference of FePO₄provides baseline of selectivity while the particle features are critical to induce different kinetic pathways and barriers, resulting in different Li to Na selectivity from 6.2 × 10²to 2.3 × 10⁴. Importantly, we categorize the FePO₄particles into two groups based on their distinctly paired phase evolutions upon lithiation and sodiation, and generate quantitative correlation maps among Li preference, morphological features, and electrochemical properties. By selecting FePO₄particles with specific features, we demonstrate 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) from a low Li source (1: 1000 Li to Na) with (95.8 ± 0.3)% purity in a single step. 

The lithium supply issue mainly lies in the inability of current mining methods to access lithium sources of dilute concentrations and complex chemistry. Electrochemical intercalation has emerged as a highly selective method for lithium extraction; however, limited source compositions have been studied, which is insufficient to predict its applicability to the wide range of unconventional water sources (UWS). This work addresses the feasibility and identifies the challenges of Li extraction by electrochemical intercalation from UWS, by answering three questions: 1) Is there enough Li in UWS? 2) How would the solution compositions affect the competition of Li + to major ions (Na + /Mg 2+ /K + /Ca 2+ )? 3) Does the complex solution composition affect the electrode stability? Using one-dimensional olivine FePO 4 as the model electrode, we show the complicated roles of major ions. Na + acts as the competitor ion for host storage sites. The competition from Na + grants Mg 2+ and Ca 2+ being only the spectator ions. However, Mg 2+ and Ca 2+ can significantly affect the charge transfer of Li + and Na + , therefore affecting the Li selectivity. We point to improving the selectivity of Li + to Na + as the key challenge for broadening the minable UWS using the olivine host.