A molecular titration strategy: utilizing a built-in electric field to measure the lithium diffusion coefficient in LiFePO4

Abstract

Electrometric titration techniques have long been used to measure the Li⁺ diffusion coefficients of electrode materials. However, the influence of electrode additives, cell assembly methods, and in particular, inaccurate assessments of the reaction area often lead to unreliable results. Here, we propose a molecular titration technique (MTT) to measure the lithium diffusion coefficient (D_Li) in LiFePO₄. This MTT alleviates the tedious electrode preparation procedure, circumvents the influence of additives, reduces the real reaction area errors, and shortens the testing time, making the testing more precise and efficient than the traditional titration techniques. In detail, [Fe(CN)₆]³⁻ solution is used to oxidize LiFePO₄ to Li⁺ and FePO₄, while the potential change rate (r_p) of the solution is recorded. Then, a built-in electric field (BIEF) electron transferring model is established, and the relationship between D_Li and r_p is formulated using the Huggins–Weppner equation. Eventually, the de-lithiation diffusion coefficient of Li_1−xFePO₄ (1 ≤ x ≤ 0) is measured to be 1–8 × 10⁻¹⁵ cm² s⁻¹ based on the recorded data and established formulae.