•Established the quaternary phase diagram FePO4-LiFePO4-NaFePO4-KFePO4 by DFT for the first time.•The competitive mechanism of alkali ions in FePO4 during extraction from a salt lake is revealed.•Na2/3FePO4 phase is the main reason for to decreased cyclability of FePO4 for lithium recovery in brines.•Proposed an innovative FePO4 regeneration process to enhance electrode performance and lifespan. The rapid expansion of lithium battery applications has resulted in a shortage of lithium resources, prompting researchers to focus on the electrochemical extraction of lithium from water resources using FePO4 as the host material. However, a large amount of alkali metal impurity ions in brine leads to irreversible capacity loss, limiting the industrial application of FePO4 materials in lithium extraction. The mechanism of alkali metal ions’ influence on FePO4 materials remains unclear. To address this issue, the quaternary phase diagram of FePO4-LiFePO4-NaFePO4-KFePO4 and the diffusion barriers of lithium/sodium ions in FePO4 were obtained for the first time based on the theoretical calculation of density functional theory (DFT). DFT and X-ray diffraction (XRD) refinement revealed that the inability to remove Na2/3FePO4 from the FePO4 is a critical issue affecting electrode recyclability. An innovative electrode regeneration process was proposed to enhance the lifespan of FePO4 material. The Na2/3FePO4 impurity was converted to LiFePO4 using K2S2O8 abstersion and 0.1 mol/L LiCl lithiation regeneration process. After five rounds of regenerations, the total lithium extraction of the material could still reach 80.18 % of the extraction of the brand-new electrode, demonstrating the multiple reuses of the host material and cost savings. These innovative discoveries can advance the industrial application of electrochemical lithium extraction from FePO4 electrode materials.