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Lithium (Li) is the most valuable metal in spent lithium iron phosphate (LiFePO4) batteries, but its recovery has become a challenge in electronic waste recovery because of its relatively low content and inconsistent quality. This study proposes an acid-free and selective Li extraction process to successfully achieve the isomorphic substitution of Li in LiFePO4 crystals with sodium (Na). The method uses low-cost and nontoxic sodium chloride (NaCl) as a cogrinding reagent via a mechanical force-induced solid-phase reaction. X-ray diffraction (XRD), energy dispersive X-ray analysis (EDAX), and X-ray photoelectron spectroscopy (XPS) characterizations demonstrated the evidence of Li/Na isomorphic substitution, while XPS spectra of Fe 2p, P 2p, and O 1s revealed that the coordination environment of these elements was not significantly altered. Density functional theory calculations further provided evidence that Na and Li share similar outer electron arrangements and coordination environments, favoring Na over Fe as a replacement for Li in LiFePO4. Additionally, the regeneration of NaCl and the recovery of precipitated Li2CO3 were simultaneously achieved with Na2CO3 as the sole reagent. This concise and efficient acid-free mechanochemical process for Li extraction is a promising candidate for feasible recycling technology of Li from spent LiFePO4 batteries. The proposed process is particularly appealing because of its high selectivity, considerable economic advantages, and environmental benefits.