We investigate the $2{}^3{S}_1–2{}^3{P}_J$ ($J=0$, 1, 2) transitions in ${}^6{\mathrm{Li}}^{+}$ using the optical Ramsey technique and achieve the most precise values of the hyperfine splittings of the $2{}^3{S}_1$ and $2{}^3{P}_J$ states, with smallest uncertainty of about 10 kHz. The present results reduce the uncertainties of previous experiments by a factor of 5 for the $2{}^3{S}_1$ state and a factor of 50 for the $2{}^3{P}_J$ states, and are in better agreement with theoretical values. Combining our measured hyperfine intervals of the $2{}^3{S}_1$ state with the latest quantum electrodynamic (QED) calculations, the improved Zemach radius of the ${}^6\mathrm{Li}$ nucleus is determined to be 2.44(2) fm, with the uncertainty entirely due to the uncalculated QED effects of order $m{\alpha }^7$. The result is in sharp disagreement with the value 3.71(16) fm determined from simple models of the nuclear charge and magnetization distribution. We call for a more definitive nuclear physics value of the ${}^6\mathrm{Li}$ Zemach radius.

Enriched lithium isotopes (⁶Li and ⁷Li) are essential in the nuclear energy industry, where ⁶Li is bombarded with neutrons to produce tritium for fusion reactions, while ⁷Li is used as a core coolant and pH regulator. Separation of ⁶Li and ⁷Li by electromigration is a promising method for producing enriched lithium isotopes that fulfill industrial needs. In this work, based on a previously proposed biphasic system electromigration routine, a three-stage system of ‘LiCl aqueous solution (anolyte)|B12C4-[EMIm][NTf₂] organic solution|NH₄Cl aqueous solution (catholyte)’ was constructed and the rules of lithium isotope separation and lithium-ion migration investigated. It was shown that the isotope enrichment effect of the catholyte was greatly affected by the experimental conditions, while that of the organic solution was less affected. As the B12C4 concentration increased, enhancement of ⁷Li enrichment in the catholyte and ⁶Li enrichment in the organic solution was observed, and α(C/O) and α(O/A) reached 0.975 and 1.018 at B12C4 of 0.5 mol/L. With the increase in current, migration time, and LiCl concentration, the isotope that was enriched in the catholyte trended from ⁷Li to ⁶Li (about 6 mA, 12 h or LiCl of 5 mol/L). Taking lithium-ion transport efficiency and lithium isotope separation effect into consideration together, a current of at least 6 mA, duration of at least 12 h, LiCl concentration of at least 1 mol/L and B12C4 concentration of 0.2 mol/L are suggested for the electromigration process. The work provides an important reference for system construction and experimental design of a biphasic electromigration separation method, which is expected to be an industrial alternative because of its environmental protection and high efficiency.