# New Trends in Separation Techniques of Lithium Isotopes: A Review of Chemical Separation Methods
## 1 Introduction
### Challenges in Development of Separation Methods for Lithium Isotopes Using Electrochemical  
#### 2.2.1 Challenges in Development of Separation Methods for Lithium Isotopes with Electromigration  
Electrochemical separation of lithium isotopes through the difference in migration rates between \({}^{6}\)Li and \({}^{7}\)Li is one of the green and promising alternatives for the separation of lithium isotopes [15]. According to media of lithium transferring, it can be divided into molten salt methods, aqueous solution methods, and organic solvent methods. However, due to the problems of corrosion resulting from high-temperature molten salt, the last two above-mentioned methods are preferred. Electromigration in aqueous solution system has good reproducibility; the electrode reactions take place in aqueous solution, so it is easy to realize multistage separation. However, due to the strong hydration of lithium ions which affects their migration, it is difficult to achieve a high separation effect. Therefore, the organic solvent method using ionic liquid, diethyl carbonate, and crown ethers was employed in recent studies. [PERSON] et al. employed electromigration from lithium-loaded organic phase aqueous solution using ionic liquids (1-octyl-3-methylimidazolium bis(trifluoromenthanesulfonyl)imide ([C\({}_{8}\)Mm][NTr\({}_{2}\)]), crown ethers (benzo 15-crown-5) and the organic sulfinomide lithium salt (Li[NTr\({}_{2}\)]) [25]. The maximum isotope fractionation of \({}^{7}\)Li upon electromigration was found to be \(-21.5\%\) vs. L-SVEC, fractionation that is attributable to the differences in the dissociation process of \({}^{6}\)Li vs. \({}^{7}\)Li ions-crown ethers complex [25]. In their next study, by also employing a new crown ether (4-nitrobenzo-15-crown-5) and a new ionic liquid (1-butyl-3-methylimidazolium bis[(trifluoromethylmethyl) sulfonyl]imide ([BMIm][NTr\({}_{2}\)]), [PERSON] et al. compared two experimental approaches: I. system lithium ions-ionic liquid-crown ether organic solution-aqueous solution, and II. aqueous solution-organic solution-aqueous solution system, for the separation of lithium isotopes using electromigration [15]. It was found that there are multiple roles for crown ether: phase transfer, selective chelation, dehydration, and retention during electromigra  
Figure 1: Overview of the ion–pair strategy for \({}^{6}\)Li separation using crown ethers B15C5 or B12C4 and [FeCl\({}_{4}\)]\({}^{-}\) as counter anion. Adapted from [24]. tion. [PERSON] et al. found a fractionation of Li isotopes, as the 8\({}^{7}\)Li values increase from 15% to 16.1, as the concentrations of crown ether increase from 0 to 0.2 mol/L [15]. Very recently, in their last electromigration study, [PERSON] et al. tested different catholytes (HCI, NH\({}_{3}\):H\({}_{2}\)O, NH\({}_{4}\)Cl and NH\({}_{4}\)HCO\({}_{3}\)) during separation of lithium isotopes with electromigration by using a lithium salt aqueous solution-organic solution-aqueous solution system [26]. In this study, the highest separation coefficient of 1.674 was obtained using a solution of 0.1 mol/L NH\({}_{4}\)Cl as catholyte [26]. Overall, the above-mentioned studies shown the potential of electromigration methodologies in eco-friendly separation of lithium isotopes. 2.2 Challenges in Development of Separation Methods for Lithium Isotopes Using Electrodialysis Methods  
Lithium ions can move by electrodialysis through certain Ionic-Liquid-i-OMs between the cathode and the anode in lithium solutions. The principle is that since the ionic mobility of \({}^{6}\)Li ions is greater than that of \({}^{7}\)Li ions, \({}^{6}\)Li can be enriched on the cathode side of a cell (see Figure 2). [PERSON] and [PERSON] used PP13-TFSI (N-methyl-N-propylpiperdium bis(trifluoromethasulfonyl)imido) ionic liquid to impregnate a highly porous Teflon film, while both surfaces of the Ionic-Liquid-i-Organic Membrane were covered by a nafion 324 overcoat or a cation exchange membrane [27]. [PERSON] and [PERSON] applied this membrane in an electrodialysis experiment [27].