Solvent Activity in Highly Concentrated Electrolyte and Reversible Li Intercalation into Graphite Electrode
Authors: Tatara Ryoichi , Moon Heejoon , Hirai Ayaka , Ueno Kazuhide , Thomas Morgan L. , Dokko Kaoru , Watanabe Masayoshi 
Published: February 2020

We reported that the equimolar mixtures of glymes and Li salts behave like ionic liquids.[1] As shown in Figure 1, triglyme (G3) can coordinate to a Li+, forming a 1:1 solvate cation of [Li(G3)]+, which acts as an independent cation. Therefore, the glyme-Li salt complexes have been categorized as “solvate” ionic liquids consisting of the solvate cations and the counter anions.[2] The concentration of Li salt in glyme-Li salt solvate ionic liquids are extremely high ~ 3 mol dm−3. In this extremely concentrated electrolyte, almost all the solvents (glyme) coordinate to Li+ cation and almost no free (uncoordinated) solvent exists, which is an unique nature of the concentrated electrolyte.[3] In this study, we estimated the activity of free solvent species in concentrated electrolyte by measuring the electrode potential of Li/Li+. As shown in Figure 2, the electrode potential in the G3-Li[TFSA] mixtures increase drastically in the highly concentrated region (> 1 mol dm-3). To interpret this remarkable increment of the potential, we assumed equilibrium reaction of Li/Li+ as following, which takes the solvation/de-solvation processes into account: [Li(G3)1]+ + e- = Li + G3. Nernst equation of above reaction can be written as: E = E0
+ 2.303RT/F log a
[Li(G3)]/a
G3. In this equation, the term of logarithm diverges infinitely if activity of free G3 is zero. The remarkably high electrode potential in the concentrated electrolyte proved that the amount of free glyme approaches zero at high concentrations > 3 mol dm−3. The amount of free solvents was also evaluated using Raman spectroscopy and will also be reported.
It is well known that ethylene carbonate is compatible with graphite anode because a good SEI layer, which prevents the cointercalation of solvents into the graphite, is formed. In contrast, the cointercalation of solvents with Li+ into graphite takes place in other common solvents such as propylene carbonate (PC) and ethers, causing destruction of the graphite crystalline structure. However, it was reported that cointercalation of these solvents, namely, PC, dimethoxy ethane, and other solvents into graphite anode can be inhibited by increasing the Li salt concentration and intercalation of Li+ into graphite anode occurs reversibly.[4] We also observed the reversible Li+ intercalation/de-intercalation reaction in extremely concentrated G3-Li[TFSA] solution, although the cointercalation takes place in the solution containing excess G3 (Figure 3).[5] The electrode potential for the formation of Li+ intercalated graphite (including desolvation process of [Li(G3)]+) is expected to increase greatly against the Li salt concentration as well as the aforementioned potential of Li/Li+, because the potential depends on the activity of not only solvate cation but also free solvent. On the other hand, the electrode potential for cointercalation depends on only the activity of solvate cation, since the reaction does not include desolvation process. Thus, in the highly concentrated region, the activity of free solvent is very small, and the electrode potential for Li-intercalated graphite formation is higher than that for cointercalation, leading to the suppression of cointercalation. In this study, it was revealed that the activity of the solvent in an electrolyte should be a key factor in controlling electrochemical reaction. Detail discussion of the relationship between the activity of free solvent and electrochemical reaction will be reported.

Acknowledgement

This study was supported in part by the ALCA program of the Japan Science and Technology Agency, and by the Technology Research Grant Program of NEDO, for which the authors are grateful.

References

[1] T. Tamura et al., Chem. Lett. 2010, 39, 753; K. Yoshida et al., J. Am. Chem. Soc. 2011, 133, 13121; K. Yoshida et al., J. Phys. Chem. C 2011, 115, 18384; K. Ueno et al., J. Phys. Chem. B 2012, 116, 11323; C. Zhang et al., J. Phys. Chem. B 2014, 118, 5144. [2] C. A. Angell et al., Faraday Discuss. 2011, 154, 9; T. Mandai et al., Phys. Chem. Chem. Phys. 2014, 16, 8761. [3] K. Ueno et al. Phys. Chem. Chem. Phys. 2015, 17, 8248 [4] S. Jeong et al.,Electrochem. Solid-State Lett. 2003, 6, A13; Y. Yamada et al., J. Electroche, Soc. 2015, 162, A2409. [5] H. Moon et al., J. Phys. Chem. C 2014, 118, 20246; H. Moon et al., J. Phys. Chem. C 2015, 119, 3957.

Figure 1