Solvation-controlled lithium-ion complexes in a nonflammable solvent containing ethylene carbonate: structural and electrochemical aspects

Abstract

The structural and electrochemical properties of lithium-ion solvation complexes in a nonflammable organic solvent, tris(2,2,2-trifluoroethyl)phosphate (TFEP) containing ethylene carbonate (EC), were investigated using vibrational spectroscopic and electrochemical measurements. Based on quantitative Raman and infrared (IR) spectral analysis of the Li bis(trifluoromethanesulfonyl)amide (TFSA) salt in TFEP + EC electrolytes, we successfully evaluated the individual solvation numbers of EC (n), TFEP (n), and TFSA (n) in the first solvation sphere of the Li-ion. We found that the n value linearly increased with increasing EC mole fraction (x), whereas the n and n values gradually decreased with increasing n. The ionic conductivity and viscosity (Walden plots) indicated that mainly LiTFSA ion pairs formed in neat TFEP (x = 0). This ion pair gradually dissociated into positively charged Li-ion complexes as x increased, which was consistent with the Raman/IR spectroscopy results. The redox reaction corresponding to an insertion/desertion of Li-ion into/from the graphite electrode occurred in the LiTFSA/TFEP + EC system at x ≥ 0.25. The same was not observed in the lower x cases. We discussed the relation between Li-ion solvation and electrode reaction behaviors at the molecular level and proposed that n plays a crucial role in the electrode reaction, particularly in terms of solid electrolyte interphase formation on the graphite electrode.

Anion Coordination Characteristics of Ion-pair Complexes in Highly Concentrated Aqueous Lithium Bis(trifluoromethane- sulfonyl)amide Electrolytes

“…This corresponded well with plots from our previous work on typical organic electrolyte systems containing LiTFSA salt. 28,36,45 The distance between the experimental plot and the ideal line did not change significantly, even while increasing the cLi up to 3.0 mol dm -3 . In the highly concentrated solutions (cLi > 3.0 mol dm -3 ) according to the Raman and DFT results in this work, Li ions directly interacted with TFSA anions to form Li + •••TFSAcontact ion-pair complexes; i.e., the dissociativity of the LiTFSA salt was significantly low.…”

“…The detailed analysis procedure is described in our previous work. 35,36 The integrated intensities of the single bands for free TFSA in the bulk that was not coordinating Li ion (termed free TFSA) and TFSA bound to Li ion (termed bound TFSA) were represented as If = Jfcf and Ib = Jbcb, where Jf and Jb are the Raman scattering coefficient and cf and cb are the concentrations of free and bound components, respectively.…”

2,2,2-Trifluoroethyl Acetate as an Electrolyte Solvent for Lithium-Ion Batteries: Effect of Weak Solvation on Electrochemical and Structural Characteristics

“…In conventional LIB systems, organic electrolytes using carbonate-based solvents [popularly, a solvent mixture of cyclic ethylene carbonate (EC) and linear dimethylcarbonate (DMC)] are necessary to achieve stable and reversible working LIBs with a graphite negative electrode. , This is because the carbonate components (particularly, EC) form a solid passivation film [i.e., solid electrolyte interphase (SEI)] via their reductive decomposition during the first charging process, allowing reversible Li-ion insertion into the graphite electrode and suppressing further decomposition of the electrolytes. − SEI formation on negative electrodes is also compassable in other organic solvents and ionic liquids when a small amount of EC or vinylene carbonate is added into their electrolyte solutions. , In this case, the EC component with low content acts as an SEI additive; in contrast, Li ions are solvated primarily by main solvents (organic solvents or ionic liquids) to form solvated Li-ion complexes in the bulk. In solvents yielding weak solvation (i.e., lower D N value), Li-X ion pairs (X: counteranion) are formed, which reduce the ionic conductivity.…”

“…21−24 SEI formation on negative electrodes is also compassable in other organic solvents and ionic liquids when a small amount of EC or vinylene carbonate is added into their electrolyte solutions. 25,26 In this case, the EC component with low content acts as an SEI additive; in contrast, Li ions are solvated primarily by main solvents (organic solvents or ionic liquids) to form solvated Li-ion complexes in the bulk. In solvents yielding weak solvation (i.e., lower D N value), Li-X ion pairs (X: counteranion) are formed, which reduce the ionic conductivity.…”

In-Operando Detection of the Physical Property Changes of an Interfacial Electrolyte during the Li-Metal Electrode Reaction by Atomic Force Microscopy

“…[29][30][31] Furthermore, the physical properties of the electrolyte provide the key to understand the solvation of Li-ions, which influence the electrochemical characteristics of the various negative electrodes as well as that of the Li metal. [32][33][34] Thus, the investigation of the interfacial electrolyte is important to understand the fundamental electrochemical reaction of the Li metal electrode.…”