Role of Mixed Solvation and Ion Pairing in the Solution Structure of Lithium Ion Battery Electrolytes

Abstract

The solution structures of organic carbonate solvents (ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), and diethyl carbonate (DEC)) as electrolyte solutions of LiPF 6 were investigated with FTIR and NMR spectroscopy and DFT computational methods. Both coordinated and uncoordinated solvents are observed by IR spectroscopy, allowing the determination of solvent coordination numbers, which a range from 2 to 5. The predominant species in solution changes as a function of LiPF 6 concentration. At low salt concentrations (<1.2 M), the predominant species is a solvent-separated ion pair, whereas at high salt concentrations (>2.0 M) the predominant species in solution is the contact ion pair. In mixed solvent systems (PC−DMC, PC−DEC, EC−DMC, or EC−DEC), the mixed solvated cations are observed in the presence of high concentrations of uncoordinated cyclic carbonate despite the much larger dielectric constant of the cyclic carbonates than dielectric constant of linear carbonate.

Lithium Salt Effects on Silicon Electrode Performance and Solid Electrolyte Interphase (SEI) Structure, Role of Solution Structure on SEI Formation

“…In contrast, LiBF 4 is less dissociated and forms more CIP or AGG with a lower solvation number. 19,20,[25][26][27] The electrolytes dominated by SSIP, 1.2 M of LiPF 6 , LiTFSI, or LiClO 4 , generate predominately LEDC and Li 2 CO 3 on the Si surface from the reduction of EC. However, the electrolyte dominated by CIP or AGG, 1.2 M of LiBF 4 , generates predominantly an inorganic rich SEI from the reduction of the BF 4 anion.…”

“…20 Thus, DMC has been selected as a model coordinating solvent since the coordinated/uncoordinated C=O bands are clearly distinguishable (Figure 7a). 19,20 Similar trends have been reported for diethyl carbonate (DEC). 20 An assumption is made that the lithium ion solvation is dominated by ionic association strength rather than the differences between cyclic or linear carbonate solvents, which is reasonable according to literature.…”

“…16,18 The change in salt concentration has been reported to result in a change solution structure of the electrolyte. 16,[19][20][21] In commercial electrolytes for LIBs where the salt concentration is typically 1-1.2 M, the solution structure is typically a lithium cation solvated by four solvent molecules and an uncoordinated counter anion. This is referred to as a solvent separated ion pair (SSIP).…”

“…[22][23][24][25] As the electrolyte concentration is increased, more PF 6 anions coordinate to the lithium cations resulting in the formation of a contact ion pair (CIP) or aggregate (AGG). 19,20 Lower average solvation numbers are observed in electrolytes with high concentrations of CIP or AGG, since the counter ion displaces solvent from the coordination sphere of lithium. We previously reported that the concentration of CIP or AGG in the electrolyte has a strong correlation with the LiF concentration in the SEI.…”

“…19,20,26,27 For instance, LiBF 4 has a stronger association strength than LiPF 6 , LiClO 4 , and LiTFSI. In PC or DMC based electrolytes, electrolytes containing LiBF 4 have been reported to have lower solvation numbers than comparable electrolytes containing LiPF 6 , LiClO 4, or LiTFSI, which implies that the BF 4 − anions more strongly coordinate the lithium cation.…”