Lithium Salt Dissociation in Diblock Copolymer Electrolyte Using Fourier Transform Infrared Spectroscopy

Abstract

Polymer electrolytes are important materials in the manufacture of all-solid-state batteries due to their ionic conductivity, achieved by doping the polymer with salt, and mechanical strength, achieved by use of a block copolymer with a rigid block. High salt concentration is advantageous to achieve high ionic conductivity, but it makes estimation of battery performance difficult due to the breakdown of dilute-solution theory, which assumes complete ion dissociation. Therefore, practical battery design would benefit from an empirical understanding of the relationship between ion dissociation and salt concentration in block copolymer electrolyte. In this study, the dissociation of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in polystyrene (PS)poly(ethylene oxide) (PEO) diblock copolymer electrolyte was investigated using Fourier transform infrared (FTIR) spectroscopy. Quantitative analysis was performed to reveal the appearance of ion pairs and interactions between the salt and the ethylene oxide moieties with increasing salt concentration. FTIR peaks associated with polymer functional groups were found to be more useful than those of the TFSI anion for understanding the chemical state of the block copolymer electrolyte. In particular, PS peaks were used to quantify polymer dilution upon salt addition and verify that the Beer-Lambert law was valid at all concentrations investigated. PEO peaks revealed conformational changes of the polymer upon coordination with lithium ions. A previously unidentified FTIR peak was discovered that relates to polymer-salt interaction. It was used to determine the extent of salt dissociation, which compares well with a Raman study of a homopolymer electrolyte. This work definitively shows that LiTFSI dissolves into the PEO phase of the block copolymer,

Investigating miscibility and lithium ion transport in blends of poly(ethylene oxide) with a polyanion containing precisely-spaced delocalized charges

“…33 For the p5PhTFSI-Li, new strong signals at 1320 cm −1 and 1280 cm −1 appear and correspond to asymmetric stretching of OvSvO unique to the PhTFSI moiety that are not present in p5PhS-Li. 33,37 In addition, the strong asymmetric stretch signal of C-F at 1190 cm −1 indicates the presence of CF 3 . 37,38 Other vibrational modes of SO 2 in sulfonamides are also detected at 1160 and 1087 cm −1 .…”

“…33,37 In addition, the strong asymmetric stretch signal of C-F at 1190 cm −1 indicates the presence of CF 3 . 37,38 Other vibrational modes of SO 2 in sulfonamides are also detected at 1160 and 1087 cm −1 . 30,38 Finally, the symmetric S-N stretch and S-N-S vibrations are observed at 790 and 745 cm −1 , respectively.…”

“…30,38 Finally, the symmetric S-N stretch and S-N-S vibrations are observed at 790 and 745 cm −1 , respectively. 37 For comparison, an overlay of the IR spectra of p5PhTFSI-Li and LiTFSI salt is shown in Fig. S21.…”

Polymer Blend Electrolytes for Batteries and Beyond

“…This method avoids any concerns of residual solvent in the blend, a concern in batteries if the solvent reacts with the electrodes. Residual solvent is a particular concern due to strong coordination that has been observed between polar solvents and ions/polyelectrolytes that makes complete removal of the solvent difficult or impossible. However, the thermal and mechanical stresses of melt mixing can cause degradation of certain polymers…”

3D Printable Composite Polymer Electrolytes: Influence of SiO2 Nanoparticles on 3D-Printability

“…Furthermore, bands from the TFSI anion are presented: SO 2 stretching at 1348 cm −1 , CF 3 stretching at 1179 cm −1 , SNS stretching at 1086 cm −1 , C-S stretching at 788 cm −1 . [ 36 , 37 ] NP–alk exhibits only the stretching vibration of the alkene C-H at 2800–3000 cm −1 . Results of the thermogravimetric analysis are presented in Figure 4 b.…”