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, essentially unaffected by PS presence. It also establishes FTIR as a useful technique for quantifying dissociation state of concentrated polymer and composite electrolytes for lithium batteries.
Highlights
The development of future energy storage systems should include the improvement of battery materials to have higher power and higher energy density with low cost and safety (Goodenough et al, 2007; Li et al, 2018)
The characteristic peaks of lithium bis(trifluoromethanesulfonyl) (LiTFSI) and poly(ethylene oxide) (PEO) in the SEO electrolyte shifted with respect to the pure components, indicating that the salt dissolved into the PEO phase of SEO
The absorbance of polystyrene peaks decreased with increasing LiTFSI content, but lacked any detectable interaction with the salt
Summary
The development of future energy storage systems should include the improvement of battery materials to have higher power and higher energy density with low cost and safety (Goodenough et al, 2007; Li et al, 2018). After it had been found that the ether groups in the amorphous phase of poly(ethylene oxide) (PEO) coordinate with cations and conduct ions by segmental motion (Wright, 1975; Borodin and Smith, 2006), there have been a great number of studies on the electrochemical properties and the transport mechanisms of polymer electrolyte systems This is, in part, because practical batteries require higher charge/ discharge rate than is currently achievable with PEO-based electrolytes (Hallinan and Balsara, 2013). Salt dissociation appears to decrease with increasing salt concentration in polymer electrolyte, resulting in the formation of triplets or higher ion clusters (Suo et al, 2016) These large agglomerates and neutral ion pairs negatively impact the diffusion and migration of the ions, making estimation or analysis of transport behavior complex or even impossible without direct knowledge of thermodynamics (via dissociation and interaction of ions with the chemical environment). This motivates an in-depth study of lithium salt dissociation and ion interaction with the chemical environment in concentrated composite electrolytes, such as block copolymer/salt mixtures
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