Elsevier

Electrochimica Acta

Volume 398, 1 December 2021, 139333
Electrochimica Acta

Ionic liquid plasticizers comprising solvating cations for lithium metal polymer batteries

https://doi.org/10.1016/j.electacta.2021.139333Get rights and content

Highlights

  • Ionic liquid cations with oligo(ethylene oxide) side chains have been prepared.

  • The effect of the side chain length on Li+ solvation was studied.

  • The effect of the ionic liquids as solid polymer electrolyte plasticizers is reported.

  • The increased solvation of the longer side chains leads to improved performance.

Abstract

Ternary solid polymer electrolytes (TSPEs) with ionic liquids (ILs) including alkyl-based ammonium cations and low coordinating anions suffer from the lack of Li+ ion coordination by the ILs compared to the immobile polymer backbone, in terms of Li+ ion transport. Thus, solvating ionic liquids (SILs) with an oligo(ethylene oxide) side chain attached onto the cation were prepared to improve the interaction between Li+ and the IL and accelerate Li+ transport in TSPEs. A variety of methods, such as pulsed field gradient nuclear magnetic resonance spectroscopy, Li metal plating/stripping and measurements of Sand's times were used to show that Li+ ion transference numbers increase with the oligo(ethylene oxide) side chain length in SIL-based TSPEs, which results in faster Li+ ion transport and translates into much slower lithium depletion at a given current, thereby delaying the onset of fast dendrite growth of lithium metal.

Introduction

Over the last decades, a strong worldwide demand for energy storage has arisen [1]. Lithium metal with its high specific capacity (3860 mAh g-1) and low standard reduction potential (-3.04 V vs. standard hydrogen electrode) is a promising negative electrode for upcoming secondary batteries to fulfill the demand [2,3]. However, the lithium metal battery (LMB) technology poses a variety of challenges that must be overcome, like the inhomogeneous electrodeposition of lithium during charge and non-uniform electrodissolution during discharge, that can lead to high surface area lithium (HSAL) [4], [5], [6]. These deposits can occur in different morphologies such as ‘needle-like’ dendrites or mossy structures. The highly reactive HSAL is especially problematic in terms of performance decay and can penetrate through the separator, inducing internal short-circuits and leading to heat generation, which can accelerate ageing and lead to thermal runaways [6], [7], [8]. It is particularly problematic since the common organic liquid electrolytes suffer from safety issues caused by their flammability, which leads to a highly hazardous mixture in LMBs [9]. Solid polymer electrolytes (SPEs) are very promising as alternative electrolytes due to their high chemical, thermal and mechanical stability [10], [11], [12]. Poly(ethylene oxide) (PEO) is the most prominent SPE. PEO/lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) can reach  10-3 S cm-1 at 80 °C due to the flexible poly(ethylene oxide) chains that are able to solvate and transport Li+ ions [13], [14], [15], [16]. However, the slow segmental mobility of the PEO chains at temperatures below 60 °C results in ionic conductivities incompatible with LMB operation [13], [14], [15]. Plasticizers are thus a promising solution to overcome this issue and increase the segmental mobility of the PEO chains at lower temperatures while also helping to obtain amorphous electrolytes. [17], [18], [19], [20] Ionic liquids (ILs) have been proposed as plasticizers due to their broad electrochemical stability window (ESW), ultra-low vapor pressure and flammability as well as their high thermal and chemical stability [21], [22], [23]. The so-called ternary solid polymer electrolytes (TSPEs) can be obtained as fully amorphous membranes with ionic conductivities of 10-3 S cm-1 at 40 °C [24,25]. However, the commonly used ILs, such as N-butyl-N-methylpyrrolidinium TFSI (Pyr1,4TFSI), suffer from drawbacks due to their lack of interaction with Li+ ions in TSPEs, which leads to low Li+ ion transference numbers (tLi) due to the strong PEO-Li+ ion interactions, and translates into higher resistance due to the formation of concentration gradients during cell operation [24,[26], [27], [28]]. These concentration gradients become steeper as the current density increases above a maximum value, which leads to full Li+ depletion at the lithium electrode during plating and triggers fast dendrite growth. This maximum current, at a given conductivity, is accentuated by high Li+ transference numbers and low anionic transference numbers (since the anion migrate in the opposite direction to the cation and does not react at the electrodes). It is why many groups are developing single-ion SPEs, either solvent-free or plasticized with molecular solvents [11,29,30]. In TSPEs, the addition of IL ions that do not participate to the Li+ ion transport reinforce this further [26,27]. The emerging solvating ionic liquids (SILs) that bear a solvating oligo(ethylene oxide) side chain onto the cation, are promising in this regard given their ability to shift the solvation sphere from the PEO chains to the SIL and thus partly “free” the Li+ ion from the PEO chains [24,27]. N-methyl-N-methoxyethyl pyrrolidinium (Pyr1,(2O)1) TFSI with a single ether oxygen side chain, is known to interact with Li+ ions and inhibit cation micelles formation in presence of Li salt [31]. The number of ether oxygen on the side chain strongly affect the Li+ ion coordination as the number of solvating units increases [32]. We reported recently that a N-methyl-N-oligo(ethylene oxide) pyrrolidinium TFSI IL with a median chain length of seven (Pyr1,(2O)7TFSI) can completely solvate a Li+ ion and enable new conduction modes in TSPE membranes, in particular via ‘vehicular’ transport (i.e. transport of Li+ ion with its solvation shell made of a single Pyr1,(2O)7 cation) [24]. However, little is known about the effect of intermediate oligo(ethylene) side chain lengths on the TSPE characteristics.

The side chain length affects the viscosity of the SIL and thus a priori both its plasticizing effect and its solvating properties. Since the performance of the TSPE membranes is an interplay between solvation and mobility, we investigated a series of oligo(ethylene oxide)-based SILs, namely N-methyl-N-oligo(ethylene oxide)pyrrolidinium (Pyr1,(2O)x)TFSI, with x the number of -(CH2)2O- units), with increasing chain length from x = 1, 2, 3, 4 and 7 for a use as plasticizers in TSPEs. In particular, we focused on influence of the oligo(ethylene oxide) side chain length on the Li+ ion transport in cross-linked PEO:LiTFSI:SIL solvent-free processed TSPE membranes. We have studied the competition between TFSI- ion and the side chain for Li+ ion solvation in binary SIL/LiTFSI electrolytes, and report here on the Li+ ion transport in TSPEs via electrochemical determination of tLi, validated via pulsed field gradient nuclear magnetic resonance (PFG-NMR), as well as on the influence of the plasticizer on the thermal stability and crystallinity for both, “freshly” prepared and “aged” membranes at 20 °C for one year. Finally, we show the performance effect of the increased cationic solvation of SILs in TSPEs in terms of Li+ ion transport and increased Sand's times in symmetric Li||Li cells.

Section snippets

Materials and synthesis of SILs

The following chemicals were used for preparing the SILs and TSPE membranes: Di(ethylene glycol) monomethyl ether (≥ 99.0%, Sigma Aldrich), tri(ethylene glycol) monomethyl ether (≥ 97.0%, Merck), tetra(ethylene glycol) monomethyl ether (≥ 98.0%, TCI Chemicals), thionyl chloride (97%, Sigma Aldrich), pyridine (anhydrous 99.8%, Sigma Aldrich), sodium sulfate (anhydrous, 99%, Sigma Aldrich), N-methylpyrrolidine (≥ 98.0%, Sigma Aldrich), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) (battery

Results and discussion

The ionic conductivity is a key descriptor of the electrochemical performance of electrolytes in LMBs and was investigated for both neat SILs and cross-linked TSPE membranes. In the following, the TSPEs are named with side chain length in subscript (e.g. O7 for Pyr1,(2O)7TFSI, see Fig. 1c and d for schematic representations of the SILS and PEO crosslinks). The labeling for the TSPEs is as follows: cl-PEO:LiTFSI:SILOX with the molar stoichiometry of PEO corresponding to the repeating -(CH2)2O-

Conclusion

Five different SILs with different oligo(ethylene oxide) side chain lengths and their corresponding cross-linked TSPEs were examined by a variety of methods for a use in LMB. The extension of the chain lengths leads to a lower thermal stability of the SILs which, however, does not affect the properties of the corresponding TSPEs. All TSPE membranes are amorphous excepted for that incorporating the longer side chain SIL, cl-20:2:1O7 which exhibits minor crystallinity, even after long term

Authors' contribution

Jaschar Atik cured and analyzed most data, conceptualized the study and wrote the original draft. Johannes Helmut Thienenkamp cured and analyzed the PFG-NMR data. E. Paillard conceptualized the study and supervised the work of J. Atik and edited the original draft. All authors participated in the formal analysis of the data and reviewed the final manuscript.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

Special thanks to Debbie Berghus (University of Münster) for the TGA and DSC measurements.

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      In contrast, the band at ≈ 748 cm−1 corresponds to Li+ ion aggregates with FSI- ions [40–42]. For Pyr1,(2O)7FSI electrolytes with Li salt concentrations up to 1.8 m (corresponding to an equimolar electrolyte of LiFSI:SIL), no significant changes are seen compared to the neat SIL in the Raman spectra, indicating a minor influence of concentration on the FSI coordination which is consistent with our previous observation [19,43] that the Pyr1,(2O)7+ cation is able to completely solvate a Li+ ion. In contrast, already in the 0.2 m LiFSI, Pyr1,4FSI spectrum, the band position and shape are both affected by the lithium salt, which can be attributed to the formation of contact ion pairs [19,44].

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