(506c) Design of a Boron-Containing Pthf-Based Solid Polymer Electrolyte for Sodium-Ion Conduction with High Na+ Mobility and Salt Dissociation

The establishment of sustainable energy sources highly depends on efficient storage devices to guarantee consistent power supply. This would allow society to overcome fossil fuels’ environmental impact and unequal global distribution. Lithium-ion batteries (LIBs) have been sufficient for the energy storage needs of portable devices and a limited fleet of electric automobiles. However, current LIBs cannot store a large-scale energy demand due to their low anode theoretical capacity (372 mA h g^−1 for graphite, the most common anode material).¹ Additionally, the uneven geolocation of lithium reserves remains a concern.²

Sodium is a promising substitute to lithium due to is abundant and widespread resources, similar electrochemical mechanisms of intercalation, and potential lower cost of associated battery materials. Although less explored than lithium-based devices, materials for sodium-based batteries have been extensively investigated for over five decades, including solid and quasi-solid electrolytes for sodium-ion conduction. These materials have the advantage of separator-free design, non-leakage, and, most importantly, the potential to suppress dendrite formation during cycling on sodium metal’s surface, which would allow the application of Na metal anodes with specific capacities of 1166 mA h g^−1, over three times higher than graphite’s.³ In particular, solid-state electrolytes have additional benefits from the lack of solvents, such as non-volatilization, and adaptability to temperature changes.

Over the years, the optimization of sodium solid-state electrolytes has inspired much research interest, but fundamental questions on ion transport still need to be further examined. In one of our recent studies on solid polymer electrolytes, we investigated how ionic mobility, Na−O coordination, and ion clustering are influenced by the nature of the polymer chain, in particular, the density of oxygens in the backbone.⁴ The polymers of interest were poly(ethylene oxide) (PEO) and poly(tetrahydrofuran) (PTHF). PEO, the standard polymer host for ionic conductivity for over 40 years, contains two carbons in each repeating unit, which leads to a high density of negative charges in its chain and consequently higher coordination numbers (CN) with ions in the electrolyte. PTHF is an alternative to PEO due to lower oxygen density in its backbone, with each repeating unit having four carbons. In our study, we used molecular dynamics to characterize two systems of interest: PEO−NaClO₄ and PTHF−NaClO₄ electrolytes.

We confirmed that the Na−O−Polymer coordination in PTHF is weaker than in PEO. In the PEO electrolyte, Na⁺ ions coordinated in their first coordination shells with approximately one oxygen from ClO₄^− and five from the PEO chain. Na⁺ in PTHF coordinated with approximately one oxygen from the chain and five from perchlorates, which indicates that coordination with more than one anion molecule was frequent. The opposing coordinating behavior observed in PEO and PTHF suggests that while ethylene oxide’s stronger coordination could potentially restrain cation motion, butylene oxide’s weaker coordination allows higher ionic interaction. Hence, very property that makes PEO dissolve Na ions well indeed also serves to restrict their motion, as confirmed by previous research.^5,6

The difference in coordination numbers resulted in their main transport mechanisms to be opposite. While in PEO, most cations coordinated with one chain during the entire simulation, in PTHF, most Na⁺ ions coordinated with three distinct polymer chains. Moreover, we demonstrated that sodium cations diffuse and move along the PEO chain at a slower pace and that interchain hopping is rarer than in PTHF. Cations coordinating with 3−5 oxygens from PEO most likely depended on the polymer chain flexibility and segmental motion to move through the system. In contrast, the decomplexation of cations coordinating with only one oxygen from the PTHF backbone could easily be achieved, which resulted in faster motion of Na⁺ in PTHF. However, the cluster analysis demonstrated that the weaker coordination of Na⁺ with PTHF’s backbone also resulted in higher interaction between cations and anions in the electrolyte. PTHF chains coordinating with less cations resulted in more Na⁺ ions free to form larger clusters with ClO₄^−, which is detrimental to the overall ionic conductivity.

These findings motivated us to focus on enhancing ionic dissociation and reducing cluster formation in PTHF electrolytes by immobilizing the anions. Hence, we experimentally produced sodium SPEs based on semi-interpenetrating polymer networks (semi-IPN) containing PTHF chains with boron moieties and PVDF for mechanical stability. The goal was to combine the anion trapping properties of boron with PTHF’s looser coordination to achieve higher cation mobility but preventing reassociation. Samples with carbon centers instead of boron were also produced to compare the influence of Lewis acid strength. Free-standing SPEs with varying NaClO₄ concentrations were obtained and evaluated by mechanical, thermal, and electrochemical analyses.

Room temperature conductivities > 10^-5 S cm^-1 were achieved, as well as a significant improvement in conductivity with an increase in salt concentration. This suggests that the electrolytes benefit from the looser coordination of Na⁺ with the PTHF chains in conjunction with the Lewis acidity of the boron centers, which facilitated ion dissociation even at higher concentrations. Relatively low activation energies (~0.2 eV) were also identified by VTF fittings of the Arrhenius curves, which indicates that the sodium-ion hopping from one oxygen to the next was facilitated by the weaker coordination with the PTHF chains. That finding was further confirmed by the study of frequency-dependent dielectric loss tangent curve (tan δ), yielding lower relaxation times in samples with strong Lewis acidity.

A transference number of 0.88 was achieved, an almost single-ion electrolyte. The improved transference number can be associated with the efficacy of the Lewis acid centers when combined with a weaker coordinating chain such as PTHF. In PTHF, the greater mobility of Na⁺ allows for more interactions of those ions with the polymer network, which contributes to the segmental motion of the chains and improves the contact of anions with the trapping centers. As a result, samples with relatively high ionic conductivities at room temperature showed transference numbers almost on the level of single-ion gel electrolytes. The stability window was also determined and found to be above the necessary for the operation of the common cathode materials for sodium-ion intercalation (1.8 V to 3.9 V). Analysis of the dielectric parameters and FTIR spectra demonstrated the improved mobility and dissociation of the sodium salt in the polymer matrix, confirming the relevance of polymers with anion-trapping capability and looser coordination for the development of improved SPEs. For the first time, to the best of our knowledge, the potential of Lewis acid centers added to PTHF’s looser coordination has been studied in depth for solid polymer electrolyte applications, indicating an important route to safer and energy-efficient solid-state sodium batteries.

References

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(2) O’donnell, L. F.; Greenbaum, S. G. Review of Multivalent Metal Ion Transport in Inorganic and Solid Polymer Electrolytes. Batteries 2021, 7 (1), 1–27. https://doi.org/10.3390/batteries7010003.

(3) Lu, J.; Jaumaux, P.; Wang, T.; Wang, C.; Wang, G. Recent Progress in Quasi-Solid and Solid Polymer Electrolytes for Multivalent Metal-Ion Batteries. J. Mater. Chem. A 2021, 9 (43), 24175–24194. https://doi.org/10.1039/d1ta06606d.

(4) Genier, F. S.; Hosein, I. D. Effect of Coordination Behavior in Polymer Electrolytes for Sodium-Ion Conduction: A Molecular Dynamics Study of Poly(Ethylene Oxide) and Poly(Tetrahydrofuran). Macromolecules 2021, 54 (18), 8553–8562. https://doi.org/10.1021/acs.macromol.1c01028.

(5) Mindemark, J.; Lacey, M. J.; Bowden, T.; Brandell, D. Beyond PEO—Alternative Host Materials for Li+-Conducting Solid Polymer Electrolytes. Prog. Polym. Sci. 2018, 81, 114–143. https://doi.org/10.1016/j.progpolymsci.2017.12.004.

(6) Mackanic, D. G.; Michaels, W.; Lee, M.; Feng, D.; Lopez, J.; Qin, J. Crosslinked Poly ( Tetrahydrofuran ) as a Loosely Coordinating Polymer Electrolyte. 2018, 1800703, 1–11. https://doi.org/10.1002/aenm.201800703.