(404d) Taking a Metal–Ligand Coordination Approach Towards High Ionic Conductivity Ca–Polymer Electrolytes

Background: It is well known that achieving high ionic conductivities with polymer electrolytes is a challenge, partly due to the complex interactions prevalent within the polymer matrix [1]. This is also the case with polymer electrolytes for Ca batteries, which are rapidly emerging as contenders for beyond Li energy storage owing to the elemental abundance of Ca metal, its low cost, comparable volumetric energy density and redox potential compared to Li metal [2].

Problem: Poly(ethylene oxide) (PEO) has been a widely investigated polymer for Li batteries as it offers excellent compatibility with metal salts; yet, the high degree of salt complexation also limits overall ionic conductivity by restricting ion movement [3]. This strategy of using ether–based polymers such as PEO has also been applied to Ca batteries with promising, but limited success. Of the few Ca polymer electrolytes reported so far, most employ ether–based backbones, and as expected, their ionic conductivities are in the range of 10^–4 S/cm [4].

Motivation: Acknowledging the fundamental limitation of ether–based electrolytes, i.e. strong coordination with the ether O and consequent low ionic conductivity, it is necessary to explore other polymers with structures more amenable to facilitating high ionic conductivity. It has been shown in recent literature that tuning metal–ligand coordination can be a suitable strategy for improving ionic conductivity, especially for multivalent polymer electrolytes [5]. Based on this, we explore and report here on the use of a N–containing polymer, i.e. polyvinylimidazole, or poly(vim), as an electrolyte for Ca batteries. Poly(vim) is an imidazole–type polymer which consists of a vinyl chain backbone with imidazole pendant groups. The pyridine N atom in these pendant groups acts as a Lewis base, and therefore, as a ligand to complex with metal ions such as Ca²⁺ [6]. While the electronegativity of N is slightly lower than that of O, the charge delocalization of the pendant imidazole group is lower than that of ether O atoms, which can decrease barriers to ion hopping, yet improve salt dissociation. Furthermore, the ability of Ca²⁺ to form physical crosslinks with the imidazole groups also aids in the formation of closely–spaced coordination sites akin to that seen using Li⁺ [7].

Methods and Results: Polymer electrolytes were prepared using a photopolymerization approach, wherein the 1–vinylimidazole monomer was mixed with Ca(TFSI)₂ salt and Irgacure photoinitiator before being exposed to UV light for 4 hours. For all salt concentrations explored (0.1M, 0.5M, and 1.0M), fourier transform infrared spectroscopy (FTIR) analysis revealed successful coordination of the salt with the polymer to varying degrees, whereas differential scanning calorimetry (DSC) confirmed the amorphous and glassy nature of these polymers, owing likely to crosslinking by Ca²⁺. Mild glass transition (T_g) temperatures were found only after 100 °C, indicating the glassy nature of the polymers at room temperature. It was also found that these polymers contained between 40% and 60% unconverted monomer, technically classifying them as ‘gel’ polymer electrolytes although vinylimidazole is not a solvent. Importantly, electrochemical impedance spectroscopy (EIS)–derived ionic conductivities for these polymer electrolytes were in the range of 0.54 mS/cm to 1.26 mS/cm, with the highest conductivity of 1.26 mS/cm obtained for a salt concentration of 0.5M. This is likely owing to the formation of ionic aggregates at 1.0M concentration, and the lack of sufficient charge carriers at 0.1M concentration, both of which result in low ionic conductivity values. Notably, these are the highest ionic conductivities reported for Ca polymer electrolytes in our knowledge, highlighting the effectiveness of leveraging metal–ligand complexation for improving ionic conductivity in polymer electrolytes. Furthermore, the activation energy required for hopping was found to be 0.19 eV for 0.5M samples, which is lower than incumbent ether–O–based Ca polymer electrolytes [8]. This is a key finding, because it indicates that ion hopping is relatively more favorable in such imidazole–based polymers compared to ether–O based ones such as PEO. We propose that this enhancement in ionic conductivity and lowering of the barrier to hopping is due to the combined effect of charge delocalization on the imidazole groups, the closely–spaced coordinating sites enabled by Ca²⁺–poly(vim) complexation, and the gel content of the polymer, which although glassy in the bulk state, still contains low–viscosity microdomains with lower barriers to ion transport. Lastly, we demonstrate the use of this polymer in a symmetric Ca/Ca cell cycled at a nominal capacity of 0.1 mAh/cm² at a current density of ~0.1 mA/cm² for over 100 cycles, wherein the overpotentials were <2 V, which is acceptable for polymer electrolytes.

Implications: This work has important implications for the Ca battery electrolyte field. First, the high ionic conductivity values obtained herein prompt the use of metal–ligand chemistry as a strategy for developing better polymer electrolytes. Second, this work serves as a bridge between traditional SPEs containing ether based backbones and more exotic poly(ionic liquid)–type electrolytes that can offer even more exciting properties.

Conclusions: We have shown that poly(vim) meets the fundamental requirement of high ionic conductivity for polymer electrolytes, and as such, can be a suitable alternative to existing ether–based polymers. Further improvement of mechanical properties and investigations of ion transport numbers are currently underway, and will be reported in the future.

References:

[1] Schauser, N. S., Seshadri, R., & Segalman, R. A. (2019). Multivalent ion conduction in solid polymer systems. Molecular Systems Design & Engineering, 4(2), 263-279.

[2] Hosein, I. D. (2021). The Promise of Calcium Batteries: Open Perspectives and Fair Comparisons. ACS Energy Letters, 6(4), 1560-1565.

[3] Mindemark, J., Lacey, M. J., Bowden, T., & Brandell, D. (2018). Beyond PEO—Alternative host materials for Li+-conducting solid polymer electrolytes. Progress in Polymer Science, 81, 114-143.

[4] Biria, S., Pathreeker, S., Genier, F. S., & Hosein, I. D. (2020). A highly conductive and thermally stable ionic liquid gel electrolyte for calcium-ion batteries. ACS Applied Polymer Materials, 2(6), 2111-2118.

[5] Jones, S. D., Schauser, N. S., Fredrickson, G. H., & Segalman, R. A. (2020). The Role of Polymer–Ion Interaction Strength on the Viscoelasticity and Conductivity of Solvent-Free Polymer Electrolytes. Macromolecules, 53(23), 10574-10581.

[6] Broekema, R. J., Durville, P. F., Reedijk, J., & Smit, J. A. (1982). The coordination chemistry ofN-vinylimidazole. Transition Metal Chemistry, 7(1), 25-28.

[7] Corder, R. D., Dudick, S. C., Bara, J. E., & Khan, S. A. (2020). Photorheology and gelation during polymerization of coordinated ionic liquids. ACS Applied Polymer Materials, 2(6), 2397-2405.

[8] Genier, F. S., Burdin, C. V., Biria, S., & Hosein, I. D. (2019). A novel calcium-ion solid polymer electrolyte based on crosslinked poly (ethylene glycol) diacrylate. Journal of Power Sources, 414, 302-307.