Solid polymer electrolytes (SPEs) have since their discovery in the 1970s been considered a promising class of Li-battery materials due to their mechanical flexibility, chemical and electrochemical stability, non-toxicity and the safety of the resulting devices – also possible to function with Li-metal anodes. SPEs have, however, not yet been realized to any great extent due to several shortcomings, primarily their inherent low ionic conductivity. This is likely associated to the limited number of polymer host materials which have been explored: the overwhelming majority of relevant publications and patents have been on poly(ethylene oxide) (PEO) materials. PEO displays some appealing properties such as low T_g and a good dissolution of many Li-salts, but are also associated with problems: semi-crystallinity, low cation transference numbers, and temperature sensitivity. Despite intense research on this category of materials during several decades, the conductivity values have not yet reached the levels which are considered necessary for most commercial applications (10^-2-10^-3 S/cm). The rapidly expanding market for electric vehicles (EVs) demand batteries with higher energy density, improved safety, and prolonged lifetime. These criteria are largely met by SPEs, and the research field could therefore experience a true renaissance if some of the inherent problems for this class of materials are properly solved. Alternative polymer materials to PEO could in this context provide a route forward.

We have been targeting polycarbonates – specifically, poly(trimethylene carbonate) (PTMC) – for Li- and Li-ion batteries. PTMC has a somewhat higher T_g than PEO, but is amorphous and display better mechanical integrity, not least at higher temperatures. The Li⁺ transference numbers are also much higher than for PEO. We have recently shown that functional Li-batteries can be constructed using PTMC as an electrolyte host for LiTFSI salt [1]. We have also shown that the ion transport and/or mechanical properties can be improved by lowering the T_g through monomer functionalization, cross-linking [2] or through co-polymerization with polyesters [3,4], rendering Li-batteries with room temperature functionality. Moreover, we have been able to significantly improve the device performance by tailoring the battery fabrication procedure using oligomer components [5] and utilized X-ray Photoelectron Spectroscopy to better understand the interfacial chemistry between the SPE and common Li-battery electrode materials [6].

References

[1] Sun, B., Mindemark, J., Edström, K., Brandell, D. Polycarbonate-based solid polymer electrolytes for Li-ion batteries. Solid State Ionics, 262 (2014) 738.

[2] Mindemark, J., Imholt, L., Brandell, D. Synthesis of high molecular flexibility polycarbonates for solid polymer electrolytes. Electrochim. Acta, 175 (2015) 247.

[3] Mindemark, J., Törmä, E., Sun, B., Brandell, D. Copolymers of trimethylene carbonate and ε-caprolactone as electrolytes for lithium-ion batteries. Polymer, 63 (2015) 91.

[4] Mindemark, J., Sun, B., Törmä, E., Brandell, D. High-performance solid polymer electrolytes for lithium batteries operational at ambient temperature. J. Power Sources, 298 (2015) 166.

[5] Sun, B., Mindemark, J., Edström, K., Brandell, D. Realization of High Performance Polycarbonate-based Li-Polymer Batteries. Electrochem. Commun., 52 (2015) 71.