Bio-based polymers for sodium-ion batteries

Electrochemical energy storage by using lithium-ion batteries (LIBs) is nowadays the solution of choice for many applications (e.g., smartphones, notebooks, cars, etc.). In this context, sodium-ion batteries (NIBs) are a good alternative thanks to sodium abundance on the Earth’s crust and its non-toxicity, especially for large stationary storage plants where record performances, weight and dimensions are not the most relevant aspects. Electrolytes for NIBs can be liquid non-aqueous solvents entrapped in a non-woven separator or solid polymers (SPEs and GPEs for gel-polymer) entrapping liquid electrolytes leading to a quasi-solid system. However, most common electrolyte compounds (liquid or polymer) are produced from the petrochemical industry and oil refinery. In this scenario, the research of new bio-based polymers for NIBs is of high interest for both industrial and research communities. A class of potential candidates for this purpose is represented by poly(glycidyl ether)s (PGs), that have structural similarities to poly(ethylene oxide) (PEO), the most used polymeric matrix for (quasi)-solid battery electrolytes. The presence of pendant ether groups in PGs inhibits crystallization that represents the major drawback for PEO and a large fraction of oxygen atoms promotes salt dissociation increasing the ion mobility. Furthermore, these polymers can be obtained from glycidol (2,3-Epoxy-1-propanol) and protected glycidol by controlled cross-linking and polymerization. Recently, we demonstrate that glycidol can be obtained as high-value bio-based product from epichlorohydrin industry waste (Epicerol process developed by Solvay®). Therefore, the preparation of PGs from bioglycidol seems to be really promising in the light of the twelve principles of Green Chemistry. Poly(ethoxy ethyl glycidyl ether) (PEEGE) and poly(glycidyl methyl ether) (PGME) were synthesized by anionic polymerization of protected form of glycidol such as glycidyl ethers, and fully characterized (NMR, GPC, TGA and DSC). Both the polymers were tested as SPEs with the addition of sodium salt and as GPEs, by adding carbonate solvents. The obtained polymer electrolytes displayed an ionic conductivity at 25°C similar to the that of a traditional PEO-based polymers (in the range 2-3× 10–7 S cm–1) and a moderate, but stable, capability to perform cell cycling at low temperature for more than 100 cycles. These results encourage the application of these polymer for sodium-ion batteries.