Expectations for the next generation of electrical vehicles batteries are continuously increasing in terms of autonomy, environmental impact, time of charge or cost. All-solid-state systems where a liquid electrolyte is replaced by a solid electrolyte are a promising opportunity for the future Li-ion batteries. The two types of solid electrolytes are based on inorganic materials or polymers. In this work, solid polymer electrolytes are studied. Poly(Ethylene Oxide) (PEO) is the gold standard electrolyte polymer with an ionic conductivity of 0.36 mS.cm^-1 at 60 °C in presence of lithium salt (e.g. lithium bis(trifluoromethanesulfonyl)imide (LiTFSI)) [1] benefiting of a low glass transition temperature, about -60 °C. One drawback is the crystallinity which decreases the ionic conductivity and consequently the electrochemical performances.

Our objective is to explore some alternative polymers to PEO in solid polymer electrolytes in particular Poly(EthyleneImine) (PEI), a nitrogen-rich polymer, structurally equivalent to PEO. The higher electrodonor character of nitrogen related to oxygen favors the salt dissociation by a stronger coordination of Li⁺ in PEI than in PEO as already modeled [2]. Branched PEI (bPEI) is an attractive candidate because totally amorphous with a low Tg about -50 °C, thermally stable (above 300 °C) with an electrochemical stability window up to 4.5V vs Li. Nevertheless, the ionic conductivity in presence of LiTFSI is still insufficient, 8.10^-4 mS.cm^-1 @ RT. Thus, the mechanical strength of the self-standing PEI polymer films must be enhanced as well as the conductivity. We considered different strategies like i) blends with more rigid N-rich polymers like polyoxazolines or ii) semi-InterPenetrated Networks (s-IPN) resulting from mobile free PEI chains entangled in a resistant mechanically network of methacrylated PEO. Another pathway consists in hybrid electrolytes by adding inorganic fillers in PEI matrix. Depending on the nature of the fillers, the acidity of their surface and their size, Li ions will be able to interact with them and their transport can be improved. Three fillers were tested and an increase in conductivity was observed at room temperature. All these routes will be detailed to demonstrate the potential contribution of nitrogen-rich polymers in Li-ion batteries.

[1] Z. Xue; D. He; X. Xie, J. of Mat. Chem. A, 2015, 3, 19218–53.

[2] P. Johansson, Polymer, 2001, 42, 4367–73.