Electrolytes for lithium and sodium ion batteries: the road from ionic liquids to deep eutectic solvents and from solid ionogels to eutectogels.

Abstract

Electrolytes for lithium and sodium ion batteries: the road from ionic liquids to deep eutectic solvents and from solid ionogels to eutectogels. An Hardy, Jonas Mercken, An-Sofie Kelchtermans, Bjorn Joos, Dries De Sloovere, Marlies K. Van Bael

In order to increase the energy density as well as the safety of alkali ion batteries, efforts over the past years have focused on solid electrolytes, mainly for Li+ conduction. Among these, the solid composite electrolyte (SCE) consists of a liquid immobilized in a solid skeleton, for example the ionogel which confines an ionic liquid within an inorganic or hybrid solid matrix. Since deep eutectic solvents have advantages over ionic liquids, such as ease of preparation, cost etc. we proposed a eutectogel (ETG) for lithium ion batteries composed of a DES (LiTFSI/NMA) in a solid silica matrix with a high ionic conductivity of 1.46 mS/cm, high thermal (130°C) and electrochemical stability (up to 4.8V) . However, these materials also are brittle, which in the final cell will influence the interfacial resistance as the contact with the electrode may be limited, besides cracking, limiting the cell performance. Therefore, two strategies were followed for improvement. First, the silica matrix was replaced by a polymer matrix, called the P-ETG, achieving 0.78 mS/cm, stability up to 4.5 V and improved fire safety in comparison to the conventional liquid electrolyte (1M LiPF6 in EC/DEC) . Depending on the polymer that is used to build the matrix, the stability could be further improved (1.5-5.0V), which allows stable cycling with high energy density NMC622 cathodes . Furthermore, insights into the DES-polymer interaction allow to optimize the conductivity from 0.4 mS/cm to 1 mS/cm at 25°C . Second, turning to sodium ion batteries, the silica matrix was organically modified with phenyl groups in ionogels, which reduces the Young’s modulus from 29 to 6 MPa. This improves the charge transfer resistance in Na/Na2Ti3O7 half cells, but also decreases the ionic conductivity somewhat to 3 mS/cm; the anodic stability was 3.9V vs. Na+/Na . The reason that the latter material was an ionogel, was that to the best of our knowledge, at the time there wasn’t any suitable sodium ion conducting DES available in literature. Therefore, its development became the subject of our research as well. A first DES consists of NaTFSI and NMA, demonstrating that high concentrations of salt are needed to improve the electrochemical (up to 4.65 V) and fire hazard stability, while compromising the conductivity, which for the most stable electrolyte is 0.3 mS/cm at 20°C . A remaining issue for this DES is that even the highest concentrated one is not stable in contact with Na metal. This is tackled by a novel composition, with optimized sodium metal compatibility and anodic stability up to 4.0 V vs. Na+/Na, and half cells with cycle life and coulombic efficiency on par with cells built with conventional carbonate-based electrolytes . The development of these novel Na+ conducting DES, paves the way to their incorporation into a solid matrix towards the formation of Na+ ion conducting eutectogels . In summary, in this presentation an overview of the group’s work of the past 5 years will be given, to show that quite a distance has been travelled on the road from ionic liquids to deep eutectic solvents as liquid electrolytes for sodium ion batteries, and from ionogels to eutectogels in both lithium ion batteries and sodium ion batteries. This has allowed to achieve improvements in cell performance regarding energy density, stability, safety and compatibility with metal anodes.