Solid Li+-carrying Membranes – SLiM

Polyethers are known as the most suitable polymers for transporting lithium cation. One issue comes from the crystallization of long chains, which considerably limits their mobility, and therefore their ion transport ability, as early as temperatures lower than about 60 °C. Use of short chains in the form of elastomers requires them to be bound as a 3-D network. Thus, it is possible to modify the ends of chains with silicon groups, which allow creating inorganic cross-links (silica-like nano-domains) by simple hydrolysis. The basic idea of SLiM project is to introduce during this cross-linkage step lithium salts bearing the same silicon groups and likely to be incorporated into silica-like cross-links, making them as many lithium cation reservoirs, homogeneously dispersed in the electrolyte. Actually, silica-like nano-domains are able both to physically block the dendrites growth and to immobilize lithium sulfonate groups. The major advantage is that the anion being grafted to the solid (and therefore immobilized) only lithium cation contributes to ionic conduction, which should be a decisive asset to inhibit the formation of dendrites, according to the current knowledge. Moreover they convey enough mechanical resistance to incorporate high amounts of solvent in the form of gels, which keep solid state behaviour (the liquid being confined within the membrane) while gaining satisfactory lithium cation conductivity.

The basic concept of SLiM project, which was to combine in a single step the incorporation of silica-like nanofillers, the cross-linkage of PEO chains and the covalent grafting of anion groups (salts of lithium) was validated. Satisfactory ionic conductivities were obtained for the gels (carbonate solvents) by playing on the degree of cross-linking, the swelling rate and the choice of salt. The mechanical and electrochemical (ionic conductivity) properties of the gels obtained from these nanocomposites can be tuned in a simple way by various synthesis parameters : length of the PEO chains, cross-linking degree (mono or bifunctional chains PEO), size of the silicon nanodomaines (possible addition of tetraethoxysilane precursor during the sol-gel synthesis ), nature of the plasticizer (organic carbonates or ionic liquids).Moreover the simultaneous functionalization of the silsesquioxane nano-domains formed during the sol-gel synthesis by ion groups (actually lithium salts, but it could be also cation groups used as receivers of anion) gives access to high transference numbers of the cation lithium.

The SLiM approach offers the advantages of controlling the nanostructure of the electrolyte membranes via synthesis parameters (from hybrid systems containing “hairy” silica-like nanoparticules to 3D PEO/polysilsesquioxane networks) and of an in situ implementation ensuring a good impregnation of electrode materials. Studies dealing with the stability of these materials over a large number of cycles, and more particularly with the possible metal dendrite formation, remain to be carried on. In addition, the developped materials open prospects for applications to electrochemical devices other than batteries, especially in the fields of the supercapacities and “flexible” electronics.

The preparation and the characterization of flexible single-ion electrolyte membranes based on PEO chains cross-linked by silica-like nanodomains, which moreover are functionalized by salt groups, led to the thesis of Mr. Mathieu Meyer, entitled “Membranes electrolytes with charge carrier Li+” defended at the University of Montpellier on 11/7/2014 and to an article published in J. Mater. Chem. ( ICGM and MATEIS partners). A second article (partner ICGM) has been submitted to New J. Chem. Two other ones are in preparation (ICGM, LEPMI and IMN partners). Works on the synthesis, the characterization and the study of the electrochemical properties of some original ionic liquid lithium salts containing anions tagged with ether oxide units led to the thesis of Mrs. Priew Eiamlamai, entitled: “Polymeric electrolytes containing ionic liquids for battery with lithium”, defended at the University of Grenoble on 2/20/2015. Two additional articles are in preparation on this part.

There is currently a strong industrial demand for all-solid-state lithium batteries. Especially, solid electrolytes offer crucial advantages in terms of safety. This demand has prompted the recent development of polymer electrolyte membranes (PEMs), which are particularly attractive because they can be flexible and shaped in different geometries. However there are some important limitations to their industrial development. As a matter of fact ion conductivity in solid state up to now remains significantly below that observed in liquid, especially at low temperature.
There is currently a burst in using room-temperature ionic liquids (liquids consisting of only ions) in polymer-in-salt systems, due to their unique properties (high ionic conductivity, negligible vapor pressure, thermal stability, non-flammability and wide electrochemical stability window). However, the simple dissolution of a lithium salt in ionic liquids always results in electrolytes in which only a fraction of the current is actually carried by Li+ ions. Moreover, extensive loadings of ionic liquid entail a loss of mechanical resistance of PEMs in relation to plasticizing effects, and some release of the ionic liquid may affect their long-term stability.
SLiM project proposes to address these issues by designing new ionic liquid lithium salts and by incorporating them into nanocomposite polymer-silica membranes arising from the end-bonding of polyethylene oxide (PEO) chains to silica-like nanofillers. PEO chains are chosen for their outstanding ability to solvate lithium cation. The silica-like nanofillers are produced in situ by sol-gel. Covalently bonded to the polymer chains, they provide mechanical strength, thus allowing high loadings of ionic liquid. Moreover, the sol-gel process permits to functionalize the surface of silica-like domains with lithium salt groups (sulfonates or bis(sulfonyl)imides), which afford lithium cations as charge carriers, while minimizing the contribution of anions, which are bonded to the surface of the nanofiller, to the ionic conduction.
In PEMs for lithium battery, ionic liquids (ILs) are usually quaternary aliphatic ammonium salts, in which a lithium salt is dissolved. In the present project, the ionic liquids will be themselves lithium salts. Moreover their sulfonate (or bis(sulfonyl)imide) anions will be tagged with ethylene oxide units. Accordingly, they are expected both to act as plasticizers of PEO chains and to contribute to the solvation of Li+. Actually, all the components (the ionic liquids as well as the functional groups in nanofillers) are built from original aromatic perfluorosulfonate and perfluorobis(sulfonyl)imide synthons developed and patented by LEPMI partner for their high ionic conduction performances.
The SLiM consortium gathers four academic partners that have got a sound experience in the fields of the synthesis and characterization of materials: sol-gel and (macro)molecular chemistry (ICG), polymer electrolyte materials (LEPMI), mechanical properties and nanostructure of composite systems (MATEIS), lithium battery materials (IMN). Both fundamental and applicative, SLiM aims at developing some new concepts in the design of nanocomposite electrolyte membranes, while deepening the understanding of dynamics of Li+ ions through nanostructured networks.