Single-ion hybrid polymer electrolytes for Li-metal battery – SELPHy

It is well known that a homogeneous dispersion of nanofillers in a polymer matrix improves several properties of the latter, in particular the mechanical properties. We have prepared several series of hybrid/composite electrolytes composed of nanofillers and ion-conductive polymer matrix, the POE. The nanofillers used were polyhedral silsesquioxane (POSS) and silica nanoparticles functionalized with short PEO (or PEG) chains and the TFSI anion. This approach allows to obtain a good dispersion of the nanofillers in the polymer matrix and thus to increase the mechanical properties and the charge fraction carried by the Li+. The physico-chemical properties in particular the dispersion state of the nanofillers and the molecular dynamics of these hybrid electrolytes have been characterized in detail, which has made it possible to theorize the relations between the chemistry/nature of the nanofillers, and their electrochemical performances leading to a selection and efficient design of these new electrolytes.

The use of TFSILi/PEG grafted POSS nanofiller or macroanion associated with a stable organic solvent, tetraethylene glycol (TEG), allowed to obtain electrolytes with an ionic conductivity of 1.9 x 10-4 S/cm and a transport number of Li+ of 0.75 at 25°C. Through a deep characterization of these new systems, we have established an empiric equation linking the ionic transport of Li+ and the size/chemistry of the macroanion which could be used for the design of new macroanions. New hybrid single-ion electrolytes (HySI) have been synthesized via sol-gel method. We designed an organic PEO network as a matrix for lithium transport, mechanically reinforced thanks to the inorganic crosslinking nodes (SiO1.5), while highly delocalized STFSI anions are grafted onto inorganic sites to produce a single- ion. These electrolytes have remarkable mechanical properties with only 3wt% of SiO1.5. A maximum ionic conductivity of 2.1 x 10-5 S/cm and a t+ comprised between 0.80 and 0.92 at 80 °C were obtained for these new hybrid electrolytes. The addition of a small amount (~ 10% by weight) of carbonates allows to multiply the conductivity by 3 without significant compromise for the mechanical strength.

The macroanion approach, mainly based on POSS-grafted with TFSI anion, makes it possible to produce stable and safe liquid electrolytes with high ionic conductivity (0.2 mS/cm) at room temperature and above all a transport number t+ > 0.75, which has no equivalent in the literature. This opens up many application perspectives. On the other hand, we have also designed for the first time conductive ionic liquids directly by lithium ions. The potential of this approach is very important since this field is completely new.

We have published 3 papers since the beginning of the project. The first one described the synthesis of a new derivative of sulfonyl(trifluoromethylsulfonyl)imide (STFSI), the vinyl-STFSI, that gives an easily access to the preparation of new monomers and polymers carrying STFSI via the Michael addition reaction (1). The second paper valorized our work on the synthesis and characterization of colloidal silica grafted with PEO of different molar masses (2). And the third one published the works on the new system composed of lithium STFSI lithium functionalized POSS-based macroanions. In this third paper, we have highlighted the relationship between the ion transport properties of Li+ and the size of the macroanion based on the combination of different analysis methods such as SAXS, DSC/TGA, EIS and PFG-NMR (3). A French patent following an extension to international patent has also been deposited on crosslinked hybrid electrolyte materials with grafted ions synthesized via a simple and original approach (5). The deep study of these electrolytes has recently been submitted for publication (4). These works were also disseminated through numerous international oral communications.
1) Michael addition” reaction onto vinyl sulfonyl(trifluoromethylsulfonyl) imide: An easy access to sulfonyl(trifluoromethylsulfonyl)imide-based monomers and polymers
Hien The Ho, Marion Rollet, Trang N.T. Phan, Didier Gigmes
European Polymer Journal, 107, 74-81 (2018)

2) Poly(ethylene oxide) grafted silica nanoparticles: efficient routes of synthesis with associated colloidal stability
Sébastien Issa, Fabrice Cousin, Marine Bonnevide, Didier Gigmes, Jacques Jestin, Trang N. T. Phan
Soft Matter, 17, 6552-6565 (2021)

3) Polyhedral Oligomeric Silsesquioxane-based macro-anions to level up the Li+ transport number of electrolytes for lithium batteries
Thi Khanh Ly Nguyen, Trang N. T. Phan, Fabrice Cousin, Didier Devaux, Sumit Mehan, Fabio Ziarelli, Stéphane Viel, Didier Gigmes, Priscillia Soudant, Renaud Bouchet
Chemistry of Materials, 34(15), 6944-6957 (2022)

4) New Crosslinked Single-ion Silica-PEO Hybrid Electrolytes
Sébastien Issa, Roselyne Jeanne-Brou, Sumit Mehan, Didier Devaux, Fabrice Cousin, Didier Gigmes, Renaud Bouchet, Trang N. T. Phan
Submitted to « Polymers »

5) Electrolytes hybrides réticulés à ions greffés
David Grosso, Didier Gigmes, Sébatien Issa, Trang Phan
French patent 2020, N°FR2007349
International patent 2022, N° WO 2022/008741

Today and for some years to come, the development of batteries with high performance and safety at a low cost is the key for the expansion of important industries and markets such as electric vehicles and renewable energies. Lithium-metal polymer battery (LMP) technology is the most attractive one. Lithium-metal as anode shows specific capacity more than ten times that of LiC6 anode used in the widespread lithium-ion battery and is considered as the best to complement the positive air (O2) or sulfur cathodes. However, solid polymer electrolyte must be operated at 80°C to provide sufficient ionic conductivity, so that mechanical properties are weak with a limited electrochemical stability window. Furthermore, as in liquids, the fraction of charge carried by lithium ions is small (transference number < 20%), limiting the battery power performances. In this context, the main objectives of our project is to develop a LMP battery that is able to operate at room temperature (RT), with a high faradic efficiency that enables long cycling (>1000) and with a very limited dendritic growth. To reach these objectives, we propose a multidisciplinary approach gathering different complementary skills to design groundbreaking single-ion nanohybrid electrolytes able to afford different antagonist properties (i.e. high ion transport at RT and high mechanical strength). These materials are composed of ionic functional nanofillers (NFs) and amorphous polymer based on poly(ethylene oxide) (PEO).
SELPHy project therefore devotes to:
• The functionalization of NFs from various families (POSS, colloidal silica, cellulose nanofibers) with amorphous PEO short chains and/or lithium salt.
• The formulation of single-ion nanohybrid electrolytes by blending functionalized NFs with an ionic conductor matrix, i.e. a crosslinked PEO based polymer.
• The depth-characterizations of nanohybrid electrolytes including NFs dispersion state, (macro)molecular dynamics and macroscospic properties (transport and mechanical properties) in the aim to establish the structure-composition-macroscopic properties relationships.
• The assembly of LMP battery prototype to qualify the new single-ion nanohybrid electrolytes.

We are totally confident that our proposed single-ion electrolytes will exhibit:
i) transference number close to 1 since the Li+ counter-ions are covalently grafted to the NFs,
ii) High ionic conductivity (i.e. 10-4 S/cm at RT) thank to the high mobility of the amorphous PEO short chains grafted to the NF surface and the use of high lithium dissociated salt
iii) Sufficient mechanical properties to encounter dendrites growth provided by the crosslinked polymer network and the NFs reinforcing capacity
iv) High electrochemical stability up to 5 V vs Li+/Li (required for the battery comprising high potential active material) due to the grafting of the anions.
v) Enhanced thermal stability for the safety thank to the presence of NFs like POSS.

SELPHy is a collaborative research project involving three academic partners and interdisciplinary as it gathers indispensable expertise in organic and polymer chemistries, nanocomposite materials, physical chemistry, electrochemistry and electrochemical storage.