Organic Ionic Plastic Crystals Nanocomposites for Safer Batteries

Rechargeable batteries allow electric energy from intermittent renewable energy sources – such as wind and solar - to be stored until needed. However, important safety concerns limit large-scale use of batteries for electric vehicles and stationary needs. New electrolyte materials with improved safety are needed to enable the next-generation of rechargeable batteries. Electrolytes play the role of conducting ions inside the battery to support the redox reactions at the electrodes. Commercial electrolytes for lithium-ion batteries are based on liquid organic carbonates due to their high ionic conductivity and good wettability with electrodes. Nevertheless, organic carbonates are highly flammable, dry out quickly and can leak out of the battery case. Solid electrolytes have been proposed as an alternative to conventional liquid electrolytes because of their intrinsic safety features. Until now, two main families of materials have been intensively investigated (Figure 1). Inorganic Glass-Ceramics exhibit high ionic conductivities (>1 mS cm-1 for some systems), but with the considerable disadvantage of their rigid interfaces and brittle material properties. On the other hand, Ion-Conductive Polymers show desirable mechanical properties but their ionic conductivity falls short of expectations (<0.1 mS cm-1 for most systems). Recently, a new class of solid electrolytes was proposed by Prof. Maria Forsyth and Prof. Douglas MacFarlane at the Australian ARC Center for Electromaterials Science (ACES). Organic Ionic Plastic Crystals (OIPCs) are crystalline materials entirely composed of small organic ions with short-range molecular motions. This structural feature gives OIPCs several unique properties such as multiple solid-solid phase transitions and plastic mechanical properties, which can greatly improve the interfacial contact with electrodes and - most importantly - facilitate ion diffusion. However, the development of OIPCs as solid electrolytes for rechargeable batteries is still in the early stages. Recent works in Prof. Forsyth’s group showed that composite electrolytes with significantly improved mechanical and ion-transport properties are obtained when OIPCs are combined with polymer fillers. These preliminary studies suggest that chemistry, aspect ratio and size of the fillers affect the structural features of OIPCs matrices at the interface with the polymer. However, a complete understanding of these composite electrolytes is still lacking. The eJUMP action goal is to develop innovative nanocomposites from OIPCs and polymer nanoparticles – which can act both as reinforcement but also add function via a purposely designed nanoparticle interfaces. The fundamental knowledge generated by eJUMP will help to establish specific design criteria for the fabrication of nanocomposite OIPC electrolytes with enhanced safety characteristic and competitive performances with respect to conventional liquid electrolytes.
The specific research objectives of the outgoing stage at ACES are:
1. To synthesize functionalized polymer nanoparticles of sizes between 30-500 nm and controlled surface chemistries as reinforcements for OIPCs nanocomposites,
2. To develop innovative OIPC nanocomposites using the functionalized polymer nanoparticles and last generation OIPCs,
3. To characterize mechanical, thermal and electrochemical properties of the OIPC nanocomposites,
4. To understand ion-transport phenomena in nanocomposite electrolytes using state-of-art characterization methods,
5. To test the most promising materials in sodium/lithium rechargeable battery prototypes.

The fellow developed reproducible protocols for the synthesis of polymer nanoparticles based on emulsion polymerization techniques. The procedures allow to control:
(I) the particle chemical composition
(II) the particle size and size distribution
(III) the functional groups present on the particle surface.
The nanoparticles were used for the synthesis of composite electrolytes based different families of OIPCs. The best performing electrolytes were selected using an array of mechanical, structural, thermal and electrochemical characterization techniques. The fellow has been trained in Solid State Nuclear Magnetic Resonance characterization techniques to study the ion dynamics in the nanocomposite electrolytes. Finally, battery fabrication activities were carried out at the BatTRI-hub prototyping facility of Deakin University to evaluate the performance of the OIPC nanocomposite electrolytes.

The use of polymer nanoparticle fillers represent a paradigm shift in composite solid electrolytes, as fillers for solid electrolytes are usually based on inorganic nanoparticles, such as alumina or silica. The surface modification of inorganic particles is often a complex task that requires several synthesis steps. The eJUMP approach simplifies the synthesis process, as polymerization in disperse media allows one-step synthesis to control not only the particle size but also the functional groups on the particle surface. In addition, the lower density of polymers allows better fillers dispersion into the OICP matrix and lower weight of nanoparticle content within nanocomposites. The polymer nanoparticles were used to prepare nanocomposite electrolytes with Organic Ionic Plastic Crystals (OIPCs) a novel class of solid state electrolytes. The effects of the polymer surfaces on the thermal behaviour and ion transport properties of the OIPCs were investigated. The action found that significantly different interfacial structures, as well as ion transport behaviours in the OIPC are observed based on the type of surface functionalization of the particles. Finally, different transport models based on the results of this projects have been proposed, which provide principle guidelines for the design of future OIPC-based highly conductive electrolyte materials.

Periodic Reporting for period 2 - eJUMP (Organic Ionic Plastic Crystals Nanocomposites for Safer Batteries)

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