The Influence of Polar Functional Groups in Hot-Melt Extruded Polymer Blend Electrolytes for Solid-State Lithium Batteries
Authors: Caradant Lea , Verdier Nina , Foran Gabrielle , Lepage David , Prébé Arnaud , Aymé-Perrot David , Dollé Mickaël 
Published: July 2022

Following the COP26 Summit in November 2021, more than hundred countries pledged to reach zero-emission by 2070 at the latest and the major car manufacturers committed to selling only electric vehicles by 2040. Currently, lithium-ion batteries (LIBs) are among the most widely used storage systems because of their high energy and power densities and long lifespan.1

The early LIBs are composed of intercalation electrodes, electronically isolated by an ion-conducting organic liquid electrolyte. However, the use of liquid electrolytes presents some disadvantages – especially in regard to consumer safety – related to short-circuits and potential leakages of the flammable liquid solvent. Moreover, in the case of lithium metal batteries, the combination of a liquid electrolyte and a high-capacity lithium metal anode leads to the uncontrolled deposition of lithium during the reduction, forming dendrites between the electrodes. A promising way to avoid this instability and improve battery safety is to replace the liquid electrolyte with an ion-conducting solid electrolyte.2

Among them, solid polymer electrolytes (SPEs) represent one of the most attractive alternatives due to their capacity to effectively conduct ions and higher mechanical resistance than their liquid counterparts.3 An important criterion for selecting polymers for use in SPEs is their ability to dissolve lithium salts through polar functional groups. Salt dissolution results in the replacement of ion-ion interactions in the lithium salt, with ion-dipole interactions in the polymer. The transport mechanisms of these ion-conducting materials differ from those of liquid electrolytes. Cation transport in polymer involves two steps which are considered to be dissociated or not, depending on the model chosen (Arrhenius or Vogel-Fulcher-Tammann, respectively). The application of one of these models provides interesting information on the ionic mobility dynamics in SPEs and, in particular, on the interplay between ionic jumps and polymer chain mobility. In the Arrhenius model, ionic jumps occur between coordinating sites, without taking into account the influence of segmental relaxation. Conversely, the VTF equation implies a strong relationship between these parameters. According to previous studies, higher segmental motions in the amorphous phase of polymers mainly provide ionic transport, which explains the limited ionic conductivity of SPEs at ambient temperature (less than 10-5 S/cm).

Another major limitation of SPEs is primarily related to their dual role as electrolyte and binder in composite electrodes, which requires contradictory requirements to be met. Indeed, SPEs must have both sufficient flexibility to allow good interfacial contact between the electrode components and sufficient rigidity to limit short circuits.

Polymer blending has emerged as an economic and effective technique to develop new SPEs which may simultaneously combine properties of each polymer and control the intrinsic properties of the resulting blend by adjusting the formulation.4 Moreover, polymer blends can be obtained by a solvent-free processing method, which reduce SPE toxicity and production time (no solvent evaporation). However, polymer blending makes both the salt dissociation processes and the ionic transport more difficult to understand as both polymers can dissolve lithium salts with their polar functional groups. Each polymer has different ionic transport properties depending on its architecture and thermal properties. Currently, no systematic survey comparing the ability of polymers with various functional groups to dissolve lithium salts in blends has thus far been conducted.

In this presentation, we will discuss the salt dissociation ability of polar functional groups in various polymer blend SPEs. These groups are limited to those that are most commonly present in SPEs : ether, nitrile, carbonate, ester, alcohol and amide.5 The blends presented have been obtained by extrusion, which allows the effect of solvents on salt/polymer interactions to be neglected. In this work, coupled FTIR, EDX and 7Li NMR analyses allow the interactions between LiTFSI and the polymer blends to be determined with a good degree of certainty. Our original study combines experimental and theoretical approaches to determine effects of polymers’ lithium salt solvating ability on polymer blend electrolyte properties and represents an advancement in understanding and optimizing polymer selection for SPEs, used in lithium-ion batteries.

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Caradant, L., Verdier, N., Foran, G., Lepage, D., Prébé, A., Aymé-Perrot, D. and Dollé, M. ACS Applied Polymer Materials
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Figure 1