Impact of Additives – Polymer Interaction on the Electrochemical Performance and Stability of Thermoplastic Vulcanizate Based Solid Polymer Electrolyte
Authors: Barcha Cédric , Foran Gabrielle , Chidiac Joseph , St-Antoine Caroline , Azimi Mona , Costalin Mehdi , Caradant Lea , Nicolle Paul , Rousselot Steeve , Prébé Arnaud , Ayme-Perrot David , Dolle Mickael 
Published: December 2024

Li-ion technology has dominated the energy storage market since 1991. However, this technology is currently approaching its limits. Firstly, the use of flammable liquid electrolytes poses safety problems, due to their limited stability at high temperatures. Additionally, the demand for increased energy density and autonomy in electric vehicles is growing [1]. The use of lithium metal as an anode (specific capacity of 3860 mAh.g-1) is one way in which higher energy density can be achieved. Unfortunately, in addition to the thermal instability of liquid electrolytes, lithium metal also gives rise to safety problems as a result of the formation of dendrites during cycling [2]. These risks can be mitigated through the use of a solid electrolyte material, such as solid polymer electrolytes (SPEs) [3]
[4]. SPEs have emerged as a promising option for advanced energy storage applications due to their inherent safety (non-volatility / reduced flammability)[5], their flexibility which make them easier to shape than other types of solid electrolytes, and their promising energy density with the bipolar configuration [6]
[7]
[8]
[9].SPEs must however overcome significant challenges to become commercially viable including their low room-temperature ionic conductivity and low mechanical strength that cannot completely inhibit dendrite growth to permit use with lithium metal electrodes.

This study presents a new class of crosslinked polymer blend electrolytes, thermoplastic vulcanizates, with increased mechanical strength that are prepared by melt processing. [10] In these blends, non-flammable additives were shown to enhance low temperature ionic conductivity and improve electrolyte/electrode interface stability. While the initial ionic conductivities of these electrolytes were modest, the introduction of lithium salt-containing solvents at 20 wt% increased conductivity by approximately two orders of magnitude. The presence of distinct phases for lithium salt dissociation prompted inquiries into the distribution of lithium salt within the electrolyte and the contribution of each phase to the observed ionic conductivity. Analysis involving ionic conductivity measurements, NMR analysis of salt distribution and ion mobility and IR spectroscopy of lithium-coordinating functional groups showed that the overall ionic conductivity is influenced by a combination of factors including lithium salt distribution, interactions with the conductive thermoplastic phase, and localized polymer mobility.

The impact of additive selection on the electrochemical performance and stability of these electrolytes during battery operation is of particular interest. The choice of additive was assessed by measuring the electrochemical stability window of the SPE(s) via potentiostatic intermittent titration technique, and stability against lithium metal with plating and stripping in Li-Li symmetric cells with and without additive. The interfacial stability of these systems was evaluated using SEM imaging and XPS.

By studying the intricate interplay between additive chemistry, battery performance, and structural evolution, we hope to have a better understanding of the processing, design and optimization of SPEs for diverse energy storage applications.

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