On the Relation between Ionic Transport Properties and Viscoelastic Response of Solid Polymer Electrolytes

Abstract

The need for high energy density and micro-scaled power sources is pushing researchers and industries towards the development of three-dimensional architecture micro-batteries and structural batteries. In this perspective, solid polymer electrolytes (SPEs) represents an appealing alternative to conventional (liquid) electrolyte. Due to their intrinsic multifunctionality, SPEs can serve as the structural matrix of fiber-reinforced composites and as the electrolyte of a battery. In addition, the improved safety features, excellent flexibility and processability, allow to cast SPEs into complex three-dimensional geometries without leakage. Experimental studies indicate that mechanical and electrochemical responses of SPEs are related. In particular, the segmental motion of the polymer molecules is crucial for both the viscoelastic properties and the ionic conductivity of the material. A limited polymer segmental motion generally results in a stiffer mechanical response and a reduced ionic conductivity, while the opposite applies when the polymer segmental motion is enhanced. This results in a trade-off between mechanical and electrical performance from a material design point of view [1] and a deformation-dependent ionic transport mechanism in the polymer during service [2]. In this work we present a coupled electrochemo-mechanical model for SPEs. The polymer is modeled as a solid through which the transport of multiple charged species takes place and is described in terms of molar concentration of ions, electric potential and displacement field. Since the molar concentration of ions in the SPE is notably non-uniform during battery service [3,4], the model explicitly accounts for concentration-dependent diffusion coefficients and viscoelastic properties. Results of a finite element method formulation are presented, and the impact of the viscoelastic mechanical behavior on the overall battery response is discussed.