Structural Batteries – the Best or Worst of Two Worlds

Abstract

The electrification of road transport is continuously progressing, and concomitantly lithium-ion batteries are developing towards higher energy densities. However, the theoretical limitations of this technology are expected to be reached in the coming years [1], leaving more demanding applications, like aeronautic ones, with high challenges regarding fully battery electric propulsion of commercial aircrafts. Structural batteries (or multifunctional energy storage), i.e. batteries which show sufficient mechanical stability to substitute conventional composite materials in load bearing structures, show a promising way to resolve this issue. Due to their multifunctional properties, structural batteries can replace interior and exterior elements of e.g. an airplane, and thus provide at best energy storage without weight increase [2].The idea of structural batteries emerged around 20 years ago, and different cell designs have been proposed including coaxial wires [3] and planar reinforced stacks [4]. Nevertheless, high degrees of multifunctionality could not be realized and the approach was never industrially applied [5]. Recently, the need to electrify the mobility sector and the prospect of massless energy storage has reincited a lot of new research activities around this topic.The main challenge at electrochemistry level for the development of structural batteries is to find an alternative electrolyte for the conventionally used carbonate-based liquid ones which provide only negligible mechanical stability. Apart from high ionic conductivities and safety, the additional requirements for the structural electrolyte include high mechanical capabilities (e.g. high Young’s modulus), good adhesion to all cell components, as well as high thermal stability. The latter is especially important for implementation of the structural batteries in composite structures, which are usually cured at elevated temperatures well beyond the stability range of conventional electrolytes. These demanding specifications render the use of conventional lithium-ion battery components formassless energy storage as futile.In this work, two different approaches for the development of electrolytes for structural batteries are discussed. The first one employs a bi-continuous electrolyte based on thermoset materials, whereas the second one uses a gel electrolyte. For the former two separate phases form percolating networks with different properties throughout the electrolyte membrane. One phase provides high ionic conductivity to achieve high electrochemical performance of the cells, whereas the other one yields high mechanical strength. The second approach uses a mono-phasic electrolyte which provides both, electrochemical as well as mechanical performance. The two approaches are further compared and discussed, focusing on their applicability for structural batteries. Important parameters like processability and feasibility for implementation into a composite structure, which are often neglected, are addressed, enabling a holistic view on this technology.