Electrochemically Activated Multifunctional Carbon Fibre Composites

Abstract

A multifunctional lightweight material combines several functions in a single material entity able to simultaneously bear mechanical loads by acting as a carbon fibre composite component while providing one or more other functionalities at the same time, e.g. storing energy. Carbon fibres are particularly attractive as they can be used both as a high-performance structural material and as an active electrochemical material. Carbon fibres allow for ion insertion into their microstructure with capacities comparable to graphite and hard carbon used in state-of-the-art lithium-ion and other alkali-ion batteries [1]. Carbon fibres are electrically conductive and can therefore replace the conventional metallic current collector. Carbon fibres are thus in this context a truly multifunctional material. A piezo-electrochemical transducer effect (PECT) [2], has also been discovered in ion inserted carbon fibres. The PECT-effect, that mechanical load changes the electrochemical potential of ion-inserted materials, could be used for sensing, where a voltage change is related to the mechanical strain in the material. When carbon fibres are inserted with alkali ions, they expand in the axial direction [3]. This expansion could be used for making linear actuators with high forces or even to create shape-morphing materials [4]. Combing the PECT effect with the expansions allow for using carbon fibres in an electrochemical cell to harvest energy [5],[6]. Such functionalities could be combined with an inherent electrochemical energy storage (battery function) capability. Imagine a solid-state high-performance structural material that senses its own state, can change shape and harvest energy – that is stored in the material itself!In this work, we will show functionalities coming from alkali ion insertion in carbon fibres that goes beyond the battery function, and how these functions can be combined in a single device. The starting point for these concepts are so called structural batteries, which are hybrid and multifunctional composite materials able to carry load while storing electrical energy like lithium-ion batteries. In such a battery, carbon fibres are used as the load carrying material thanks to their excellent strength and stiffness properties, but also as the active negative electrode. They also include a multifunctional matrix system – a structural battery electrolyte (SBE), the material holding the fibres together to make a solid-state load carrying material, but which at the same time is ionically conductive and chemically stable in the voltage window used. It also includes a multifunctional positive electrode – a composite electrode based on conventional ion inserting materials like lithium iron phosphate (LFP) supported by electrochemically active carbon fibres for load carrying and electrical conduction. This can then also be used for inherent strain sensing in the material without any added mass or wiring. The material can also be made to shape-morph with electrical control at low voltages and harvesting energy by converting mechanical strain to electrical power.