Multifunctional automotive structural composites for energy storage

Automobile manufacturers are increasingly using carbon fibre composite materials and electric/hybrid power technologies to reduce greenhouse gas emissions and increase fuel economy. The conventional approach is to design vehicles with composite body panels and energy storage devices (e.g. batteries) as separate systems. Multifunctional automotive structural systems in which the batteries are integrated/embedded within the composite material has the potential to reduce the weight and save space. However, the application of multifunctional energy structures for next-generation vehicles needs a detailed evaluation of their mechanical and electrical properties under various mechanical loading conditions. The aim of this MEng thesis is to experimentally investigate the effect of embedding lithiumion polymer (LiPo) batteries on the mechanical properties and energy storage capacity of carbon fibre composite structures under static and fatigue compression loads. Batteries were embedded inside carbon-epoxy laminate and inside the foam core of sandwich composites with carbon laminate face sheets. The effect of the number and spatial orientation of the batteries within these two types of composites was investigated. The optimum configuration of multiple batteries was determined to provide maximum electrical energy storage capacity with the minimum adverse effect on the mechanical properties. The thesis presents a literature review which critically evaluates published research work investigating the electrical and mechanical responses of batteries used in electric/hybrid vehicles as a guideline for assessing their performance when embedded within composite materials. Then, a detailed review of published research on composite structures containing embedded batteries is presented, including an assessment of manufacturing methods, mechanical properties, failure mechanism and electrical performance under various loading conditions. The main research gaps based on the published literature are identified. The influence of embedding single or multiple LiPo batteries into a monolithic carbon-epoxy laminate is evaluated for both static and fatigue compression load conditions. Experimental testing revealed that the compression properties (modulus and failure stress) and fatigue strength of the laminate are significantly reduced when batteries are embedded. The deterioration to the compressive properties are attributed to several factors, including the compressive properties of the battery being much lower than the laminate material; the geometric stress concentration created inside the composite by the battery; fibre discontinuities in the laminate; and the resin-rich region at the battery-composite interface. The reduction to the compression properties is dependent on the number and configuration of batteries. Aligning multiple batteries in the load direction causes a lower reduction to the compression properties than when aligned transverse to the load direction. It is also discovered that the electrical performance/energy storage capacity of batteries is not affected during compressive fracture at high load of the multifunctional laminate material, which indicates the LiPo battery used in this study is mechanically and electrically robust under compression loading. The mechanical and electrical behaviour of sandwich composites containing LiPo batteries embedded within the foam core is investigated under in-plane and through-thickness (flatwise) compression loads. Embedding the battery into the core of the sandwich composite did not have an adverse effect on the in-plane compression properties because the laminate face skins are not affected. The embedded batteries also retained excellent energy storage capacity under high in-plane compressive loads. Embedding batteries in core of the sandwich composite resulted in the improvement to the through-thickness compression properties, although under high strains the batteries were damaged. In summary, this MEng thesis presents an original experimental assessment of the advantages and drawbacks of fully integrating LiPo batteries into structural laminates and sandwich composites for potential next-generation lightweight energy storage automotive structures.