Properties and performance of energy storage structural composites

<p>The rapid increase in the number and types of electric road vehicles (EVs) needs to be supported by the use of lighter-weight materials as well as better electrical energy storage systems. Carbon fibre reinforced polymer (CFRP) composite materials and lithium-ion polymer (LiPo) batteries are expected to assist the global automotive manufacturing industry meet the demand for more weight and energy efficient EVs. Some EVs are presently using CFRP composites and Li-ion batteries for separated functions; with the composite for structural applications and the batteries for energy storage. However, this approach is not optimised for mass and volume savings, and therefore the specific energy density of the EVs is not optimal. Multifunctional CFRP composites which can simultaneously carry structural loads and store and supply electrical energy are one potential solution for future EVs.</p> <p>The aim of this PhD project is to advance the development of energy storage composites with embedded pouch LiPo batteries and structural battery composites for potential future use in EVs. The project uses experimental testing, finite element (FE) modelling and parametric FE analysis to understand the effect of embedded LiPo batteries on the mechanical and thermal properties of energy storage carbon fibre laminates and sandwich composites. The project also explores the suitability of carbon fibre composites as a battery package material.</p> <p>A comprehensive review of published research into energy storage composites with embedded batteries and their applications is presented in Chapter 2 of this PhD thesis. The review concludes that there are outstanding research challenges that must be solved to enable the practical implementation of composites with embedded batteries into EVs. Some of the challenges are addressed in this PhD project.</p> <p>The composite materials considered in this PhD project are CFRP laminates and CFRP/polymer foam core sandwich composites. The composites with embedded batteries were made using vacuum assisted resin infusion and wet hand lay-up techniques. The effect of LiPo batteries embedded inside these composites on the mechanical properties under different loading conditions including tension (Chapter 3) as well as compression, impact and compression-after-impact (CAI) (Chapter 4) are investigated. Experimental testing and FE analysis reveal that the embedded batteries significantly change the mechanical properties and impact properties of CFRP laminates. Tensile modulus and strength are reduced by up to ~45% and ~60%, respectively. Impact energy absorption is increased by up to ~80%. The change in the laminate properties is due to two major reasons: i) the need to remove stiff and strong composite material for the insertion of the significantly lower (by ~330x) stiffness LiPo battery and ii) the creation of an internal geometric stress concentration due to the low stiffness of LiPo battery compared to the laminate material. In contrast, the mechanical properties of sandwich composites are not significantly altered because the batteries are embedded in the foam core, which has similar properties to the LiPo battery. FE models are developed to predict the change in properties of laminates and sandwich composites due to embedded batteries, and the numerical accuracy of the models assessed using experimental test data. The energy storage performance of embedded LiPo batteries before, during and after mechanical and impact loading of the composites is also investigated.</p> <p>FE models are used to perform parametric analysis to understand the tension and compression properties of CFRP laminates with distributed arrays of embedded batteries for future automotive vehicles (Chapter 5). FE parametric analysis expands the understanding of the effect of multiple embedded batteries (up to 400) on the tension and compression properties of CFRP laminates. The analysis reveals that a flat CFRP laminate with a distributed array of embedded batteries has the potential to replace certain battery packs currently used in EVs. However, a curved CFRP laminate with a distributed array of embedded batteries has lower mechanical properties, and therefore is more suitable to be used as additional energy storage systems.</p> <p>The thermal properties of the composites with embedded LiPo batteries are studied experimentally and using FE analysis (Chapter 6). The thermal performance of LiPo battery is different when embedded in CFRP laminates compared to sandwich composites. CFRP laminate acts as a heat sink which helps dissipate heat generated by embedded batteries. The rise in the battery surface temperature is reduced by up to ~60%. In comparison, the foam core traps heat radiated from embedded batteries in sandwich composites. The rise in the battery surface temperature is increased by up to ~10%. Despite the temperature rises within sandwich composites, the LiPo batteries stay within their operating temperature range.</p> <p>CFRP laminate for use as a battery packaging material is elevated to determine their barrier layer properties compared to conventional aluminium packaging (Chapter 7). The permeability properties against water and oxygen vapour as well as water and liquid electrolyte absorption properties of CFRP laminate are investigated. The laminate has inferior water vapour and oxygen permeability properties compared to aluminium packaging. That is, the laminate allows too much water vapour and oxygen to permeate the battery stack. Therefore, it is expected that the battery properties are to be degraded (e.g. higher internal resistance, lower capacity) and the ageing mechanisms can be accelerated (e.g. battery life is shortened) when using CFRP for the packaging. The flexural properties of the composite packaging material are negatively affected by the absorbed water. The reductions to the flexural modulus and strength are about 10% and 15%, respectively. However, absorbed water does not adversely affect the tension and compression properties. Tension, compression and flexural properties are not affected by prolonged exposure to the battery electrolyte.</p> <p>The research presented in this PhD thesis provides important new insights, experimental results and FE models to assist with the design, fabrication and performance assessment of energy storage laminates and sandwich composites for next-generation EVs.</p>