Thermal management of lithium-ion batteries using graphene coated nickel foam saturated with phase change material

Abstract

Lithium ion (Li-ion) batteries are an integral part of electric vehicles and hybrid electric vehicles because of their high energy and power density. These batteries suffer from a high temperature rise during operation, thus affecting their life span and efficiency. It is necessary for electric vehicles (EVs) and hybrid electric vehicles (HEVs) to have a highly efficient thermal management system to maintain high powered lithium ion batteries within permissible temperature limits. In this thesis, an efficient thermal management system for high powered lithium ion batteries using a novel composite (nickel foam-paraffin wax) is designed and investigated experimentally. The results have been compared with two other cases: a natural air cooling mode and a cooling mode with pure phase change materials (PCM). The results indicate that the safety demands of lithium ion batteries cannot be fulfilled using natural air convection as the thermal management mode. The use of PCM can dramatically reduce the surface temperature within the permissible range due to heat absorption by the PCM undergoing phase change. This effect can be further enhanced by using the nickel foam-paraffin wax composite, showing a temperature reduction of 31% and 24% compared to natural air convection and pure PCM, respectively. The battery surface temperature decreases with the decrease of porosity and the pore density of the metal foam. To maximize the advantages of porous structures, effective thermal conductivity calculations are critical in designing suitable thermal management systems. In this thesis, a mathematical model to determine the effective thermal conductivity of graphene coated metal foams is reported. Possible effects of foam porosity and the filling medium inside the foam are studied and modeling is based on the 2D hexagonal structure of the graphene coated metal foams. The ligament of the foam is of a cylindrical shape and nodes are treated by square geometry. A comparison study for thermal conductivity is formulated between the empirical model and experimental results. It is found that the effective thermal conductivities calculated by the model are in good agreement with the experimental results, in which the deviation is less than 2%. Finally, thermal management of Li-ion batteries is accomplished using a novel material (Graphene coated nickel (GcN) foam saturated with phase change material (PCM)). The growth of graphene on nickel foam is carried out using chemical vapor deposition. The thermal conductivity of the pure paraffin wax is enhanced by 23 times after infiltrating it into the GcN foam. The melting and freezing temperatures of the GcN foam saturated with PCM are increased and decreased respectively as compared to pure paraffin wax. The latent heat and specific heat of the GcN foam saturated with PCM is decreased by 30.41% and 34.10% respectively as compared to pure PCM. The thermal management for Li-ion batteries is also compared among five materials: nickel foam, GcN foam, paraffin wax, nickel foam saturated with PCM and GcN foam saturated with PCM. The battery surface temperature under a 1.7 ampere discharge current using graphene coated nickel foam saturated with PCM dropped by 17% as compared to using nickel foam.

Date of Award	2017
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology