Fiber Metal Laminates (FML), produced in both monolithic and sandwich configurations with glass-, basalt- and carbon-reinforced composites, were investigated for application in fire-resistant lithium battery boxes. Different resins, including recyclable and bio-based systems, were tested to improve sustainability; cores of recycled PET (RPET, 150 g/dm³, 10 mm) were considered. The study focused on the effect of core introduction on mechanical performance, with the dual goal of reducing weight and achieving stiffness values compliant with automotive OEM standards for lithium battery housings. Results demonstrated that sandwich structures improved stiffness up to 12-fold compared to monolithic laminates, while preserving the corrosion resistance of the outer aluminium layer and the flexural strength of the laminates after 670 h of Neutral Salt Spray (NSS) exposure.