The global acceleration of electrification and renewable energy integration has placed lithium-ion batteries (LIBs) at the centre of sustainable energy systems. However, as large volumes of batteries from electric vehicles and stationary storage approach the end of their operational life, their end-of-life (EoL) management has become a critical technical, economic, and environmental challenge. The growing diversity of recycling and second-life options has made the decision of whether a battery should be reused, repurposed, or recycled increasingly complex, with limited standardised guidance to support such choices. This thesis aims to address this gap by developing a transparent and adaptable decision-support framework for optimal EoL pathway selection for Battery Energy Storage Systems (BESS).