This paper investigates the performance degradation of lithiumion cells, with a specific focus on energy efficiency throughout their operational life. A novel metric, termed State of Efficiency (SOE), is introduced to quantify energy losses during charge-discharge cycles. Utilizing empirical battery aging data, the evolution of SOE is analyzed, revealing a linear decay trend. Furthermore, the influence of operational variables, including temperature and discharge rates, on SOE is examined. The findings contribute to a refined understanding of efficiency dynamics in lithium-ion cells, providing insights for enhanced battery management strategies.