Li-ion batteries are critical in enabling environmentally sustainable modernization of our society. These systems have an excellent performance and convenience with multiple applications, but elevated temperature or abuse can cause thermal hazard limiting the adoption. In a bid to increase the energy density, batteries have been packed with higher active material content, making the issue of thermal safety even more grave at present. The thermal safety characteristics of a Li-ion battery are dynamic and dependent on the state of charge, operating condition, and ageing history of a battery. The present study aims to comprehensively understand the thermal safety under these scenarios using combination of accelerated rate calorimetry (ARC) experiments with a physics-based model for simulating thermal runaway. We approach the issue of thermal stability fundamentally by modelling the material level electrochemical reaction kinetics characteristics and coupling them with cell level behavior. This can highlight the hazardous electrochemical interactions in a cell and estimate the severity of a catastrophe at cell level.