The need to shift to cleaner energy sources is imperative. Battery technology is considered a highly promising technology to successfully bring about this shift. It has already been implemented in numerous ways and features in our day-to-day lives; from mobile phones to homes. Recently, concerns regarding their safety have increased and as a result, governments have boosted research efforts in this area, with the added urge to work collectively with industry partners and regulatory bodies. These cells are prone to undergo catastrophic failures as a result of a series of exothermic reactions (thermal runaway) that can be triggered by several methods. Many research efforts have been made to understand this phenomenon from various perspectives: material selection, mechanical design, mitigation or preventative measures. This thesis shows how we can begin to comprehend this complexity and apply it to advancing existing battery safety assessment techniques. Through thermal analyses and multi-scale X-ray CT imaging, the correlations between heat generation and battery architecture are addressed. In this work, for the first time, differential scanning calorimetry was used to measure heat signals from full cells, high aspect ratio battery samples were imaged and a custom-built calorimeter chamber was developed to provide operando images and heat measurements of cells undergoing thermal failure. The results obtained from the methodologies and techniques established in this work have advanced our understanding of how various battery material morphologies and architectures behave under certain stresses. In turn, these findings can aid not only in the development and manufacture of safer lithium-ion batteries but also in the standardisation of testing standards, and improvement of failure mitigation strategies.