Submarines face an ongoing (technical) battle to improve the operational effectiveness by increasing the submerged endurance and range. Installing lithium-ion batteries on new or refitted diesel-electric submarines has become increasingly interesting based on their relatively high energy density and specific energy. The characteristics compared to the currently implemented lead-acid batteries are known, meaning that the implementation of lithium-ion batteries on submarines can increase the submerged range and endurance based on their better specifications. Additionally, lithium-ion batteries require less maintenance and provide a relatively longer life expectancy compared to lead-acid batteries. However, lithium-ion batteries can develop a thermal runaway: a process which exponentially generates heat, leading to the risk of an explosion and fire. A thermal runaway can be initiated based on internal failure mechanisms and external causes, such as exceeding critical temperature limits. Understanding of the thermal behaviour of lithium-ion batteries is essential to reduce the probability of a thermal runaway. The probability of initiating a thermal runaway can be further reduced by auxiliary systems such as a cooling system and a battery management system. The main objective of this research is therefore to investigate the implications on preliminary submarine system design based on the thermal behaviour of lithium-ion batteries and to quantify the thermal behaviour and design implications. The relevance is that by quantifying the thermal behaviour, the cell temperatures can be estimated. As a consequence, awareness of the risk of a thermal runaway is established, while at the same time input for the dimensioning of a cooling system is provided. An analysis has been performed concerning the safety of lithium-ion batteries. The causes, the characteristics and the prevention methods regarding a thermal runaway have been studied in this research. The four stages of a thermal runaway are discussed, as well as the generation of toxic and flammable gasses. Moreover, the sources of heat generation in a lithium-ion cell have been described. The heat generation typically consists of reversible and irreversible heat components, of which the relative contributions have been described based on the charge or discharge rate. To support preliminary submarine design, a thermal model of a lithium-ion battery module has been created. The first model that has been created describes the electric behaviour of a lithium-ion battery cell that is typically implemented in marine applications. EST-Floattech has provided technical specifications of the lithium-ion pouch cell that is implemented in their modules. The second model determines the generated heat based on the electrical model and cell specific properties. The third model simulates a lithium-ion battery module, where heat is generated by multiple cells and heat transfer rates with the surroundings are modelled. A lumped thermal capacity approach has been implemented and cooling is modelled to provide insight in typical cooling rates regarding thermal management. Conclusions have been drawn regarding the temperatures of the cells in the module based on three operational profiles. Based on a submerged sprint, C-rates up to 1.0C can be sustained without cooling while remaining below the critical temperature limit of 55°C. For consecutive cycles during a covert transit or covert surveillance, cooling is typically necessary. Only at C-rates below 0.3C a covert transit can be sustained. Typical cooling rates vary between 60 W and 185 W, where the effects of cooling are most significant for covert transit and covert surveillance. Module optimisation provides increased cooling rates while increasing the energy density and specific energy. This results in a decreased mass and volume of the module, resulting in a significant amount of extra space in the entire battery system on board of a submarine. The largest improvement in thermal management can be realised by choosing the right conductive filler. Moreover, the cooling capacity has a significant influence on the cell temperatures. Cell temperatures remain relatively constant after the fourth cell in a row, meaning that modules are typically not limited by the number of cells.