For the proper design and evaluation of next‐generation lithium‐ion batteries, different physical‐chemical scales have to be considered. Taking into account the electrochemical principles and methods that govern the different processes occurring in the battery, the present review describes the main theoretical electrochemical and thermal models that allow simulation of the performance of lithium‐ion batteries, including different materials and components (electrodes and separators) and battery geometries. As the separator plays an essential role in the performance and safety of lithium‐ion batteries, the recent theoretical simulation work for this battery component are shown, with particular emphasis on morphology, dendrite growth, ionic transport, and mechanical properties. Further theoretical simulations and modeling of this battery component are still required for improving performance, taking into consideration varying geometric parameters such as pore size, porosity, and tortuosity as well as the optimization of the lithium diffusion process and ionic conductivity value. Theoretical simulations of battery separators will play an essential role in the new generation of lithium‐ion batteries, allowing the improvement of their performance while reducing experimental probes and time.