Rechargeable lithium-ion batteries (LIBs) are now playing crucial roles in power supply and energy storage systems. Among all types of rechargeable batteries available nowadays, LIBs are one of the most important ways to store energy because of their high energy density, high operating voltage, and low rate of self-discharge. Nonetheless, the performance of LIBs could be improved by different design parameters, such as the size of solid particles in the battery composite electrodes. Therefore, this study aims to investigate the effect of the composite electrode particles size on the electrochemical and heat generation of an LIB. A Newman's electrochemical pseudo two-dimenisonal model was used to model the LIB cell. Reversible heat produced through electrochemical reactions was calculated as well as irreversible heat originating from internal resistances in the battery cell. Our results show that smaller sizes of electrode solid particles improve the thermal characteristics of the battery, especially in higher charge and discharge currents (C-rate). Furthermore, as the solid particle sizes decrease, the battery capacity increases for various C-rates in charge and discharge cycles.