Silicon/graphite composite electrodes are promising because of their high capacities, and much research has been conducted to speed up the commercialization of lithium-ion batteries with silicon/graphite electrodes. However, most of the research focuses on electrochemical and mechanical behaviors of the composite electrodes, and thermal behavior analysis of silicon/graphite electrodes is scarce. It is necessary to study the thermal behavior because it hugely affects the performance and safe operation of lithium-ion batteries. This study for the first time develops an electrochemical-thermal model for silicon/graphite electrodes based on a multi-material framework, which can separate the electrochemical and thermal behaviors of each electrode material. Using the model, the thermal characteristics of silicon/graphite electrodes are investigated. The research reveals the relationship between heat, characteristics of active materials, and their content in the composite electrode. At the same C-rate, an electrode with a higher silicon content experiences a higher temperature rise. Thermal peaks representing the phase transition processes of graphite are observed during (de)lithiation, which can be potentially used to detect the aging of silicon-based batteries in service. We further analyze the contributions of different heat sources. The heat generation of graphite converges on the beginning stage of delithiation followed by huge heat generation from silicon. In contrast, the two active materials are lithiated simultaneously, and graphite plays a dominant role during its phase transition processes. For a composite electrode with a mass ratio of silicon to graphite of 0.2 at a moderate C rate (2C), ohmic heat generation is the major contributor to heat generation accounting for 41% of the total heat generation, followed by reversible (36.1%) and irreversible (22.9%) heat generation. This model paves the way for experimental work regarding the thermal characteristics of silicon/graphite composite electrodes and can be potentially used for the thermal analysis of large-format batteries with silicon/graphite electrodes in the electrical vehicle industry.