Flexible lithium-ion batteries (LIBs) have a strong oncoming consumer market demand for use in wearable electronic devices, flexible smart electronics, roll-up displays, electronic shelf labels, active radio-frequency identification tags, implantable medical devices, and so forth. This market demand necessitates research and development of new flexible LIBs to fulfill the electrical energy and power requirements of these next-generation devices. In this study, we investigate the performance of a new flexible LIB made from semi-solid electrodes. The semi-solid electrodes are made by adding a mixture of electrode active material and conductive material to the organic liquid electrode. This gives dense and viscous slurry so that all solid particles can move by acting pressure, shear, or bending forces to the battery. To study the performance of this battery we develop a 3D heterogeneous mathematical model that considers all necessary transport phenomena including the charge and mass transfer and electrochemical reactions at the continuum mechanics level on the reconstructed 3D structure of semi-solid electrodes. The finite element analysis (FEA) method was used to solve the governing equations using the comsol multiphysics software package. The model is validated using experimental data for the flexible LIB made in the lab. Based on the developed model, several studies are conducted to understand the effect of the battery discharge rate and the operating temperature on the battery capacity. These studies recommend an operational range for the temperature and discharge rate for this flexible LIB.