Understanding the thermal and electrochemical behaviour of lithium-ion batteries (LIBs) under different operating conditions is essential for enhancing their performance and safety. In this study, an electrochemicalthermal model is developed for an 18650-type NCM523/graphite LIB and validated through experiments at various C-rates and ambient temperatures. The model is then employed to analyse heat generation characteristics, electrochemical non-uniformity, and the differences between charging and discharging under varying Crates, environmental temperatures, and effective heat transfer coefficients. The results indicate that ohmic heat is the primary contributor to total heat generation, while the proportions of polarization and reversible heat decrease with increasing charge-discharge rates. Electrochemical reaction non-uniformity intensifies at higher rates, with the negative electrode exhibiting greater non-uniformity during charging than discharging. Furthermore, local current density distributions reveal the propagation patterns of electrochemical reactions within the electrodes. At the end of charging, lithium ions deintercalate from the region near the separator in the negative electrode and migrate deeper into the electrode. These findings provide valuable insights for optimizing battery thermal management, improving fast-charging capabilities, and enhancing battery safety and lifespan.