Lithium-ion batteries (LiBs) are widely used in electric vehicles due to their high energy and power density. The operating temperature has a significant impact on the thermal performance and longevity of LiBs. The thermal performance of an air-cooled battery module containing 16 (4S4P) high-energy density LiBs has been investigated through a series of experiments and numerical simulations. At varying transverse and longitudinal cell spacing, airflow rates, ambient temperatures, and discharge C-rates, the thermal performance of a battery module with aligned battery cells was analyzed. For the thermal performance evaluation, the average temperature rise, temperature non-uniformity, and maximum temperature of the module’s battery cells are utilized. During discharge cycles, the rate of temperature increase is linear but becomes nonlinear at the end of the discharge cycle. In the current architecture of the battery module, a minimum space utilization ratio of 0.38 is necessary to limit maximum temperature and temperature non-uniformity to safe battery thermal management temperatures. The thermal performance was significantly affected by the airflow rate. Increasing airflow rate decreases temperature but increases pressure drop substantially. The maximum cell temperature is greatly affected by the inlet air temperature, increasing from 62.8 °C to 76.6 °C when the inlet air temperature is increased from 30 °C to 45 °C. At high ambient temperatures (over 40 °C), LiB temperatures exceed permissible limits, and air cooling alone is inadequate. This study examines the thermal performance of an air-cooled battery module working at high temperatures.