The batteries are energy storage device that stores intermittent energy from renewables to supply when it is desired. The marvelous aspect of today's battery industry remains to be the advancement and evolution of high-energy battery technologies. Batteries have moderate energy and power. Energy density measures how much energy is delivered by the battery in proportion to its weight and is measured in terms of Watt-hours (product of capacity (Ah) and voltage) per kilogram (Whkg−1) or per liter (WhL−1). The passage of reaching higher energy density is divided into two ways: pushing the battery's capacity further or lifting the voltage up. Because of its superior energy density, the batteries are the choice in various electrical portable devices, grids, and hybrid/electric vehicles. Energy density is very critical for the prolonged operation of the devices. This chapter provides a brief introduction to the thermodynamic aspects of high-energy-density batteries. © 2022 Elsevier Inc. All rights reserved