Energy Density vs. Power Density

Energy density is the amount of energy a battery has per unit volume or mass. Another term used to describe energy density per volume is volumetric density and gravimetric energy density refers to the energy per mass. Most often though, energy density usually refers to volumetric energy density and specific energy refers to the gravimetric energy density. Energy density is an important battery metric as it helps to ascertain battery fitness for different applications. Higher energy density means that for the same volume, more energy is stored in a battery. For electric vehicles where volume is limited, this is a sought after quality as the car will have longer range. For grid applications, this is also highly advantageous. Power density is how quickly energy can be pulled from a given volume of a battery. This directly relates to applications such as accelerating in cars and other applications requiring power bursts.

It is however hard to maximize both of these in a single product because the electrochemical factors that impact both of these work inversely to each other. These factors in electrodes include electrode thicknesses, porosities, and particle size. The lower the electrode porosity, the higher the energy density because more active material is present in the same volume. However, with fewer pores and open routes for electron passage, there is increased resistance in quickly accessing the active material which reduces the power density. The same applies to thicker electrodes and particle size which result in higher energy density and lower power density due to increased resistance.