Energy density (ED): Specific ED, Gravimetric ED & Volumetric ED

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Gravimetric Energy Density logo
Gravimetric Energy Density logo

FAQ - Energy Density

What is the energy density?

The Term “energy density” usually refers to the volumetric energy density, which is the energy per volume in [Wh/L].

Energy-density-Formula. The Term "energy density" usually refers to the volumetric energy density, which is the energy per volume unit.

The volumetric energy density, is a certain amount of energy per volume in [Wh/L].

volumetric-energy-density-definition

The specific energy density is often used to express the gravimetric energy density. Therefore, the specific energy density is given in [Wh/kg] or in [kWh/m3].

specific-energy-density-formula

The volumetric energy refers to the volumetric energy density.

The volumetric energy refers to the volumetric energy density.

The gravimetric energy refers to the gravimetric energy density.

The gravimetric energy refers to the gravimetric energy density.

The gravimetric energy refers to the gravimetric energy density.

The gravimetric energy refers to the gravimetric energy density.

Gravimetric-energy-density-vs-Volumetric-energy-density: An optimized volumetric energy density is desired when limited space is available (like in electric cars). 

An optimized gravimetric energy density is desired when the application needs to be light-weight (like an aerial vehicle or electric airplane).

Gravimetric-energy-density-vs-Volumetric-energy-density: An optimized volumetric energy density is desired when limited space is available (like in electric cars). An optimized gravimetric energy density is desired when the application needs to be light-weight (like an aerial vehicle or electric airplane).

Energy Density - Introduction

Energy-density-Formula. The Term "energy density" usually refers to the volumetric energy density, which is the energy per volume unit.

The energy density describes the amount of energy that can be stored in a battery per mass or per volume.

Thus, the amount of energy per kg (Wh/kg) or the amount of energy per liter (Wh/L). There is a “volumetric energy density“, short “energy density” and a specific energy density also known as gravimetric energy density

Some people call the energy density in short as “Energy” like the “specific energy”. However, this can be confusing as the energy in physics is a “state” and not a density.

The volumetric energy density

volumetric-energy-density-definition
  • Energy density, or volumetric energy density, reflects the amount of energy stored per volume in liters (Wh/L). 

The gravimetric energy density

gravimetric-energy-density-definition
  • Gravimetric energy density or specific energy density (both are the same), defines the amount of energy contained in the battery in per mass weight (Wh/kg)
specific-energy-density-formula

Gravimetric energy density vs volumetric energy density

Gravimetric-energy-density-vs-Volumetric-energy-density: An optimized volumetric energy density is desired when limited space is available (like in electric cars). An optimized gravimetric energy density is desired when the application needs to be light-weight (like an aerial vehicle or electric airplane).

For electric cars, volumetric energy density is more important that specific energy density because battery space is limited. For electric aviation and aviation in general, the gravimetric energy density is relevant because gravity has to be overcome constantly. 

Products that require long run times at moderate loads are optimized for high-energy density; the ability to deliver high current loads can be rather neglected. Electric cars require lithium-ion batteries with a high-energy density, since a passenger car usually wants to drive for a long time at a constant rate.

Gravimetric energy - gravimetric density

Every so often, the term “gravimetric energy” and “gravimetric density” refers to gravimetric energy density.

However, the term “energy” [Wh] is physically not the same as the energy density [Wh/kg]. Therefore, the terms should not be confused. 

The gravimetric energy refers to the gravimetric energy density.

Volumetric energy

The volumetric energy refers to the volumetric energy density.

Occasionally, volumetric energy refers to the volumetric energy density.  However, the term “energy” [Wh] is physically not the same as the energy density [Wh/kg]. Therefore, the terms should be used carefully. 

Volumetric energy density logo
Volumetric Energy Density logo

Energy densities in Batteries

Battery-Pack

The energy density of a battery pack often is specified in e-car marketing.This figure takes into account all the components of a battery pack. This energy density of the “battery pack” is useful for the end user because this energy density is also effectively used in an electric car or other device. The high-energy density in LIB battery packs is responsible, among other things, for the good environmental balance in passenger cars.

"Bare" battery cell

In development and in the trade, the energy density of the “bare cell” is often specified. This is higher than the energy density of the battery pack because “inactive battery components”(components which do not hold energy) such as a BMS, metal components, sealing materials, adhesives, thermal pastes and insulation materials that do not store energy are not included in the energy density

In development and in the trade, the energy density of the “bare cell” is often specified. This is higher than the energy density of the battery pack because “inactive battery components”(components which do not hold energy) such as a BMS, metal components, sealing materials, adhesives, thermal pastes and insulation materials that do not store energy are not included in the energy density

The following battery components are included here: active electrode materials, conductor films, electrolytes, separator, and a simple cell envelope.

Active material

The highest energy density is usually given by researchers who investigate and develop a specific material. Here, for the most part, only the mass of the ACTIVE electrode material under investigation is considered (without additives, electrolytes, separator, and active counter-electrode).

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Energy density in batteries: Specific, Gravimetric & Volumetric Energy Density

This is justified, since in science one wants to investigate and compare individual materials.

However, for the researchers and readers of the research results, it is necessary to always describe exactly which cell components were considered in calculations for energy density, power density, and capacity.

References & Readings

  1. Energy Density on Wikipedia, retrieved November 2022.
  2. Comparison of commercial battery types, Wikipedia, retrieved March 2023.

  3. Wang, CY., Liu, T., Yang, XG. et al. Fast charging of energy-dense lithium-ion batteriesNature 611, 485–490 (2022). https://doi.org/10.1038/s41586-022-05281-0
  4. Doron Aurbach, Elena Markevich, and Gregory Salitra
    Journal of the American Chemical Society 2021 143 (50), 21161-21176

    DOI: 10.1021/jacs.1c11315

  5. Battery Definitions and what they mean, batteruniversity.com, retrieved March 2023.

  6. Batteries with high theoretical energy densities, Energy Storage Materials, Volume 26, April 2020, Pages 46-55.

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