Power Density vs Energy Density

Jump to: Power Density vs Energy Density in Batteries | Impact of Battery Chemistry | Impact of Battery Design and Engineering

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Power density vs energy density is an important consideration when it comes to battery science. Batteries that have high energy density, (can store a lot of energy) may not be able to deliver that energy quickly. Conversely batteries with high power density (can deliver a lot of power) may not be able to deliver that energy for a long period of time. First let’s consider the definitions of both properties:

Property	Definition	Unit
Energy Density	The amount of energy stored per unit of mass or volume	J/kg, Wh/kg, J/L, Wh/L
Power Density	The amount of power that can be delivered per unit mass or volume	W/kg, W/L

Energy vs Power Formula

The units for both energy and power can be explained by the following relationship formula:

To realise power density and energy density you need to divide by mass or volume.

Power Density vs Energy Density in Batteries

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In the context of batteries, energy density and power density are both key performance indicators. Energy density indicates how much energy a battery can store. Power density determines how fast that stored energy can be released. These properties are not proportional, as one increases the other does not necessarily also increase.

When designing a battery, we must consider the energy and power requirements. Different components and architectures will suit different applications. Ideally, any given battery will have maximum energy and power density in order to give the required power for as long as possible. However, for high power applications such as use in power tools, high power density should be prioritized. Whereas, for applications where battery life is a priority such as in mobile phones, high energy density is the priority. The power required by the phone may be little in comparison to the power tool and therefore not a priority.

High power density batteries are able to discharge stored energy rapidly. Their internal energy release mechanisms allow for rapid charge transfer. This in turn means that charging is also likely to be rapid. This is a significant advantage for application such as electric vehicles where owners want recharge times to be comparable to standard refuelling. Battery designers and engineers must therefore know the desired application of their battery to understand how to create the ideal battery for the role.

Impact of Battery Chemistry

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Different battery chemistries have varying balances of energy density and power density. Although not the only factor influencing these properties, battery chemistry remains one of the most important. The graph below shows a rough schematic of the balance of power density and energy density of different battery chemistries. Lithium containing technology spans impressive energy densities and power densities. However, one type of density often comes at the sacrifice of the other. Pre- and post-lithium battery technology shown here do not manage to span the same range in properties as lithium-based battery technology. This page discusses developments in lithium-ion battery technology but the future of battery technology is not likely to be limited to lithium.

Impact of Battery Design and Engineering

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Batteries can be designed with high energy density or high power density performance in mind. A breakdown of the strategies used to prioritize either property allows use to understand the design-function relationship of battery technology. Understanding the role of each component and how their micro- and macro-scale properties impact the overall battery cell is crucial in the drive for efficient energy storage. Components such as the separator and electrolyte will have the same requirements in whichever property is prioritized. The separator (if needed) should always be as thin as possible and the electrolyte should always be as conductive as possible.

Battery Design Parameters

The key lithium-ion battery design parameter strategies to consider are highlighted in the table below:

Component	Energy Density	Power Density
Electrodes	High mass of dried electrode coating Low electrode porosity Medium/large particle sizes Low conductive additive content Minimum possible binder content to maximize active material ratio	Low mass of dried electrode coating High electrode porosity Small/medium particles sizes High conductive additive content
Current Collectors	Thinner Coated to improve adhesion	Thicker Coated to reduce resistance

Cathode Design

There is a clear inverse relationship between the ratio of power and energy within a battery and the cathode capacity. As the power density decreases relative to the energy density the cathode capacity increases. This means that more charge is stored per area unit of cathode, often referred to as areal capacity. In other words, as energy density is made a priority, the capacity within the cathode tends to increase.

Areal capacity is calculated using the total area of the cathode and the cell capacity. To achieve high areal capacity and energy density, a high mass of dried electrode coating is used per area. High power cells use low areal capacities with low mass of electrode coating.

n/p ratio

The n/p ratio is the ratio of negative to positive electrode capacity. In battery cell design with safety as a priority, a high n/p ratio is preferred. A high n/p ratio means the anode capacity is larger than the cathode capacity. This is so that the anode can reach its full state of charge without reaching its capacity for lithium-ions. If the anode does reach its full capacity of lithium-ions this can trigger unwanted side reactions such as uncontrolled liquid electrolyte decomposition and other exothermic reactions that lead to thermal runaway and battery failure.

Electrode Thickness

Battery cells with high energy density will prioritise maximizing the size of the anode as this stores the lithium-ions in the charged state. The more lithium-ions stored the higher the energy density. The cathode active material is the source of the lithium-ions and therefore should also be maximised. Therefore for high energy density batteries both the anode and cathode are relatively thick whilst still maintaining a high n/p ratio for safety. High power batteries will typically have thinner electrodes to decrease the length of diffusion pathways.

The comparison of an example high energy density and high power density allows us to consider the impact of electrode thickness. The high energy density battery here has a much thicker anode. However, surprisingly the cathode for the high power density battery is thicker than for the high energy density battery. We must consider other material properties to explain this and understand that electrode thickness is a rudimentary indicator of energy/power performance.

Feature	High Energy Density		High Power Density
Material(s)	Lithium Nickel Cobalt Aluminum Powder	Graphite + Si	Lithium Iron Phosphate (LiFePO4) Powder	Graphite
Material Capacity / mA hr g-1	199	347	119	211
Porosity / %	13	22	26	25
Density / g cm-3	4.8	-	3.6	-

Electrode Material Capacity

Increasing the thickness of the material is a good way to ensure high energy density but it also depends on the density of the material. The capacity of the electrode depends on both the volume or area of active material as well as its density. High power density batteries can have comparatively thick electrodes but the material may be less dense, storing less energy per area or volume. In the example above whilst the cathode in high power density battery is thicker it has a lower capacity due to the materials lower density and increased porosity.

Electrode Porosity

Electrode porosity considers both particle size and packing, including electrode morphology and microstructure. The electrode fabrication process ultimately determines electrode packing and the voids left between particles. Whereas the particle size of the materials used is predetermined by the synthesis protocol and manufacturing conditions. The porosity of the active material itself can also be considered, characterized by its density and crystal structure. Discharge capacity increases with increased porosity therefore, high power density batteries tend to have higher porosity. Porosity typically reduces the resistivity of lithium-ion diffusion and charge transfer which facilitate the rapid discharge. Polymer binders are sometimes used to lock in the active electrode material and increase conductivity however, they must prioritize ionic transport and structural flexibility.

In contrast, high energy density batteries will have much lower porosity. Battery designers opt for dense packing to maximise the amount of active electrode material in a given area. Due to increased thickness of high energy electrodes in comparison to high power, polymer binders are used to ensure electrode stability prioritizing mechanical strength and cohesion. However, only the minimum effective amount of binder is used, in order to preserve a high proportion of active material within the electrode.

Electrode Particle Size

For high power density battery cells, smaller particles of active materials are used. This reduces the electrode resistances and lithium-ion diffusion pathways, allowing for sharp bursts of discharged power. For high energy batteries a larger particle size is required, in particular particle sizes that have improved packing efficiency. This reduces electrode porosity and increases electrode capacity. For example, flake particle morphology of LFP has reduced particle porosity to 14% at high loadings which increased volumetric energy density by 28% compared to conventional LFP.

Electrode Additives

In high power density batteries, achieving high electrical conductivity is critical. To enhance electrode conductivity, conductive additives are incorporated into electrode slurries. Common additives include carbon black, graphene materials, and carbon nanotubes. In contrast, high energy density batteries contain conductive additives only when essential, as these materials do not contribute directly to energy storage.

Current Collectors

High energy density batteries have taken advantage of the developments made to produce much thinner current collectors. Microscale battery grade copper and aluminum foils are now commercially available. This allows the electrode active material to remain at high proportions. Thicker current collectors reduce cell resistance, and improve heat transfer out of the cell. As a result, high power density batteries are designed with thicker current collector foils. Coatings on current collectors are also used to reduce cell internal resistance.

Cathode Active Materials

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