Energy Density vs. Power Density

Energy density and power density are two key metrics used to evaluate battery performance. They are optimized to varying extents depending on the intended application because they describe different aspects of how a battery behaves.

In the simplest terms:

• Energy density tells us how much energy can be stored in a battery

• Power density tells us how quickly that stored energy can be delivered

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Increasing Energy Density

Energy density is how much usable energy is stored per unit mass (gravimetric energy density, Wh/kg) or per unit volume (volumetric energy density, Wh/L). A practical way to think about energy density is: How long can this battery run my device before it needs to be recharged?

In applications like portable electronics (phones, tablets etc.) and electric vehicles, space and weight are limited. Higher energy density allows for longer driving range or longer device usage without making the product heavier or bulkier. Focusing on gravimetric energy density, the equation for energy density is:

Where: Q = capacity (Ah), V = voltage (V), m = mass (kg)

This relationship shows that increasing either capacity or operating voltage can increase energy density.

1. Capacity 

   Capacity is the total charge a battery can store which is primarily determined by the number of ions that can reversibly intercalate into and out of the electrodes during cycling.

   
   

At the electrode level, areal capacity (charge per unit area) can be improved by increasing the thickness of the electrode, reducing the porosity, or increasing the active material loading. These interventions work by packing more active material into the same footprint.

Material choice also matters. Theoretical capacity depends on factors like the morphology, crystal structure, and redox mechanism. Higher capacity materials show a lot of promise, however, it should be noted that they also often introduce new engineering challenges. So, achieving high energy density is not just about choosing the highest-capacity material, but also about managing the mechanical and electrochemical consequences.

Here is an example:

Silicon vs. Graphite: Silicon has about 10 times higher theoretical capacity than graphite, which is the conventional anode material in Lithium ion batteries. However, Silicon undergoes volume expansion of about 300% during lithiation, which can lead to particle cracking, loss of electrical contact, and unstable SEI formation, which causes rapid capacity fade.

2. Voltage 

   Voltage is related to the Gibbs free energy difference between the reactions at the electrodes. Using electrodes with larger free energy difference, and consequently higher voltage, can increase the energy density. However, this also comes with some tradeoffs: Higher voltage can accelerate electrolyte decomposition, increase electrode dissolution, and increase thermal instability.  

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Increasing Power Density

Power density is how quickly energy can be pulled from a battery relative to its volume or mass. Practically, this relates to mechanisms such as accelerating in cars, power tools, drones, and other applications requiring rapid bursts of energy. Gravimetric power density (W/kg) is calculated as: 

Where: V = voltage (V), I = current (A), m = mass (kg)

As such, increasing voltage and current are the best bets to increase power density. We touched on ways to increase voltage already so how do we increase current? Going by Ohm's law, I = V/R, the current that can be drawn from a battery is limited by its internal resistance, so high resistance results in less power delivered per unit mass.

1. Resistance

Resistance can be lowered by: 

• Nano-structuring: This can help reduce the resistance in charge transfer at the electrodes by increasing the number of active sites on the electrodes.

• Catalytic coatings: Incorporating these coatings can help lower the activation energy of the redox reactions, reducing resistance. 

• Using thinner separators and thinner electrodes: This facilitates faster transport of ions because they have a shorter distance to travel.

• Increasing the porosity: With more pores and open routes for ion passage, there is decreased resistance in quickly accessing the active material, which increases the power density to a point.

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Where Does Mass Come in?

In both energy and power density equations, mass is present as the denominator. This is because reducing inactive mass is ubiquitously important. The total mass of a battery includes components like the separator, current collectors, electrolyte, and casing, which are all electrochemically inactive. Even though these components don't directly store energy, they strongly influence the overall energy and power density of the cell.

Common strategies to reduce inactive mass include using thinner current collectors, lighter cell casings, lower binder content in electrodes, thinner separators, and reduced electrolyte volumes. However, these optimizations must be approached carefully, being mindful of the trade-offs. Casings that are too light may compromise mechanical integrity and increase the risk of cell puncture. Separators that are too thin can increase the likelihood of the cell shorting. Electrolyte starvation can cause mass-transport limitations and poor rate capability.

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Fundamentally,

There are relatively direct pathways to improving either energy density or power density in isolation. However, this is rarely the case in practice and both are typically needed to some degree. Many of the design levers that improve one metric can negatively affect the other, creating an inherent tradeoff. Successful battery engineering is therefore about balancing both metrics based on the intended application.