Energy Density vs. Power Density

Energy density and power density are two of the most important metrics used to compare batteries. They matter to varying degrees depending on the intended application of the battery because they describe different aspects of battery performance.

In the simplest form:

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

• Power density tells us how fast energy stored in a battery can be delivered

---

Increasing energy density

Energy density is how much energy a battery can store 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 power 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 longer driving range or longer device usage without making the product heavier or bulkier. Focusing on gravitational energy density, the equation for energy density is:

Energy density = (Q×V)/m

Where, 

• E_density = energy (Wh/kg)

• Q = capacity (Ah)

• V = voltage (V)

• m = mass (kg)

Thus, to obtain a higher energy density, you need to increase the capacity or/and voltage

1. Capacity 

   Capacity is the total charge a battery can store which is typically determined by the number of ions that can reversibly intercalate into the electrodes. Areal capacity (charge per unit area) can be improved by increasing the thickness of the electrode, and reducing the porosity because there is more active material packed in the same volume.

   
   

   Moreover, materials have different theoretical capacities due to various factors such as the morphology, crystal structure, and redox mechanism. However, there are often trade offs with higher capacity materials which makes the choice of electrode less intuitive.

   
   

   Here is an example:

   Silicon vs. Graphite: Silicon has about 10 times the theoretical capacity of graphite which is conventionally used in li-ion batteries. However, Silicon experiences volume expansion of about 300% leading to cracking, loss of electrical contact, and unstable SEI formation, which causes rapid capacity fade.

2. Voltage 

   Voltage is determined by the Gibbs free energy difference between the reactants. Using electrodes with larger free energy change can increase the energy density, however it also comes with some tradeoffs: Higher voltage can accelerate electrolyte decomposition, increase electrode dissolution, and increase thermal instability.  

---

Increasing power density

Power density is how quickly energy can be pulled from a given volume/mass of a battery. Practically, this relates to mechanisms such as accelerating in cars, high energy bursts in a drill, and other applications requiring high power. Power density is calculated as: 

Where,

• P= Power density (W/kg)

• V = voltage (V)

• R = resistance (ohm)

• m = mass (kg)

As such, lowering resistance or increasing voltage is the best bet to increase power density. 

1. Resistance

Resistance can be lowered by: 

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

• Catalytic coatings: Incorporating these coatings can help lower the activation energy of the reaction 

• 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 reduces the power density.

---

Where does the Mass come in?

In both energy and power density equations, mass is present as the denomenator, but hasn't been touched on yet. This is because reducing 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 contribute to energy storage or power generation, they strongly influence the overall energy and power density of the cell. As such, reducing the mass of inactive components is generally desirable when designing for both high energy density and high power density.

Common strategies include using thinner current collectors, lighter cell casings, lower binder content in electrodes, thinner separators, and reduced electrolyte volumes. However, as is often the case in battery engineering, 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.

Fundamentally,

There are straightforward paths to increasing either the energy density or power density of the cell independently, however, as you may have realised, some of these levers have opposite effects on energy and power. Successful battery engineering is about balancing both metrics based on the intended application of the battery.