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Energy and power density definitions.

Energy density refers to the energy in a given mass (or volume) in Wh/L or Wh/kg, and power density refers to the power in a given mass (or volume) in W/L or W/kg. The difference between the two is similar to the difference between energy and power. The energy density of a battery is higher than that of a capacitor, but the power density of a capacitor is higher than that of a battery. This difference is due to the fact that batteries are able to store more energy, while capacitors release energy more quickly. In general, energy density represents the ability to store energy, and power density is the speed at which energy is emitted. The following figure compares the energy density and power density of some mainstream energy storage systems.

Energy density basis.

The key thing to understand about battery energy density is that there are two different ways to measure it - volume measurement and weight measurement. While the two approaches are sometimes conflated, they have clear differences and describe different information about the battery's performance capabilities.

Volumetric energy density:

For space-constrained applications, the energy density within a specific physical volume is most important. Consumer electronics and passenger cars are two good examples. Consumer electronics - Users value the thinness and lightness of phones, tablets and laptops because they can easily be carried in a bag or pocket; But at the same time, users aren't willing to compromise on functionality: a device that can run applications quickly and last a long time between charges requires a lot of energy to get things done. Therefore, the more energy you can store in a small volume, the better. Passenger cars - Drivers want electric vehicles to have as much range as possible. To increase the driving range of electric vehicles using conventional lithium-ion batteries, the battery pack must be made larger, which means the car itself must be larger. It's no coincidence that the longest range electric cars on the market today are full-size sedans: they're long enough to fit large battery packs under the car. However, these large cars are not suitable for most consumers. In addition, other popular electric vehicle types, such as SUVs and pickups, are already quite large, but their range may not meet the needs of every driver due to their low aerodynamic efficiency. Since the vehicle itself cannot easily be made larger to accommodate more batteries, the battery unit itself needs to provide more energy in the available space. While weight is also important, better volumetric energy density is a top priority for these use cases.

Weight Energy density:

For applications such as drones or aerospace, weight is absolutely necessary. In these cases, a larger battery can be accommodated through good design, but the increase in weight will always limit performance, so the lighter the battery, the better. In extreme cases, such as batteries in satellites, each additional kilogram of weight adds thousands of dollars to launch costs. Other application limitations are more general. For example, in heavy truck transport, there are legal maximum weight limits to avoid damaging the road. For semitrailers, the calculation is simple: the smaller the weight of the battery, the more payload can be transported, and the higher the revenue per trip the truck makes. In other use cases, such as high-performance sports cars, lighter cars handle better and accelerate faster. For such applications, weight energy density (also known as specific energy) can be a bigger pain point.

Why increase the energy density?

The energy density of batteries has been slowly but steadily increasing over the past few decades, and lithium-ion batteries are now so dense that they can power electric vehicles for hundreds of miles. With so much progress already made, why do we need better batteries? The truth is, when it comes to energy storage, there is always room for better energy density. With better energy density: Passenger cars can become smaller and lighter, offer more interior space to accommodate legroom or cargo, handle and accelerate better, providing drivers with greater efficiency and longer driving range.

1. Consumer electronics devices can become thinner and lighter, run more powerful processors, add more features, provide better graphics, faster interfaces, and better screens.

2. Drones can fly faster and higher, carry larger loads and have greater endurance.

3. Heavy trucks can travel longer routes and carry larger payloads. Companies are racing to come up with cars, laptops or drones that outperform their competitors, and energy density is one of the biggest bottlenecks to improving performance. In short, people always need a higher energy density.

In addition to these immediate performance benefits, increasing energy density has potential advantages:

Battery cost - The basic floor on the cost of a battery for an individual cell is determined by its list of materials: What is the battery made of? As the name suggests, a battery with a higher energy density requires less material. lithium metal batteries minus an anode, for example, do not require graphite, which is a major component of the weight and cost of electric vehicle batteries. While battery costs are not just related to raw materials, reducing costs is one of the possible long-term benefits of more energy-dense batteries.

System cost - An increase in energy density may also reduce the overall system cost. All else being equal, a smaller battery pack requires less: less steel and other structural materials, less wiring, fewer cooling system components, and so on. While the savings are a small percentage of the overall system cost, they are not insignificant.

New applications - The potential benefits of higher energy density are not limited to improving existing applications. From lead-acid batteries to lithium-ion batteries, advances in battery technology have created entirely new uses for batteries, such as electric vehicles and drones, and the next generation of batteries with higher energy density has also opened up the possibility of new types of applications. One prominent example is the electric taxi or personal aircraft (EVTOL), which is currently in the early stages of development and would benefit greatly from batteries with higher energy density. Whether or not this particular application becomes widespread, higher energy density is a key factor for future innovation.

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