How To Increase Battery Energy Density?

While lithium-ion batteries have shown significant advancements in the last twenty years, their energy density and rapid charging abilities remain restricted. Consequently, they may not be the best choice for applications demanding high energy densities like long-distance electric vehicles, electric airplanes, or cutting-edge consumer electronics. Enhancing the energy density and fast charging features of batteries will undoubtedly involve innovation in materials and systems, but this development process is time-consuming. Today, I will demonstrate how to optimize energy density through the utilization of superior active materials and thicker electrodes (both positive and negative electrodes).

Active and inactive materials

Lithium-ion batteries are mainly composed of active and inactive materials. As with most batteries, there are active and inactive materials in both the negative and positive electrodes of the battery. Active materials - electrochemically active ingredients that directly participate in electrochemical reactions, allowing batteries to store and release energy. Inactive materials – components that provide the necessary infrastructure for battery function

There are two main ways to maximize energy density:

1) Use active materials that can store more energy;

2) Increase the ratio of active materials to inactive materials in the battery.

Negative active material

The most common active material in traditional negative electrodes is graphite. Graphite has been used in lithium-ion batteries for decades and its properties are well understood. However, graphite pales in comparison to the theoretical capacity of alternative materials such as silicon or lithium metal.

The most energy-dense batteries today use small amounts of silicon mixed with graphite to increase the capacity of the negative electrode. But metallic lithium has the highest specific capacity among all negative electrodes in lithium-based systems.

Positive active material

To maximize the energy of the positive electrode, the capacity of the positive electrode must match that of the negative electrode. Nickel-rich cathodes, such as nickel-rich manganese-cobalt (NMC) or nickel-cobalt-aluminum (NCA), are high-performance cathodes. They have relatively high capacity and voltage, which means greater energy storage capacity per gram of material. Lithium iron phosphate (LFP) is another popular cathode chemistry; it has lower energy than high-performance cathodes such as NMC and NCA and is often used to optimize cost.

Positive and negative loads: need to be balanced

In conventional batteries, the energy density of a battery can be increased by adding more positive and negative active materials to each layer of the battery (increasing electrode loading) and making the electrodes denser (lowering porosity). As a result, cells can store more energy per layer with fewer layers per cell. Because each layer contains lightweight inactive materials, thicker electrodes can increase the energy density of the battery to a certain extent.

Why not create a single-layer battery with ultra-thick, ultra-dense positive and negative electrodes to minimize the proportion of inactive materials?

While making ultra-thick and ultra-dense single-layer cells may increase energy density, extracting the energy becomes more difficult. As electrode thickness and/or density increases, lithium ions encounter bottlenecks as they leave the positive electrode or enter the negative electrode due to the increased distance they travel and the tortuous path they take from one electrode to the other. This higher tortuosity and longer travel path limits the battery's power—how quickly it can deliver or receive energy.

This fundamentally explains the trade-off between energy and power: optimizing a battery's energy and power often results in a degradation of the other, making it challenging to serve applications such as electric vehicles that have both energy and power requirements. . Due to the need to balance the positive and negative electrodes and maintain acceptable power performance, the thicker and denser the electrodes of traditional lithium-ion batteries, the lower the yield, typically around 5mAh/cm2.

Specifically, today's most advanced lithium-ion battery cells can reach approximately 750Wh/L and 275Wh/kg. However, volumetric energy density and gravitational energy density have begun to reach theoretical limits. We need next-generation technologies to achieve higher energy densities. 

Reference:https://www.quantumscape.com/resources/blog/energy-density-active-materials-electrode-loading/