High-energy-density batteries are at the forefront of modern energy storage solutions and play a vital role in powering everything from electric vehicles to drones and portable electronics. These batteries are characterized by their ability to store large amounts of energy in a compact, lightweight form factor, making them ideal for space- and weight-constrained applications. In this article, we will explore what makes batteries so energy-dense, identify current leaders in the field, and discuss the science behind their high performance, focusing on the materials and technologies that enable such amazing energy storage capabilities.
What are high-energy-density batteries?
High-energy-density batteries are those that can store a large amount of energy relative to their weight (gravimetric energy density) or volume (volumetric energy density). In simple terms, high-energy-density batteries are able to provide power for longer or store more energy in a smaller or lighter package. This is particularly important in applications such as electric vehicles (EVs), drones, and portable electronics, which need to maximize energy storage capabilities while minimizing size and weight.
Which battery has the highest energy density?
Currently, the batteries with the highest energy density are lithium metal batteries (particularly solid-state lithium-ion or lithium-sulfur (Li-S) batteries). These batteries are still in the R&D phase and their commercial viability is still being explored, but they have shown great potential.
Among commercially available batteries, lithium-ion batteries (Li-ion) are the current standard, offering high energy density compared to other commercial options. The nickel-cobalt-manganese (NCM) and nickel-cobalt-aluminum (NCA) chemistries in Li-ion batteries are particularly known for their high energy density. The best Li-ion batteries can have energy densities of 250–300 watt-hours per kilogram (Wh/kg).
The highest energy-dense commercial batteries are solid-state Li-ion batteries, which are expected to have energy densities of about 50%–100% higher than conventional Li-ion batteries (up to 500 Wh/kg) due to the use of a lithium metal cathode and a solid-state electrolyte.
Why do Li-ion batteries have high energy density?
Li-ion batteries work by the movement of lithium ions (Li⁺) between the positive and negative electrodes during charging and discharging. This ion movement, along with the flow of electrons, generates and stores electrical energy. During charging, lithium ions are extracted from the cathode material (usually graphite) and move through the electrolyte to the negative electrode. These ions are inserted into the negative electrode material (such as lithium cobalt oxide, LCO) to store energy. At the same time, electrons flow to the positive electrode through an external circuit. When discharging, lithium ions move from the negative electrode back to the positive electrode through the electrolyte. This movement of ions generates current, which powers the device.
Based on the working principle of lithium-ion batteries, we can see that its high energy density mainly comes from the unique properties of lithium ions themselves, high-capacity positive and negative electrode materials, and efficient electrolyte materials that promote ion transport.
1. High specific energy of lithium
Lithium is the lightest metal, and its small atomic size enables more energy to be stored per unit mass (high weight energy density). This means that lithium-based batteries can store more energy with less weight than metals used in other battery types (such as lead in lead-acid batteries or zinc in alkaline batteries).
2. High-capacity electrode materials
The negative and positive electrode materials used in lithium-ion batteries are optimized to maximize the number of lithium ions that can be stored without compromising the battery’s cycle life or safety.
Graphite is the most common positive electrode material, which is able to efficiently store lithium ions. Silicon-based cathodes have even higher capacities than graphite, but they often have volume expansion issues. For the anode, lithium-ion batteries typically use high-capacity anode materials such as lithium cobalt oxide (LCO) and lithium nickel manganese cobalt oxide (NMC). These materials are able to store large amounts of energy.
3. High ionic conductivity (electrolyte)
Electrolytes with high ionic conductivity facilitate the movement of ions between the positive and negative electrodes, thereby increasing energy density.
Lithium-ion batteries typically use liquid electrolytes that facilitate the rapid movement of lithium ions between electrodes. This efficient ion transport minimizes energy losses during charging and discharging. The solid electrolytes used in lithium-ion batteries are more efficient than the liquid electrolytes used in many other battery types, allowing for higher energy storage density (volume energy density) in a given volume.
How to improve the energy density of lithium-ion batteries?
Improving the energy density of lithium-ion batteries (Li-ion) is a key area of research because it directly affects the battery’s performance in terms of capacity, operating time, and size. The energy density of lithium-ion batteries is primarily determined by the materials used in the negative electrode, positive electrode, and electrolyte. The following are strategies to improve energy density from these three perspectives:
1. Negative electrode improvement
The negative electrode material plays a vital role in determining the overall capacity of lithium-ion batteries. The energy density of the negative electrode depends on its ability to efficiently store and release lithium ions.
High nickel negative electrode: Increasing the nickel content in layered oxide negative electrodes (such as NMC, NCA) can significantly improve energy density because nickel can store more lithium ions than cobalt.
NMC: NMC has become a popular choice due to its good balance between energy density, cost and safety. Further optimization of its composition and structure can bring performance improvements. Materials with higher nickel content (such as NCM 811 containing 80% nickel) can improve energy density.
2. Positive electrode improvement
The positive electrode material is equally important in determining the energy storage capacity of the battery. Traditionally, graphite is used as the positive electrode material, but there are various ways to improve the positive electrode to increase energy density.
Silicon-based cathode: Silicon has a much higher theoretical capacity than graphite (about 4,200 mAh/g, compared to 372 mAh/g for graphite), which means that silicon is able to store more lithium ions. However, silicon cathodes are prone to volume expansion and mechanical degradation during charge and discharge cycles, which can reduce the cycle life of the battery. Solutions to this problem include using silicon-carbon composites, nanostructured silicon, or silicon oxides to mitigate the expansion problem.
Lithium metal cathode: The theoretical capacity of lithium metal cathodes is even higher than that of silicon (about 3,860 mAh/g), but dendrites formed during cycling can cause safety risks and shorten cycle life. Solid electrolytes and advanced surface coatings are potential solutions.
3. Electrolyte improvement
The electrolyte in lithium-ion batteries facilitates the flow of lithium ions between the positive and negative electrodes. Its composition can affect the energy density, stability, and safety of the battery.
Solid electrolytes: Solid electrolytes (such as ceramic or sulfide-based electrolytes) can replace liquid electrolytes. They have high ionic conductivity, higher safety (non-flammable), and the potential to use lithium metal cathodes, thereby achieving higher energy density.
Ionic liquids and polymer electrolytes: Ionic liquids are non-flammable, high temperature resistant electrolytes that help improve battery safety and stability while potentially achieving higher energy density. Polymer electrolytes are flexible and can be tailored to the needs of solid applications, helping to improve safety and potentially increasing volumetric energy density.
As the demand for more efficient and longer lasting energy continues to grow, developing high energy density batteries remains a key area of innovation. From advances in anode and cathode materials to the potential of solid electrolytes, researchers are pushing the limits to improve battery performance. CHUNQU’s high energy density semi-solid battery uses advanced high nickel NMC anode materials, silicon carbon cathode materials and coated separator technology to achieve a maximum energy density of up to 350 Wh/kg. This makes it ideal for applications such as drones, humanoid robots and consumer electronics that require lightweight and high performance power sources, where weight is a key factor.