Electric vertical take-off and landing (eVTOL) aircraft are under development around the world, primarily aimed at flying taxis to avoid the ever-increasing congestion in major cities.
In all-electric aircraft, weight is critical. Today lithium ion (Li-ion) is widely used in applications where lightweight batteries are required, but it is simply not light enough to meet the needs of most electric plane applications including eVTOL. Furthermore, Li-ion has already reached maturity, and with only small incremental improvements with each iteration, it is unlikely to ever meet these aviation needs. Therefore a new battery chemistry is needed and this is seen by many to be lithium-sulfur (Li-S).
OXIS Energy has been developing lithium-sulfur since 2004 and cells are already being integrated into many different applications, including aircraft. The battery technology is extremely light, at around half the weight of lithium-ion, or looking at this another way, is able to store double the amount of energy for a given weight. Comparisons are usually made in watt-hours per kilogram (Wh/kg). At the lower end of the scale, a lead-acid battery that helps start a car engine achieves about 50 Wh/kg. Typical lithium-ion cells store between 100 and 265 Wh/kg, depending on various other performance characteristics. At the top end, OXIS recently developed a prototype lithium-sulfur pouch cell that achieves over 470 Wh/kg and expects to reach 500 Wh/kg within a year. Furthermore, with a theoretical limit of 2,700 Wh/kg, 600 Wh/kg can be expected by 2025.
A lithium-sulfur cell is composed of the following components:
- The cathode is the positive electrode consisting of an aluminum foil current collector coated with a mixture of carbon and sulfur. Sulfur (as you might imagine in “lithium-sulfur” cells) takes part in the electrochemical reactions, but as it is an electrical insulator, carbon is added as a conductor. In addition, a small amount of binder is added to hold the carbon and sulfur together.
- The lithium anode is the negative electrode, which acts as both the current collector and takes part in the reaction with the sulfur from the cathode.
- An electrolyte containing lithium salts facilitates the electrochemical reaction by allowing the movement of ions between the two electrodes.
- A separator, surrounded by the electrolyte, prevents the two electrodes from touching and causing a short circuit.
These components are packaged in foil as pouch cells, which in turn are packaged into battery modules. For a large vehicle such as an eVTOL aircraft, many modules are connected to create the battery system required to power a high-voltage motor for the entire flight, including an energy reserve for emergencies.
One of the challenges for eVTOL is the high power needed, particularly during the take-off and landing phases of flight. Therefore, for any technology to be successful, it must not only store the energy in a relatively lightweight package, but also deliver that energy at high power rates. As different applications differ in their requirements, OXIS has developed two versions of its cell chemistry:
- The high-energy cell is designed to minimize weight, but it is limited to relatively low power. Here all components within the cells have been optimized to reduce weight. The high-energy cell has been developed for applications such as High Altitude Pseudo Satellites, which slowly charge during the day and slowly discharge at night.
- The high-power cell increases the power capability, but with an increased weight, although this is still significantly less when compared to a lithium-ion cell. One of the Li-S research areas is to continue to increase the energy density of the cell alongside the power density, meeting the power requirements of eVTOL at a minimum weight.
Achieving both the power and energy requirements at a low battery weight is essential for the aircraft to even take off and land safely. For improving commercial success, on the other hand, it is important to maximize how many times the aircraft can make the flight before a new battery is required. This “cycle life” is one of the greatest challenges for all batteries, lithium-sulfur included. As a Li-S cell is charged and discharged, small internal variations cause it to be plated and stripped unevenly. Over time, this gradually causes moss-like deposits on the anode — these can become electrically disconnected from the anode, leaving less lithium available for the electrochemical reaction. This slowly reduces the capacity of each cell until the battery needs to be replaced.
To this end, OXIS is developing solutions to this degradation problem to produce a cell that can operate over many cycles. In particular, OXIS is designing a coating for the lithium-metal anode with thin layers of ceramic materials to prevent degradation. These ceramic materials not only need to be mechanically and chemically robust, but must also not impede the electrochemical reaction within the cell.
Of course, high performance in the aircraft is not just down to the cells: when cells are integrated into batteries, careful consideration must be given to the case, the control electronics, the connections, and the thermal management system. Here OXIS uses advanced composite materials to provide light, robust, flameproof enclosures.
The control electronics, in the form of the battery management system (BMS), optimize the battery performance and further ensure safety, controlling and protecting the battery. The BMS includes various algorithms, or software components — such as for measuring the energy remaining in a battery, known as the state-of-charge (SoC) algorithm.
With many battery chemistries, the SoC can be estimated by simply measuring the voltage, which decreases as the remaining energy decreases. However, this is not the case with lithium-sulfur. Indeed, for around half the discharge, the voltage barely changes. Furthermore, the voltage characteristics differ between charging and discharging. Therefore, more sophisticated algorithms have had to be developed, including Kalman-type filters and artificial intelligence (AI)-based approaches such as neural networks. Knowing the remaining energy accurately then allows greater use of the battery, providing greater range whilst ensuring the essential safety reserve.
As with all cell chemistries, no two cells are identical. This piece-to-piece variation, coupled with the small difference that cells experience within a battery, cause individual cells to be at different levels of SoC. This can cause some cells to degrade faster, and for the battery to not achieve its maximum capacity. Cells are therefore kept at similar SoC levels using “balancing” whereby resistors are connected in parallel with cells, controlled by one of the software components within the BMS.
Batteries also need effective thermal management as both charging and discharging creates heat. eVTOL are particularly challenging as the power rates are high compared to many other applications, so greater cooling is required. Liquid cooling is used in many battery-powered applications, but this can add significant weight to the aircraft. The ideal solution is air cooling, with careful thought given to the position of the battery to allow the outside air to easily flow onto or into the battery system.
The uninterrupted operation of the battery is critical to the eVTOL aircraft and any problems could have catastrophic consequences. Therefore, redundancy must be designed into any such solution. One element of this is the architecture of the battery itself whereby the battery is split into “strings” of modules, allowing power to continue to be delivered from the battery in the event of part of the battery failing. Although capacity would be reduced, this would at least allow the eVTOL to perform an emergency landing.
OXIS has already manufactured tens of thousands of Li-S cells at its headquarters near Oxford, U.K. The company is now scaling this up to meet the current demand with two new facilities. One of them will produce two of the key cell components, the electrolyte and the cathode active material, and is situated in Port Talbot, Wales. Another will assemble the cell components into pouch cells in Minas Gerais, Brazil, at a site owned by Mercedes-Benz Brazil. Both factories are planned to be operational by 2023, producing the many cells required for electric aircraft.