62 techniques, which will help high-volume manufacturing. Unlike cylindrical cells, FlexFrame cells can fit neatly next to one another with minimal wasted volume. This is a key step in improving the overall pack energy density relative to a cylindrical cell format. The energy density at cell level is not just about chemistry; packaging also plays an important role. One goal of any cell format is to minimise the amount of dead weight and wasted volume taken up by inactive materials, such as the cell casing, and maximise the proportion of the cell’s active materials – the part of the cell that stores energy. Optimising this packaging efficiency is key to enabling high cell-level energy density. Because the lightweight pouch cell material and low-profile frame add minimal overhead, the QSE5 cell format developed by QuantumScape with the FlexFrame packaging can deliver energy density above 800 Wh/L in a 5 Ah cell, with room for improvement in a higher-capacity design. Modelling “What I’ve seen in the last five years is lots of novel material breakthroughs, but I’m also highly sceptical about the commercialisation; the tradeoff of lifetime, power and cost,” says Kieran O’Regan, co-founder and chief operating officer at About:Energy. The company analyses the performance of a wide variety of battery cells. It has developed simulation and modulation tools using data from the analysis. “For me, it’s the incremental improvements over the last five, 10 and 15 years that have made a huge difference, particularly with maritime and aerospace. There are so many different paths to improve energy density, from the materials to components, cells, packs and software. It is really holistic. It’s all of those topics moving together,” says O’Regan. “What we do is supply data and simulation tools to help battery developers move more quickly, whether that’s cells or BMS software. We help people design cells, packs and BMS, rather than develop materials. Collecting data for development is expensive and time-consuming, so we have our own labs and we test them. It can take years. Understanding degradation is not a process you can hurry,” O’Regan continues. “A lot of companies use the same batteries, which is surprising as they are very different; for example, the cells from Molicell are used, from eVTOL to satellites. But the way companies build that into a cell or a pack is very different. “We facilitate the power versus energy density decision to be made by the customer, and they are the ones that make the trade-off – how many cells to improve the power, but it’s heavier, so you reduce the energy density. Our tools are used to make those trade-offs across all of that.” Materials “Some of the big trends are about optimising the electrodes in the cell. The main way is adding silicon to the graphite anode, but it does have limitations on lifetime, and 40% of silicon in the anode is perhaps the sensible limit,” says O’Regan. “At this point there is more voltage hysteresis, and that makes BMS and prediction very hard. It can be a drop-in replacement in manufacturing, but it can be an issue with the BMS and the lifetime. That’s where simulation can be used very effectively to use models to provide the lifetime over five, 10 and 15 years, and find a way to make those trade-offs a lot quicker. “On the cathode, it’s NMC, and now companies are moving to higher nickel chemistries. We are seeing a slow transition to new chemistries for LFP, solid state, niobium, sodium, but they are not necessarily more energy dense. Lithium sulfur is also on the horizon, but it’s more early-stage. “The electrolyte is one of the main ways to make the cell more energy dense, and there are new coatings and materials to mitigate the degradation of the cell. A lot of that is through repeated trial and error,” O’Regan adds. Amprius Technologies is working with an eVTOL maker on a custom battery cell using its 100% Silicon Anode technology. This is based on silicon wires grown on the copper current collector, and the custom cell is in internal qualification with the customer. The 400 Wh/kg cell is capable of a continuous discharge rate of 10 C to power the motors during take-off, cruise and landing without sacrificing performance. In addition, it has fast charging capabilities, allowing it to reach an 80% charge in six minutes or less, to speed up turnaround times. These will be used in pre-production units of the eVTOL in 2025. Panasonic Energy, one of the major cell manufacturers supplying e-mobility system makers, is working with Sila Nano May/June 2024 | E-Mobility Engineering A high-power silicon anode material (Image courtesy of Sila Nano)
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