Battery technology of the future
Guest article by Claus Bünnagel, chief editor of ’busplaner’ magazine
Over the coming years battery energy density is expected to increase dramatically, which will result in the final breakthrough of electro-mobility. It is anticipated that cobalt-free battery cells and solid-state batteries will be market-ready. Consequently, in a few years both urban buses and travel coaches will be electrically powered.
Electro-mobility is poised to make its final breakthrough on the volume car market as well. Even the most sceptical observers will have realised that by now. Over the last five years there has been remarkable progress in battery technology. In mid-2019 the major battery manufacturer CATL announced that its next NMC-523 battery cell generation (523 = five parts lithium, two parts cobalt and three parts magnesium) would achieve an energy density of 304 Wh/kg. In the mid-term the Chinese company’s aim is to switch to NMC-811 battery cells (eight parts lithium, one part each magnesium and cobalt) in order to save on cobalt, a critical raw material. Their energy density will probably be similar, at 300 Wh/kg. Meanwhile, Panasonic and Tesla are understood to have reduced cobalt amounts to 3 per cent.
Svolt Energy Technology, formerly the battery business arm of the Chinese carmaker Great Wall, has taken this technology one step further and announced its first cobalt-free lithium ion (NMx) battery cell and four-element lithium-ion (NMCA) battery cell. In the case of the NMx battery cell, material costs are expected to fall by 5 to 15 per cent and unit costs by up to 5 per cent. NMCA battery cells are also expected to perform better than their NCM-811 counterparts in many areas. According to Svolt they are more durable, withstand higher temperatures and are safer, resulting in higher battery capacity. In the case of the NMCA cathodes, aluminium is added to the usual nickel, cobalt and manganese compound. The amount of nickel can exceed 90 per cent, thereby further reducing controversial materials such as cobalt.
The energy density of battery cells in buses is increasing, and progress is being made with the chemical composition of cells. Thus Akasol, a battery company from Hesse that supplies Daimler Buses and Volvo Buses among others, already has 33 kWh modules on the market featuring an energy density of around 143 Wh/kg. Further progress is assured: a third generation featuring round battery cells is due on the market in 2021 and will achieve 50 kWh per battery back. Providing it can handle the extra battery weight, and with a slightly enlarged battery compartment, a Mercedes-Benz eCitaro could then carry batteries with a capacity of 600 kWh, enabling a minimum vehicle range of 240 km. Battery charging could also take place at rates of up to 500 kW. With roof-mounted charging rails and an inverted pantograph, the batteries could receive intermediate charging giving a range of more than 300 km. Thus in a few years vehicle range will likely no longer be a problem, even over long distances. The issue of weight, however, still remains to be solved. One could shift the ever-increasing weight of batteries to the bus floor, for instance, which would make it possible to create a lightweight superstructure – as already demonstrated by Proterra and Ebusco.
The prospects for solid-state batteries are even more promising, and they will soon be optional equipment on the eCitaro. However, the experts agree that progressive solid-state batteries will not be market-ready until 2025, at which point energy density will rise significantly again. In this case liquids are substituted by solid-state electrolyte which can achieve higher voltage and is able to withstand higher temperatures. As a result, charging and discharging of solid-state batteries is quicker and they store more energy per kilo.
The battery interior consists mainly of solid electrolyte and a lithium and phosphorous sulphide. Embedded within are small tin pellets measuring approximately 30 microns, half as thick as a human hair. When the battery is charged lithium ions are deposited in the tin pellets. The problem is that the lithium is forced into the lattice structure of the tin. The pellets grow in volume and expand, causing the surrounding electrolyte material to fracture. The resulting cracks hinder the movement of lithium ions through the electrolyte, which significantly reduces the performance of the solid-state battery
Researchers at the Paul Scherrer Institute (PSI) in Villigen, Switzerland, were able to observe the mechanical processes in a solid-state battery with greater precision than previously possible. They discovered that upon discharging, the battery appears to repair itself. When the lithium ions migrate out of the tin pellets the cracks in the surrounding electrolyte close again. The next step in their research is to find other electrolyte materials that react to the expansion of the tin pellets to a smaller extent. Substituting liquid electrolyte with solid material has other benefits too. It makes it possible to realise lithium-metal instead of graphite-based anodes, and this would herald significant progress in energy density. It would also save energy during production and drastically reduce CO2.
emissions. Currently, the drying process is complex and energy-consuming. With the anode this is no longer the case if solid electrolyte material is employed, because lithium metal in the form of a film can be used. Another advantage is a potential reduction in toxicity. Initial successes, by TeraWatt Technology in California for example, show that the right path has been taken to gradually achieving one’s goal. In August the company announced the creation of a solid-state battery with an energy density of 432 Wh/kg. By comparison, Panasonic’s most advanced batteries in the Tesla Model 3 achieve 247 Wh/kg. The company aims to first use these solid-state batteries in consumer electronics, and later on to install the Tera 4.0 model, capable of 500 Wh/kg, on board vehicles.
Battery development can be expected to make great strides in the years to come. With battery energy density destined to reach 500 Wh/kg and beyond, it is already conceivable that coaches will be electrically powered during the next decade. With batteries weighing only two tonnes a minimum range of 400 km would thus be feasible. As drivers are obliged to take a 45-minute break after 4.5 hours behind the wheel, the batteries could be recharged to at least 80 per cent capacity during this time. Charging technology is naturally progressing too. Thus the CCS initiative CharIN, a group comprising Audi, BMW, Daimler, Mennekes, Opel, Phoenix Contact, Porsche, TÜV SÜD and VW, part of the High Power Commercial Vehicle Charging Task Force (HPCVC), is currently working on high-performance charging technology for electric utility vehicles with a charging capacity of 1 to 3 MW.
There is a technology with the potential to take current developments one step further and revolutionise electro-mobility. StoreDot, a battery technology developer in Israel, wants to be able to recharge vehicles in five to ten minutes in the future. The anode on its Flash Battery is completely different to the type used on conventional lithium-ion batteries. Instead of graphite, it uses metalloids such as silicon, tin and germanium. And that is not all: it is nano-sized and protected by an organic film during the ion insertion process. In combination with a highly conductive, thermally optimised cathode and with electrolyte comprising bio-organic peptide nanodots this smoothens the ion transport and facilitates ultra-fast charging without the risk of dreaded dendrite formation. Low cell resistance levels mean that virtually full charging capacity is available without any excessive cell ageing taking place.
At 4,200 mAh/g, the charging volume theoretically exceeds that of a cell with a graphite anode by a multiple of five – a huge leap forward. At the same time, at 200 to 240 Wh/kg, the energy density achieved is relatively high. At StoreDot they are confident that the battery can be fully charged 1,500 times. A minimum of 450,000 km should thus be feasible for the entire life cycle of the battery.