What drives the car of tomorrow?

^{Web special Fraunhofer magazine 1.2023}

Mobility of the future equals electromobility: a simple equation, and to see the proof, you need only cast your eye out to the street. In fact, global sales figures for electric vehi­cles − i.e., battery-powered and hybrid vehicles, plug-in hybrids and vehicles with fuel cells − increased more than tenfold between 2010 and 2021, from just about 838,000 to more than 9,345,000 vehicles. Germany was well at the head of this trend: although less than 6,000 electric vehi­cles were sold nationwide in 2010, the figure had jumped to 750,000 by 2021. The pioneering field of e-mobility faces a big challenge in the form of batteries; every stage brings up issues to resolve, from manufacturing to disposal. Researchers at institutes and developers at automotive manufacturers are working hard to increase the energy density of the batteries − which would, in turn, increase the vehicles’ range − through novel material combina­tions and new cell designs, improving the batteries’ safety and lifespan and reducing production costs.

Photovoltaics in the mobility sector?

E-cars are only genuinely environmentally friendly when they are powered by electricity from renewable sources. As such, the ideal solution would be to generate energy at the point where it is needed, while the car is on the move. And capturing solar energy on car rooftops has already become more than just a nice idea. “We are working with a number of vehicle manufacturers to integrate solar cells into the car body,” confirms Dr. Martin Heinrich. A group manager at the largest solar research institu­tion in Europe, the 1,400-employee-strong Fraunhofer Institute for Solar Energy Systems ISE, he has made it his mission to “produce as much of the required energy as possible on the vehicle itself, independently of any other source.” As part of the “3D” project, the researchers set out to manufacture curved solar modules − this shape would make it possible to integrate them inconspicuously into car rooftops. The greatest challenge here was the lamination process required to make the photovoltaic modules wind and weather proof. The question was, how can manufacturers work with the bent modules while still guaranteeing the required uniform temperature and homogeneous pressure across the entire surface?

This problem has been solved, at least for some specific vehicles. For example, the automotive manufacturer Mercedes has already integrated the project’s solar roof into its EQXX show car − invisibly, of course. The car’s 117 solar cells supply a 12 volt battery, which then powers the full-dashboard display, among other things. This appreciably reduces the load on the high-voltage battery − the solar energy from the roof contributes a total of 25 kilometers to the car’s 1,000 kilometer range. But car rooftops have space for even more cells: “If you were to integrate 366 invisible solar cells onto a car roof, they would generate 2,000 kilometers of range in the course of a year, according to initial rough estimates − on a good summer’s day, you could get as much as 10 kilometers,” Dr. Heinrich suggests. The overall potential is enormous: if you were to take the rooftop space of all the vehicles in Germany, you would have a surface area for generating solar power that’s double the size of Liechtenstein. 

The exact amount of energy generated depends on how much sunlight hits the modules during journeys and parking time, so Dr. Heinrich’s colleagues are researching how much sunshine falls on Germany’s roads. As part of the project PV2Go, researchers have developed sensors used by more than 60 volunteers across Germany on their roofs of their cars – they have been driving around the country since the start of 2022. If the car starts moving, the sensor collects data that is accurate to the nearest second, sending it to the server via a wireless connection. “We are using this data to develop and validate a model that predicts solar irradiation on a particular driving route, depending on the time of day and the season,” explains Fritz Haider, a scientist at Fraunhofer ISE. In the long-term, solar car drivers could then select the route that would produce the most solar power from their vehicles’ roofs.

Fraunhofer ISE is also involved in another initiative aimed at generating energy close to the point where it will be consumed. ASFINAG, the Austrian public corpo­ration that manages the country’s highways, has teamed up with Fraunhofer ISE and other partners in project PV-Süd to investigate the possibility of covering partic­ular stretches of the highway with photovoltaic module roofs. The best place for these systems would be in close proximity to the consumers, for example, at service sta­tions with charging facilities for e-cars. The design phase is already complete, and construction of a demonstrator near the “Im Hegau” service station on the A 81 is set to start in spring 2023. “To keep costs down and facilitate the building of other structures, we have deliberately opted to use commercially available photovoltaic modules,” relates Dr. Heinrich. However, there are some very specific challenges to overcome. To avoid endangering road users, the modules cannot splinter upon breaking. The Fraunhofer researchers will next explore the level of energy the sys­tem generates and whether the photovoltaic construction can withstand wind and weather, including snow. Pho­tovoltaic modules can also be mounted on the sides of highways, on noise protection barriers. The Fraunhofer ISE team is already investigating what conditions would be needed for this arrangement to work, how the modules could be installed on existing walls and how the sound-ab­sorbing materials and photovoltaic modules can be combined when building new walls. 

Longer lifespan for pouch batteries

Safe, reliable and highly efficient batteries are the core of electromobility, its make-or-break point. However, as anyone who owns a smartphone will tell you, lithium-ion batteries have a limited lifespan. The battery works reli­ably, but then its capacity suddenly plummets. Pouch cells are a common form of lithium-ion battery that are often used in electric cars; however, they are prone to cell aging, which arises due to factors such as the uneven loads that affect the batteries during charging. The parts of the cells that are close to the electrical contact point tend to expand more than the parts that are further away. Temperatures in these cell areas are also significantly higher, which results in quicker aging. This means that maintaining an equal load during charging could extend the battery’s lifespan – that’s the Fraunhofer ISE researcher’s theory at least. They and their partners in project OrtOptZelle are working to find out whether the theory holds true. “First, we want to gain a better understanding of the local volume fluctuations and pressure distribution in the battery cells. To do that, we clamp the cell between two metal plates with built-in pressure sensors and investigate the effects that occur during charging,” explains Dr. Luciana Pitta Bauermann, a project manager at Fraunhofer ISE. “Based on the data from the pressure sensors, we compress the cell mechan­ically, so that the entire battery has to expand to an equal extent. To do this, we use metal plates that have been milled at incremental depths to create tiers, meaning that batteries are under more pressure at the back than the front. This reduces the ionic resistance on a local­ized basis, which in turn compensates for the unequal expansion during charging.” To find out whether the tiered plates produce the desired effect, the research team charged and ran down the cells until they broke down, measuring the charging capacity all the while so that they could determine how the cells were aging. Then they completed a kind of post-mortem analysis, whereby the cells were dismantled and underwent chemical testing to show what had happened in the different cell parts.

“At the moment, we cannot precisely quantify to what extent the tiered plates affect the batteries’ lifespan. However, we hope to increase the lifespan by up to 10 percent. The advantage of the compression should become apparent after a loss of just 5 percent of the cell capacity,” explains Dr. Bauermann. The project’s findings so far already indicate that in general, the lifespan of com­pressed cells is much higher than that of uncompressed cells. In a subsequent project, the researchers plan to collaborate with an industry partner to replace the heavy, rigid metal plates with plastic sheeting, which would be lighter and thus more suitable for every day use. 

Higher energy density, improved safety

The limited range continues to be one of the main points of criticism leveled at e-cars. How exactly is a car like that going to take its passengers on a long road trip for a well-earned vacation? The Fraunhofer ISE research team is also tackling the question of range, teaming up with industry partners in project FliBatt – the German name is short for “solid lithium batteries with non-woven materials” – and using this as an opportunity to improve safety. In this initiative, the team is replacing the standard lithium-ion batteries, which are based on a liquid electrolyte, with solid-state batteries. This makes it easier to use metallic lithium as the anode material instead of graphite, which is the most common choice at present. These changes increase the overall cell’s volumetric energy density by around 85 percent. What’s more, batteries with a liquid electrolyte can short-circuit; if this effect spreads, the heat it generates can vaporize and ignite the liquid electrolyte. This thermal runaway process may well be unlikely, but the risk does remain. However, with solid-state batter­ies, this kind of maximum credible accident (MCA) is impossible.

That said, there are still many unresolved issues in the manufacturing process for solid-state batteries. For example, the aluminum and copper sheets that the elec­trodes are deposited on and the separator between the cathode and anode are structural necessities, but their extra weight is costly in terms of energy storage. As it stands, the layer thickness is also limited by the technical restrictions of the production process, as very thin sheets tear quickly. “We are solving this problem by reversing the usual manufacturing process,” reveals Dr. Andreas Georg, a project manager at Fraunhofer ISE. “Instead of depositing the electrodes on the metal sheets and then connecting them to the cell via a separator, we begin by manufacturing the separator and applying the electrodes as a coating.” Finally, the metal arrester sheets are evap­orated. In principle, for a process like this, thicknesses of around the single micrometer scale would suffice. Until now, however, sheets with a thickness of 10 to 20 microm­eters have been required. This method could not only save material and costs, but also reduce the weight. 

Stress tests for batteries

Solid-state batteries may well result in just a smidgen more safety in the future, but this does not mean the lith­ium-ion batteries that are currently used so much present any kind of safety risk – after all, they are equipped with built-in protection mechanisms during manufacturing. However, the problem is that they not only have to ensure safety during normal operations, but also in the event of accidents. The Fraunhofer Institute for High-Speed Dynamics, Ernst-Mach-Institute, EMI, provides the facilities for the required crash testing. “We conduct crash tests for e-mobility batteries at all three levels: the cell level, i.e. the smallest components of the battery; the module level; and in the future, for entire battery packs as well,” says Dr. Sebastian Schopferer, group manager for Measurement Technology at Fraunhofer EMI. At the cell level, the researchers are primarily testing dynamic material behavior. One discovery that the Fraunhofer team have made here is that the behavior of a cell that undergoes rapid deformation during an accident is fun­damentally different from that of a cell that undergoes a slower form of stress. “If a crash occurs, even a slight deformation could be enough to bring about critical con­ditions,” explains the expert. To perform the tests, the researchers drive a variety of differently shaped punches into the cells at varying speeds, precisely measuring the cells’ resistance and deformation. This allowed them to determine the conditions that can trigger a thermal runaway, whereby cells break down with a chemical chain reaction that emits high levels of heat and gas and can cause fires or even explosions. In addition to mechanical characterization, the team is also testing the thermal runaway process itself, along with the related internal cell safety mechanisms. To test the reliability of these safety mechanisms in the event of a breakdown, the researchers have developed a high-speed X-ray device – a novel piece of equipment that is unrivaled on the mar­ket. There were more challenges to overcome than just the high frame rate, although at 1,000 to 10,000 frames per second, the rate required for capturing high-speed processes is by no means negligible. To protect the X-ray device from explosions and damage, the team developed a robust protection chamber that the X-rays could pen­etrate, which was then used to house the battery cells during testing. At the module level, the researchers are concentrating on questions such as whether thermal insulation can prevent cell-to-cell propagation of thermal runaway. They have also recently expanded their portfo­lio to include crash testing for entire battery packs with capacities of up to 50 kilowatt hours. The new building where the researchers can conduct these tests by smash­ing impactors and batteries together was only opened in December 2022. “In the neighboring department, the Crash Center, we have learned what stresses affect the batteries during accidents. However, the center cannot be used to carry out tests with charged batteries, as the facility is not set up for that,” Dr. Schopferer points out. “This means we have to extract the stresses that affect the batteries from the overall vehicle crash tests, and then reproduce these stresses with our own equipment.”

European battery cell manufacturing? It’s time to make the idea a reality!

At present, most battery cells are manufactured in China – and as we have seen in recent years, these kinds of dependencies can be quite problematic. That means that increasing industry resilience needs to be a top pri­ority. But how is the European battery cell manufacturing scene doing? Researchers at the Fraunhofer Institute for Systems and Innovation Research ISI are collecting the data needed to answer this question in BEMA2020 II, a project initiated by the German Federal Ministry of Edu­cation and Research (BMBF). “The European market for battery cells is experiencing a lot of momentum,” reveals Dr. Lukas Weymann. “Many battery cell manufacturers have reported significant growth, while start-ups and established automotive manufacturers are also getting involved in mass production of battery cells.” In 2022, European manufacturers could produce a maximum battery cell capacity of a little over 100 gigawatt hours; however, by 2030, this figure is set to reach 1,700 gigawatt hours. “The challenge does not lie in getting more stake­holders involved in constructing battery cell factories in Europe in the coming years. In fact, the critical question is, how can we make the existing plans for mass produc­tion in Europe a reality in the near future and with suffi­cient quality levels?” asks Dr. Weymann of Fraunhofer ISI. 

Research on the road to gigafactories

“When it comes to battery cell design and new materials for the batteries, Germany and Europe are very well-po­sitioned; however, we are lagging behind in the area of large-scale production,” confirms Dr. Thomas Paulsen, a group manager at the Fraunhofer Research Institution for Battery Cell Production FFB. Fraunhofer FFB has set out to tackle precisely this challenge, by acting as an open research platform for scaling product and process inno­vations. Asia is leading the pack in terms of battery cell production and it has no interest in sharing its knowledge with Europeans. “We want to build up experience in the mass production of battery cells and – most impor­tantly – act as an open research platform for exchanging these learnings,” explains Dr. Paulsen. Fraunhofer FFB was launched in 2019 as part of the FoFeBat project on researching battery cell production, which is funded by the BMBF. If all goes to plan, then the Münster-based Fraunhofer facility will become an institute in 2024. The first two battery production steps (out of a total of 19) are already underway. These involve preparing the anode slurry, a battery mixture that may consist of graphite, binding agents, conductive additives and aqueous sol­vents, and coating and drying the electrodes.

Construction is also nearly complete for the FFB PreFab, a building with 3,000 square meters of research space that could theoretically achieve production capac­ities of up to 200 megawatt hours per year. Why “theoret­ically”? Because, to reach these figures, all the equipment would have to work faultlessly around the clock. And that is not the goal of the FFB. Instead, the aim is to research production processes and environments with a view to making production cheaper, quicker and more environ­mentally friendly. The process of drying the electrodes is a good example here. At the moment, the process functions much like a pizza oven, meaning that it is incredibly energy intensive. This is why the researchers are working on an alternative drying process based on infrared beams, which are a far more energy-efficient option. Lasers could also be used for the drying procedure. “We know that this method works at a small scale, but not whether it could be scale up for a gigafactory. In FoFeBat, we are filling the gap that currently exists in Germany between the proto­type stage and the gigafactory. We are also breaking new ground, in that we are consciously entering very high technology readiness levels,” enthuses Dr. Paulsen. The FFB Fab, which is set to be completed by 2026, will have 20,000 square meters of floor space and a theoretical production capacity of 6.8 gigawatt hours. 

Just one more step to go before gigafactory-scale production

The FoFeBat project is focusing on battery cells with high technology readiness levels, thus choosing battery technologies that are already very advanced. By con­trast, the Fraunhofer Project Center for Energy Storage and Management Systems ZESS, which is operated by the Fraunhofer Institute for Ceramic Technologies and Systems IKTS, for Surface Engineering and Thin Films IST and Manufacturing Technology and Advanced Materials IFAM, is concentrating on newer battery technologies. “We want to bring solid-state batteries from the labo­ratory scale to the pilot plant scale, which means going from a three to a six in terms of technology readiness levels,” says Dr. Julian Schwenzel, director of Fraunhofer ZESS. “As such, we just have one more step to go before gigafactory-scale production: we have to actually develop the process technology.” Because the manufacturing processes for solid-state and liquid batteries are very dif­ferent. With solid-state cells, for example, the electrolyte has to be incorporated into the manufacturing process right from the beginning, whereas for liquid batteries, it is added at the end of the process. “At Fraunhofer ZESS, we generally work on three different solid-state battery technologies: polymer batteries, thiophosphate-based batteries and oxide batteries,” relates Dr. Schwenzel. The primary advantage of the polymers is that they are easy to work with. The researchers primarily use novel polymers that remain stable when undergoing temperature changes and can conduct electricity. They have even submitted a patent application for a new polymer. “However, we have yet to discover a polymer that works as an all-rounder – instead, we have to work out precisely which polymer suits each application,” says Dr. Schwenzel.

He sees much cause for excitement in the thiophos­phate-based batteries. “They are the champions when it comes to conductivity.” However, there are still some issues to be resolved even here: Thiophosphates are very sensi­tive to damp and have to be processed in a dry room – which would present a challenge for any plans for large-scale production. That’s why the researchers are working to reduce these production steps and increase sustain­ability. The third runner in the race, the oxide batteries, is still very much on the starting blocks. The necessary high-temperature processes are difficult to control, so the ZESS researchers have quite a lot of work left to do before solid-state batteries can knock the established liquid batteries down from their top slot in the market.

Battery cell production meets the digital transformation

Asian countries have gained a head start in battery cell production – so how can Germany and Europe catch up? “Solid mechanical engineering is not going to be enough there,” asserts Joachim Montnacher, head of the Energy business unit at the Fraunhofer Institute for Manufac­turing Engineering and Automation IPA. “The digital transformation is our only chance of regaining the ground we have lost,” he adds. “In specific terms, by digitalizing, and thus also optimizing production.” The key issues are the economic viability of the manufacturing process and the quality of the resulting cells. This is because the more cells roll off a factory’s production line, the higher the reject pile grows. “If a gigafactory were to produce a billion cells per year and 10 percent of those goods were defective, you would be rejecting 100 million cells – that would be an enormous waste of materials,” explains Prof. Kai Peter Birke, head of the Center for Digitalized Battery Cell Manufacturing ZDB at Fraunhofer IPA. The researchers hope to improve these figures by digitalizing the entire value chain, in projects such as a DigiBattPro 4.0. “We can validate our approaches on a large production line with mass-scale capabilities in a live operational setting with the help of our industry partner VARTA,” reports Florian Maier of Fraunhofer IPA. “We can even rent out the entire production line in order to conduct our own experiments over the course of a full production shift. This means we can use a real-world setting to prove that our digitaliza­tion approach is suitable for the industry sector.”

The aim here is to use the data to predict the quality of the finished battery during earlier production steps, in what is known as a predictive quality process. This will facilitate immediate removal of low-quality cells. “That is the top of the pyramid that we are trying to climb,” explains Mr. Maier. “However, we must first lay the foun­dations, that is, automatic data collection. After all, no human being could manually enter the data for the tens of thousands of cells that would be manufactured every hour.” The researchers hope to optimize the repeatability of the production process using cloud systems, real-time data processing and, in the long term, even digital twins – and to make German and European battery cell produc­tion competitive in the process. “The train hasn’t left the station,” affirms Prof. Birke. “We still have a chance to regain technology leadership in the next generations of battery cells. However, the next decade will be focused on production, not new cell technologies.” 

The alternative option: fuel cells

Fuel-cell-powered vehicles constitute another contender amid the ranks of electric vehicles – and thus also a beacon of hope for the mobility of tomorrow. These cars combine hydrogen fuel with air to form water and electrical energy, which then powers the electric engine. However, they must overcome much the same challenges as battery-powered vehicles. The sector is facing enor­mous pressure in terms of costs, which means that fuel cells have to become cheaper. The installation space is also limited, in trucks as much as regular cars – so the modules have to be as small as possible. And if they are to be deployed in trucks, then their lifespan must be increased even further.

In project SinterGDL, researchers at Fraunhofer IFAM in Dresden are working to reduce the costs of fuel cells for trucks, while also delivering a more compact cell design. The goal of the project is to develop a novel PEM stack unit that is both cheap and compact. The core element of the unit is the gas diffusion layer (GDL), which facilitates the supply of equal amounts of hydrogen and air over the entire surface area – which can be as big as a letter-size sheet of paper – and the removal of heat, water and elec­ tricity. “Crucially, instead of manufacturing the GDL out of carbon, we make it entirely out of metal,” reveals Dr. Olaf Andersen, head of department at Fraunhofer IFAM. “By doing this, we are making it easier to scale manufac­turing processes up to high volumes, as well as reducing the production costs for the components. Metal components are also easier to install and recycle.” This is because the researchers are using processes from the paper industry, which are suitable for mass production; however, they are not only working with cellulose fibers, fillers and additives, but also metal powder. The end product looks like a sheet of paper, but the metal powder it contains gives it a gray color. It is also up to 200 micrometers thicker than paper, which generally only gets up to 80 microm­eters. Next, the team burns out the cellulose at tempera­tures of up to 600 degrees Celsius, in a special inert gas atmosphere that prevents the metal from oxidizing. The purpose of this step is to cleanly remove all organic com­ponents, so that only the metallic parts remain. These are then sintered at 1,250 degrees Celsius, which causes the metallic elements to fuse tightly together. Because the researchers can adjust the metallic GDL’s properties to a great extent, for example, by using different kinds of fibers or altering the size distribution of the pores in the GDL as it forms, they hope that, on top of enabling cost-effec­tive, mass-scale production, they will also improve the GDL’s performance parameters and thus reduce the installation space needed for the fuel cells. Fraunhofer is sowing fresh seeds of hope for fuel cells as an alternative power source in electric vehicles.