23 february 2023
Researchers around the world have been working on the solid-state battery for years, and some would even go as far as to consider it the “Holy Grail” of electromobility. This should come as no surprise: solid-state batteries are non-flammable and offer longer ranges and faster charging speeds than their current lithium-ion counterparts. In this article, we find out where things are at with the solid-state battery, when it can be expected to arrive and whether it will be able to meet the great expectations with which it is awaited.
Lithium-ion batteries are a success story. They integrated in all our mobile devices, and without them, electric mobility would never have been possible in the first place. Their performance has improved over the years: they charge ever more quickly and store even higher amounts of electricity. And still, the technology has its limitations, which are mainly due to its consistency: today’s lithium-ion batteries are essentially liquid based. To be precise, they have a liquid electrolyte, through which the lithium ions migrate from one electrode to the other. During charging, the ions move from the cathode to the anode; when discharging, they migrate in the opposite direction. Today’s anodes consist of graphite, but metallic lithium would seem to be a more promising candidate. “This has a considerably higher energy density, so it would offer a better range without the need for bigger batteries,” says Thomas Schmaltz. Together with his colleagues at the Fraunhofer Institute for Systems and Innovation Research (ISI), materials scientist Schmaltz has drafted a roadmap for solid-state batteries.
Solid instead of liquid
For years, researchers have tried to combine the liquid electrolyte with a lithium anode. Without success, however, because of the ever-present risk of the formation of dendrites: needle-like structures that can lead to a short circuit or reduce the battery’s storage capacity. Scientists are therefore pursuing the goal of replacing the liquid electrolyte with a solid material that is more compatible with the promising lithium anode. The expectation is that this kind of solid-state cell will offer a longer range while also making the battery safer, because there will no longer be any need for the flammable liquid.
Expectations of the new technology are high. Volkswagen CEO Oliver Blume, for example, holds out the prospect of a range boost of 30 percent and a charging time fifty percent less than that required by today’s batteries. The car company has already invested over 400 million dollars in the battery start-up QuantumScape, whose aim is to bring a solid-state cell to the point where mass production becomes possible. All other major manufacturers – from the Stellantis Group to Toyota, Nissan, Ford, BMW and Mercedes – are also investing large sums and placing great hopes in this development.
© BMW Group, Solid
The BMW Group and Ford believe in solid-state batteries as the future heart of electric vehicles. To this end, they are cooperating with the US manufacturer Solid Power.
And Mercedes has already deployed the first solid-fuel cells in its electric buses. The electrolyte of the batteries used in the eCitaro consists of polymers: in other words, plastics. The batteries, which Mercedes obtains from a French manufacturer, do indeed offer longer ranges but currently still come with some limitations: for example, they must be heated to their “feel-good temperature” of 50 to 80 degrees before they are ready for operation. Moreover, their fast-charging capability is lower than that of today’s batteries. In the case of city buses, which run continuously on fixed routes every day and can be charged overnight, the greater range makes up for these disadvantages. However, as things currently stand, these batteries are not yet an option for use in cars.
Sulphides for fast charging
Enhanced fast-charging capabilities could be provided by solid-state batteries whose electrolytes consist of sulphides: compounds of sulphur, lithium and other materials such as phosphorus. This is a technological path that BMW, for example, has struck out along in cooperation with US company Solid Power. The Bavarian carmaker aims to demonstrate its first solid-state cell vehicle before 2025, with the expectation that the technology will be ready for series production by the end of the decade.
Until then, however, there is still a lot of development work to be done. This is because sulphides also present certain stumbling blocks and challenges. If they come into direct contact with the lithium anode, there is a risk of a chemical reaction that can impair the performance of the battery. “Here, you have to work with particular coatings to prevent this,” explains Thomas Schmaltz. Moreover, although sulphides are less flammable than liquid electrolytes, if they come into contact with water, toxic and flammable hydrogen sulphide is released. The battery cells therefore need to be be packaged particularly well and securely.
© Krisztian Bocsi Bloomberg, Getty Images
Lithium-ion batteries are currently the standard in electric vehicles. Here, a Volkswagen employee connects a corresponding battery to an electric motor.
It is for these reasons that some manufacturers, such as QuantumScape, are turning away from sulphides or combining them with other materials. But one thing is true of all solid-state candidates: while the production methods for lithium-ion batteries have been established for decades and are gradually being optimised, such methods do not yet exist for their solid-state counterparts. Production lines and machines will need to be designed and factories built.
Evolution, not revolution
When solid-state batteries come onto the market towards the end of the decade, it therefore follows that they will complement today’s batteries rather than directly replacing them. “Because of their initially higher prices, they’re likely to be used mainly in sports cars or other high-end vehicles,” says Schmaltz.
With higher production volumes, costs will soon drop, but, even then, they will still have to compete with today’s battery technology. This is because the latter’s development potential in terms of range and charging speed is also far from exhausted, even if the leaps in performance will become increasingly smaller. Thomas Schmaltz and his colleagues predict that solid-state batteries will account for just two percent of the battery market by 2035. “Solid-state batteries are the next logical step in battery evolution, but they aren’t going to be a total game changer,” says Schmaltz. This is partly because it is unlikely that the new batteries will be better in all respects than today’s. “As a rule, an improvement in one performance parameter comes at the expense of another,” says Schmaltz. In other words, if you increase energy density and charging speed, this can lead to a reduction in service life. But this will on the other hand open up the possibility of developing batteries in a highly targeted way for specific applications.
Batteries are becoming more diverse
Thomas Schmaltz is, in any case, confident that solid-state batteries will add more diversity to the battery range: in the case of buses or cars for urban traffic, the solution could be inexpensive and more highly developed polymer batteries, whereas sulphide solid-state cells might be the thing for longer journeys. It is, however, also possible that they will actually open up completely new dimensions for electro mobility. Where today’s batteries will at best be adequate for use in small aircraft and over shorter distances, thanks to their higher energy density, they might make it possible for larger aircraft also to completely jettison conventional engine technology.
Growing battery diversity
The range of batteries is already expanding. For instance, more and more car manufacturers are also turning to cheaper iron phosphate batteries. These have a slightly lower energy density than classic batteries, but are somewhat more durable and do not require expensive and critical materials such as cobalt. Even more readily affordable sodium batteries could soon be added to the range, as announced by the Chinese battery giant CATL. “Due to the fact that their current performance is relatively poor, however, they are likely to be used primarily in small cars or electric scooters,” says Thomas Schmaltz from Fraunhofer ISI. So, the perspective for the near and distant future looks like this: more and more battery variants will become available for different budgets and applications – some of them with liquid at their core, others solid.
