Kyle Proffitt
August 29, 2024 | The 2024 Solid-State Battery Summit, held earlier this month in Chicago, kicked off with a heavy-hitting lineup of EV manufacturers discussing the future of solid-state batteries as they relate to transportation. We heard from Mercedes, Ford, Toyota, Stellantis, and BMW about the challenges and promises offered by solid-state batteries.
The overarching theme was that the future is bright, with many potential solutions that will continually improve performance, safety, longevity, and cost of batteries for EVs. However, these original equipment manufacturers (OEMs) seem to agree that we aren’t quite there yet in picking the winners of solid-state chemistry and design and that automotive applications and pack-level scaling present special challenges. Although hesitant to disclose too much about their favorite new technologies, they provided useful insights about where things might be headed.
Mercedes: Creeping Lithium
Tobias Glossmann, Principal Systems Engineer with Mercedes, started the conference with a focus on the promises and challenges associated with using lithium metal as anode material, ultimately condoning standardized dynamic testing protocols. He asked the rhetorical question why Mercedes cares about lithium metal. “It’s about extraordinary vehicles,” he answered. A lot of that is about energy density, and lithium metal provides a theoretical opportunity for 10-fold improvement. “You can imagine for a high performance vehicle there’s not much room in that vehicle that’s packed with technology, and the lower mass is really important,” he said.
However, Glossmann wanted to discuss the challenges of working with lithium metal, drilling down to the fundamental atomic arrangement. He reminded the audience that “we should never assume that the same element results in the same structure and properties,” pointing to the examples of carbon existing as both graphite and diamond. Similarly, when lithium deposits (plates) on an anode surface, “we don’t know much about this metal that’s coming out—grain size, defects in the structure, solid-electrolyte interphase, crystal faces… it will probably depend on the operating conditions,” he said. “The structure of my anode might depend on the weather or the mood of my driver.” That wouldn’t be ideal.
Glossmann explained the special challenges that lithium metal presents, starting with the thesis statement that “lithium is a metal that creeps.” He said that cycling a lithium metal battery involves a buildup and release of stress, like breathing. In a standard stress-strain curve, he explained, there is an elastic portion where material reversibly deforms, followed by an inelastic portion of irreversible change. However, lithium metal “really diffuses at room temperature,” Glossmann said. “While you’re on that elastic curve, [lithium] will also move… if you push on it, it will move around; it will follow the stress.”
That matters, he says, because the results of measurements can depend on so many variables, including whether you measure right after you charge or if you wait a while first. This all culminated in his pronouncement that the “test protocols really, really matter.”
The result of this concern was the creation of a consortium of several academic and industry experts to develop standardized protocols for material characterization and battery performance assessment, who together published guidelines. They realized that “sometimes, certain load cycles can hide issues,” adding that “usually it’s very difficult to know the worst case” in terms of the conditions that create issues. Glossmann continued: “For example, high temperature is good for lithium metal because it becomes softer, but also things are more reactive; high stack pressure may help, but too much, maybe something breaks.” To address this challenge, the consortium ultimately proposed dynamic stress testing for their guidelines, a set of more realistic EV-like cycling conditions to deal with more realistic lithium metal structure.
With regard to how soon we will see the technology in cars, Glossmann deflected on firm predictions, only saying, “It’s coming. I just want to make sure it’s not coming into the market from someone who doesn’t have all the details under control.”
Ford: From Prototypes to Commercial Success, Be Transparent
Celia Cunningham, research engineer at Ford Motor Co., talked about solid-state battery prototypes. She informed the audience that there would be no discussion of the specific types of batteries Ford is working on. Instead, her goal was to help direct and shepherd those working on solid-state batteries with regard to moving their technology toward commercialization.
She discussed the framework of technology readiness levels (TRLs), ranging from 1: idea formulated through 10: widespread adoption, no recalls. In between these numbers is the movement from small prototypes to full-size batteries to GWh-scale production. “I’d encourage you to be aware of where you are on this track,” Cunningham said.
She had advice for how to demonstrate that a battery is working well. “I would recommend not making up your own test but using an established industry standard; I’d recommend looking at the US Advanced Battery Consortium, a partnership between Stellantis, Ford, and GM, to develop and demonstrate advanced battery technologies in hybrids and EVs.” This group works with national labs in the DOE to benchmark and test emerging technologies, and she showed tables with DOE goals for specific advanced battery characteristics such as energy density, cost, and cycle life.
“There’s a lot of figures of merit to pay attention to,” Cunningham said. “I’d recommend picking a couple that you’re focusing on. For example, if you want to look at cells, you might want to hit a 350 Wh/kg specific energy with a cost like $100/kWh.” Although she didn’t want to discuss specific goals they might have for a supplier, she said that using these metrics, “we hope to have industry goals that we can all agree on.”
Cunningham also talked about the complications with solid-state. “Especially with solid-state batteries, cell pressure and temperature are really important,” she said. “High pressures might help your electrolyte perform better, but it might be impractical or expensive to actually apply that; compression systems in vehicles are not negligible.”
A point of interest for Cunningham is how materials are assembled. “One of the underrated things about solid-state electrode development is the distribution of those composites, how things are connected, how charge transfers through all these different components,” she said. “It’s not so much a materials property but a process definition about how you’re mixing and distributing all these things together to make those composites.”
For solid-state batteries, Cunningham said that diagnostics are complicated. “It’s really hard to pull apart a solid-state battery, you can’t just pull apart those interfaces.” “Using your electrical devices is a great start” for interrogating these interfaces, she said, and gave concrete examples.
She closed with more advice. “Be transparent about your scope, where you are in the development process and cycle, what you need to move forward, what metrics you’re using.” She advised using “electrochemical signatures to tell a story about what’s going on in your cell; prove that it performs well or prove that if it doesn’t perform well, you understand why and what’s going on with that.”
Toyota: Diversified Approach, Solid-State Electrolyte Focus
Rana Mohtadi, Senior Principal Scientist for Toyota, also discussed the path to high performance solid-state batteries. Of note, Toyota has publicly stated their work on solid-state batteries with expectations of commercialization as early as 2027. Mohtadi began, though, by highlighting Toyota’s stated desire not to put all their eggs in a single basket, saying that their vision includes battery EVs as well as plug-in hybrids and vehicles running on hydrogen and fuel cells. “We do not believe that one technology is going to solve all of our problems as we move toward zero CO2 emissions in the future,” she said.
She pointed to the battery manufacturing plant being built in Liberty, NC—Toyota’s first automotive plant globally—set to begin production in 2025, as evidence of the company’s increased support of hybrid electric vehicle (HEV) and battery electric vehicle (BEV) technologies, with 4 lines dedicated to HEVs and 2 for BEVs.
Mohtadi highlighted bipolar battery stacking as an avenue for advancement. In this arrangement, all individual cells are connected in series through a single current collector contacting two electrodes, reducing inactive and packaging materials and translating to increased energy density. This design is more commonly associated with solid-state systems, But Mohtadi says it is just as applicable in liquid systems.
She then discussed her lab’s basic research, which she described as “TRL 0 to 1 and 2”. For her work, she said that battery improvements must translate to vehicle improvements. “When you think of a material design… we always link the fundamental phenomena associated with the materials that we design and work on all the way to the vehicle.”
Much of her laboratory’s focus is on the electrolytes, both in liquid and solid-state systems, and ion conductivity is a major parameter. Ultimately, for the success of SSBs, she feels that the electrolytes are key. “We feel that there are clear pathways that will allow us to improve how fast lithium can move,” she said. “One of the reasons we work on solid-state electrolytes is because you don’t have the solvation/desolvation issues associated with the liquid systems, and therefore you have the opportunity to fast charge/discharge without worrying about lots of hurdles along the way,” she added.
However, while ionic conductivity is a baseline requirement for a solid-state electrolyte, there are many other parameters, and Mohtadi said that unfortunately, none of the progress with superionic solid-state conductivity has met all the requirements; “all of these families fall short for meeting KPIs (key performance indicators) when it comes to finally implementing a battery onboard a car.” Her spider-web diagram showed sulfides, oxides, halides, hydrides, and polymers, but none meeting all of the desired goals for an electrolyte.
Despite not quite reaching perfection, she says they have stumbled upon a new class of solid-state inorganic electrolytes that meet “at least 4 of the KPIs.” These materials are patented, and no structures were disclosed, but plots demonstrated their superconductivity for lithium ions. Perhaps even more importantly and difficult to achieve, the electrolytes are compatible with the active materials. These SSEs showed 99.69% coulombic efficiency in solid-state lithium metal half cells for over 150 cycles. She also showed data where the new SSEs performed well in full lithium metal anode NMC622-based cells without coating or protection of the cathode or lithium metal anode.
Finally, she brought it all back to the vehicles. She is member of the same consortium with Glossmann and co-author of the guidelines for next-gen battery research; she encouraged the audience to read it, because the “dynamic conditions can really exacerbate the degradation of the cell” and need to be appropriately considered.
Stellantis: Full Breadth of Options to Reduce Carbon Footprint
Ramin Rojaee, Cell Technologist of Stellantis, wanted to talk about next-gen chemistries in the automotive industry. Though perhaps not a household name, Stellantis designs, manufactures, and sells automobiles across 14 brands, including Chrysler, Dodge, Jeep, Maserati, and Citröen. Collectively, Rojaee says that Stellantis plans to offer more than 75 BEV models.
Part of the mission of Stellantis, their Dare Forward 2030 plan, is to reduce the company’s carbon footprint by 50% relative to 2021 levels and carbon net zero by 2038. Rojaee pointed out that transitioning to EVs is actually not enough, though. “By transitioning internal combustion engine (ICE) vehicles to BEV, you’ll be able to reduce the global warming potential (GWP) index, mostly CO2 in this case, by up to 60%,” he said. To get to that carbon net zero goal, “we also need to adopt sustainable battery technologies.”
Next-gen chemistries, including lithium-sulfur, solid-state, sodium-based, and low-cost Li-ion (LFP primarily) technologies are part of the move toward achieving the Dare Forward goals, and Rojaee provided an overview of each, highlighting some of the opportunities and challenges of each. He said Stellantis is also focused on critical minerals, referring to the minimization of the associated “total peace costs” and considering these minerals in comparisons.
Rojaee was the only OEM representative significantly discussing Li-S batteries. A Li-S battery “goes through a series of conversion reactions that convert S8 to Li2S and 16 Li ions with different steps of intermediate polysulfides.” These batteries have the benefits of abundant, low-cost materials, contain no Ni, Co, or Mn, and have high theoretical specific capacity (1675 mAh/g vs. typical 270 mAh/g for Li-ion), but they suffer from low nominal voltage, limited electrical conductivity, and short cycle life. Additionally, they have a 2-step voltage profile that could complicate battery management system design, Rojaee said. Although promising, they are at a low state of maturity.
Rojaee continued to walk through the pros and cons for solid-state batteries and sodium-ion batteries, providing qualitative tables comparing different options for electrolytes, cathode and anode materials, and composites, across a range of metrics. Ultimately, he said, “It’s like a puzzle where you need to use multiple solutions to get your battery to work better.”
BMW: Solid-State Opportunities, Pack-Level Considerations
Julian Bigi, Senior Battery Technology Engineer with BMW, reiterated the theme that with SSBs there is no goldilocks just-right solution for all possibilities, only tradeoffs.
He provided some useful baseline statistics for consideration during his talk. For an i4 series EV, “40% of the cost of the vehicle is the battery pack; 80% of the cost of the battery pack is the cells.” He also said that the hope for SSB technologies is that you’ll “get a boost across a range of indicators… higher energy density, but also safer cells”.
Bigi showed an internal roadmap BMW uses to think about the current state of different battery technologies and expectations for their future. While they keep specific numbers to themselves, Bigi said that “for the state of the art, we’re talking about high-nickel, or at least mid-to-high nickel NMCs, some silicon content in the anode” translating to “on the order of 700 Wh/L” for a cylindrical cell. That’s the liquid electrolyte lithium-ion cell, and the expectation is that these will continue to see improved energy density in time. As a contrast, they see LFP with a pretty flat line and lower overall energy density; no major improvements are expected there.
There were cost-saving (lower energy density) and performance-optimizing (higher energy density) variants of the basic lithium-ion battery curve, each showing similar slopes of improvement. “The cathode active material is the most expensive material in that cell, so if you can go to cobalt free or maybe reduce your nickel content, you’ll save on costs there; these are LMR (lithium and manganese-rich) or LMNO (lithium manganese nickel oxide) materials,” Bigi said. In contrast, you can shoot for performance with even higher nickel content cathode and more silicon in the anode. However, “in that case, it’s going to come with certain challenges: the expansion of the silicon, maybe there’s safety issues associated with high nickel,” Bigi said.
SSBs were presented with a unique curve. “We put it in the future, but we also put it at a high energy density, and the slope is different. You might see bigger gains in energy density over a shorter period of time,” Bigi said. A point he wanted to highlight, though, is that it’s a moving target, and expectations for future SSBs need to be measured not against current state-of-the-art lithium cells but against their future variants.
Bigi pointed to NMC as an example with limits; although moving from 88% to 95% nickel may improve energy density, we can only go so far. In contrast, “the promise of a technology like solid-state is to open up new active materials—cathodes, anodes, new technologies that aren’t reaching limits.”
A factor Bigi highlighted was that for EVs, the pack can present unique challenges with new materials. This can be associated with “more complex packs and assembly, which can affect cost and energy density.” More than the other OEMs, Bigi gave a detailed description of how the expansion in solid-state cells, requiring compression, can have major effects at the pack level. In his example, a simple cell requiring 10 bars of pressure, which must be applied uniformly across all cells, across the pack, translates to 4 tons of force. Then, he said, “you need that compression layer to expand and contract to compensate for that volume—1,000 cycles.” He suggested that the expansion must not deform the frame and reminded everyone that additional materials add to cost and offset energy density gains. In other words, it’s challenging.
He explained that BMW is dealing with these challenges by doing basic laboratory, small-scale research, but also learning how to scale. Bigi described their battery cell competence center in Munich “that’s more focused on materials—cell-level, lab-level.” East of the city, he said, they have a much larger cell manufacturing and competence center. “There we focus on scaling up.” So, while they have suppliers and do not manufacture batteries for their EVs, “we work with them very closely; we need to understand their challenges, their opportunities, so we can guide them or so we know where opportunities exist.”
