By Kent Griffith
August 30, 2021 | Billion dollar start-ups, unicorns, are becoming increasingly common in the lithium battery sector as companies vie for a share of the exponentially growing electric vehicle market. The annual Solid-State Battery Summit was held virtually in August 2021 and featured keynote presentations from the CTOs of two such unicorns and leaders in the emerging business of solid-state batteries: Tim Holme from QuantumScape and Josh Buettner-Garrett from Solid Power.
QuantumScape went public via a special purpose acquisition company, or SPAC, in November 2020 at a valuation of $3.3 billion. Solid Power also recently went public via a SPAC in June 2021 at a valuation of $1.2 billion. Both companies are ultimately attempting to commercialize batteries that utilize lithium metal as the anode material and contain a solid-state separator. The Solid-State Battery Summit provided a platform to share their latest progress and the technical, manufacturing, and commercial outlooks with the industrial and academic battery communities.
While there are myriad definitions of a solid-state battery, fundamentally the liquid electrolyte that separates the cathode and anode should be replaced with a solid electrolyte. Three classes of solid electrolytes have emerged as commercial front-runners: polymers, oxides, and sulfides. In general, proponents of solid-state batteries are targeting batteries with higher energy density and lower flammability/higher safety while trying to at least break even on cost, charging time, and low temperature performance. As Dr. Buettner-Garrett pointed out, “Replacing the liquid electrolyte with a solid will not improve energy density on its own. It needs to be paired with higher voltage cathodes, higher capacity anodes, and/or higher electrode loadings.” The highest energy density anode is pure lithium metal, a material that has largely evaded successful commercialization owing to its proclivity to form metallic projections known as dendrites that cross the separator and short-circuit the battery, leading to capacity degradation and fires. Not only does the conventional liquid electrolyte do little to hinder the growth of lithium metal dendrites, it is also a flammable liquid that acts as a fuel and contributes a substantial amount of the energy released as heat in a battery fire.
Ultimately, the choice of solid electrolyte and related design considerations will strongly influence the accessibility of the goals of solid-state batteries, and no electrolyte simultaneously satisfies all desired criteria. Polymers and sulfides have favorable mechanical properties and are more easily processable than their brittle ceramic oxide counterparts. However, typical polymer electrolytes do not remove the flammability or thermal stability concerns and have poor ionic conductivity at ambient temperature. Meanwhile, sulfide electrolytes are chemically and electrochemically reactive, requiring more stringent handling conditions and passivation strategies within the cell. Oxides, though they are the most electrochemically stable class of electrolytes, suffer from manufacturing and mechanical challenges and are still sensitive to moisture in the air.
Rise of the Anodeless Cell
For its approach, QuantumScape is aiming for the limits of energy density, at least with existing cathode technology, by designing an anodeless cell. As the name suggests, this technology is constructed without an anode active material and utilizes only the lithium atoms that exist within the cathode. Upon charging the cell, lithium deposits onto the anode current collector, a thin sheet of copper foil, and subsequently cycles back into the cathode on discharge of the battery. QuantumScape was founded in 2010 as a spin-out company from Stanford University. They have over $2 billion in capital investment with more than 400 employees and an IP portfolio of over 200 patents and patent applications. Whereas state-of-the-art lithium-ion battery cells are achieving volumetric energy density of more than 700 Wh/l and gravimetric energy density approaching 280 Wh/kg, QuantumScape envisions 1000 Wh/l and 350–400 Wh/kg with their new lithium metal solid-state battery technology.
Admittedly, the QuantumScape battery is not “all-solid-state” because it contains a liquid between the solid electrolyte and the cathode to facilitate contact between the ceramics. While the cost of solid-state batteries is yet to be determined, Holme noted several cost advantages such as the elimination of the anode bill-of-materials and decreased manufacturing costs associated with the lack of anode production lines or the formation and aging steps.
QuantumScape has disclosed cell data from a variety of cells and testing conditions, including single-layer pouch cells and four-layer and ten-layer electrode stacks. Among the performance shown at the Solid-State Battery Summit was 90% capacity retention at 1000 cycles under 1C/1C and 100% depth-of-discharge cycling at 30 °C with 3.2 mAh/cm2 areal capacity for an anodeless 70 mm ´ 85 mm single-layer pouch cell under 3.4 atm pressure.
Holme noted that reaching 800 cycles in a 300 mile range EV corresponds to 240,000 mile lifetime. In terms of high-rate, the same format cell can reach 80% state-of-charge in 15 minutes. As an indicator of the scale of the R&D required to develop their new technology, QuantumScape has over 3000 channels for battery cycling that are in use nearly 24/7. One practical challenge for solid-state batteries is the typical requirement for external applied pressure, in some cases from the literature this pressure is prohibitively large for EV applications.
To examine the pressure dependence of their cells, QuantumScape is performing cycling under zero applied pressure and find that they can still reach 1400 cycles with more than 80% capacity retention in small pouch cells. More pressure testing is required but these are promising early results. Looking toward the future, Holme sees several key remaining tasks including improving the quality, consistency, and throughput of the separator during processing; increasing the layer count in multilayer cells; and ramping and improving volume manufacturing processes.
Solid Power Insights
Solid Power was founded in 2012 as a spin-out company from the University of Colorado Boulder and has grown to more than 70 employees. On the balance of processability and stability, Solid Power has chosen to move forward with a sulfide solid electrolyte. Buettner-Garrett notes that although the powders must be handled in a dry room, consolidated films are considerably less air sensitive, and the high conductivity and roll-to-roll manufacturability are worth the effort.
The anode strategy of Solid Power differs from QuantumScape. For their lithium metal cell, Solid Power do include a lithium seed layer as constructed. Furthermore, they are developing parallel product lines and thus have a cell with a silicon-based anode as well. Buettner-Garrett reported that the silicious anode has greater than 50% silicon content, including electrolyte weight, and cells with this anode reach 320 Wh/kg and 740 Wh/l today with a roadmap toward 390 Wh/kg and 930 Wh/l with innovations such as thicker electrodes and decreasing the solid electrolyte separator thickness from 25 µm to 15 µm. Moreover, the silicon-based cells do not require pre-lithiation nor a lithium reserve.
In his plenary, Buettner-Garrett offered a hint to their dual-product development approach, noting that avoiding dendrites with lithium metal anodes requires chemical compatibility and extreme homogeneity, while lithium dendrites with silicon are a relative non-issue. Today’s prototypes use industry standard and commercially mature intercalation cathodes, but Solid Power is keeping its foot on the accelerator and pursuing high-capacity conversion-type cathodes for future use.
Both Solid Power and QuantumScape are scaling production and looking toward the middle of the decade for full automotive qualification and adoption. Holme, a self-described optimist, added some perspective, “People argue about whether a [non-polymer*] solid-state battery will get introduced in 2024 or 2025 or 2026 and that seems to be where a lot of the estimates cluster. Well, I’d just like to point out that the future doesn’t end at 2026. There’s a long future beyond that and a battery solves a really fundamental human need, which is mobile energy.” Solid-state batteries are a hard technical problem, but they offer an alluring route toward higher safety and higher energy density if the challenges can be overcome.
*Blue Solutions, another presenter at the Solid-State Battery Summit, has a commercialized lithium metal polymer battery in use in the EV sector. It has a PEO-based solid polymer electrolyte and operates at an internal temperature of 80 °C.