Ceramics 101: The QuantumScape Separator in Context

QuantumScape's solid-state lithium-metal battery technology is enabled by our proprietary ceramic solid-electrolyte separator. While you may be familiar with everyday examples of ceramics, like a coffee mug, high-tech ceramics are likely less well-known. In this blog, we'll review the basics of ceramics - what they are, how they're made, how they're used - and the relationship between our ceramic material and the broader high-tech ceramics industry.

What is a ceramic?

A ceramic is an inorganic non-metallic solid made up of either metal or non-metal powders that have been blended together and hardened by heating to high temperatures. At the molecular level, ceramics generally have a polycrystalline[1] structure, unlike glass, which has no orderly microscopic structure.[2] There is a vast spectrum of ceramic and hybrid materials, so these should only be considered rules of thumb, but generally, this family of materials shares several characteristics:

• Hard and strong under compression
• Chemically and thermally stable
• Good electrical insulators

These properties make ceramics attractive for a wide variety of everyday and high-tech applications. For example, specialized ceramics are used for their hardness and compressive strength in dental prosthetics, for their thermal properties in the heat shields on spacecraft, and as insulators in capacitors such as the multilayer ceramic capacitor (MLCC) used for short-term energy storage in devices like smartphones and cars.

Certain properties make ceramics very attractive candidate materials for next-generation batteries. For example, the thermal stability of ceramics means far less risk of combustion or deformation at high temperatures compared to other materials, such as plastics, which soften, melt, and even catch fire under comparatively low heat. These fires are one of the main safety risks of conventional lithium-ion batteries.

Beyond thermal stability, ceramics tend to have excellent hardness, which means they resist deformation, and compressive strength, meaning they are very good at carrying loads. Bricks are a great example of this: they can carry the weight of large buildings without crumbling. At the same time, ceramics are often described as brittle in the technical sense; however, brittle is not synonymous with fragile or inflexible. Brittle means the material doesn't plastically deform[3] before it breaks, not necessarily that it breaks at a low level of stress.[4]

Most everyday ceramics, like a plate or brick, don't bend at all and will break if you try too hard. But when made extremely thin, ceramics can become flexible enough to bend without reaching a breaking point. This is because the amount of strain the material must withstand when bending is proportional to the distance between the bending surface and the center of the material. For a very thin ceramic, this distance is tiny, which means that the stress it experiences is quite small.

This combination of properties is the key to our battery design and scale-up strategy. On the microscopic scale, the incredible hardness of the ceramic enables it to resist lithium-metal dendrites. At the same time, the macro-scale flexibility of the separator allows it to be handled and processed in a factory setting, which is a requirement for mass production of EV batteries.

A finished QuantumScape ceramic separator is capable of bending.

How are ceramics made?

Ceramics have been manufactured for tens of thousands of years and represent some of the oldest traces of human civilization. While ceramics are diverse in their composition, properties and uses, ceramic production generally has a few significant similarities. The base material is typically a combination of a solid, such as a mineral or metal powder, and a liquid that holds the solid together; in clay, water serves this purpose, but some other ceramics use added chemicals, or binders, when necessary. This mixture is shaped into the intended final form, described as a green body to indicate it has not yet been heated, and put in a kiln, where it is exposed to high temperatures in a heat-treatment process called sintering.

The heat treatment process removes the liquid and binders out of the green body and fuses the individual grains of solid material together, which makes the final ceramic hard and allows it to keep its shape. The heat treatment step typically takes place at high temperatures (≥800 °C). To maximize throughput and energy efficiency, continuous-flow kilns have been developed to extract heat during the cooling process and recycle it back to heat the next wave of green bodies. These kilns also allow for highly efficient processing and are well suited for mass manufacturing. They can also be heated using electricity, which allows for renewable energy to power the sintering process.

Continuous-flow kiln on QuantumScape's Phase 1 engineering line

High-tech ceramics applications

Advanced ceramics are used in a broad set of high-tech applications. An excellent example is multilayer ceramic capacitors (MLCCs). MLCCs are stacked layers of ceramics made with precisely calibrated electrical properties. The ceramic layers in each MLCC must be made incredibly thin, from a few microns to less than one micron, smaller than the diameter of a red blood cell. With such thin ceramics, even minute defects must be very carefully prevented. Nevertheless, MLCCs are manufactured on a huge scale, with trillions produced each year. They are relatively low-cost products, available on the open market for a few cents apiece.[5]

The structure of an MLCC, showing the stacked layers of ceramic material. Adapted fromPassive Components Blog.

Ceramics for MLCCs are produced using a method known as tape casting, a technique first developed in the 1940s. Tape-cast ceramics are commonplace in many applications, including, fuel cells and filtration membranes.[6] The process begins with a powder that is mixed with a liquid binder to create a viscous liquid known as a slurry. This slurry is coated onto a long carrier surface, forming one continuous sheet that is thinner than a human hair. This long continuous sheet is called the tape, hence the name tape casting. In MLCC manufacturing, the capacitor electrodes are then printed onto the ceramic tape, the tape layers are stacked, the parts are cut out individually, and the whole block is sintered together and becomes one solid ceramic piece.

How we make QuantumScape solid-state separators

The QuantumScape ceramic separator is manufactured using many of the same well-established processes used to manufacture MLCCs and other ceramic materials and components. Our approach is similar to how lithium-ion battery cathodes are made: a slurry is coated onto a carrier surface; the tape is dried and heat treated using our continuous-flow kiln, making our separator pieces that are then stacked together with cathodes, and packaged into the final battery cell. This basic description glosses over many of the precise details, which are key to making a separator that delivers good performance, and we are constantly innovating and refining our process to improve.

A QuantumScape ceramic tape coated onto a carrier surface

While the production of QuantumScape's separator is similar to that of MLCCs, ceramics for batteries present a different set of requirements: battery cells have a different architecture than capacitors and the composition of the material is distinct from that found in MLCCs. Most importantly, the material must be a lithium-ion conductor that is stable against metallic lithium. However, because the global ceramics industry is so well established, we are able to leverage many high-volume tools and techniques from this industry into our own manufacturing processes.

One key to making battery-grade materials is keeping defect rates and non-uniformities low. Doing so requires an understanding of which non-uniformities matter and which don't. As a result of our years of research on our ceramic solid-electrolyte separator, we believe we have gained a deep understanding of this aspect of ceramic manufacturing.

Summary

Ceramics are a broad family of materials that can be made to have a wide array of properties, depending on the chemical composition and manufacturing processes used. Advanced ceramics are widespread in modern high-tech applications but have not yet been used at scale in electric vehicle batteries. Electrochemistry and ceramics have mostly operated independently, with few practitioners conversant in both subjects, so we've built a team of experts in both fields to develop and manufacture the ceramic solid-electrolyte separator at the foundation of our solid-state lithium-metal battery. We believe this interdisciplinary approach gives us a world-leading understanding of ceramic solid-state electrolytes' behavior and the confidence that we can scale up and commercialize our technology.

[1] A polycrystalline structure consists of many small crystals with different orientations. The individual crystals are known as grains and are connected at grain boundaries to the other grains in the polycrystalline structure.

[2] The lack of coherent crystals or grains in glass is referred to as an amorphous structure.

[3] A good example of plastic deformation is a paperclip: when a thin metal wire is bent into shape, it stays bent, rather than springing back out into a straight piece of wire. Contrast this with elastic deformation in something like a rubber band: when stretched out and then released, the rubber band returns to its original shape.

[4] Carbon fiber is another example of a material that is brittle in the technical sense but also incredibly strong.

[5] https://www.mouser.de/ProductDetail/TDK/C1005X7R1E473K050BC?qs=LcTL%2F5vFEzGTbQ5wp%2F3zOw%3D%3D&mgh=1&vip=1&gclid=CjwKCAjw6raYBhB7EiwABge5KsahmCJGg4q4EB_xTqYVpsa7cFgquOUjwS0sxTClFxsmWVkl7XJdvhoCJkkQAvD_BwE

[6] https://www.ikts.fraunhofer.de/en/departments/electronics_microsystems_biomedicine/hybrid_micro_systems/ceramic_tapes_and_tape_casting.html

Forward-Looking Statements

This article contains forward-looking statements within the meaning of the federal securities laws and information based on management's current expectations as of the date of this current report. All statements other than statements of historical fact contained in this article, including statements regarding the future development of QuantumScape's battery technology, the anticipated benefits of QuantumScape's technologies and the performance of its batteries, and plans and objectives for future operations, are forward-looking statements. When used in this current report, the words "may," "will," "estimate," "pro forma," "expect," "plan," "believe," "potential," "predict," "target," "should," "would," "could," "continue," "believe," "project," "intend," "anticipates" the negative of such terms and other similar expressions are intended to identify forward-looking statements, although not all forward-looking statements contain such identifying words.

These forward-looking statements are based on management's current expectations, assumptions, hopes, beliefs, intentions, and strategies regarding future events and are based on currently available information as to the outcome and timing of future events. These forward-looking statements involve significant risks and uncertainties that could cause the actual results to differ materially from the expected results. Many of these factors are outside QuantumScape's control and are difficult to predict. QuantumScape cautions readers not to place undue reliance upon any forward-looking statements, which speak only as of the date made. Except as otherwise required by applicable law, QuantumScape disclaims any duty to update any forward-looking statements. Should underlying assumptions prove incorrect, actual results and projections could differ materially from those expressed in any forward-looking statements. Additional information concerning these and other factors that could materially affect QuantumScape's actual results can be found in QuantumScape's periodic filings with the SEC. QuantumScape's SEC filings are available publicly on the SEC's website at www.sec.gov.