At the end of 2020, an event that attracted a lot of attention from the energy storage industry was the listing of Quantum Scape, a company that developed and produced lithium metal solid-state batteries (hereinafter referred to as QS). This company, which has been operating in a stealth mode for 10 years, suddenly surfaced through a public offering, thanks to the full support of its backer Volkswagen. Its stock price surged from $13 at the beginning of its listing to a peak of $132 in just one month, a tenfold increase. Its market value has also surged from $4 billion at the beginning of its listing to over $40 billion. Why is this company receiving so much attention? Let's take a look at its history and technology.

QuantumScape was founded in 2010 by a research group at Stanford University, headquartered in Silicon Valley, California. The company's goal is to develop and produce lithium metal solid-state batteries. What is the difference between this type of battery and the lithium batteries currently available on the market? The following diagram shows the structure of a regular lithium battery, which has a total of 5 layers of materials from top to bottom, including negative electrode conductive copper foil, negative electrode graphite/silicon material, porous polymer separator, positive electrode material, and positive electrode conductive aluminum film. The positive and negative electrode materials have a lattice structure similar to a bookshelf, which can be used to store lithium ions. When the battery is charged and discharged, lithium ions move back and forth through the middle separator between the positive and negative electrode materials, while electrons generate current through the wires connected to the outside of the battery. However, there are gaps in the stacking of particles inside the positive and negative electrode materials, making it impossible for lithium ions to move in the empty gaps without a medium. Therefore, liquid electrolytes are added to the positive and negative electrode materials to provide a moving medium for lithium ions. Due to its ability to penetrate into any crevice, liquid perfectly solves the problem of lithium ion mobility channels.

The structure of lithium metal solid-state batteries is shown on the left side of the figure below. During production, there are only four layers of materials from top to bottom, which are: negative electrode conductive copper foil, solid separator, positive electrode material, and positive electrode conductive aluminum foil. Compared to traditional lithium batteries mentioned above, there is a layer less negative electrode graphite/silicon material. This is because when charging, the lithium ions move to the negative electrode side, allowing the lithium metal to exist in its independent form (as shown in the green area on the right in the figure below), and there is no need for a storage bookshelf. Therefore, people remove the bookshelf, which can save a lot of space and make the battery smaller in size under the same energy, thereby increasing the energy storage density. Due to the instability of lithium metal in contact with almost all liquid electrolytes, liquid electrolytes can no longer be used and solid electrolytes can be used instead, which is why it is called lithium metal solid-state batteries.

Compared to traditional lithium batteries, the two biggest advantages of metal lithium solid-state batteries are:

1. The energy density can be doubled compared to existing lithium batteries, which means it can double the range of electric vehicles (as shown in the following figure);

2. The use of solid electrolytes in the battery completely eliminates liquid organic solutions and solves the problem of liquid electrolytes burning and exploding when heated.

The research on lithium metal solid-state batteries has been ongoing for thirty to forty years, but over the centuries, there has been no mature technology put into production application. One of the biggest challenges is solid-state electrolytes (also known as solid-state separators), which need to meet the following three requirements:

1. Its lithium ion conductivity should be close to or exceed that of existing liquid electrolytes;

2. There is no chemical or electrochemical reaction with the lithium metal negative electrode;

During the process of generating lithium metal at the negative electrode during charging, sharp lithium dendrites will be generated, and the strength of the separator should be sufficient to resist the puncture of lithium dendrites.

So QS Company spent most of its ten years in stealth researching and developing solid-state diaphragms. From the ten year history chart released by the company below, it can be seen that their milestone progress from 2010 to 2017 was in the field of solid-state separators. The ability to maintain a 10 year stealth entrepreneurship model is thanks to the full support of Volkswagen. QS has been establishing a partnership with Volkswagen since 2012, and until its listing in 2020, Volkswagen had invested a total of $200 million in QS and promised to add an additional $100 million in 2021 to establish a solid-state battery production line.

QS first disclosed their technology when it went public in 2020, allowing people to know about their research and development progress. Before its launch in 2020, QS had verified that their single-layer batteries could maintain 90% of their initial energy storage after 1000 cycles of charging and discharging at a rate of 1C under room temperature pressure (see figure below), far exceeding the current technical requirements for electric vehicle batteries. This data is very exciting, and QS has also announced that they have partnered with Volkswagen to start the construction of the production line, which is expected to start production in 2025. Investors rushed towards the public's attention and pushed QS's stock price up tenfold within a month.

However, after the frenzy, investors calmed down and some began to question the feasibility of converting QS research and development results into actual products. After all, what they have achieved success is only a single-layer battery, while a real electric vehicle battery needs to be a multi-layer battery, which is then stacked into a thicker battery pack, which seems a bit too far from a single-layer battery. The stock price of QS then began to slowly decline, eventually falling back to the rational range of $40. However, QS quickly provided further test results at the quarterly financial report presentation in early 2021: successfully verifying that multi-layer batteries can still maintain 90% energy storage after 800 cycles under the same testing conditions. This result gave investors hope again, causing the stock price to rise again to the range of $60-70.

Although QS did not disclose much information about its core technology, solid-state separators, in the released materials, one of the photos gave people a glimpse of the rough shape of its solid-state separator: a solid-state separator based on ceramic materials, with a thickness of approximately 30 to 50 microns, can be bent, indicating that polymer materials should be added to increase toughness. Although QS did not disclose any technical details, we can investigate it ourselves. So we studied the patents applied by QS and learned more detailed technical information about its solid-state diaphragm.

Generally speaking, the simplest solid separator is a pure ceramic separator, but pure ceramic separators are too brittle and can only reach a minimum thickness of about 100 microns, making them easy to break even if they are thin. A thickness of 100 microns is unacceptable for power batteries because the conductivity resistance is too high. In order to meet the conductivity requirements of power batteries, the thickness of the solid-state separator cannot exceed 50 microns at most. We have learned from patents that QS has approximately two research and development ideas for solid-state separators. The following diagram is one of the ideas. Ceramic particles with a diameter of about 20 microns are mixed with polymers to form a thin film, and then the thickness of the film is compressed to about 20 microns, ensuring that the ceramic particles can be evenly distributed into a single-layer structure. Finally, the polymer on both sides of the film is polished off to expose the ceramic particles, which become channels for conducting lithium ions in the solid membrane. The surrounding polymer acts as a bonding agent to prevent the membrane from breaking.

However, there is a problem with the above approach. On the side facing the negative electrode lithium metal, there will still be a portion of the surface made of polymer materials that cannot resist the penetration of lithium dendrites, which can cause the diaphragm to rupture. So QS has another idea, as shown in the following figure. First, use polymer materials to create a horizontally and vertically staggered scaffold, and then lay ceramic particles flat on the scaffold. Process under high temperature and pressure, burn off all polymer materials, and turn ceramic particles into a layer of ceramic thin film. One side of this film is flat and smooth, while the other side has grooves left by the polymer support. Finally, use polymer materials to fill the side with grooves, forming a solid diaphragm with one side full of ceramics and the other side full of polymer materials. The intersecting interfaces are riveted together through grooves - this is not the riveting technique of ancient Chinese woodworking! This approach solves the problems of diaphragm thickness and toughness, which is likely to be the diaphragm technology currently used by QS.

Finally, many people ask a question: which is more promising, lithium metal solid-state batteries or silicon negative electrode lithium batteries? What we can see is that companies with more experience in developing silicon negative electrode lithium batteries are a promising and stable development direction in the near future. Lithium metal solid-state batteries have many attractive advantages, but their research and production still face huge risks and challenges. We are pleased to see companies like Volkswagen investing huge amounts of money to develop lithium metal solid-state battery technology. If this technology is realized one day, it will be a blessing for all humanity.

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Reference:

1. QuantumScape official website https://www.quantumscape.com/

two Compositeelectrolytes， US Patent Application No. 20200067137.

three Solidelectrolyte separator bonding agent， US Patent Application No. 20200176743.

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Author Introduction:

Dr. Xie Wei, Bachelor and Master of Materials Science from Tsinghua University, and Ph.D. in Chemical Engineering from the University of Texas (Austin) in the United States. Mainly engaged in the development of energy storage batteries, has held important positions in multinational corporations and startups, led multiple research and development projects funded by the US Department of Energy, and won the 2013 US Annual 100 Best Research and Development Technology Award. Published 17 papers in top journals in materials science and energy storage, served as a reviewer for 5 international journals, and has applied for 17 international invention patents.

Introduction to ZH Energy Storage Company:

Shenzhen ZH Energy Storage Technology Co., Ltd. is committed to the research and development, promotion, and application of energy storage technology, aiming to help achieve China's goal of "carbon neutrality" through the application of electrochemical energy storage technology. In the early stages of development, the company focused on providing technical support and consulting services to the Chinese energy storage market by leveraging its accumulated industry experience and outstanding research and development capabilities in the field of energy storage. At the same time, the company focuses on conducting research and analysis on the Chinese energy storage market, and developing or introducing the most advanced and effective energy storage technologies for the Chinese market.

Company's technical research and development direction: water-based energy storage batteries, lithium-ion battery materials, fuel cells, ion exchange membranes, coatings and adhesives, membrane separation technology.

Domestic business: technical cooperation, academic exchange, technical lectures on company technology research and development, research and development consulting, and guidance on paper writing.