The company claims a breakthrough in solid-state lithium metal batteries | Ars Technica

2021-11-12 09:34:42 By : Ms. Annie Wu

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Scott K. Johnson-December 9, 2020 at 9:05 PM UTC

To some extent, all modern lithium-ion batteries are a compromise. The original concept is a "lithium metal" battery, which can hold more energy in the same volume. There is only one small problem: they always self-destruct. But this week, a closely watched battery technology company announced that it believes it has solved the problem. If the content displayed by the company is accurate, it will be a big deal.

First, a brief introduction. Lithium batteries transport lithium ions from the cathode to the anode during the charging process, and collect energy when the ions return to their original position in the cathode material. This requires a separator in the middle that only allows lithium ions to pass through, and a conductive electrolyte. In lithium metal batteries, the shuttle lithium ions only form pure metal lithium on the anode side. But lithium tends to form branched needle-like structures called dendrites, which can pierce the cathode and cause a short circuit in the battery. Since the liquid electrolyte used in these batteries is flammable, bad things can happen when they are short-circuited.

The solution is to use graphite anodes. The ordered structure of graphite provides a good lodging place for lithium ions, which can be safely accommodated in the room during the charging process. This greatly reduces the risk of dendrite formation. But this kind of graphite can occupy nearly half of the battery's volume without adding additional energy storage. This allows the battery to work safely, but will reduce its performance.

One strategy for improving lithium-ion batteries is to create a solid electrolyte material. This is attractive because it replaces the flammable components of the battery, and because it can reduce unwanted chemical reactions between the electrolyte and other battery materials, which can cause degradation over time. QuantumScape has been studying solid electrolytes for about ten years after the project was spun off from Stanford Laboratories and received some funding from the US Department of Energy. (Although the company started to adopt a different technology that was unsuccessful.) But in addition to these benefits, QuantumScape said its solid electrolyte material can also prevent dendrite formation.

In press releases and video calls, QuantumScape demonstrated the batteries it made and provided a lot of test data. There is an obvious warning that the test has not been independently verified, which is enough to allow any battery nerd's eyebrows to escape the forehead.

The test cells are not exactly what the final product looks like (more on that later), but they are complete cells. They are pouch batteries with a single-piece cathode layer, solid electrolyte layer, and anode contact foil. They did not describe the cathode, but judging from the answer to the later question, it is likely to be a nickel-manganese-cobalt cathode material-currently the most common material for electric vehicles. However, the solid electrolyte should be suitable for any type of cathode.

During the charging process, lithium ions leave the positive electrode material, pass through the solid electrolyte layer, and are plated on the negative electrode foil to form pure lithium. This does cause the thickness of the battery to expand slightly during charging, but QuantumScape says this behavior is easy to adapt because it only expands in one direction. Some attempts of lithium metal batteries require adding a layer of lithium on the anode side for better stability, but this type of battery also skips this cheating.

After removing the anode material from the equation, these batteries have approximately 1,000 watt-hours per liter. In contrast, the best lithium-ion batteries available today can be up to over 700 years old.

Some attempts at solid electrolyte materials require elevated temperatures (above 60°C) to maintain stability, which is not the case here. Tests have shown that batteries operate safely in the temperature range of -30°C to 45°C, although they do lose some of their ability to operate at the coldest temperatures.

These batteries can also handle significant charging rates, charging from 0% to 80% in just 15 minutes. Graphite anodes are a major bottleneck for safe and fast charging, and the solid electrolyte layer seems to be able to prevent the formation of dendrites, thereby pushing the charging rate farther. This seems to be quite sustainable, because even this charging rate can survive 1,000 charging cycles with a capacity loss of only about 10%. This means that the battery life should be comparable to today's lithium-ion batteries.

QuantumScape seems to go further than typical laboratory research. The company said it identified the ceramic material for its solid electrolyte about five years ago and has been working on the manufacturing process ever since. However, there are still obstacles between this and vehicles running on these batteries. Although the test battery shown by QuantumScape contains a single battery sandwich the size of a playing card, the company said that its actual battery cell will be more like a deck of cards with many stacked layers. QuantumScape needs to design this unit.

From then on, the company will need to expand its material production scale. This is not just creating more problems. QuantumScape needs to make more efforts while maintaining perfect quality. Although the company's CEO Jagdeep Singh evasively answered questions about defect tolerance, it seems that any inconsistency may cause dendrites to form and destroy cells.

However, QuantumScape has already received a huge investment. Just a few weeks ago, it went public after merging with an investment group. (This is a good time to show promising data...) And it has reached a cooperation agreement with Volkswagen, which will extend to the manufacturing stage, so it will not navigate to an industrial-scale jump on its own.

As for how long this will take, Singh explained that although the company does not require any aggressive manufacturing processes, you cannot fully order this type of machine for two-day delivery. But if QuantumScape reaches its milestone, it intends to introduce its products to the electric vehicle market in 2024 or 2025.

The promise of new battery technology usually requires caution, but it is fair to say that the technology seems to be more mature than usual. It also seems to check all the boxes instead of just providing an exciting feature and (diluted) trade-off. Unless the test data is falsified in some way or is extremely unrepresentative, this is a compelling technique. Maybe other companies working in this field are doing similar things, but it hasn't been made public yet. But in any case, if a battery like this crosses the finish line, it will be a major leap beyond the gradual progress of recent years.

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