The Problem with Sulfides

 

July 2, 2021

The Problem with Sulfides

 

July 2, 2021

We are often asked about the basic chemistry of solid-state battery materials. Specifically, what’s the composition of your solid electrolyte, and why did you choose that material over others?

The electrolyte has a straightforward role in all types of lithium-ion batteries — it carries lithium ions from one side of the battery to the other. In legacy lithium-ion batteries, this substance is a liquid that fills the whole cell. A thin plastic sheet keeps the two sides of the batteries apart, and the lithium atoms flow in the liquid, through holes in the sheet as they carry their energy during charge and discharge.

In a solid-state battery, the liquid (the electrolyte) and the plastic sheet (the separator) are replaced by a single material: the solid electrolyte. Solids are usually worse than liquids at letting lithium atoms move through them,[1] for much the same reason that it’s harder to swim through ice than through water. Still, there are a few solid materials that let lithium “swim” through them. Some of the most common are sulfides.

It’s important to note that at QuantumScape, we’ve been working for over 10 years to invent and manufacture the best EV batteries possible. We have carefully examined every solid material we could think of — running over three million lab tests — with particular attention to sulfides. Although at first sulfides seemed to have some positive features,[2] after extensive investigation, we concluded that sulfides are not suitable as separators.

So, why did we give up on sulfide-based solid separators? There are three main reasons:

  1. Sulfides don’t prevent dendrite formation and require external systems to maintain high temperatures and pressures, adding weight, bulk, and, most importantly, cost to the battery pack.
  2. Sulfides break down chemically when used in high-performance batteries.[3]
  3. Sulfides produce hazardous gas when they come into contact with moisture.

Problem 1: Dendrites

The first and most important reason we don’t think sulfides work as solid separators is the lack of evidence that they prevent dendrites. Dendrites are root-like structures of pure lithium that can grow from the anode[4] of a lithium-metal battery cell. They tear apart the cell from the inside as they grow. When they reach all the way to the cathode,[5] they cause the battery to short-circuit and fail. Dendrites can form even in legacy lithium-ion battery cells, which can cause fires and even explosions.

There are several compromises a cell with a sulfide-based separator can try to work around this problem:

  • Use a graphite or silicon anode: Using a graphite or silicon anode[6] instead of lithium metal may prevent dendrites, but it also reduces the driving range, cost, and charge time advantages[7] over legacy lithium-ion technology, as the lithium-metal anode is what creates the improvements. Several other solid-state companies are taking this approach. For example, Solid Power announced a new silicon anode pilot line a few months after it released data showing its cells were unable to maintain acceptable rates of power using pure lithium-metal anodes without the help of high temperatures. Samsung also published results using a carbon-based anode with the addition of silver, which is too costly to be practical.
  • Put under immense pressure: Sulfide-based separator batteries can appear to deliver acceptable results if they are put under enormous pressure. For example, Samsung tested its sulfide-based separator batteries under 20-40 atmospheres of pressure, Solid Power reported testing at 70-90 atmospheres,[8] and a lab at Harvard recently published test results with a sulfide separator at more than 750 atmospheres — the pressure at nearly five miles under the sea, more than enough to crush a nuclear submarine. Pressure on that scale may be possible in a lab, but anything over 10 atmospheres is likely to be impractical in an EV battery pack.
  • Operate at high temperatures: Lithium is softer at high temperatures,[9] making dendrites less likely to form.[10] However, high temperatures require complex and expensive thermal management systems, and keeping the battery hot drains energy and hurts battery life.
  • Run on low power: Dendrites are less likely to form when the battery runs at low rates of power.[11] However, limiting a battery’s power means you can’t charge it quickly, one of the key requirements for EVs to compete effectively with combustion-engine vehicles. Lack of quick charging is an Achilles heel for most solid-state batteries.


Problem 2: Instability

Another problem with sulfide separators is the instability of the chemicals themselves. For example, even if sulfides could prevent lithium dendrites,[12] they’d still react with lithium metal, filling up the anode with chemical junk[13] that limits power output and shortens the battery’s lifespan. Given how important lifespan and power are to drivers, the instability of sulfides will likely limit their attractiveness for passenger EVs.

Sulfides don’t just react with pure lithium; they also react with the chemicals typically found in the cathode, such as the nickel-rich battery materials used in high-performance EVs. Although this problem can be reduced by adding other chemicals,[14] there is still a risk that the battery will begin to degrade on both the anode and the cathode sides if it’s pushed too hard.

Batteries with sulfide-based separators are, therefore, quite sensitive and must be operated with substantial performance limitations. This makes them unattractive to EV drivers, who want to charge quicker and accelerate faster without worrying about hurting their batteries.

Problem 3: Safety

This is, in some ways, the most serious problem. As mentioned above, sulfides are very reactive. One of the things sulfides react with is water, and when they come into contact with water or humidity, they produce a gas known as hydrogen sulfide.[15] Hydrogen sulfide gas is extremely toxic, flammable, and can be explosive — it was used as a chemical weapon in World War I and has killed dozens of workers in industrial accidents over the years.

Keeping water away from sulfide materials during manufacturing is a serious, complex, and expensive challenge — even a small amount of humidity in the air can lead to deadly levels of hydrogen sulfide buildup. And a manufacturing defect or car accident could lead to the battery cells being exposed to water while in use, potentially causing a fire or poisoning the car’s occupants.

One of the key benefits of solid-state batteries is the promise of improved safety but using a sulfide-based solid separator creates a new set of very serious risks.

QuantumScape’s Advantages

In contrast to sulfide-based approaches, QuantumScape’s sulfide-free ceramic separator has been shown to prevent the formation of dendrites under practical conditions; this previous blog post covers the conditions under which we test our batteries. Our separator is stable against the high-performance lithium-metal anode and does not produce dangerous byproducts if it touches water.

These advantages improve performance, safety, and manufacturability, and are ultimately why QuantumScape is not using a sulfide separator. Though sulfides looked promising when we started our work a decade ago, and we investigated them thoroughly, we believe the obstacles faced by sulfide separators are incredibly challenging, and perhaps even insurmountable.


[1] Battery scientists refer to this concept as “ionic conductivity.” The higher the conductivity, the lower the resistance.

[2] The properties that got scientists interested in sulfides were their relatively high ionic conductivity and the ability to process them at lower temperatures.

[3] More precisely, they have a narrow range of voltage stability.

[4] The negative side.

[5] The positive side.

[6] When there is extra material added to the anode to help store the lithium ions and prevent dendrites from forming, this material is called the “host.” Batteries with hosted anodes include all legacy lithium-ion batteries and most other solid-state batteries that we are aware of.

[7] This is because the hosted anode takes up space, adds materials and manufacturing costs, and represents a bottleneck in the flow of lithium ions that puts a fundamental limit on charge speed.

[8] This data was disclosed by Solid Power CEO Doug Campbell at an investor presentation held by Key Bank.

[9] Sulfide tests are often reported at temperatures between 60-80 °C.

[10] Elevated temperatures also reduce the internal resistance of the cell, covering up deficiencies in the design of the cell.

[11] Battery researchers call this concept “current density.”

[12] We have found no evidence that they can.

[13] The technical term is “electrochemical decomposition by-products.”

[14] Many sulfide-based approaches deal with the poor stability of sulfides by coating the cathode material with less reactive materials. However, such additions can increase the internal resistance of the cell, reducing its power capability.

[15] H2S


Forward-Looking Statements

The information in this press release includes “forward-looking statements” within the meaning of Section 27A of the Securities Act and Section 21E of the Securities Exchange Act of 1934, as amended. All statements, other than statements of present or historical fact included in this press release, including, without limitation, regarding the development, timeline and performance of QuantumScape’s products and technology are forward-looking statements.

These forward-looking statements involve significant risks and uncertainties that could cause the actual results to differ materially from the expected results. Most of these factors are outside QuantumScape’s control and are difficult to predict. Information about factors that could materially affect QuantumScape is set forth under the “Risk Factors” section in QuantumScape’s most recent quarterly report on Form 10-Q filed with the Securities and Exchange Commission on July 29, 2021 and available on the SEC’s website at www.sec.gov.

Except as otherwise required by applicable law, QuantumScape disclaims any duty to update any forward-looking statements, all of which are expressly qualified by the statements in this section, to reflect events or circumstances after the date of this press release. Should underlying assumptions prove incorrect, actual results and projections could different materially from those expressed in any forward-looking statements.


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