Distinguishing charge rates for next-generation batteries

 

November 23, 2021

Distinguishing charge rates for next-generation batteries

 

November 23, 2021

A critical factor drivers consider when looking at electric vehicles is how quickly they can be charged (we covered other factors in an earlier blog post). Unfortunately, battery experts can use confusing jargon when talking about this concept, so here we’re going to break down key charging concepts and how they translate to the real world.

What is a C-rate?

The C-rate is the unit battery experts use to measure the speed at which a battery is fully charged or discharged. For example, charging at a C-rate of 1C means that the battery is charged from 0-100% in one hour. A C-rate higher than 1C means a faster charge; for example, a 3C rate is three times faster, so a full charge in 20 minutes. Likewise, a lower C-rate means a slower charge: C/5 (or 0.2C) would be five times slower than 1C, amounting to a five-hour charge.

C-rate also is a relative metric. Many important aspects of battery behavior (such as resistance to dendrites) depend on the absolute current density, the amount of electrical current that goes through the battery divided by the area of its layers. For a given C-rate, the current density will be a function of the loading of the cathode, which is closely related to its thickness. Thus, when comparing C-rates, it’s important to ensure the cathode loading is at commercially relevant levels: for EVs, this is typically in the range of 2.5-5 mAh/cm2.

What do EV drivers want?

To be a viable alternative, EVs should perform at least as well as their internal-combustion engine (ICE) counterparts: deliver high levels of power during acceleration, operate in all driving conditions, and require minimal time to recharge. 

In an ideal world, a battery would perform at extremely high C-rates all the time. However, the higher the C-rate, the more difficult it is for a battery to deliver reliable performance. Higher C-rates increase the rate of degradation in the battery, reducing range and shortening the vehicle’s lifespan. Faster charging can also cause dangerous dendrites to form. These hurt the battery’s lifespan, can lead to cell failure, and in some extreme cases have been known to cause fires. Consequently, EV battery chargers do not sustain charging rates higher than 2C for longer than a few minutes before the charging rate is reduced to avoid causing damage to the battery. 

This means that today’s best EV batteries can still only be charged relatively slowly compared to the few minutes it takes to fill a combustion engine gas tank. Of course, EVs have the advantage that they can be recharged at home and overnight, and so fast-charging is usually more desirable when taking trips longer than a single charge (typically around 300 miles) would allow. And the good news is that most batteries regularly charged at slow C-rates will retain their original range for longer than similar batteries repeatedly charged at fast rates.

However, we believe that mass-market EV adoption will require a more comparable experience ICEs. To achieve this, battery technology must be improved to enable a car to charge at substantially faster rates so that drivers can “fill” their batteries in minutes instead of hours. This is a key reason why certain next-generation battery technology is so important.

Not all battery technology is equal

If current battery technology risks dendrite formation if charged too quickly, are next-generation battery technologies going to solve this problem? Unfortunately, it has been found that not every new battery technology is capable of delivering improvements to fast-charging performance. For example, in our blog post on sulfides, we explained why we believe that many of the newly announced solid-state lithium-metal batteries based on sulfide electrolytes are unlikely to deliver improvements in charging performance over conventional lithium-ion batteries. Similarly, due to the twin issues of dendrite formation and resistance growth, we believe most liquid electrolyte-based lithium-metal batteries are also unlikely to be able to deliver fast charge performance. Though they appear to successfully charge at relatively slow rates, such as C/5, we believe these fundamental limitations to many of the sulfide- and liquid-based technologies are likely to put super-fast charging speeds out of reach.

In contrast, our ceramic solid-electrolyte separator enables the use of a lithium-metal anode in its fully charged state. This lithium-metal anode replaces the graphite anode, one of the key bottlenecks to fast charging in current EV batteries. The test results of our batteries using our solid-state lithium-metal anodes show better than 80% energy retention after 800 charging cycles with repeated 1C rates of charge and discharge, the equivalent of over 240,000 miles for a car with a 300-mile range. In short, charging our lithium-metal batteries at a relatively high 1C rate does not cause a dramatic drop in range over the battery’s lifespan.

We believe the data we have shown demonstrates that our fundamental technology, when scaled to a size that is useful for EVs, can deliver the fast-charging speeds that make EVs competitive with ICE vehicles. Our ultimate goal is to make a battery for EVs that can recharge from a low state of charge to 80% in 15 minutes, empowering drivers to switch to zero-emissions cars without having to tolerate the inconvenience of slower recharging currently associated with today’s EVs. We recognize that to make an impact in the real world, we need batteries that don’t compromise on power, charge time, or battery life, and that’s precisely why we design and test our batteries using C-rates that are more reflective of real-world conditions.

FURTHER READING

Dendrites

Many next-generation lithium-metal battery technologies replace the conventional graphite anode with a lithium-metal anode, which allows the battery to store a greater amount of energy in the same volume. But dendrite formation in lithium-metal batteries is a key reason such batteries have not yet been commercialized at scale.

Dendrites are root-like structures of pure lithium formed within a separator that begin at the anode and grow toward the cathode while the cell is charging. They tear apart the cell from the inside as they grow. When a dendrite reaches all the way to the cathode, they cause the battery to short-circuit and fail. Dendrites are also known to form in legacy lithium-ion battery cells, which have caused fires and even explosions.

It has been observed that for lithium-ion and lithium-metal batteries, higher rates of charge increase the likelihood of dendrite formation exponentially. However, lowering the charge rate in a new battery technology means limiting its ability to be a viable alternative to ICEs or conventional battery-based EVs.

Things to note when reviewing C-rate performance data

During the battery development process, scientists and experts in a laboratory can control the conditions under which their batteries are tested, allowing them to gain insights into fundamental behaviors. However, such lab tests aren’t standardized or always straightforward, making it difficult to interpret the findings or compare the results with other technologies. A few things to look out for are:

  • Charging slowly and discharging quickly is a well-known method among scientists to modify the charge-discharge protocol to evaluate certain batteries that are vulnerable to dendrite formation. Charging slowly at low C-rates (e.g., C/4, C/5) is less stressful on the battery internally and reduces the possibility that dendrites will form, and when they do form, discharging at high C-rates (e.g., 1C, 2C) can help shrink them (although they can never be entirely stopped). For example, a battery that reaches 800 cycles with a C/5 charge rate and a 1C discharge rate must charge for five hours for every one hour of driving (discharge). However, such laboratory test schemes are the inverse of what drivers actually want to do: charge for a short time at a high rate (e.g., less than an hour) and discharge at a lower rate (e.g., drive 300 miles over several hours). 
  • Reporting cycle life at low C-rates can conceal the battery’s inability to prevent dendrites in real-world conditions. Solid-state electrolytes are considered by many the only way forward to enable the lithium-metal anode. QuantumScape has demonstrated an inorganic ceramic separator design that can replace the liquid electrolyte/polymer separator combination of conventional lithium-ion batteries. This separator enables a lithium-metal battery that can prevent dendrite formation under real-world operating conditions. However, not all solid-state electrolytes can prevent lithium dendrite growth under such conditions. As we’ve explained, we have conducted significant work on other types of solid electrolytes, like sulfides, and concluded that they can’t prevent the formation of dendrites under repeated cycling (800 cycles) at high C-rates with thick cathodes.
  • Replacing the lithium-metal anode with another hosted material, like silicon, is another known workaround to prevent dendrite formation. But silicon can create another set of challenges that also limit the battery’s practical applications. For example, the substantial volume changes that the silicon particles undergo during charge and discharge can cause a battery’s internal components to crack, resulting in reduced cycle life. To address this, some battery manufacturers apply external pressure to keep silicon intact, but the mechanisms used to apply this pressure reduces the energy density of the battery pack.

[1] https://insideevs.com/news/507489/tesla-model3-charging-faster-ccs2/


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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