Understanding Battery Safety
June 17, 2024
As lithium-ion batteries have become ubiquitous, their safety risks have increasingly been a focus of public concern.
White paper: A deep dive into QuantumScape’s fast-charging performance
January 27, 2022
White paper: A deep dive into QuantumScape’s fast-charging performance
January 27, 2022
Fast charging is increasingly important to buyers of electric vehicles, but high-energy legacy lithium-ion batteries are still limited in how fast they can recharge. These limitations are largely due to fundamental constraints of battery design. QuantumScape’s technology has been designed to overcome many of these constraints, to unlock a step-change in fast-charging performance that has profound implications for EV adoption and the potential to win over a segment of drivers who might otherwise hesitate to make the switch.
The limitations of legacy batteries
Legacy lithium-ion batteries are versatile; they are used today in everything from electric vehicles to power tools. However, the requirements for these distinct applications are very different, and these different requirements necessitate fundamental tradeoffs between power, energy and cycle life. When it comes to charging speed, three features drive these tradeoffs: electrode loading, anode material and temperature.
Electrode loading
Batteries are often divided into two general categories: power cells and energy cells. For applications like handheld power tools, where high-power output is the priority, cells are typically constructed using electrodes with lower mass of active material (electrode loading) and higher porosity to enable higher electrolyte content. This allows for better transport of lithium ions across the battery cell, but at the expense of energy density.
For electric vehicles, on the other hand, the priority has been to maximize range by optimizing the amount of energy a cell can store. This typically means constructing electrodes with higher mass loading and lower porosity, allowing for more energy-storing active material. However, this limits the transport of lithium ions through the electrode, resulting in inferior power performance.[1] For EV drivers, this means longer charge times and worse driving experience in power-demanding situations, like fast acceleration or driving uphill.
This power penalty is the result of several factors. As the electrode thickness increases, the rate of lithium ions leaving the cathode or entering the anode is restricted, due to increases in the distance the lithium ion must travel and the tortuosity, or twistedness, of the path it takes to move from one electrode to another. A thick cathode is like a dense, overgrown forest; the lithium cannot easily move through it in a straight line. In addition, a lithium atom moving along a very long and twisted path loses a greater percentage of its energy as heat, which reduces the battery’s efficiency. Not only that, but at very high charge rates this heat can reach unsafe levels if not properly controlled.
Electrode loading plays another role in affecting charge rates. The more capacity a cathode offers, the more lithium must cross from the cathode to the anode to fully charge the battery. The current density signifies the magnitude of this flow of lithium ions. To charge a battery with a thicker cathode in the same amount of time as a battery with a thinner cathode requires the former to be capable of handling a higher current density; the other components of the battery (the electrolyte, separator and anode) have limited ability to pass high current densities. In particular, the graphite anode of lithium-ion batteries has proven to be a substantial bottleneck to enabling fast charging speeds in energy-dense cells.
Graphite and other anode materials
Electrode loading can restrict fast charging in legacy lithium-ion batteries, and the electrode material also plays a role. In legacy lithium-ion batteries, most of the material in the anode is graphite. Lithium ions intercalate into the graphite host during charge and are stored there until the battery is discharged. However, the rate at which lithium can diffuse into the graphite host material is limited by the fundamental properties of the materials themselves.
When charging speeds exceed the diffusion rate of lithium into graphite, lithium begins plating on top of the graphite particle rather than diffusing into the atomic lattice of the graphite.[2] This plating process reduces the available lithium and lowers the battery’s capacity. Because the lithium metal reacts with the liquid electrolyte to form chemical reaction side products on the surface of the graphite, it forms a barrier that makes it increasingly difficult for lithium ions to diffuse into the graphite. This increases cell resistance and accelerates cell degradation. In the worst case, lithium plating on graphite can lead to the formation of dendrites, root-like structures of lithium that can grow from the anode to the cathode and cause a short-circuit, resulting in failure and potentially presenting a safety hazard
Other anode materials, such as silicon, theoretically offer the possibility to handle higher rates of charge.[3] However, silicon swells and contracts dramatically during charge and discharge, causing the silicon particles to pulverize, compromising cycle life. Attempting to control this expansion can increase the cost of the anode. Other materials have found specialty applications: for example, lithium titanium oxide (LTO) is capable of good performance over many cycles at high rates of charge, but the material’s extremely poor energy density makes it unsuitable for passenger EVs. Because of the inherent limitations of the known alternatives, graphite typically makes up most of the anode active material in today’s EV batteries.
Temperature and fast charging
One of the critical advances in EV fast charging over the past decade is a better understanding of how temperature affects graphite’s ability to handle higher current densities. Lithium-ion batteries are typically capable of accepting higher charge rates when heated to 35-45 °C. Many modern battery management systems will be programmed to notice when the driver has entered a fast-charging station into the vehicle’s navigation system and preheat the battery before arrival so that when the driver plugs in, the battery is ready to accept higher than usual charge rates.
Higher temperatures reduce the battery’s internal resistance, allowing lithium ions to slip more easily into the graphite anode material without plating on the surface. However, in today’s liquid-based lithium-ion batteries, elevated temperatures also promote side reactions between the liquid electrolyte and the electrode materials, which consume lithium and leave behind reaction side products that permanently increase the battery’s internal resistance and decrease its capacity.
Together, these effects can seriously damage the lifespan of the battery. While higher temperatures can yield faster charging, legacy lithium-ion batteries must sacrifice cycle life to deliver fast charging speeds. Fortunately, if a driver only makes the occasional fast-charging stop on a road trip, this effect is negligible. Still, drivers who want to fast charge their vehicles every time will eventually notice that it permanently reduces their vehicle’s range.
In summary, repeated fast-charging results in degraded cycle life due to multiple effects: loss of lithium due to reactions on the surface of the graphite particles in the anode, as well as accelerated degradation stemming from the elevated temperatures that result from the high current densities required for fast charge in energy-dense cells.
The chart below illustrates the fundamental tradeoff between an energy cell that delivers longer driving range and a power cell that can deliver faster-charging speeds. For a given cell chemistry, battery designs can optimize for one but only at the expense of the other. New battery technologies aim to expand this frontier outward to improve range and charging speed simultaneously.
The QuantumScape platform
Today, QuantumScape published data on the repeated fast-charge performance of our battery technology over hundreds of cycles, demonstrating what we believe is a step-change improvement with the potential to unlock a host of benefits for EV drivers. The charts below tell the story. The first chart shows the time it takes the tested cells to charge from a state of charge (SOC) of 10-80%, at 25 °C and 45°C. The room-temperature cells can reach 80% in less than 15 minutes, and the cells at 45 °C are even faster. The second chart illustrates the charging curve used for these tests: the cells are fast-charged at a peak C-rate of 4C.
The real value of this breakthrough is unlocking the ability to repeatedly fast charge over hundreds of cycles without incurring significant losses to discharge energy retention. This is clear when looking at the cycle life of our single-layer cells under fast-charge conditions. This data demonstrates well above 80% energy retention over 400 cycles in multiple cells under both room- and elevated-temperature conditions (25°C and 45°C), while charging from 10-80% in under 15 minutes:
These cells are built with separators of commercially relevant areas (70×85 mm) and cathode loadings (3.3 mAh/cm2). At 4C rates, this translates to a peak current density of ~13.3 mA/cm2, which to our knowledge, far exceeds anything shown by any competing next-generation lithium-metal cell, whether based on liquid or solid-state electrolytes. For reference, we have included the cycle life performance of cylindrical lithium-ion cells from a commercial EV under similar temperatures[4] and fast-charge test conditions; the cells degrade rapidly, falling below 80% of their initial discharge energy after only a dozen cycles, and at such high rates of charge, it carries a risk of catastrophic failure (thermal runaway):
Our ceramic solid-electrolyte separator is the core innovation that enables QuantumScape cells to achieve such a dramatic increase in fast-charging capability. This component resolves several of the limitations of legacy lithium-ion batteries in one fell swoop:
- Lithium plating: Batteries with graphite anodes must limit charge rates to prevent lithium plating that causes capacity fade, resistance growth and, in the worst case, dendrite formation. Our solid-electrolyte separator, which has demonstrated resistance to lithium dendrites even at high current densities, enables QuantumScape batteries to use lithium metal as the anode. This allows us to turn lithium plating, one of the key constraints on fast-charging in legacy lithium-ion batteries, into an advantage for fast charging. Other lithium-metal approaches have not shown the ability to resist dendrites at these rates of power.
- Stability to lithium metal: Our separator material has demonstrated exceptional stability against lithium metal, limiting the formation of reaction side products that can increase internal resistance and consume lithium. This property results in higher power capability and good cycle life.
- Electrode thickness: Because of our battery’s anode-free design, there is no host material. This means that the lithium diffusion distance is substantially shortened; lithium does not have to be transported across the anode but rather plates directly on the separator surface. In addition, this design offers anode energy density close to the theoretical maximum.
- Temperature: When legacy lithium-ion batteries are cold, lithium diffuses more slowly into the host material. Legacy batteries must limit charge rates when the batteries have not been pre-heated to prevent lithium plating. In contrast, QuantumScape batteries use lithium plating as the mechanism of action to deliver hundreds of fast-charge cycles even at room temperature. Our tests show that current state-of-the-art energy cells fail after a limited number of such cycles.
With this step-change improvement, QuantumScape’s battery technology is capable of expanding the performance frontier, enabling improvements to both vehicle range and charging speed simultaneously.
Commercial implications of fast charging
The term range anxiety was first used in print in 1997 to describe drivers’ reactions to driving the then-new General Motors EV1.[5] Ever since, the primary focus of EV battery development has been on improving the ability of EVs to drive longer and longer distances on a single charge. However, range anxiety is closely related to another concern: charging anxiety. Drivers are accustomed to refueling their vehicles quickly and continuing their journey. Many drivers regularly travel hundreds of miles for work or to visit friends or family, and millions of drivers who live in apartments don’t have the option of overnight charging at home.
For these people, long waits to recharge the car can be a headache or even a dealbreaker that stops them from considering EVs at all. Although early EV adopters have been willing to work around the inconvenience, we believe universal adoption of electric vehicles will be much more difficult to achieve if drivers are asked to make these compromises.
Beyond the simple inconvenience, current fast charging requires economic tradeoffs as well. As discussed above, even today’s fastest-charging EVs require drivers to sacrifice the health of their battery if they want to reach their destination quickly. So while some EVs today are capable of fast charging, with peak charge rates as high as 3C, drivers always have to be mindful of the degradation that comes with too much fast charging.
This is a particular problem for businesses such as logistics providers, taxi firms or rideshare services. These companies need their vehicles on the road as much as possible, and fast-charging batteries would help. But they also can’t afford to incur too much battery degradation since that burdens their bottom line with extra depreciation costs. For these fleet operators, fast charging with good cycle life is not just a question of convenience but a matter of competitive survival.
QuantumScape’s step-change improvement to charging performance offers the potential to help narrow the gap between EVs and combustion-engine vehicles. The example of a road trip might help to illustrate the benefits:
In this example, the tradeoff between charging time and range is easy to see. An EV battery pack that prioritizes energy and range requires fewer stops along the way but takes longer to refill its battery pack. A car that prioritizes charging speed over range saves a bit of time but needs more charging stops to reach its destination. In contrast, a QuantumScape-powered vehicle could potentially deliver both more extended range and faster-charging speeds simultaneously, enabling drivers to reach their destination significantly faster: in this example, as much as an hour faster over a 560-mile journey.
The time-saving advantages of fast-charging QuantumScape batteries would be even more pronounced in vehicles with lower efficiency, such as SUVs or pickup trucks. Over thousands of miles of travel, the ability to charge from 10-80% in less than 15 minutes will shave hours off travel time, substantially alleviating a significant pain point that makes many drivers reluctant to make the switch to EVs. The exceptional fast-charging performance demonstrated here differentiates QuantumScape technology from existing technology in tangible ways that make a difference to car buyers. The ability to cut wait times and increase travel speeds is always an attractive value proposition.
Outlook
Given the limitations of existing battery technology, we believe EVs will continue to lag combustion-engine vehicles on key functionality measures, limiting EV adoption and leaving a substantial market segment unaddressed. QuantumScape’s solid-state lithium-metal technology potentially provides a pathway to dramatically improved fast-charging performance without compromising energy density or cycle life, and this is a key reason we believe it represents the future of electric transportation.
[3] This is because the electrochemical reaction with silicon occurs at a higher voltage (~0.4 V vs Li/Li+) than carbon (~0.15 V vs Li/Li+), providing a wider margin between the intercalation reaction voltage and plating voltage (0 V vs Li/Li+), allowing for more overvoltage before plating occurs. Increasing overvoltage is a result of increasing current (V=IR).
[4] The Panasonic 2170 cylindrical cells were tested at an ambient temperature of 25°C and 45°C. But because of self-heating, during the 4C charge pulse the cell temperature increased to about 45°C and 65°C, respectively.
[5] Richard Acello’s article in the San Diego Business Journal titled “Getting into gear with the vehicle of the future” is apparently the first time the term was used in print, although Acello reports it as part of a quotation from an interview subject, leaving open the question of who coined the phrase.
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.
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Interpreting QuantumScape’s Safety Test Results
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. In this blog article, we’ll look at battery safety testing and do a deep dive into a couple of key test results from energy-dense 24-layer QuantumScape prototype cells based on our Alpha-2 design.
Understanding Battery Safety
June 17, 2024
As lithium-ion batteries have become ubiquitous, their safety risks have increasingly been a focus of public concern.
Interpreting QuantumScape’s Safety Test Results
June 17, 2024
. In this blog article, we’ll look at battery safety testing and do a deep dive into a couple of key test results from energy-dense 24-layer QuantumScape prototype cells based on our Alpha-2 design.
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