Safety and energy density are prime motivators as researchers seek to improve lithium batteries.
The ongoing electrification of everyday items has resulted in the proliferation of batteries, and spurred continued development for automotive and grid use. Lithium-ion batteries still dominate the rechargeable-battery landscape, with solid-state versions prolonging that position, but other lithium variants aim for greater safety while raising energy capacity.
Battery researchers must balance performance against cost, reliability, manufacturability, and a host of other practical considerations. And while lithium remains king of the hill for now, what will be the next big battery technology isn’t clear. In all cases, battery development considers a battery’s primary electrochemical reactions, including the materials forming the electrodes and electrolyte. Improvements are expected for lithium-ion batteries, but the search continues for safer and less-expensive substances. Here, multiple chemistries may see success because one type may suit a particular use case better than another.
“If you look at electric vehicles as a big consumer of high-voltage batteries, that landscape has changed rapidly in the last couple of years,” said Puneet Sinha, senior director, global head of battery at Siemens Digital Industries Software, which just joined the Global Battery Alliance. “Energy density in the last 15 years has improved 3 times, and battery costs have come down by 90%.”
Lithium-ion battery weaknesses
Battery research continues simply because all battery systems have weaknesses, and engineers are constantly trying to overcome them. Batteries are classified as primary (non-rechargeable) and secondary (rechargeable). Given their prevalence and potentially greater environmental friendliness, secondary batteries have exploded in popularity. Lithium-ion batteries are now the rule, having succeeded older nickel-cadmium technology.
In some ways, lithium is an ideal battery material because it involves a small, reactive, lightweight ion packing high potential in a small volume. Energy-per-volume (volumetric) and energy-per-weight (gravimetric) are two ways of measuring energy density, and energy density at the very least determines the weight of a battery.
“These two measurements are correlated, but it’s not the case that if you improve one the other will improve automatically,” noted Xiao Hu, senior principal application engineer at Ansys. “It depends on how dense the material is. You could have pretty good energy density in terms of volume, but the material may be heavy.” In essence, the higher the density, the smaller the necessary battery, making them suitable for small portable electronic devices.
Another downside is the reactivity of lithium. Side reactions deplete lithium over time, and more importantly, the growth of lithium dendrites can lead to explosive reactions with electrolytes or, if the dendrites are too long, result in the battery shorting out. This reactivity affects construction of the current generation of batteries, as well as providing motivation for newer systems.
And while dendrites are the best-known cause of thermal runaway, they’re not the only one. “The cause is typically some kind of abuse — mechanical, electrical, thermal,” said Hu.
In addition, the proliferation of portable equipment drives the weight and volume focus even though costs may be higher than alternatives. But interest in stationary batteries — particularly for grid storage — opens the door for systems that don’t need to be light enough to wear on one’s arm.
Anatomy of a rechargeable battery
A battery consists of three fundamental components:
Technically, the portion of the electrolyte adjacent to the anode is the anolyte, while the portion adjacent to the cathode is the catholyte. Each of these components affects how the battery operates.
Of these, the cathode is a critical determiner of capacity. “It’s the cathode chemistry that determines the energy you’re going to put in the battery,” said Sinha.
Fig. 1: Battery structure. Charging the battery involves moving charges between the cathode and the anode under a voltage coercing that movement. Once charged, those particles want to move back, but can’t if there’s an open circuit. Closing the circuit allows the particles to move back from the anode to the cathode as the battery discharges. An added separator helps to keep dendrites from punching through on lithium-ion batteries. Source: Bryon Moyer/Semiconductor Engineering
Lithium-ion batteries have a well-known possibility of igniting. One cause has been the formation of lithium dendrites, or spiky filaments, that can grow in the anode. As they grow into or through the electrolyte, the lithium can react with the electrolyte. The dendrite also can push all the way through to the cathode, shorting out the battery and permitting a high, uncontrolled current and thermal runaway.
The current generation of batteries has addressed this issue in at least two ways. The most obvious is the addition of a separator in the middle of the electrolyte to resist dendrites pushing through. In addition, the anode typically consists of a graphite matrix that holds lithium ions in little gaps. “Most of the batteries that you see in the market — whether for your cell phone or vehicle or any green energy storage — have graphite- or carbon-based anodes,” Sinha explained.
When charging, the ions move into the anode’s graphite matrix through a process called intercalation, where the ions nestle into the crystal structure and find a place to stay. But that process takes time, and if charging proceeds too fast, then the lithium ions pile up too quickly to work into the anode. Instead, they plate out, leading to dendrite formation. This is one reason standard lithium-ion batteries can’t be charged too quickly. “They’re cheap and reasonably robust but have limitations on how fast you can charge, because it starts growing dendrites if you try to charge very fast,” Sinha said.
The benefit of the graphite anode is that the lithium ions are kept apart from each other, discouraging dendrite formation in the first place. The downside is a reduction in energy density since much of the weight now consists of graphite, which is electrically inert. Another challenge is that, depending on the atoms making up the matrix and those acting as charged particles, the process of slipping into the matrix may cause that matrix to expand, contracting again during discharge. For some materials, this change in size can pose a serious challenge.
In summary, several factors can limit battery performance:
Moving to a solid-state electrolyte
Traditionally, electrolytes have been liquid, and ideally are mediums that permit the flow of charges while not interacting with them. Commercial electrolytes can be water-based (aqueous) or non-aqueous, and may involve some organic substance.
The organic electrolytes employed in lithium-ion batteries contribute to the safety concern. The electrolytes are volatile, and the reason fires occur is due to all the gases, which stem from the decomposition of that electrolyte liquid. The separator must permit ion flow while blocking metallic lithium but it doesn’t protect against reactions with the electrolyte if temperatures rise too high. Some manufacturers are working with polymers to form something less liquid, such as an electrolyte gel.
“For lithium to move in a solid medium is a lot tougher than when it has to move in a liquid,” Sinha noted. “A lot of companies are working on what they call a semi-solid electrolyte, some kind of a gel.” Work on such so-called lithium-polymer (LiPo) batteries dates back to the 1970s.
Commercial battery development
New LiPo versions are also raising the maximum voltage. “Traditional lithium-ion and lithium polymer (LiPo) batteries long used in consumer devices have a max charge voltage of 4.2 V,” said Ken Helfrich, chief product officer at Orca. ‘The existence of lithium chemistries that allow for higher voltages has been known for many years, but the breakthrough toward commercialization has come from advances in polymers. New high-voltage lithium polymer (LiHv) batteries quickly gained popularity in aerial drones, and now, since battery lifetimes have improved, they are filtering down into other devices including wearables.”
These changes, though slight, affect battery-charging circuits, as well. Those designed to top out at 4.2 V cannot serve new modified versions having a higher voltage. As an example, Orca’s new power-management IC (PMIC) is flexible with respect to top voltage. “We’re offering very wide end-of-charge voltage termination capabilities, because a lot of the battery chemistries that are being adopted today are increasing well above the [previous 4.2 V] standard,” said Andrew Baker, CEO and co-founder at Orca.
While it’s intuitive to picture ions floating across a liquid or semi-liquid electrolyte, they also can penetrate some porous solid materials, such as select ceramics. One benefit of this approach is that a solid-state electrolyte also acts as a separator, removing one component from the battery while maintaining safety. The lack of volatility further improves safety. “Every battery supplier is actively working on a solid-state battery,” noted Sinha.
A solid electrolyte also makes it possible to use a metallic lithium anode, giving the resulting battery much better energy density than traditional versions. Metal anodes always have been desirable, but their tendency to form dendrites has made graphite or other matrices necessary. With dendrite formation no longer a threat, lithium anodes should be possible.
This means batteries can be made much thinner and lighter, improving their suitability for portable electronics and, in particular, vehicles. “Solid state is one of the popular choices for the automotive industry,” said Hu. Their higher energy density means that the smaller batteries can store as much energy as older, larger ones. The shorter distance between the cathodes and the lack of dendrites can dramatically improve charging times.
Instead of intercalation, the use of metallic lithium means that the anode is plated in lithium when charging. That plating is removed during discharge. If a cathode is preloaded with the necessary lithium in a discharged new battery, the battery ships with no anode, and charging the battery plates the location where an anode would be — only to disappear when fully discharged.
Fig. 2: A solid-state metal-anode lithium battery. When discharged, it lacks an anode. When charged, metallic lithium is plated to create an anode. Source: Bryon Moyer/Semiconductor Engineering
Removing the anode simplifies and reduces the cost of the manufacturing process. Building a metallic anode rather than relying on plating would be more difficult since one would need only a small amount, and thin lithium foils are hard to work with. So eliminating the anode altogether can have an outsized impact on cost.
Solid-state lithium batteries are being commercialized by companies such as QuantumScape and Solid Power. Samsung also has announced a solid-state model that targets electric vehicles with a claimed 600-mile range and a nine-minute charging time. China is also working hard on this. “They’re very advanced, and they make some very bold claims,” said Hu.
These improvements have the potential to make solid-state technology attractive in the market, but it’s early enough that challenges could crop up as the new approach proves itself out. “The solid-state lithium-ion battery is closer to production than some of the novel ideas that the startup companies are coming up with,” added Hu.
Bipolar batteries
The battery depicted in Figure 1 is more accurately called a cell. A real-world battery is likely to have multiple cells in what’s called a pack. Stacked in series, as is common, they provide a voltage that’s a multiple of the individual cell voltage. Run in parallel, you get more current at the cell voltage.
Fig. 3: In a single-electrode configuration, cells must be wired externally to form a pack. With monopolar electrodes, cells are arranged side by side, but every other cell is flipped around so that current collectors have the same electrode on each side. They operate in parallel. With bipolar electrodes, the cells are still side-by-side but in series, and none is flipped. Here, a substrate has one electrode on one side, and the other on the other side. It’s effectively the same as single-electrode but without the need to wire externally, saving space. (Source: Bryon Moyer/Semiconductor Engineering)
When stacking the cells, they’re typically wired together using separate components outside the cells. In this configuration, all cells have single electrodes (as shown in Figure 3). If doing a parallel configuration, pairs of cells can be arranged back-to-back (with one cell flipped), and the current is then pulled from the respective collectors. This arrangement is called a monopolar electrode, because each current collector has the same type of electrode (anode or cathode) on each side of it.
The series configuration can be significantly improved if the cells are stacked next to each other and abut the electrodes with a conductive substrate that keeps the electrodes apart. This behaves just as the single-electrode arrangement, but it lacks the external wiring and can be assembled much more compactly. Using thin films, many layers can be formed into a thin battery, such as in pouch batteries.
Then, in a bipolar electrode configuration the substrates separating the cells have opposite electrodes on each side, in contrast to the monopolar structure. This has been successfully implemented with lead-acid batteries but still requires some work for lithium-based batteries.
Conclusion
Lithium remains king of batteries for now. All the changes envisioned for lithium batteries should keep the element front and center for powering portable equipment for a few years at least, but some researchers foresee bigger changes and new chemistries that don’t involve lithium.
This first part in a two-part series has focused on battery basics and some of the upgrades that lithium-ion batteries are undergoing. The second part in this two-part series will address the longer-term promise of stored energy both for portable devices and immovable energy storage.
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