Ionic liquids give push to next-gen solid-state lithium metal batteries. Credit: Tokyo Metropolitan University
Since their first commercialization, rechargeable Li-ion batteries have dominated the portable electronicsmarket for the last three decades. But as we look for better solutions with higher energy density, scientists have been turning to solid-state lithium metal batteries.
Li metal batteries potentially have a much higher energy density than their Li-ion counterparts, but technical issues keep solid-state lithium metal batteries from making their way into demanding applications. It is difficult to achieve good contact between electrodes and solid electrolytes. Any surface roughness on either side leads to high interfacial resistance, which plagues battery performance.
Researchers at Japan’s Tokyo Metropolitan University have been developing new ways of improving the contact between the cathode and solid-state electrolyte in solid-state lithium metal batteries. And now, they have succeeded in creating a new quasi-solid-state cathode, with significantly reduced problematic resistance between key components. The new quasi-solid-state lithium cobalt oxide (LiCoO2) cathode contains a room-temperature ionic liquid, which is salt in a liquid state.
By adding an ionic liquid, their modified cathode could maintain excellent contact with the electrolyte.
The addition of an ionic liquid to the cathode material fills structural voids and provides a better interface with the solid electrolyte. Credit: Tokyo Metropolitan University
Ionic liquids consist of positive and negative ions; they can also transport ions and fill tiny voids at the cathode and the solid electrolyte interface. With the voids filled, the interfacial resistance was significantly reduced. Ionic liquids are not only ionically conductive but almost non-volatile and usually non-flammable.
The team demonstrated a prototype battery featuring this novel, quasi-solid-state cathode that showed impressive stability, with 80% capacity retention after 100 charge and discharge cycles at an elevated temperature of 60°C.
Though finding a better ionic liquid that doesn’t degrade as easily remains challenging, the idea promises new directions in solid lithium battery development for practical applications.
Californian company Amprius Technologies has announced the shipment of the first batch of its 450 Wh/kg, 1150 Wh/L lithium-ion battery cells to an industry leader of a new generation of High-Altitude Pseudo Satellites (HAPS). The company claims these are the most energy-dense lithium batteries commercially available today.
The batteries’ impressive performance is the result is Amprius Technologies’ silicon nanowire anode (Si-Nanowire platform), which offers a unique combination of performance metrics, including fast charge (under 10 minutes), high power (10C rates), high energy density (over 400 Wh/kg) and long life (over 500 cycles). The company was able to achieve 450 Wh/kg just a few months after announcing the 405 Wh/kg product in November 2021. In December, we also learned about the 370 Wh/kg version, which can be recharged to 80% from 0% state of charge in just about 6 minutes.
“This advancement from the 405 Wh/kg product highlights the acceleration of our roadmap towards delivering products with unrivaled performance,”said Jon Bornstein, COO of Amprius Technologies. “Our proprietary Si-Nanowire platform and the comprehensive solutions we have developed enable unparalleled performance and continue to sustain our product leadership.”
This shipment represents the culmination of collaborative development and testing for this latest design win. Currently, Amprius Technologies, which has been in commercial manufacturing since 2018, produces the battery cells at a limited scale at its facility in Fremont, California. The company has embarked on constructing its first high-volume manufacturing facility located in the United States. A mass production site will be selected in the first quarter of 2022.
Amprius Technologies’ batteries deliver up to 100% higher energy density than standard lithium-ion batteries. This means our cells provide more energy and power with much less weight and volume.
Research & development
Building on research at Stanford University, Amprius Technologies continually explores new ways to improve battery technology and manufacturing processes. Amprius Technologies’ batteries have established breakthrough performance with new cells approaching 500 Wh/kg over hundreds of cycles.
Real world applications
Amprius Technologies has demonstrated scalable manufacturing and revolutionary performance in real world applications. The company’s game changing battery is a proven solution for advanced products and mission critical applications.
Researchers at the University of Technology Sydney (UTS) have designed a molecule to boost the performance of lithium-oxygen batteries to give electric vehiclesthe same driving range as petrol-fuelled cars.
Lithium-oxygen batteries employ cutting-edge technology aimed to deliver maximum energy density through breathing air to generate electricity.
To date, however, they have been beset by challenges, including low discharge capacity, poor energy efficiency, and severe parasitic reactions. This new all-in-one molecule can simultaneously tackle those issues.
According to the researchers, their new discovery resolved several existing obstacles and created the possibility of developing a long-life, energy-dense lithium-oxygen battery that was highly efficient.
“Batteries are changing fundamentally,” said UTS Professor Guoxiu Wang, who led the research team. “They will facilitate the transition towards a climate-neutral society and open up new industry opportunities for a country like Australia that is rich in the fundamental elements for building batteries.
They will also help utilities improve power quality and reliability and help governments around the world achieve net-zero carbon emissions.”
The study reports a lithium-oxygen battery operated via a new quenching/mediating mechanism that relies on the direct chemical reactions between a versatile molecule and superoxide radical/Li2O2. The battery exhibits a 46-fold increase in discharge capacity, a low charge overpotential of 0.7 V, and an ultralong cycle life greater than 1400 cycles.
“Our rationally designed and synthesized PDI-TEMPO molecule opens a new avenue for developing high-performance Li-O2 batteries,”Professor Wang said. “The capacity of next-generation lithium-oxygen batteries to extend the driving range between charges would be a significant leap forward for the electric vehicle industry. We are confident our all-in-one molecule can dramatically improve the performances of lithium-oxygen batteries and enable new generation lithium-oxygen batteries to be practical.”
StoreDot, an Israel-based electric vehicle extreme fast charging (XFC) battery startup, today announced that it has advanced technology that extends the life span of batteries, making them highly effective not only during the vehicle life span, but also for second-life applications.
The technology combines the electrochemistry system of the company’s silicon dominant cells to ensure that there is minimal drop-off in performance even as the battery ages.
StoreDot reports that a robust performance is maintained even after 1,000 cycles and 80% capacity, the point at which rival lithium-ion fast-charging technologies start to rapidly deteriorate in performance. Even after 1,700 cycles, long after the accepted industry norm, StoreDot claims its batteries can maintain 70% of their original capacity, making them effective in second-life usage for less dynamic applications such as in energy storage and grid load balancing systems.
Dr. Doron Myersdorf, StoreDot CEO, said:
StoreDot is well known for creating extreme fast charging technologies and helping drivers overcome charging anxiety, which is currently the biggest barrier to EV ownership.
But we believe in advancing the entire battery eco-system, ultimately delivering an optimum solution to sustain the transition to full EV electrification. This latest development is proof of that.
We now have the ability to hugely extend the life of our batteries, long after their vehicle service life. This not only has benefits for the drivers of EVs, allowing them to maintain performance of their vehicles for many more years, but also in second-life applications.
Not only will this transformative development encourage more people to drive EVs, but this technology has huge benefits for sustainability, too, reducing the need to retire and recycle an expensive component that can now serve in critical second-life applications.
StoreDot says it’s in advanced talks with global car makers.It also says it’s on track to deliver mass-produced XFC batteries, which provide a 50% reduction in charging time at the same cost, by 2024.
In early September 2021, StoreDot announced that it produced the first 4680 cylindrical cell, that it claims can charge in only 10 minutes.
In November, StoreDot claimed it had become the first company to produce XFC cells for electric vehicles on a mass production line. And on December 1, 2021, StoreDot announcednew patented technology that uses a background repair mechanism to allow battery cells to regenerate while they are in use.
Lithium-ion batteries, those marvels of lightweight power that have made possible today’s age of handheld electronics and electric vehicles, have plunged in cost since their introduction three decades ago at a rate similar to the drop in solar panel prices, as documented by a study published last March.
But what brought about such an astonishing cost decline, of about 97 percent?
Some of the researchers behind that earlier study have now analyzed what accounted for the extraordinary savings. They found that by far the biggest factor was work on research and development, particularly in chemistry and materials science. This outweighed the gains achieved through economies of scale, though that turned out to be the second-largest category of reductions.
The new findings are being published in the journal Energy and Environmental Science, in a paper by MIT postdoc Micah Ziegler, recent graduate student Juhyun Song Ph.D. ’19, and Jessika Trancik, a professor in MIT’s Institute for Data, Systems and Society.
The findings could be useful for policymakers and planners to help guide spending priorities in order to continue the pathway toward ever-lower costs for this and other crucial energy storage technologies, according to Trancik. Their work suggests that there is still considerable room for further improvement in electrochemical battery technologies, she says.
The analysis required digging through a variety of sources, since much of the relevant information consists of closely held proprietary business data. “The data collection effort was extensive,” Ziegler says. “We looked at academic articles, industry and government reports, press releases, and specification sheets. We even looked at some legal filings that came out. We had to piece together data from many different sources to get a sense of what was happening.” He says they collected “about 15,000 qualitative and quantitative data points, across 1,000 individual records from approximately 280 references.”
Data from the earliest times are hardest to access and can have the greatest uncertainties, Trancik says, but by comparing different data sources from the same period they have attempted to account for these uncertainties.
Overall, she says, “we estimate that the majority of the cost decline, more than 50 percent, came from research-and-development-related activities.” That included both private sector and government-funded research and development, and “the vast majority” of that cost decline within that R&D category came from chemistry and materials research.
That was an interesting finding, she says, because “there were so many variables that people were working on through very different kinds of efforts,” including the design of the battery cells themselves, their manufacturing systems, supply chains, and so on. “The cost improvement emerged from a diverse set of efforts and many people, and not from the work of only a few individuals.”
The findings about the importance of investment in R&D were especially significant, Ziegler says, because much of this investment happened after lithium-ion battery technology was commercialized, a stage at which some analysts thought the research contribution would become less significant. Over roughly a 20-year period starting five years after the batteries’ introduction in the early 1990s, he says, “most of the cost reduction still came from R&D. The R&D contribution didn’t end when commercialization began. In fact, it was still the biggest contributor to cost reduction.”
The study took advantage of an analytical approach that Trancik and her team initially developed to analyze the similarly precipitous drop in costs of silicon solar panels over the last few decades. They also applied the approach to understand the rising costs of nuclear energy. “This is really getting at the fundamental mechanisms of technological change,” she says. “And we can also develop these models looking forward in time, which allows us to uncover the levers that people could use to improve the technology in the future.”
One advantage of the methodology Trancik and her colleagues have developed, she says, is that it helps to sort out the relative importance of different factors when many variables are changing all at once, which typically happens as a technology improves. “It’s not simply adding up the cost effects of these variables,” she says, “because many of these variables affect many different cost components. There’s this kind of intricate web of dependencies.” But the team’s methodology makes it possible to “look at how that overall cost change can be attributed to those variables, by essentially mapping out that network of dependencies,” she says.
This can help provide guidance on public spending, private investments, and other incentives. “What are all the things that different decision makers could do?” she asks. “What decisions do they have agency over so that they could improve the technology, which is important in the case of low-carbon technologies, where we’re looking for solutions to climate change and we have limited time and limited resources? The new approach allows us to potentially be a bit more intentional about where we make those investments of time and money.”
David Chandler MIT Technology
More information: Determinants of lithium-ion battery technology cost decline, Energy and Environmental Science (2021). DOI: 10.1039/d1ee01313k
Journal information: Energy and Environmental Science
Iron-flow technology from ESS is being deployed at scale in the U.S.
The world’s electric grids are creaking under the pressure of volatile fossil-fuel prices and the imperative of weaning the world off polluting energy sources. A solution may be at hand, thanks to an innovative battery that’s a cheaper alternative to lithium-ion technology.
SB Energy Corp., a U.S. renewable-energy firm that’s an arm of Japan’s SoftBank Group Corp., is making a record purchase of the batteries manufactured by ESS Inc. The Oregon company says it has new technology that can store renewable energy for longer and help overcome some of the reliability problems that have caused blackouts in California and record-high energy prices in Europe.
The units, which rely on something called “iron-flow chemistry,” will be used in utility-scale solar projects dotted across the U.S., allowing those power plants to provide electricity for hours after the sun sets. SB Energy will buy enough batteries over the next five years to power 50,000 American homes for a day.
“Long-duration energy storage, like this iron-flow battery, are key to adding more renewables to the grid,” said Venkat Viswanathan, a battery expert and associate professor of mechanical engineering at Carnegie Mellon University.
ESS was founded in 2011 by Craig Evans, now president, and Julia Song, the chief technology officer. They recognized that while lithium-ion batteries will play a key role in electrification of transport, longer duration grid-scale energy storage needed a different battery. That’s because while the price of lithium-ion batteries has declined 90% over the last decade, their ingredients, which sometimes include expensive metals such as cobalt and nickel, limit how low the price can fall.
The deal for 2 gigawatt-hours of batteries is worth at least $300 million, according to ESS. Rich Hossfeld, chief executive officer of SB Energy, said the genius of the units lies in their simplicity.
“The battery is made of iron salt and water,” said Hossfeld. “Unlike lithium-ion batteries, iron flow batteries are really cheap to manufacture.”
Every battery has four components: two electrodes between which charged particles shuffle as the battery is charged and discharged, electrolyte that allows the particles to flow smoothly and a separator that prevents the two electrodes from forming a short circuit.
Flow batteries, however, look nothing like the battery inside smartphones or electric cars. That’s because the electrolyte needs to be physically moved using pumps as the battery charges or discharges. That makes these batteries large, with ESS’s main product sold inside a shipping container.
What they take up in space, they can make up in cost. Lithium-ion batteries for grid-scale storage can cost as much as $350 per kilowatt-hour. But ESS says its battery could cost $200 per kWh or less by 2025.
Crucially, adding storage capacity to cover longer interruptions at a solar or wind plant may not require purchasing an entirely new battery. Flow batteries require only extra electrolyte, which in ESS’s case can cost as little as $20 per kilowatt hour.
“This is a big, big deal,” said Eric Toone, science lead at Breakthrough Energy Ventures, which has invested in ESS. “We’ve been talking about flow batteries forever and ever and now it’s actually happening.”
The U.S. National Aeronautics and Space Administration built a flow battery as early as 1980. Because these batteries used water, they presented a much safer option for space applications than lithium-ion batteries developed around that time, which were infamous for catching on fire. Hossfeld says he’s been able to get permits for ESS batteries, even in wildfire-prone California, that wouldn’t have been given to lithium-ion versions.
Still, there was a problem with iron flow batteries. During charging, the battery can produce a small amount of hydrogen, which is a symptom of reactions that, left unchecked, shorten the battery’s life. ESS’s main innovation, said Song, was a way of keeping any hydrogen produced within the system and thus hugely extending its life.
“As soon as you close the loop on hydrogen, you suddenly turn a lab prototype into a commercially viable battery option,” said Viswanathan. ESS’s iron-flow battery can endure more than 20 years of daily use without losing much performance, said Hossfeld.
At the company’s factory near Portland, yellow robots cover plastic sheets with chemicals and glue them together to form the battery cores. Inside the shipping containers, vats full of electrolyte feed into each electrode through pumps — allowing the battery to do its job of absorbing renewable power when the sun shines and releasing it when it gets dark.
It’s a promising first step. ESS’s battery is a cheap solution that can currently provide about 12 hours of storage, but utilities will eventually need batteries that can last much longer as more renewables are added to the grid. Earlier this month, for example, the lack of storage contributed to a record spike in power prices across the U.K. when wind speeds remained low for weeks. Startups such as Form Energy Inc. are also using iron, an abundant and cheap material, to build newer forms of batteries that could beat ESS on price.
So far, ESS has commercially deployed 8 megawatt-hours of iron flow batteries. Last week, after a six-month evaluation, Spanish utility Enel Green Power SpA signed a single deal for ESS to build an equivalent amount. SB Energy’s Hossfeld, who also sits on ESS’s board, said the company would likely buy still more battery capacity from ESS in the next five years.
Even as its order books fill up, ESS faces a challenging road ahead. Bringing new batteries to market is notoriously difficult and the sector is littered with failed startups. Crucially, lithium-ion technology got a head start and customers are more familiar with its pros and cons. ESS will have to prove that its batteries can meet the rigorous demands of power plant operators.
The new order should help ESS as it looks to go public within weeks through a special-purpose acquisition company at a valuation of $1.07 billion. The listing will net the company $465 million, which it plans to use to scale up its operations.
Lithium-metal batteries (LMBs), an emerging type of rechargeable lithium-based batteries made of solid-state metal instead of lithium-ions, are among the most promising high-energy-density rechargeable battery technologies. Although they have some advantageous characteristics, these batteries have several limitations, including a poor energy density and safety-related issues.
In recent years, researchers have tried to overcome these limitations by introducing an alternative, anode-free lithium battery cell design. This anode-free design could help to increase the energy density and safety of lithium-metal batteries.
Researchers at the National Institute of Advanced Industrial Science and Technology recently carried out a study aimed at increasing the energy density of anode-free lithium batteries. Their paper, published in Nature Energy, introduces a new high-energy-density and long-life anode-free lithium battery based on the use of a Li2O sacrificial agent.
Anode-free full-cell battery architectures are typically based on a fully lithiated cathode with a bare anode copper current collector. Remarkably, both the gravimetric and volumetric energy densities of anode-free lithium batteries can be extended to their maximum limit. Anode-free cell architectures have several other advantages over more conventional LMB designs, including a lower cost, greater safety and simpler cell assembly procedures.
To unlock the full potential of anode-free LMBs, researchers should first figure out how to achieve the reversibility/stability of Li-metal plating. While many have tried to solve this problem by engineering and selecting more favorable electrolytes, most of these efforts have so far been unsuccessful.
Others have also explored the potential of using salts or additives that could improve the Li-metal plating/stripping reversibility. After reviewing these previous attempts, the researchers at the National Institute of Advanced Industrial Science and Technology proposed the use of Li2O as a sacrificial agent, which is pre-loaded onto a LiNi0.8Co0.1Mn0.1O2 surface.
“It is challenging to realize high Li reversibility, especially considering the limited Li reservoir (typically zero lithium excess) in the cell configuration,” the researchers wrote in their paper. “In this study we have introduced Li2O as a preloaded sacrificial agent on a LiNi0.8Co0.1Mn0.1O2 cathode, providing an additional Li source to offset the irreversible loss of Li during long-term cycling in an initial-anode-free cell.”
In addition to employing Li2O as a sacrificial agent, the researchers proposed the use of a fluoropropyl ether additive to neutralize nucleophilic O2-, which is released during the oxidation of Li2O, and prevent the additional evolution of gaseous O2 resulting from the fabrication of a LiF-based electrolyte coated on the surface of the battery’s cathode.
“We show that O2– species, released through Li2O oxidation, are synergistically neutralized by a fluorinated ether additive,” the researchers explained in their paper. “This leads to the construction of a LiF-based layer at the cathode/electrolyte interface, which passivates the cathode surface and restrains the detrimental oxidative decomposition of ether solvents.”
Based on the design they devised, Yu Qiao and the rest of the team at the National Institute of Advanced Industrial Science and Technology were able to realize a long-life 2.46 Ah initial-anode-free pouch cell. This cell exhibited a gravimetric energy density of 320 Wh kg-1, maintaining an 80% capacity after 300 operation cycles.
In the future, the anode-free lithium battery introduced by this research group could help to overcome some of the commonly reported limitations of LMBs. In addition, its design could inspire the creation of safer lithium-based rechargeable batteries with higher energy densities and longer lifetimes.
Last week, the US International Trade Commission (ITC) proposed a 10-year import ban on South Korean battery producer, SK Innovation, after the conclusion of an IP lawsuit filed by fellow South Korean battery maker, LG Chem. This decision to ban imports essentially cuts material supply from two factories with a combined capacity of almost 22GWh (9.8GWh and 11.7GWh respectively), expected to commence production in 2022 and 2023. However, there is still an option of local material sourcing, though there are limited opportunities to source the required materials, such as active cathode materials domestically within the USA at the scale required.
Roskill’s analysis shows that in 2020, the USA accounted for 1% of the global cathode materials market, which is forecast to increase to around 5% by 2030. The legislation passed by the US ITC, however, maintains SK innovation’s ability to supply battery cells to Volkswagen’s MEB line in North America for two years and Ford’s F-150 for four years, in addition to supplying spare parts for Kia models. Considering sales/production levels of Ford and Volkswagen in USA, Roskill estimates SK Innovation’s potential market size to be 9GWh through to February 2023, falling to 3GWh until February 2025, as potential to supply VW’s requirements expires. As a result, it seems unlikely for SK Innovation to invest further capital and time developing and commissioning its two USA based factories, only to achieve production of battery cells for 2-3 years at 14% planned utilization rate.
The removal of 22GWh of pipeline production capacity would represent a 10% decrease in total giga-factories capacity in North America in 2023, while EV demand in North America is expected to triple in the next five years and requires nearly 75GWh in installed battery capacity. As a result, the ITC’s decision, if not reversed or altered, would negatively impact the supply of Li-ion batteries for EV applications in the USA. The absence of SK Innovation would also place greater reliance on other battery makers in the USA, including Tesla/Panasonic, LG Chem and Envision AESC.
Roskill publishes annual Market Outlook reports for lithium-ion batteries and for a range of commodities across the lithium-ion battery supply chain, including lithium, cobalt, nickel sulphate and graphite. To see our full range of analysis, click here.
Join Roskill’s Lithium Mine to Market Conference to gain insight into the key drivers of the lithium market in 2021 and beyond. To register, click here.
Contact the authors
This article was written by Egor Prokhodtsev and Kevin Shang. Please get in touch below if you wish to discuss further
Electric planes could be the future of aviation. In theory, they will be much quieter, cheaper, and cleaner than the planes we have today. Electric planes with a 1,000 km (620 mile) range on a single charge could be used for half of all commercial aircraft flights today, cutting global aviation’s carbon emissions by about 15%.
It’s the same story with electric cars. An electric car isn’t simply a cleaner version of its pollution-spewing cousin. It is, fundamentally, a better car: Its electric motor makes little noise and provides lightning-fast response to the driver’s decisions. Charging an electric car costs much less than paying for an equivalent amount of gasoline. Electric cars can be built with a fraction of moving parts, which makes them cheaper to maintain.
So why aren’t electric cars everywhere already? It’s because batteries are expensive, making the upfront cost of an electric car much higher than a similar gas-powered model. And unless you drive a lot, the savings on gasoline don’t always offset the higher upfront cost. In short, electric cars still aren’t economical.
Similarly, current batteries don’t pack in enough energy by weight or volume to power passenger aircrafts. We still need fundamental breakthroughs in battery technology before that becomes a reality.
Battery-powered portable devices have transformed our lives. But there’s a lot more that can batteries can disrupt, if only safer, more powerful, and energy-dense batteries could be made cheaply. No law of physics precludes their existence.
And yet, despite over two centuries of close study since the first battery was invented in 1799, scientists still don’t fully understand many of the fundamentals of what exactly happens inside these devices. What we do know is that there are, essentially, three problems to solve in order for batteries to truly transform our lives yet again: power, energy, and safety.
THERE ISN’T A ONE-SIZE-FITS-ALL LITHIUM-ION BATTERY
Every battery has two electrodes: a cathode and an anode. Most anodes of lithium-ion batteries are made of graphite, but cathodes are made of various materials, depending on what the battery will be used for. Below, you can see how different cathode materials change the way battery types perform on six measures.
The power challenge
In common parlance, people use “energy” and “power” interchangeably, but it’s important to differentiate between them when talking about batteries. Power is the rate at which energy can be released.
A battery strong enough to launch and keep aloft a commercial jet for 1,000 km requires a lot of energy to be released in very little time, especially during takeoff. So it’s not just about having lots of energy stored but also having the ability to extract that energy very quickly.
Tackling the power challenge requires us to look inside the black box of commercial batteries. It’s going to get a little nerdy, but bear with me. New battery technologies are often overhyped because most people don’t look closely enough at the details.
The most cutting-edge battery chemistry we currently have is lithium-ion. Most experts agree that no other chemistry is going disrupt lithium-ion for at least another decade or more. A lithium-ion battery has two electrodes (cathode and anode) with a separator (a material that conducts ions but not electrons, designed to prevent shorting) in the middle and an electrolyte (usually liquid) to enable the flow of lithium ions back and forth between the electrodes. When a battery is charging, the ions travel from the cathode to the anode; when the battery is powering something, the ions move in the opposite direction.
Imagine two loaves of sliced bread. Each loaf is an electrode: the left one is the cathode and the right is one the anode. Let’s assume the cathode is made up of slices of nickel, manganese, and cobalt (NMC)—one of the best in the class—and that the anode is made up of graphite, which is essentially layered sheets, or slices, of carbon atoms.
In the discharged state—i.e., after it has been drained of energy—the NMC loaf has lithium ions sandwiched between each slice. When the battery is charging, each lithium ion is extracted from between the slices and forced to travel through the liquid electrolyte. The separator acts as a checkpoint ensuring only lithium ions pass through to the graphite loaf. When fully charged, the battery’s cathode loaf will have no lithium ions left; they will all be neatly sandwiched between the slices of the graphite loaf. As the battery’s energy is consumed, the lithium ions travel back to the cathode, until there are none left in the anode. That’s when the battery needs to be charged again.
The battery’s power capacity is determined by, essentially, how fast this process happens. But it’s not so simple to turn up the speed. Drawing lithium-ions out of the cathode loaf too quickly can cause the slices to develop flaws and eventually break down. It’s one reason why the longer we use our smartphone, laptop, or electric car, the worse their battery life gets. Every charge and discharge causes the loaf to weaken that little bit.
Various companies are working on solutions to the problem. One idea is to replace layered electrodes with something structurally stronger. For example, the 100-year-old Swiss battery company Leclanché is working on a technology that uses lithium iron phosphate (LFP), which has an “olivine” structure, as the cathode, and lithium titanate oxide (LTO), which has a “spinel” structure, as the anode. These structures are better at handling the flow of lithium ions in and out of the material.
Leclanché currently uses its battery cells in autonomous warehouse forklifts, which can be charged to 100% in nine minutes. For comparison, the best Tesla supercharger can charge a Tesla car battery to about 50% in 10 minutes. Leclanché is also deploying its batteries in the UK for fast-charging electric cars. These batteries sit at the charging station slowly drawing small amounts of power over a long period from the grid until they are fully charged. Then, when a car docks, the docking-station batteries quick-charge the car’s battery. When the car leaves, the station battery starts recharging again.
Efforts like Leclanché’s show it’s possible to tinker with battery chemistries to increase their power. Still, nobody has yet built a battery powerful enough to rapidly deliver the energy needed for a commercial plane to defeat gravity. Startups are looking to build smaller planes (seating up to 12 people), which could fly on relatively lower power-dense batteries, or electric hybrid planes, where jet fuel does the hard lifting and batteries do the coasting.
But there’s really no company working in this space anywhere near commercialization. Further, the kind of technological leap required for an all-electric commercial plane will likely take decades, says Venkat Viswanathan, a battery expert at Carnegie Mellon University.
REUTERS/ALISTER DOYLE A two-seat electric plane made by Slovenian firm Pipistrel stands outside a hangar at Oslo Airport, Norway.
The energy challenge
The Tesla Model 3, the company’s most affordable model, starts at $35,000. It runs on a 50 kWh battery, which costs approximately $8,750, or 25% of the total car price.
That’s still amazingly affordable compared to not that long ago. According to Bloomberg New Energy Finance, the average global cost for lithium-ion batteries in 2018 was about $175 per kWh—down from nearly $1,200 per kWh in 2010.
The US Department of Energy calculates that once battery costs fall below $125 per kWh, owning and operating an electric car will be cheaper than a gas-powered car in most parts of the world. It doesn’t mean electric vehicles will win over gas-powered vehicles in all niches and domains—for example, long-haul trucks don’t yet have an electric solution. But it’s a tipping point where people will start to prefer electric cars simply because they will make more economical sense in most cases.
One way to get there is to increase the energy density of batteries—to cram more kWh into a battery pack without lowering its price. Battery chemist can do that, in theory, by increasing the energy density of either the cathode or the anode, or both.
The most energy-dense cathode on the way to commercial availability is NMC 811 (each digit in the number represents the ratio of nickel, manganese, and cobalt, respectively, in the mix). It’s not yet perfect. The biggest problem is that it can only withstand a relatively small number of charge-discharge life cycles before it stops working. But experts predict that industry R&D should solve the problems of the NMC 811 within the next five years. When that happens, batteries using NMC 811 will have higher energy density by 10% or more.
However, a 10% increase is not that much in the big picture.
And, while a series of innovations over the past few decades have pushed the energy density of cathodes ever higher, anodes are where the biggest energy-density opportunities lie.
Graphite has been and remains far and away the dominant anode material. It’s cheap, reliable, and relatively energy dense, especially compared to current cathode materials. But it’s fairly weak when stacked up against other potential anode materials, like silicon and lithium.
Silicon, for example, is theoretically much better at absorbing lithium ions as graphite. That’s why a number of battery companies are trying to pepper some silicon in with the graphite in their anode designs; Tesla CEO Elon Musk has said his company is already doing this in its lithium-ion batteries.
A bigger step would be to develop a commercially viable anode made completely from silicon. But the element has traits that make this difficult. When graphite absorbs lithium ions, its volume does not change much. A silicon anode, however, swells to four times its original volume in the same scenario.
Unfortunately, you can’t just make the casing bigger to accommodate that swelling, because the expansion breaks apart what’s called the “solid electrolyte interphase,” or SEI, of the silicon anode.
You can think of the SEI as a sort of protective layer that the anode creates for itself, similar to the way that iron forms rust, also known as iron oxide, to protect itself from the elements: When you leave a piece of newly forged iron outside, it slowly reacts with the oxygen in the air to rust. Underneath the layer of rust, the rest of the iron doesn’t suffer from the same fate and thus retains the structural integrity.
At the end of a battery’s first charge, the electrode forms it’s own “rust” layer—the SEI—separating the uneroded part of the electrode from the electrolyte. The SEI stops additional chemical reactions from consuming the electrode, ensuring that lithium ions can flow as smoothly as possible.
But with a silicon anode, the SEI breaks apart every time the battery is used to power something up, and reforms every time the battery is charged. And during each charge cycle, a little bit of silicon is consumed. Eventually, the silicon dissipates to the point where the battery no longer works.
Over the last decade, a few Silicon Valley startups have been working to solve this problem. For example, Sila Nano’s approach is to encase silicon atoms inside a nano-sized shell with lots of empty room inside. That way, the SEI is formed on the outside of the shell and the expansion of silicon atoms happens inside it without shattering the SEI after each charge-discharge cycle. The company, valued at $350 million, says its technology will power devices as soon as 2020.
Enovix, on the other hand, applies a special manufacturing technique to put a 100% silicon anode under enormous physical pressure, forcing it to absorb fewer lithium ion and thus restricting the expansion of the anode and preventing the SEI from breaking. The company has investments from Intel and Qualcomm, and it also expects to have its batteries in devices by 2020.
These compromises mean the silicon anode can’t reach its theoretical high energy density. However, both companies say their anodes perform better than a graphite anode. Third parties are currently testing both firms’ batteries.
TESLA In 2020, the new Tesla Roadster is set to become the first electric car that offers 1,000 km (620 miles) on a single charge.
The safety challenge
All the molecular tinkering done to pack more energy in batteries can come at the cost of safety. Ever since its invention, the lithium-ion battery has caused headaches because of how often it catches fire. In the 1990s, for example, Canada’s Moli Energy commercialized a lithium-metal battery for use in phones. But out in the real world, its batteries started catching fire, and Moli was forced to make a recall, and, eventually, file for bankruptcy. (Some of its assets were bought by a Taiwanese company and it still sells lithium-ion batteries the brand name E-One Moli Energy.) More recently, Samsung’s Galaxy Note 7 smartphones, which were made with modern lithium-ion batteries, started exploding in people’s pockets. The resulting 2016 product recall cost the South Korean giant $5.3 billion.
Today’s lithium-ion batteries still have inherent risks, because they almost always use flammable liquids as the electrolyte. It’s one of nature’s unfortunate (for us humans) quirks that liquids able to easily transport ions also tend to have a lower threshold to catching fire. One solution is to use solid electrolytes. But that means other compromises. A battery design can easily include a liquid electrolyte that’s in contact with every bit of the electrodes—making it able to efficiently transfer ions. It’s much harder with solids. Imagine dropping a pair of dice into a cup of water. Now imagine dropping those same dice into a cup of sand. Obviously, the water will touch far more surface area of the dice than the sand will.
So far, the commercial use of lithium-ion batteries with solid electrolytes has been limited to low-power applications, such as for internet-connected sensors. The efforts to scale up solid-state batteries—that is, containing no liquid electrolyte—can be broadly classified into two categories: solid polymers at high temperatures and ceramics at room temperature.
SOLID POLYMERS AT HIGH TEMPERATURES
Polymers are long chains of molecules linked up together. They’re extremely common in everyday applications—single-use plastic bags are made of polymers, for example. When some types of polymers are heated, they behave like liquids, but without the flammability of the liquid electrolytes used in most batteries. In other words, they have the high ion conductivity as a liquid electrolyte without the risks.
But they have limitations. They can only operate at temperatures above 105°C (220°F), which means they aren’t practical options for, say, smartphones. But they can be used for storing energy from the grid in home batteries, for example. At least two companies—US-based SEEO and France-based Bolloré—are developing solid-state batteries that use high-temperature polymers as the electrolyte.
CERAMICS AT ROOM TEMPERATURE
Over the last decade, two classes of ceramics—LLZO (lithium, lanthanum, and zirconium oxide) and LGPS (lithium, germanium, phosphorus sulfide)—have proven almost as good at conducting ions at room temperature as liquids.
Toyota, as well as the Silicon Valley startup QuantumScape (which raised $100 million in funding from Volkswagen last year), are both working on deploying ceramics in lithium-ion batteries. The inclusion of big players in the space is indicative that a breakthrough might be nearer than many think.
“We are quite close to seeing something real [using ceramics] in two or three years,” says Carnegie Mellon‘s Viswanathan.
A balancing act
Batteries are already big business, and the market for them keeps growing. All that money attracts a lot of entrepreneurs with even more ideas. But battery startups are difficult bets—they fizzle even more often than software companies, which are known for their high failure rate. That’s because innovation in material sciences is hard.
So far battery chemists have found that, when they try to improve one trait (say energy density), they have to compromise on some other trait (say safety). That kind of balancing act has meant the progress on each front has been slow and fraught with problems.
But with more eyes on the problem—MIT’s Yet-Ming Chiang reckons there are three times as many battery scientists in the US today than just 10 years ago—the chances of success go up. The potential of batteries remains huge, but given the challenges ahead, it’s better to look at every claim about new batteries with a good dose of skepticism.
A scanning electron microscope image of lithium titanate (lithium, titanium, oxygen) “nanoflowers.”
Lithium-ion batteries work by shuffling lithium ions between a positive electrode (cathode) and a negative electrode (anode) during charging and in the opposite direction during discharging.
Our smartphones, laptops, and electric vehicles conventionally employ lithium-ion batteries with anodes made of graphite, a form of carbon.
Lithium is inserted into graphite as you charge the battery and removed as you use the battery.
While graphite can be reversibly charged and discharged over hundreds or even thousands of cycles, the amount of lithium it can store (capacity) is not enough for energy-intensive applications.
For example, electric cars can only travel so far before they need to be recharged. In addition, graphite cannot be charged or discharged at very high rates (power). Because of these limitations, scientists have been on the hunt for alternative anode materials.
One such promising anode material is lithium titanate (LTO), which contains lithium, titanium, and oxygen. In addition to its high-rate capability, LTO has good cycling stability and maintains empty sites within its structure to accommodate lithium ions. However, LTO conducts electricity poorly, and lithium ions are slow to diffuse into the material.
“Pure LTO has moderate usable capacity but can deliver power quickly,” said Amy Marschilok, an associate professor in the Department of Chemistry and an adjunct faculty member in the Department of Materials Science and Chemical Engineering at Stony Brook University—where she also serves as deputy director of the Center for Mesoscale Transport Properties (m2M)—and Energy Storage Division manager and scientist in the Interdisciplinary Science Department at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory.
“High-rate battery materials are appealing for applications where you want to use stored energy quickly, over minutes—such as electric vehicles, portable power tools, and emergency power supply systems.”
Marschilok is part of an interdisciplinary Brookhaven Lab–Stony Brook team that began collaborating on LTO research in 2014. In their latest effort, they increased the capacity of LTO by 12 percent by adding chlorine through a process known as doping.
“Controlled doping can change the electronic and structural properties of a material,” explained Stanislaus Wong, distinguished professor in the Department of Chemistry at Stony Brook University, where he is also the principal investigator in charge of the student-based team comprising the Wong Group.
“In my group, we are interested in developing and using chemistry to direct favorable structure-property correlations.
For LTO, the incorporation of dopant atoms can increase electrical conductivity and expand the crystal lattice, such that the channel for lithium ions to migrate becomes wider. Scientists have been testing out many different types of dopants, but chlorine has not been explored as much.”
To make “chlorine-doped” LTO, the team used a solution-based method called hydrothermal synthesis. In hydrothermal synthesis, scientists add a solution containing relevant precursors (materials that react to form the desired product) in water, place the mixture in a sealed vessel, and expose it to relatively modest temperatures and pressures for a certain duration. In this case, to enable a scaling up of their procedure, the scientists selected a liquid-based titanium precursor instead of the solid titanium foil that had been previously used in these types of reactions.
Following the hydrothermal synthesis of both pure LTO and chlorine-doped LTO for 36 hours, they performed additional chemical processing steps to isolate the desired materials.
The team’s imaging studies using scanning electron microscopy (SEM) at the Electron Microscopy Facility of Brookhaven’s Center for Functional Nanomaterials (CFN) revealed that both sample types were characterized by “flower-shaped” nanostructures.
This result suggested that the chemical treatment did not destroy the original structure.
“Our novel synthesis approach facilitates a more rapid, uniform, and efficient reaction for the large-scale production of these 3-D nanoflowers,” said Wong. “This relatively unique kind of architecture has a high surface area, with flower-like “petals” radially disseminating from a central core. This structure provides multiple pathways for lithium ions to access the material.
By varying the concentration of chlorine, lithium, and precursor; the purity of the precursor; and the reaction time, the scientists found the optimal conditions for making highly crystalline nanoflowers.
At the CFN, the team performed several characterization experiments based on how the samples interact with x-rays and electrons: x-ray diffraction to obtain crystallinity information and chemical composition, SEM to visualize morphology (shape), energy-dispersive x-ray spectroscopy to map the distribution of elements, and x-ray photoelectron spectroscopy (XPS) to confirm chemical composition and derive chemical oxidation states.
“The XPS data are key in this study because they prove that titanium—which ordinarily exists in LTO as 4+, meaning four electrons have been removed—is reduced to 3+,” said Xiao Tong, a staff scientist in the CFN Interface Science and Catalysis Group.
“This change in chemical state is significant because the material transforms from an insulator to a semiconductor, increasing electrical conductivity and lithium-ion mobility.”
With the optimized samples, the scientists performed several electrochemical tests. They found that chlorine-doped LTO has more usable capacity under a high-rate condition in which the battery discharges in 30 minutes. This improvement was maintained over more than 100 charge/discharge cycles.
“Chlorine-doped LTO is not only better initially but also remains stable over time,” said Marschilok.
To understand why this improvement occurred, the team turned to computational theory, modeling the structural and electronic changes that arise from chlorine doping.
“When we do basic science experiments, we need to understand what we observe to see how the material is functioning and gain insights on how to improve the material’s performance,” explained Ping Liu, a chemist in Brookhaven’s Chemistry Division who was led the theoretical studies.
“Theory is a very effective way to achieve such mechanistic understanding, especially for complex materials like LTO.”
In calculating the most energetically stable geometry of LTO with chlorine doping, the team found that chlorine prefers to substitute sites where oxygen sits in the LTO structure.
“This substitution throws one electron to the system, causing electronic redistribution,” said Liu. “It causes titanium, which interacts directly with chlorine, to be reduced from 4+ to 3+, consistent with the experimental XPS results.
We also did calculations that showed once chlorine is substituted for oxygen, more lithium can be inserted into LTO during discharge. Chlorine is bigger than oxygen, so it provides an enlarged tunnel for lithium transport.”
Next, the team is studying how the microscopic structure of the 3-D nanoflowers affects transport. They also are exploring other atomic-level substitutions in both anode and cathode materials that may lead to improved transport.
“Improving both the electronic and ionic conductivity through one process is often challenging,” said Marschilok. “But beyond improving the performance of any one material, at m2M, we’re always thinking about designing model studies that can show the scientific community ways to develop new battery materials in a comprehensive way.
The combination of material synthesis, advanced material characterization, and computational theory, as well as the collaboration between Stony Brook and Brookhaven, are strengths of m2M’s work.”
This research—published in a special issue on “Low Temperature Solution Route Approaches to Oxide Functional Nanoscale Materials” in Chemistry–A European Journal—was funded as part of m2M, an Energy Frontier Research Center supported by the DOE Office of Science, Basic Energy Sciences. The scientists performed the theoretical calculations using computational resources at the CFN and the Scientific Data and Computing Center, part of Brookhaven’s Computational Science Initiative.
The CFN is a DOE Office of Science User Facility. Some co-authors were also supported by the National Science Foundation Graduate Research Fellowship and the William and Jane Knapp Chair for Energy and the Environment atStony Brook University.