[ad_1]
Since its development in the 1990s, graphite has been the anode material of choice for battery manufacturers producing lithium-ion (Li-ion) batteries. However, as graphite hits its energy-density limits, silicon (Si) anodes have the potential to provide significant improvements in gravimetric and volumetric energy density, according to industry players.
The anode is an essential part of a Li-ion battery, along with the cathode, separator and electrolyte. By replacing graphite with silicon in the anode, Li-ion battery manufacturers could significantly increase energy density to deliver longer runtimes and increase battery life for a range of applications, from consumer electronics to electric vehicles (EVs). Theoretically, Si anodes have 10× higher energy density than graphite anodes.
“Si anodes have been heralded as one of the key future technology drivers for the performance of Li-ion batteries,” said Francis Wang, CEO of NanoGraf Corp., a spinout of Northwestern University and Argonne National Laboratory. “Silicon has 10 times the gravimetric energy density of graphite anodes. They make Li-ion batteries more energy dense and, in some cases, more power dense. What that means for applications like your iPhone or EV is that the runtimes are that much greater.”
Wang estimates about a 20-30% increase in driving distance in EVs by using a Si anode in a Li-ion battery.
In addition, Si anodes can also deliver faster charging and do it more safely thanks to several new innovative technologies. But there are still several inherent challenges with Si anodes, including first cycle (or charge) efficiency, volume expansion, swelling and cycle life.
Battery manufacturers like Amprius Technologies, Enovix and NanoGraf report that they have developed unique architectures that solve some of these design challenges, with commercialization of their solutions underway.
“[Energy density] is the number one problem in consumer electronics devices,” said Enovix Corp CEO and president Taj Talluri. “[Si anodes] provide much more energy in the same space, so
go much longer and that is what everyone wants.”
Processors, displays and cameras all have improved over the last decade with much higher performance, while the battery technology has not kept up at the same rate, he added.
“If you use the processors, the memories and the displays at their full capability in your phone or any other device, your battery goes down really fast,” Talluri said. “If you can provide a battery with much higher energy density, the number of opportunities is huge.”
Why silicon anodes?
Graphite has been the material of choice for the anode in Li-ion batteries since its development in the 1990s. While there is general agreement that Si anodes can theoretically store more than twice the lithium than a graphite anode, there have been major limitations in Si anode development, including first cycle efficiency, expansion and cycle life.
“People have known that silicon can hold much more lithium than graphite and get a much higher energy density battery, but there are problems when silicon gets lithium deposited on it,” Enovix’s Talluri said. “Unlike graphite, where the lithium actually goes into the spaces in the atomic structure, silicon actually combines with it, which makes the silicon become much bigger that it literally swells out.”
“Silicon has this material expansion problem,” agreed Amprius Technologies’ CTO Ionel Stefan. “That is why it is so difficult to use when it absorbs or stores lithium. The technical term is ‘conversion reaction.’ It expands up to three times its initial volume. If the battery expands that much, it won’t survive.”
Stefan believes most Li-ion cells today contain some silicon, but “instead of completely replacing graphite with silicon, the tendency is to add just a little bit [in the single digits] or embed silicon into other materials into matrices of inert material.”
At higher percentages, the negative effects of silicon become visible and that results in cell expansion and shorter life cycle, he added.
“If you used the current [battery] architecture and just replaced the graphite with silicon and made a similar battery for a smartphone, it would produce so much pressure that it will pop the back cover off,” Talluri said.
This is why nobody has been able to replace graphite with silicon so far. That is, until now, he said.
As manufacturers began to hit a wall when attempting to improve graphite’s energy density, awareness of Si anodes increased. Since 2015, there has not been much progress in energy density with graphite electrodes, Stefan said, with small incremental progress of a few percentages a year through cell engineering rather than materials science.
“For batteries, progress is usually in the direction of more energy density, longer life and faster charge, as well as being safer and cheaper,” Stefan said. “Graphite has been the state-of-the-art material since the invention of the Li-ion battery in the 1990s, and it has pretty much reached as good as it gets.”
Over the years, manufacturers have typically added silicon to graphite anodes at very small percentages—generally at less than 5%. But even with a small percentage of silicon, Li-ion battery manufacturers witnessed gains in energy density better than that of graphite anodes. Thanks to technology improvements, battery cells now use anywhere from 5-100% Si anodes.
New architectures
Not all Si anodes are alike. Si-anode developers have diverged in terms of how they produce these anodes.
For example, Amprius developed a silicon nanowire anode; Enovix created a new 3D cell architecture that uses anodes, cathodes and separators laser-patterned and stacked side-by-side together with a stainless-steel constraint; and NanoGraf designed a silicon alloy material architecture with a protective coating.
“Instead of trying to solve the problem by masking it, we are engineering the silicon into the shape of silicon nanowire, and secondly, we anchor each nanowire directly to the current collector foils,” Amprius’ Stefan said. “This makes the anode structure more robust mechanically, and it doesn’t expand at the cell level, which is the most difficult issue for silicon. The second part is that it is pure silicon, so we don’t have any binders or any inert material added to the electrode so that makes it the highest capacity in reality, not in theory.”
Amprius claims the first 500 Wh/kg battery with the highest energy density in the industry.
Also addressing the silicon expansion problem, Enovix developed its 3D silicon Li-ion cell architecture that uses a 100% active silicon anode. The architecture stacks the electrodes, which consists of thin strips of silicon, anodes and cathodes, together with the electrolyte and separators, wrapped with a mechanical constraint or stainless-steel cage to hold it tight. The stainless-steel constraint around the cell limits the battery from swelling.
The significant expansion of the Si anode during charging creates high pressure, so the electrodes are also reoriented to face the small side of the battery to decrease the constraining force. The small surface area significantly lowers the pressure needed to constrain the Li-ion cell, limiting swelling to as little as <2% cell thickness after 500 cycles. In addition, the integrated constraint keeps the particles under constant stack pressure, which limits electrical disconnects and cracking.
The new architecture also addresses first charge efficiency issues. At first charge, Si anodes trap some of the lithium at formation. Enovix developed a “pre-lithiation” process during manufacturing, which adds additional lithium to top-off the lithium trapped at formation. It works in this architecture because it only has to travel a short distance to permeate the anode.
Cycle life is another problem, Talluri said. After a battery is charged/discharged for 500 cycles, typically the energy density will drop to about 80% and it will keep dropping over time, he added.
“Because we have so much energy density now, we are able to use that to make the right tradeoff between energy density and cycle life,” Talluri said. “Cycle life and energy density are coupled because the more you increase the cycle life, the less energy density you get and the less cycle life, the more energy density.”
If a device is charged only once every two to three days, the cycle life will be much longer and removing the need to charge the device every day completely changes the user experience, he added.
NanoGraf uses a metal-doped silicon oxide core combined with a proprietary surface coating to stabilize the solid electrolyte interface. The company has developed what it considers to be a second generation of silicon oxide, which mitigates and solves the key challenges of low first cycle efficiency, high CVD carbon coating costs/scalability and swelling.
“Up until now, Tier I battery manufacturers have leveraged SiOx [silicon oxide] in small quantities [5-7%] as an additive to traditional graphitic anodes to boost energy and power density,” Wang said. “However, battery manufacturers have struggled to go to higher additive percentages due to relatively low first cycle efficiency (FCE), high CVD carbon coating costs/scalability and swelling.”
The battery has demonstrated loading levels of beyond 20% in standard 18650-cell form factors, according to Wang. He also commented that first cycle efficiency of first-gen Si anodes is about 74%, but cell producers would like to see that in the 90 percentile.
Developing drop-in ready products for battery cell manufacturers also plays a role in the adoption of the new technologies. Si-anode suppliers are working to ensure that their products can drop into a cell producer’s infrastructure. Both Amprius and NanoGraf call their technologies drop-in ready.
Wang added that it is important that battery cell producers do not have to create or change anything about their manufacturing processes, designs or form factors.
Designing for safety
While Si anodes do address energy-density issues in Li-ion batteries using graphite, it is also important to point out that certain safety issues remain in Li-ion batteries, such as thermal runaway. Some Si-anode manufacturers, however, have developed new technologies to help solve these safety issues as they develop their Si-anode platforms.
For example, Enovix reported a new level of safety with its BrakeFlow technology. In the event of a short circuit, the BrakeFlow technology limits the short area by regulating the current flux from other areas of the battery to the short, protecting the battery from overheating and inhibiting thermal runaway, which often causes fires.
Enovix’s nail penetration test shows the battery only swelling slightly, compared with a standard Li-ion battery, which caught fire and exploded in seconds. The test used a fully charged cell at 4.35 V with an impact speed of 20 cm/s using a 6-cm long steel rod.
Amprius has also developed a new design to prevent thermal runaway. The company uses a gel polymer electrolyte that prevents short circuits. In a third-party nail penetration test of Amprius’ battery cell, the cell did not go into thermal runaway. The voltage decreased 266 mV to about 3.900 V, and there was no temperature increase 10 minutes after the nail penetration occurred.
Cost and production
Cost remains a challenge for Si-anode development until battery manufacturers can scale-up production. Most manufacturers are targeting applications that absolutely need the benefits gained from Si anodes. Certain market segments like military/aerospace are willing to pay premium prices until manufacturers can scale their technologies and products.
For example, Amprius has had commercial production since 2018 with customers including the U.S. Army, Airbus and BAE Systems. The company claims a very high energy density performance for its 100% Si-anode Li-ion cell at up to 500 Wh/kg and 1,300 Wh/L for a much faster run time. The cell also delivers a fast charge rate capability of 80% charge in about six minutes and is about half the weight and volume of commercially available Li-ion cells. The company said its next-gen cells are poised to power aviation and eventually EVs.
In March, Amprius announced plans to build a 775,000-square-foot facility in Brighton, Colorado. With a target operational date of 2025, the factory will be built in phases starting with an initial 500 megawatt-hours (MWh) with the potential of up to five gigawatt-hours (GWh), which will significantly expand its current manufacturing capacity. The initial phase of 500 MWh will be funded in part by a $50 million cost sharing grant from the U.S. Department of Energy’s Office of Manufacturing and Energy Supply Chains. Amprius is one of the first companies to receive funding from President Biden’s Bipartisan Infrastructure Law to expand domestic manufacturing of batteries.
Amprius also is bringing about 10% more capacity online at its facility in Fremont, California. Once the Colorado facility comes online in 2025, the company will have increased its capacity by a 100× compared to today.
Nanograf also targets applications that require much higher energy density. The company currently addresses military applications, although consumer electronics and EVs are on the roadmap. Production scale-up is underway, with a small-scale, 50-ton facility in Chicago.
The 17,000-square-foot office and manufacturing facility will start its battery materials production in Q4 of 2023, with a planned phase-one capacity of 35 tons per year—enough to produce 24 million battery cells. NanoGraf previously produced its proprietary Si-anode battery materials in Japan, and moved onshore to support its contracts with the U.S. military and the U.S. battery supply chain as part of President Biden’s administration goals for the U.S. to become a global leader in EV and battery innovation.
About two years ago, the Department of Defense funded NanoGraf to produce the most energy-dense battery for applications like radios to offer longer runtime in the battlefield. The result was the development of a standard 18650 Li-ion cell with an energy density of 3.8 Ah, at 800 Wh/L, enabling communications equipment to last about eight hours longer in the battlefield. The benchmark for a Li-ion battery is 3.5 Ah.
With a new round of funding, the company has currently demonstrated a 4.0-Ah cell at 810 Wh/L battery. A 4.3-Ah cell, with an energy density of 870 Wh/L, is in the research and development phase.
Enovix is currently producing two Si-anode Li-ion batteries. One targets wearables like smartwatches, while the other is designed for mobile devices like laptops.
The wearable battery is in production at the company’s factory in Fremont, California, and has received UL1642 and IEC62133 certifications. Enovix’s Talluri said this is the first year the company is producing batteries in the thousands.
Although Enovix is currently only producing standard batteries, it has the capability to customize the batteries to fit the customer’s form factor. “For us to get to really high volumes, we need to make those custom batteries that our customers want,” Talluri said.
Enovix has ordered new equipment for its high-speed R&D line in Fremont that is expected to arrive by November to help increase the speed of its battery customization. The company is also building a factory in Penang, Malaysia, which will also address customization for different shaped and sized batteries. The factory is estimated to come online around April 2024 and is expected to produce millions of batteries by the end of next year.
The post Silicon Anodes Improve Li-ion Batteries appeared first on EE Times.
[ad_2]
