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Energy Storage

In the battery materials world, the anode’s time has come

After focusing on improving the cathode, battery material developers are turning to novel anode materials

by Alex Scott
April 7, 2019 | APPEARED IN VOLUME 97, ISSUE 14

09714-feature1-satellite.jpg
Credit: Airbus Defence and Space
Amprius’s silicon anode in a lithium-ion battery helped power Airbus's Zephyr S pseudosatellite, depicted here, for more than 25 days above 21 km, setting a new endurance and altitude record for stratospheric flight.

It was 2007. Apple CEO Steve Jobs announced the iPhone, J. K. Rowling finished her seventh and final Harry Potter novel, and the worst financial crisis since the 1930s was about to hit.

It was also the year that Gene Berdichevsky, an engineer and employee number 7 at Tesla, the electric car pioneer, began questioning why gains in recent years in the energy density of lithium-ion batteries had fallen from 7–8% to 3–4%. With returns from improvements in battery cathode performance beginning to taper, Berdichevsky began to consider the next bottleneck—the poor energy density of the traditional graphite anode.

Tens of start-ups and established materials firms eventually began asking the same question. Many came to the same conclusion as Berdichevsky: that silicon or lithium would be ideal as an anode material. In theory, they are able to hold roughly 10 times the number of electrons as graphite, leading to lithium-ion batteries with 20–40% higher energy density.

The catch is that the anode also absorbs a large number of lithium ions during charging. Graphite handles them well, but a silicon anode swells more than 300%, causing its surface to crack and energy storage performance to drop rapidly. Lithium-metal anodes don’t present an expansion problem, but they are expensive and present other technical problems.

After more than 10 years of R&D, several material developers think they have solved the expansion issues associated with using silicon in anodes, and they are starting to bring their materials to the market. Substantial business challenges lie ahead, not least a potential intellectual property showdown because so many companies are developing technologies in such a narrow field. But it’s clear that the anode’s time has come.

Anode material developers are well aware that the market potential is big and getting bigger as lithium-ion battery use grows in portable devices, electric cars, and grid energy storage. The anode is worth 10–15% of the total cost of a lithium-ion battery, according to Chloe Holzinger, an energy storage analyst with Lux Research. The global anode material market could be worth $10 billion by 2025, she says.

The research is following the money, says Jeff Chamberlain, former head of Argonne National Laboratory’s battery development activities and now CEO of Volta Energy Technologies, a venture capital firm investing in battery technologies. “There is no surprise that people are seeking to improve the lithium-ion battery, and the anode is the place,” he says.

For Berdichevsky, especially, the journey to an improved anode has been long. He left Tesla in 2008 to return to school at Stanford University, earning an MS in energy-dense anode materials. He cofounded and became CEO of Sila Nanotechnologies in 2011 to develop a commercial silicon anode.

The conventional wisdom is to replace, say, 10% of the graphite in a battery anode with silicon metal or oxide, improving density without introducing too much swelling. Sila is taking a different approach.

The company has created a nanocomposite of covalently bonded nanostructures of which 50% are silicon and the rest undisclosed nongraphite materials. The composite is porous but encapsulated with a sealed outer layer that prevents electrolyte penetration into the composite, protecting it from damage during charge and discharge. The composite is contained in a porous scaffold structure so it is able to expand and contract without puncturing the coating, Berdichevsky says.

“We are not adding our silicon nanocomposite to graphite. We are replacing all the graphite in the anode,” he says.

Berdichevsky estimates that Sila’s material has an energy storage capacity four or five times that of graphite, enabling the energy density of a lithium-ion battery to increase by 20–40%.

“A lot of the magic is in how we do the processing. We expect it will be cheaper than graphite at very large scale on a dollar-per-capacity basis,” Berdichevsky says.

The California-based firm raised $70 million from investors last year to commercialize the technology, bringing the total invested in the firm to $125 million. Sila expects its anode materials to be used in batteries for portable devices sometime this year. “By year-end, the material could be powering millions of devices,” Berdichevsky says.

We are not adding our silicon nanocomposite to graphite. We are replacing all the graphite.
Gene Berdichevsky, CEO, Sila Nanotechnologies

Along with other anode material developers, Sila has an eye on the ballooning auto battery market. The firm has a partnership with the German carmaker BMW, which is investing heavily in electric cars. Berdichevsky is targeting large sales volumes starting in the mid-2020s.

For Nexeon, an Abingdon, England–based silicon anode material start-up, the race to commercialization is more akin to a marathon than a sprint. Now in its 13th year, Nexeon has experienced some lows, including the 2014 mothballing of a $5 million pilot plant in Abingdon when the firm’s nanopillar-structured silicon proved problematic to manufacture at scale and low cost.

Sweet spot

Silicon-based anodes offer more energy density than graphite and more stability than lithium.

Source: Prog. Mater. Sci. 2014, DOI: 10.1016/j.pmatsci.2014.02.001; C&EN research.
Note: (mA h)/g is a measure of the amount of charge (electrons) per gram of material.

But Nexeon has taken the lessons learned from that experience and bounced back, Chief Engineer Bill Macklin says.

Nexeon is developing two anode materials. NSP-1 features a powdered silicon compound with particles ranging in size from a few to about 10 µm. Its use is limited to about 10% loading by weight in a graphite anode to avoid expansion problems. Compared with graphite, it promises to increase anode capacity by about 30%, leading to a corresponding increase in battery cell energy density of up to 15%.

To offer even more energy density, Nexeon is developing NSP-2, a silicon compound featuring engineered porosity at the particle level for use in concentrations far higher than 10% to yield an increase in cell energy density of up to 30% versus graphite. Nexeon is a year into a 3-year project to develop NSP-2 in association with specialty chemical firm Synthomer and University College London. Synthomer is developing a polymer binder for use with Nexeon’s silicon anode material, while the university is undertaking materials characterization.

The idea is that, by the end of the project, you have customer validation, scale, demonstrated performance, and intellectual property, Macklin says. “These inputs then form the basis for an investment.”

Wacker Chemie, which holds an option to take a minority stake in Nexeon, is commercializing its own silicon anode material in lithium-ion button batteries that will debut later this year. The big German silicon producer estimates that its technology could enhance the energy density of such a battery by about 20%.

“Most of the market is using silicon suboxides sprinkled in a small percentage into existing graphite. Our approach is to use a silicon-dominated anode,” says Christian Hartel, the Wacker board member responsible for R&D. The company is developing a series of strategies to increase energy density, including coatings to protect silicon against swelling and contraction.

Moving from button batteries to automotive batteries could take years, Hartel acknowledges. This is partly because the materials that Wacker has developed may have to be modified substantially to maintain a high level of performance over many charging cycles, he says.

Sila, Nexeon, and Wacker have yet to put a product on the market. In contrast, the Stanford University spin-off Amprius, formed in 2008, is already selling its silicon-graphite compounds commercially in a mix of around 10% silicon and 90% graphite, according to Lux Research’s Holzinger. The company didn’t respond to C&EN’s requests for an interview.

Amprius’s technology features a shell that encapsulates silicon nanowires. The firm claims its approach overcomes the inherent instability of silicon-containing anodes, enabling their use over hundreds of charge cycles.

Amprius also has a 100% silicon anode that Airbus successfully tested in lithium-ion batteries for its Zephyr S pseudosatellite, which travels in the stratosphere rather than in space for applications such as surveillance and navigation. The batteries have an energy density of over 435 (W h)/kg—substantially higher than that of commercial lithium-ion batteries in use today.

Manufacturing technologies are much more complex for anodes with high silicon loading, though, so all-silicon anodes for the automotive market are still some ways off, Lux’s Holzinger says.

Also pursuing an all-metal anode is the lithium-sulfur battery maker Oxis Energy, though in its case the metal is lithium. Lithium-sulfur batteries are known to suffer from fading performance after a number of recharge cycles, but if Oxis can solve this problem, it will be on course to outperform the energy density of the best lithium-ion batteries.

A lithium-metal anode has the highest specific capacity of any anode material, at 3,862 (mA h)/g, says Oxis’s chief technical officer, David A. Ainsworth. When paired with an optimized sulfur-based cathode, it will allow Oxis’s Li-S battery to achieve an energy density of more than 425 (W h)/kg, he says, compared with about 200 (W h)/kg for a lithium-ion battery.

Equipping the battery with a lithium sulfide electrolyte protects the lithium-sulfide anode from degradation because the electrolyte instantly forms a film on the anode, Oxis says. With a melting point of more than 900 °C, this coating protects the lithium even at extreme temperatures, the firm says.

Oxis raised $60 million in recent weeks to build its first manufacturing facility. The plant, which will be located in Brazil, will have the capacity to make 2 million battery cell pouches per year. The pouches are set to go into batteries used by the aviation and electric vehicle sectors.

Lux’s Holzinger doubts that lithium-sulfur battery developers can solve the problem of performance fade. A possible indicator as to how competition between lithium-sulfur and silicon-containing lithium-ion batteries will play out is Airbus’s decision to ditch the use of lithium-sulfur batteries in its pseudosatellites and opt for Amprius’s silicon anode battery. “We are no longer working with lithium-sulfur as we did not see the performance we require,” Airbus says.

Yet another firm, the Massachusetts Institute of Technology spin-off SolidEnergy Systems, sees a way to keep the benefits of a lithium-metal anode without the potential drawbacks of lithium-sulfur chemistry.

SolidEnergy claims it has gotten around the issues associated with lithium-metal anodes—such as sharp, dendritic lithium structures that form on the anode surface—by developing a lithium-ion battery that is anodeless. During its first charge cycle, lithium is back plated onto a copper current collector, activating the battery.

The technology also features an electrolyte that is stable in contact with lithium. The system promises double the energy density of a standard lithium-ion battery. It is also the world’s lightest rechargeable battery, claims SolidEnergy’s CEO, Qichao Hu. The firm plans to break ground this year on a large-scale production facility at an undisclosed site.

Despite such plans, industry experts contend that lithium-metal anodes are years away from mass-market introduction. Given lithium-ion batteries’ history of catching fire, such as those made in recent years by Samsung, battery makers have become risk averse, Volta Energy’s Chamberlain says. The most likely scenario will be the gradual adoption of silicon blended with graphite in anodes, rather than a jump to 100% silicon or lithium, he says.

Overshadowing these marketplace developments is uncertainty about intellectual property rights relating to silicon anode technology because so many patents have been filed. About 1,100 patents relating to silicon anodes were filed in 2016 alone, and filings are increasing annually, according to the technology market research firm IDTechEx. Samsung was the most prolific in 2016 with almost 250 patents filed, followed by LG Chem, Panasonic, Sony, and Nexeon.

A potential issue for Sila is that the firm’s technology appears to be similar to that of Amprius, Lux’s Holzinger says. Sila’s “patents describe a core shell morphology that is very similar to that of Amprius. This could be a problem for Sila given that Amprius also filed in the US but filed its patent first,” she says.

A broader challenge for many silicon anode material developers is that after so many years of research they could soon come under financial pressure—if they are not already—to actually start selling their products. Just as the technological breakthroughs are emerging, industry consolidation could be on the way. The winners and losers will likely emerge in the next few years.

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Comments
Vincent Pluvinage (April 7, 2019 12:40 PM)
Your article is interesting and indeed timely, as the battery industry started using silicon oxide additives in several commercial cells a few years ago (mainly in cylindrical cells of 18650 format), mixed in small weight percentage (less than 5%) with graphite. However, the industry consensus is that silicon oxides have too many drawbacks (oxygen atoms add to weight and reduce silicon capacity, plus pose safety risks at higher concentration). However, your article fails to mention the SiNANOde material, first started at Nanosys and later at OneD Material. Not only the broad patent filings (with priority dates ranging from 2004 thru 2011) pre-date most (if not all) of the IP of its competitors, but the technology was already licensed in 2014 for production (by a battery maker) and now scaling up from 10 to 100 ton per year. Performance is important and necessary, but not sufficient condition to broad adoption. In fact, availability of large manufacturing capacity and competitive costs, plus compatibility with existing equipment and cell designs are critical. The top 8 largest battery makers control more than 85% of the world production of lithium ion batteries, and are risk averse for good reasons. The transition to new anode materials will necessarily combine existing high quality commercial graphites with safe and cost effective silicon materials that can progressively, safely and costs effectively allow the increase in silicon to graphite weight percentage in the anode from just below 10% to over 20%. This transition will take years. This is not software: material science at large scale requires patient learning curves for each order of magnitude volume increases.

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