Fighting Has Broken Out Over How Long Lithium Metal Can Sit on the Shelf
For years, they were the Don Quixotes of the battery world — tinkerers in pure lithium metal romantically seeking to install them in batteries, only to find them catching fire and the answers they thought would fix them a mirage. Then, over the last six months, these People of La Mancha became the toast of Silicon Valley and Wall Street, winning high-dollar valuations in SPAC deals and courted by virtually every major automaker in the world.
Now, a new paper published Monday at Nature Energy has triggered a heated debate in the battery community, asserting that in certain cases, lithium-metal batteries suffer a catastrophic loss of lifetime capacity. The issue is calendar life — how many years a battery can be useful, regardless of whether the vehicle in which it’s installed is driven or not. The paper, written by nine authors at Stanford University led by Yi Cui, a materials scientist, found that in extreme testing, lithium-metal batteries lose up to 25% of their lifetime capacity just sitting around.
Major automakers including General Motors and Volkswagen are relying on pure lithium-metal batteries to make next-generation electric vehicles ultra-affordable and allow them to crack the mass market. But the paper waves a gigantic red flag, suggesting that researchers need to heed this potential flaw in order to ensure that lithium-metal batteries work for a full guaranteed lifetime. “If you lose 25%, it’s game over,” said Venkat Viswanathan, a professor at Carnegie Mellon. “At 20%, you already lose the useful life of the battery.”
On Twitter, where battery debate has gravitated in recent months, some researchers heavily criticized the paper. The primary assertions are that the test was carried out under excessively adverse conditions and that the protocol didn’t actually test calendar life. A third claim was that the result was highly dependent on what type of electrolyte and cell format were used.
Cui, the paper’s primary author, declined to respond to emailed questions.
Among those to hotly challenge the paper was Shirley Meng, a materials science professor at the University of California, San Diego, who tweeted, “Can I make a clear statement here — the conclusion in this paper is NOT correct. The testing protocol is not the right one for ‘calendar’ life.” I asked Meng whether she meant that the paper is wrong in all cases, or only with some electrolytes. “Their electrolyte and setup is not optimized,” she responded, “so we are really looking at [a] worst-case scenario. In addition, the fast fading is only [in the] first 24 hours.”
But Matt Lacey, a battery scientist with Scania Group, an arm of VW that makes big trucks and buses, tweeted that the battery field, fixated on issues like creating fast-charging batteries that will take vehicles long distances, had failed to pay adequate attention to calendar life. Citing a paper he wrote in 2016, he said he had noticed a similar phenomenon in a different kind of battery — one using a lithium-sulfur cathode. It, too, lost 25% of its capacity, though it had recovered. He said he hoped that researchers would now pay more attention to what happens to idle batteries.
Recent months have seen a new trend toward data transparency in the battery industry, a field previously renowned for intense and paranoid secrecy. Yet, amid the comparative disgorgement of revelations from many prominent battery companies, none seems to have released any data about calendar life, suggesting that at least some of them may not have been testing for it.
I checked with a handful of battery companies. All said they were free of any calendar-life problems. Qichao Hu, CEO of SES, a lithium-metal startup whose batteries appear likely to be used by the middle of the decade by GM, told me that his batteries have had some calendar-life loss but “significantly less… [than] what the paper is pointing out.” He said he couldn’t disclose specifically how much loss.
Here are some other responses:
・QuantumScape (largest investor is VW): CEO Jagdeep Singh said the company’s solid-state separator has proven to be stable in contact with its lithium-metal anode. One test the company had run “[corresponded] to about seven months of calendar time,” Singh said in an email. “That’s seven months of exposure of the ceramic to metallic lithium without a dramatic rise in anode-side impedance or loss of capacity.”
・Cuberg (acquired by Northvolt, the Swedish battery-maker): CEO Richard Wang said his company had not done tests comparable to Stanford’s, but that “we have some practical commercial data showing this is not a major issue for our electrolyte. For example, some customers have been keeping our cell prototypes (in uncontrolled/unconditioned conditions) for many months, or in some cases, more than a year at this point.”
・Solid Power (investors include Ford): Spokesman Will McKenna said in a statement, “Lithium inventory loss as a result of reforming a solid electrolyte interface (SEI) layer does not occur in Solid Power’s solid-state architecture.”