Scientists have disputed a claimed breakthrough in one of the most promising fields of advanced battery research, casting fresh doubt on efforts to leapfrog current lithium-ion technology.

The questions have been raised about an advance announced by Clare Grey, a prominent battery researcher at Cambridge University, who created a stir in October with a paper published by the prestigious peer-reviewed journal Science. In the paper, Grey described a breakthrough with “lithium-air,” a theoretical technology that, if it could be made to work, could possess more than five times the energy density of current lithium-ion batteries, and roughly the same density as gasoline.

Such a system would solve the shortcomings of current lithium-ion batteries, which are costly and weigh too much to allow electrics to compete with equivalently priced gasoline-propelled cars. Electric cars to be launched over the next few years from GM, Tesla, and others will cost $35,000 and go 200 miles, but if lithium-air were solved, such vehicles would go much farther, and cost much less.

“New battery could power electric car from London to Edinburgh on a single charge,” a distance of 400 miles, read the headline at the UK’s Daily Telegraph, reporting on Grey’s Oct. 29 announcement.

But in two dissents published at Science on May 6, researchers at seven universities and national laboratories in the US, China, and Australia contend that Grey’s paper contained errors, and that her claims could not be replicated. Grey replied in the same issue of the magazine, but did not appear to contest the substance of their objections.

A battery’s energy is derived from the composition and action of its two electrodes. In this case, the theory is that air would be made to flow in and out of a battery, serving as one of the two electrodes, and essentially weightless. The second electrode would be highly energetic metallic lithium. Combined, such a system would produce a light, energetic battery that would challenge the gasoline-powered internal combustion engine head to head.

A principal challenge faced by researchers, however, has been that lithium-air batteries have refused to recharge more than a few tens of times; the system has only grudgingly released the oxygen necessary for the air flow after it’s been absorbed. Leading US labs, discouraged by failed attempts to resolve such problems, have stopped trying.

But in her paper, Grey and six researchers in her Cambridge group proposed resolving the recharge problem (paywall) by involving two compounds as mediators— lithium iodide and water—along with fluffy carbon.

No one appears to have previously combined all three compounds, at least for the purpose of a lithium-air battery. The result, Grey’s group reported, was a battery that charged and recharged 2,000 times—a remarkable achievement, if true. A decade more of work would be required to resolve other problems with lithium-air, but one big obstacle was lifted.

Both dissents, however, say that the compound does not have the claimed impact. They say the special additive used by Grey’s group, lithium iodide, does not produce sufficient energy to force release of the air from the lithium hydroxide, and thus solve the recharging problem.

“The breakthrough is not a breakthrough, and we are in a sense no further along in lithium air than we were,” Venkat Viswanathan, a professor at Carnegie Mellon University and the lead author on one of the dissents, told Quartz.

Grey has not responded to an email requesting a comment.

Here is the dissent led by Viswananthan. He wrote it with Vikram Pande, also at Carnegie-Mellon; K.M. Abraham at Northeastern University; Alan Luntz at the Stanford Linear Accelerator Center; Bryan McCloskey at the University of California, Berkeley; and Dan Addison at Liox Power. Here is Grey’s response to it. And here is the second dissent, written by Yue Shen and Wang Zhang at Huazhong University; and Shu-Lei Chou and Shi-Xue Dou at Australia’s University of Wollongong. Here is Grey’s response to their dissent.