A soon-to-be-released study from a supercapacitor manufacturer claims lithium-ion batteries could be reaching their physical energy density limits, calling their future usefulness into question.

Using a “first-principles analysis,” Wolfgang Mack, vice president of business development at Menlo Park-based Capacitor Sciences, argues lithium-ion technology has reached 87 percent of its commercially achievable cell limit for energy density.

“The point of diminishing returns for the contemporary chemistries of lithium-ion batteries is evident,” he said. “To achieve the remaining 13 percent of the commercially achievable cell limits will be costly and slow, with limited returns on investments.”

Mack said he was inspired to carry out the research after seeing a slide presented by Tesla that showed lithium-ion volumetric energy density doubling every 10 years, a trend the car company highlights as being important for lighter, longer-range cars. 

According to Mack’s research, the energy density of lithium-ion batteries did indeed double in the period 1995 to 2005. But between 2005 and 2015, the energy density of the technology has gone from 580 to 676 watt-hours per liter, an increase of just 16.6 percent, he said.

Based on their electrochemical potential, Mack estimates the lithium-ion chemistries used in standard 18650-size cylindrical cells could theoretically reach an energy density of around 1,180 watt-hours per liter or 400 watt-hours per kilogram.

But the commercially achievable limit is more likely to be around two-thirds of this, equaling 800 watt-hours per liter or 260 watt-hours per kilo, he said.

The two-thirds calculation was based on the observation that this ratio holds true for theoretical limits and the current highest-efficiency products in the PV and fuel cell industries.

“Both solar PV and fuel cells are mature technologies and have reached a point of diminishing returns for increased efficiency,” Mack claimed.

Based on his research, Mack said the evolution of lithium-ion batteries is rapidly coming to an end. He also pointed out his analysis did not take into account the effects of capacity degradation due to charge-discharge cycles, temperature extremes or high-power operation.

Furthermore, he noted, safety remains an issue for lithium-ion chemistries, particularly when used for high-power applications.

“Fundamental breakthroughs in cathode chemistry and anode materials are required to significantly increase energy density and specific energy, and until that time, we should expect only incremental performance improvements,” he concluded.

Unsurprisingly, Mack believes it is time to start looking for alternatives to lithium-ion, including supercapacitors.

Capacitor Sciences is currently seeking funding to commercialize a technology that it says could provide 10 times the energy density and 100 times the power density of lithium-ion, while costing just $100 per kilowatt-hour once volume production is underway.

The company is using organic chemicals to overcome the charge-discharge cycle stress that has dogged products from other supercapacitor hopefuls, such as EEStor.

Interestingly, EEStor’s implosion half a decade ago was associated with the kind of grand claims about future potential that Capacitor Sciences is now attributing to the lithium-ion sector.

And while it is legitimate to expect limitations on any form of technology development, Mack’s analysis appears to contain shortfalls in a number of areas. 

For example, it is unclear why the commercially achievable limit on lithium-ion technology should necessarily be two-thirds of the theoretical boundary, just because that is what seems to be the case in the domain of PV or fuel cells.

Furthermore, some of the limits uncovered by Mack relate to the design of 18650 cells specifically. There is no mention of whether they could be overcome through different lithium-ion cell designs, such as pouch cells.

But perhaps the most significant failing of Mack’s study, according to experts consulted by GTM, is that it focuses solely on technical considerations and does not take into account wider commercial realities. 

Julian Jansen, energy storage research manager at Delta Energy & Environment, a Scotland-based research and consulting company, said: “I would argue that while he may be correct in his assumptions, he misses the point. Currently, lithium-ion batteries are the easiest available, to an extent cheapest and ultimately the technology [most] chosen by the market to quickly deploy energy storage for those applications which today are commercially viable.”

In addition, said GTM Research energy storage analyst Brett Simon: “If Li-ion battery costs fall far enough, systems can simply be oversized to meet project goals and still be cheaper than another technology.”