Feb 08 2021

Are Hemp Batteries Better Than Lithium-Ion?

You know the rule for headlines – if they ask a question, the answer is almost certainly “no”. This is no exception, but this post is really about bad science news reporting. A reader asked me about this story – “Hemp Batteries are Eight Times More Powerful than Lithium, Scientists Discover.” I didn’t believe it for a second. Clearly I needed to dig into the actual scientific discovery, if there were one, but generically, battery news is usually misleading because they typically fail to put the limited incremental advance into any context. My first question is – what do they mean by “more powerful”? Are they referring to energy density, power density, by mass or volume, and in any case what are the other critical features, like charge-discharge cycles? Usually there is a deal-breaker in there that gets shorted in the coverage.

Here is the paper on which it is reporting – Interconnected Carbon Nanosheets Derived from Hemp for Ultrafast Supercapacitors with High Energy. The study was published in 2013, so it is hardly news. I could not find any further research since then. There is a report from 2019 that the scientist involved, David Mitlin, patented this process and is developing a research facility in upstate NY. What is available gives the reporting the vibe of corporate press releases, probably fishing for funding, rather than genuine news.

The actual science here may be useful, there is just far too little evidence to tell. The original study heated the bast from from hemp waste (hemp is an industrial form of marijuana) to create “graphitic carbon nanosheets”. They then studies there electrical conductivity and energy storage capacity, essentially uses them as capacitors – not batteries. I don’t think this is a small detail, and I wish popular reporting would stop conflating batteries and capacitors. They are not the same thing. You can read the original paper for details, but it appears the resulting nanosheets have good conductivity at a range of temperatures.

Focusing on the “more powerful” claim, when discussing large energy storage for electric cars, for example, what we need to know is energy density and power density. Power density is how much power can the energy storage device deliver for its mass. Li-ion batteries are already plenty powerful, which anyone who has experienced the acceleration of a Tesla can testify. More power density is therefore not very useful at this point, for this application. Energy density is the far more important criterion – how much energy can be stored in a given mass or volume. As I discussed recently, the energy density of a Li-Ion battery is pretty good – 100-265 Wh/kg. The best carbon nanofiber supercapacitors have a reported energy density of 73 Wh/kg.

What is the energy density of a supercapacity made from the hemp-derived graphitic carbon nanosheets? They report 19 Wh/kg at 20C and up to 40 at 100C – but the assembled supercapacitors were 12 Wh/kg. That is one-sixth as energy dense as carbon nanofiber and 22 times less than Li-Ion batteries.  So what was the “8 times more powerful” claim based on? An at-home “study” done by some Youtuber, not anything actually published.

The selling point of the hemp-derived carbon nanosheets is not that they are superior to graphene, but that they can be produced 1000 times more cheaply. Fair-enough – mass producing graphene with the desirable properties on an industrial scale at a reasonable price is still a hurdle for this technology. We are seeing lots of exciting things in the lab, but translating them to products is still difficult. Finding a cheaper alternative, even if performance is less, could be worthwhile. But let’s be clear about what we are talking about. An energy storage device with 22 times less energy density than current Li-Ion batteries is not viable for cars, period. This is a dealbreaker. It might be useful, however, for grid storage where weight is less of an issue. For grid storage factors such as cost and the availability of raw material are more important. We also don’t know where this technology might lead, although I am a bit concerned that I can’t find anything published in the last 7 years.

Also, there does not appear to be anything special about hemp, this was just the only material they tested. There appears to be a subculture that, for some reason, wants to believe that hemp is a magical material with thousands of uses. I am not doubting its usefulness as a natural fiber, sure, but there are lots of plants out there and some may be better suited to this specific application.

Graphitic carbon nanosheets that can be cheaply mass produced from a renewable resource may also have other uses. Sometimes industries work backwards – rather than filling a need, they take a resource and find a use for it.

It’s disappointing that the reporting on this was confusing and counterproductive. The real story is not that impressive. It may lead to something, but probably it’s a long-shot. The bottom line being reported is certainly not true and contributes to the overall sense that many in the public have that you “cannot trust scientists” or that science itself is often misleading, when it is just bad science reporting.

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