Don't believe in miracles

The above was one of the closing points in an excellent talk I saw earlier today by Dr Jef Ongena, the chairman of the Energy group at the European Physical Society. Based on the email announcement for the talk I was expecting to hear mostly about current progress in nuclear fusion research, but what we got was partly a thought-provoking and fiercely critical overview of the European approach towards renewable energy, and partly a call for better public awareness and consideration of wider context. It really deserved a larger audience than it had so I thought I’d write a bit about it!

“Europe alone cannot save the world” was the first key message, giving examples such as Germany: around 1 trillion Euros has been committed to the Energiewende, and has so far brought about very little meaningful reduction in Germany’s CO2 emissions, which themselves contribute only ~2.5% of the world’s emissions (interestingly enough, the decade following the collapse of the GDR saw German CO2 emissions drop by a quarter, simply because of the loss or modernisation of East German industry). Globally, this reduction is insignificant, because emissions from countries such as India and China have increased to a greater extent. More than this, as a result of this policy, German energy prices are among the highest in the world.

Dr Ongena was also keen to point out that at least a portion of the EU’s reduced emissions are effectively an accounting trick – we import more goods from China rather than produce them here, meaning the associated emissions appear as Chinese emissions and not European. It’s worth reading the EPS Energy Group’s position paper on this topic.

What was most interesting for me was the stark look at what the consequences of an energy system based 100% on renewable energy would be. At this point I am reminded of this excellent post on the situation in Scotland, since this is exactly what the Scottish government is aiming for in the short-term (the blog at which that post resides, Energy Matters, is excellent in general by the way). The biggest issue with renewable energy such as solar and wind is the intermittency; wind is unpredictable, and the demand for solar is typically out of phase with the consumption, so some energy storage is essential. But how much?

It’s also one thing to cope with diurnal (day/night variation) in production/consumption, but if you plan for a future in which solar is a large or the major part of the energy production mix, then the huge seasonal variation in energy production becomes a big problem.

Dr Ongena gave an example of a study looking at German energy production as it would look in 2050 based on current plans. Unfortunately, I didn’t note down the reference, but the short version is that coping with the seasonal variation in energy production and consumption would require of the order of 33 TWh of storage capacity for Germany alone! To put this into context, if this energy was to be stored using batteries, 27 cubic kilometers(!!) of space would be needed to store the batteries themselves. How much space is that? Well, I calculated that myself, and it would take an aircraft hangar tall enough to fit an Airbus A380, this big:

What 900 km<sup>2</sup> looks like

Or, about 2,000 buildings the size of the Boeing Everett Factory, the largest building in the world by volume. I won’t even try to estimate the impossible cost of such a solution.

This idea of “knowing your numbers” was I think the main scientific point in the talk, which pleased me greatly: this is something I think is really important, and is something I try to prioritise in any teaching that I do. Dr Ongena gave a few interesting factoids – for example, that the energy consumed by satellite TV boxes in Belgium is something like 17 times more than the energy consumed lighting all the country’s roads – but the main point was about the power density of energy production.

Power density, as in power per unit area, tells you about the land area needed to produce energy, and in this respect all renewable energies are considerably more “dilute” than conventional (fossil fuel, nuclear) technologies. I’ve since found some good references for this, notably this one, so I won’t write anything else on this except to say that it is often and easily forgotten that wind power, for example, requires 400-500 times the land area to provide the same power as nuclear (and optimistically about 50 times for solar). It’s long been a mystery to me as to why some types of environmental destruction (e.g., large scale pumped hydro or vast fields of massive on-shore wind turbines) are apparently preferable to others (e.g., storing relatively small amounts of nuclear waste, or fracking).

Although it was a talk with relatively few crumbs of comfort for the future energy landscape (fusion power was not even discussed), it was still rather refreshing to get such a brutal reality check and plenty of food for thought. Unfortunately, my suspicion is that politics and emotions will always trump science in any decision-making process, but I would be happy to be proved wrong on that.

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Energy density and Li-S batteries

Yesterday I created a new Shiny app for estimating the gravimetric energy density of Li-S cells based on ten different parameters of the materials that make up those cells. You can find the page describing it the app here (or you can jump straight to the app here.

I wrote some accompanying text giving some background to the app – more specifically about the gap between theory and practice and why wild promises surrounding new battery technologies never seem to come true. That page got a bit long, so I’ve split off that text into this post. It’s still a bit long, but I hope it can be of some interest!

Gravimetric energy density, or more properly specific energy – the amount of energy stored for a given mass – is one of the most important characteristics of a battery for any portable application, whether it’s for a laptop or an electric vehicle. It’s especially important for the latter, because the energies required to move something as heavy as a car over distances of hundreds of kilometers currently require exceptionally heavy batteries. This is one of the main motivations for research into new rechargeable battery chemistries which have much higher theoretical energy densities than Li-ion batteries, the current state-of-the-art.

The theoretical specific energy (hereafter referred to simply as energy density) of the lithium-sulfur (Li-S) battery system, for example, is given in various papers, review articles, news articles, etc, to be about 2,600 Wh/kg. This number comes from simply converting the Gibbs free energy of formation of Li2S (-432 kJ/mol) into the units of Wh/kg, and isn’t actually based on anything relating to the construction of a battery (it doesn’t consider, for example, an electrolyte, without which a battery can’t function). The real energy density of a battery is the energy released from the electrochemical reaction divided by the masses of everything in that battery – both electrodes, the separator, the electrolyte, the current collectors, and the packaging.

Most often, the number of interest is the energy density on the cell level, that is, the energy density of a single cell. This is of course much lower than the theoretical energy released from a perfectly efficient reaction of the reactants in that cell, because all the components besides the active materials (sulfur or lithium, in this case) do not contribute to the energy density, even if the battery can’t work without them. However, this fact hasn’t stopped a large number of researchers and journalists describing the system from writing things like:

The theoretical energy of the Li-S battery is 2,600 Wh/kg, which is much higher than for Li-ion batteries, currently 150-180 Wh/kg.

This is enormously misleading, and I am of the opinion that even unintentionally making these inappropriate comparisons – and the implicit wild promises of magic technology to come – does not do the reputation of the field any favours. Making cells with high energy densities is very hard: there are two companies that I know of (Sion Power and OXIS Energy) which have been developing these batteries long before the current rush of academic interest and currently produce real cells with energy densities of >300 Wh/kg. Both are claiming to be able to deliver 400 Wh/kg in the near future. For comparison, the highest energy Li-ion battery in production that I’m aware of is the 243 Wh/kg Panasonic NCR18650B.

I’m also of the opinion that many researchers in the field do not really appreciate what a remarkable achievement companies like Sion Power and OXIS have made in producing 300+ Wh/kg cells that can actually be recharged for more than a few cycles. One of the key conclusions I’ve come to in the time that I’ve been working in this field is that the more you work to make a cell that will actually have a high energy density, the more you realise the system really doesn’t want to work nicely under those conditions.

Making a high-energy density battery is hard

What I mean by “a cell that will actually have a high energy density” is one where the dead weight – the weight in the cell which is not active lithium or sulfur, in this case – is minimised as far as possible. The electrolyte and current collectors are big contributors to this, but other electrode additives and any excess on the part of one of the electrodes also contribute. More importantly, the electrochemistry of the lithium-sulfur battery is extremely sensitive to many of these factors in ways that Li-ion batteries simply aren’t.

As far as I can see, the most serious issues regarding long-term rechargeability (cycle life) are a direct result of the instability of the negative electrode and destruction of the electrolyte. In most academic work when results from test batteries are reported, the electrode “loading” (the amount of sulfur on the positive electrode per unit area) is usually low, and the electrolyte and negative (lithium) electrode are in huge excess. This minimises the effect of capacity loss due to these serious issues. This is not an issue in itself: you can deliberately test cells in this way, so as to look at the stability of the positive electrode itself (in what we would typically call a half cell). However, in most cases it is not obviously deliberate: even though there are a number of papers now which have demonstrated how fundamentally important the electrolyte volume is, most papers in the recent past do not even report how much electrolyte was used. And from my own experience, I have so far only reviewed one or two articles on Li-S batteries where I have not had to ask the authors to include the electrolyte volume (or more specifically, the electrolyte/sulfur ratio). The thickness of the negative (lithium) electrode is reported even less frequently, and is also important. This situation of unreported experimental parameters does seem to be slowly improving, though.

It is also, simply, more convenient to make test batteries like this. For example, it is harder to coat thicker positive electrodes. Very thin Li foil (e.g. tens of µm thick) has also only recently become available, is difficult to work with and is relatively expensive. It is also not trivial to work with realistically small electrolyte-to-active material ratios for such test batteries either, because the volumes are usually very small (perhaps less than 30 µL, for batteries with a few mAh of capacity). For all these reasons, the rapid capacity fade that comes with having the combination of a low electrolyte volume, a thick positive electrode and a thin negative electrode, is usually not seen.

It is common now to see journal articles reporting test Li-S batteries as completing hundreds and hundreds of cycles with little capacity fade. This frequently comes with the implication, intended or not, that a major hurdle with the system has been cleared and we are well on our way to long-lived, high energy batteries with a dizzying range of applications. More disturbingly, I have seen a number of papers report relatively unremarkable results along with a statement to the effect of:

“We estimate that this corresponds to an energy density of 750 Wh/kg in a complete cell

or something similarly unsubstantiated, where this projection is made on the basis of assuming masses of other components which may not be realistically achievable. I do not know if those who would write this or similar things actually believe it, but certainly many readers would, and would assume it to be the truth, not least because these sorts of statements pass peer review. I know the urge and need to promote and spin one’s research is a strong one, but this is a bad habit that really needs to be kicked.

This all sounds very bad!

I’m not trying to play down the potential of the lithium-sulfur system at all, in case that’s what it looks like. As much as I believe that wild, unrealistic promises are likely to eventually kill of interest from funding agencies and industry when those promises can’t be fulfilled, I also believe that direct and brutal criticism of these promises may also achieve the same result. I’m not going to say that a 500 Wh/kg Li-S battery is impossible. On the contrary, I think it’s quite realistic! I could tell you some values we need to reach in order to get there, although I couldn’t tell you how to actually get to them (that’s what research is for!). I will say that I’m fairly sure it’s not going to happen without better awareness of the limitations of our experiments and what conclusions can be drawn from them. It’s just as Feynman said (quoted on the top “Science” page on this site):

The first principle is that you must not fool yourself - and you are the easiest person to fool.

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It’s a word I see a lot in materials chemistry articles, and one I’m increasingly disliking because it is so casually used. It’s something of a cliché now to describe the synthesis or preparation of some material, at the very least in the field I work in, as “facile”, instead of, say, “simple”.

Anyway, I saw it again today in an article, used in a similarly generous context, and out of interest decided to quickly look up the dictionary definition via Google:

1. ignoring the true complexities of an issue; superficial. “facile generalizations”

Somehow it seems like it might be more appropriate some of the time. Maybe I should start my own glossary of battery science clichés?

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It might be more than just Tesla's most important car

Everyone’s getting very excited about the announcement of the Tesla Model 3, and for good reasons – it’s a Tesla with a 215 mile range and a $35,000 price tag. This is pretty much in Nissan Leaf country, for a car with twice the driving range – so I’m not all that surprised that more than 250,000 people have put down a hefty deposit to pre-order one.

I’ll admit straight off that it’s not for me – I’d still rather have my diesel Golf (no, it’s not one of those ones) – but the Model 3 is a big deal for the future of Tesla, the future of electric cars, and maybe the future of the battery industry. Many observers have been debating whether this is the make or break moment for the company, and it might well be – its losses increased to almost $900m last year, and the company’s share price decreased 40% before it was restored after Tesla announced last month that it expects to finally become profitable this year.

I doubt Tesla will actually go bust, but if it somehow does, or if the Model 3 doesn’t meet expectations, I think it could seriously shake public confidence in electric cars in general. Again, the Model 3 being a flop seems unlikely given Tesla’s image and reputation for customer service, but concerns have surfaced about the build quality and reliability of their other cars:

It’s one thing to have a quirky, problematic car that sells 20,000 units per year to wealthy people who probably own at least one backup vehicle. It’s quite another when Tesla scales up to its 2020 projection of 200,000 U.S. Model 3 buyers, who may not have the luxury of being so forgiving.

I probably seem pessimistic and down on electric cars in general, and I don’t hide the fact that I wouldn’t want to own any all-battery-powered car as they exist today (and for the forseeable future). I would rather consider a hybrid like the Golf GTE, but even that is rather expensive for a car which in practice is no more fuel-efficient than my diesel one (yes, I know I’m oversimplifying a bit). I am looking on with interest, though. There’s a lot to like about electric cars, but too many drawbacks for me at the moment – but a company like Tesla who can capture the headlines and early adopters has to encourage innovation across the industry. The announcement of the Model 3 at least makes me think that one day I’ll be properly convinced.

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Vanishing traces of winter

Today is Easter Monday, and I think it’s the first day of the year so far that it’s actually felt like spring to me, even if there is still ice on the lake. This photo is from the “wilderness trail” (vildmarksleden) at Fjällnora near Uppsala – which is a pretty good hiking path considering the relatively flat landscape in this part of Sweden!

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