[timerol]

This particular Li2O battery is a little under 700 Wh/kg, with the theoretical maximum being 11k Wh/kg, compared to gasoline's 13k Wh/kg. It's an incredible accomplishment that they have managed to get such a reaction reasonably stable. Minor improvements to the battery cited in the paper would be beyond the theoretical limits of existing commercial lithium chemistries.

> The results shown in fig. S9 indicate that this solid-state Li-air battery cell can work up to a capacity of ~10.4 mAh/cm2, resulting in a specific energy of ~685 Wh/kgcell. In addition, the cell has a volumetric energy density of ~614 Wh/Lcell because it operates well in air with no deleterious effects (supplementary materials, section S6.3)

[jillesvangurp]

Especially when considering that most of that 13 Wh/kg for petrol is typically delivered as waste heat. You can get a decent estimate of how bad it is comparing miles per kwh for an EV to miles per gallon for a typical petrol car. It's about 3-4 miles per kwh vs. about 20 miles per gallon. EVs just use their kwh a lot more efficiently than petrol cars. Because batteries and electrical motors are just really efficient.

An 11 wh/kg battery would result in a battery that delivers about 5-6 times more miles per kg of battery than petrol. You get weight parity around 3-4 kg. If you factor in the weight of the engine (they can be quite heavy) it gets a little better. Of course the weight matters far less than people think. The amount of energy needed to move a vehicle does not necesseily scale linearly with weight of the vehicle. Which is why a heavy cyber truck and much lighter / smaller EVs can have miles per kwh metrics that aren't that far apart. Same with petrol cars. Halving the weight doesn't given them twice as much range. Heavy batteries are not that big of a deal. Unless you put them in a plane. Weight matters a lot in planes.

So, a battery like this would be amazing news for battery electric planes that currently fly with 200-300 wh/kg batteries (at best). 11kwh/kg would be a 70x improvement in energy density. That's a lot of range. Even a small fraction of that would be a massive improvement. 700wh/kg more than doubles the range already.

I think we'll see batteries break 1kwh/kg next decade or so. 500 wh/kg is already on its way to production. So, a doubling is only a modest step up. At 1kwh/kg, most GA flight will become electric. 3-6 hours of range with dirt cheap electricity turns a 100$ hamburger into a Starbucks coffee run. That's game over for ICE engines in small planes.

[norrsson]

Usual disclaimer regarding the energy density of gasoline: current gasoline engines have an efficiency of 10-30% while EVs are around 90%.

So to reach similar kWh/g we're looking at ~3k Wh/kg

[thot_experiment]

I think your point still stands but modern engines cluster toward the higher end of that range and some do exceed the 30% efficiency mark, hybrid drivetrains can approach 40%.

[fpoling]

Indeed. Tesla Model 3 consumes about 50MJ of battery energy per 100 km. Toyota Prius consumes about 4.5 liters of gasoline. That gives roughly 150 MJ. So a electrical car consumes 3 times less or about 33% of energy of one of the best hybrid drivetrain.

[jillesvangurp]

Is that fresh out of the factory or after a few years of service? I think a lot of the metrics around hybrids are a bit optimistic. In the same way that official metrics for EV ranges are usually a bit more than is realistic.

Hybrids running on battery are about as efficient as an EV. When you enter the highway, they turn into ordinary ICE engines. If you use your prius exclusively for traffic in your neighborhood, you only use petrol to charge the battery, which is efficient and about as good as it gets with a hybrid. Unless you can plug it in of course. It won't use any petrol at all in that case.

[Tade0]

I've been driving my hybrid(compact estate) for over 7 years now and there's no noticeable change in fuel economy.

That being said, both the figures mentioned are to me a little bit optimistic.

I don't know about Teslas, but my fuel economy presents itself like this(figures are in litres per 100km):

-City driving: ~5 + ~100ml to bring the engine to working temperature. Checks out to 7 on a 7km drive and falling with distance.

-Highway, so maintaining real 120-140km/h (speed limit around here), 6.3-6.5. Absolute worst was 7.8 during a snowless -20°C night.

-Backroads doing 70-90km/h, average trip speed 50km/h, and here is where I think hybrids shine - 4.0-4.2.

-Hypermiling record: 3.7 as I was steadily rolling at 20-30km/h to a highway onramp a few kilometres away.

Overall fuel economy is nice, but what I like about this car the most is the ease of manoeuvring on the parking lot and very little vibration when the engine is running.

[fpoling]

I bought Toyota Yaris Hybrid in 2012. Toyota at that time claimed it should be 4.4 l/100. My average after 4 years before I sold it was 5.1. So my usage was just 15% off. And this was in Norway with a lot of driving in mountains.

On the other hand with electrical cars that I rented and from what I heard from friends the real range was shorter by 20% or more.

[AtlasBarfed]

Ice hybrids can run on the Atkinson cycle, which fas been explained to me as diesel range from gasoline.

I believe this is because the electric motor can handle the torque variation so the engine runs in a more consistent optimized torque band

[jillesvangurp]

The most optimal thing is to just use ICE engines as a generator/range extender that tops up the battery. Most hydrogen vehicles actually work like that as well: the fuel cell is basically used as a generator to top up the battery.

Some of the Chinese plugin hybrids are starting to do this. Those basically are EVs with a generator bolted on to extend the range to something crazy like well over 1000 miles. These vehicles are far simpler to build and mechanically a lot simpler. And the generator is of course an optional extra if your battery is big enough. More like a safety blanket for the range anxious.

Edison trucks (a Canadian startup) is doing putting generators on their trucks. They've built and designed their own electrical logging truck designed for the extremes of that business. Pretty awesome vehicle. Effortless pulls huge loads over unpaved roads. Basically, it has no gears so it is super simple to operate and it drives like a car. A pretty sporty one even. The generator is just there for when they go out into the middle of nowhere where they can't charge the truck. It's purely a range extender.

If you think about it, you could just buy an off the shelf generator, shove it on a trailer and hook it up to your EV. If you have a EV pickup, you can just sacrifice some space in the back. This isn't that hard. You only need a few kw of output from the generator. A simple, small, and cheap one would be good enough.

The reason companies like Toyota are trying to sell you a much harder to maintain and much more complicated solution instead is not because they haven't thought about it but because they have a large sunken investment in ICE engine manufacturing that they want to milk for a bit longer.

[bagels]

Not that any of the tech is practical for every day cars or how they're used, but F1 engines with the multiple energy recovery systems are up to 52% efficient if they are to be believed (we'll ignore the rules subterfuge around tricking the fuel sensors, injecting oil, and who knows what else).

[lukevp]

Is there a similar volumetric equivalent measurement or is it all about energy density by weight? Like, if the batteries are lightweight but massive, that would also be a bit of a problem since the structure to safely transport a large volume could be expensive and heavy.

[sn0wf1re]

Looks like the created cell is 614 Wh/L from the above comment. Gasoline is ~2.2kWh/L [0]. So my take is that even with the created cell the density is not going to be an issue with car or grid batteries -- only <4 times the size even at this non-theoretical cell. Who knows how the packs will be configured though as I am sure airflow will be a design consideration when making larger packs.

[0] This uses the 3kWh/kg that was provided above and a density of gasoline of .75g/mL

    units
    You have: 0.7429 g/mL * 3 kWh/kg
    You want: kWh/L
 * 2.2287

[adrian_b]

That 3 kWh/kg estimated by the poster above corresponds to an abysmal efficiency of an internal-combustion engine, of less than 25%.

Modern cars with good high-compression engines have efficiencies over 40%.

A fuel cell with hydrocarbons could reach efficiencies of 60% or more.

So no lithium battery can reach volumic energies or specific energies comparable to what can be achieved with hydrocarbons.

The reason to use lithium rechargeable batteries is to obtain a better total efficiency of using energy, not the hope that it is possible to match the densities achievable with energy stored in hydrocarbons.

Among lithium rechargeable batteries, the lithium-air batteries should achieve the best energy per mass, perhaps also per volume.

Usually the weak point of metal-air batteries is the power per mass or the power per volume, because the reaction with air is slow, therefore the electrical current density in the electrodes is low, so to obtain a given amount of power requires great areas for the electrodes.

[adrian_b]

A lithium-air battery (in general all metal-air batteries) is likely to have lower efficiencies for a complete cycle than other lithium-based batteries, perhaps not much above 80%, if not even less. The lower efficiency is caused by one of the reactants being a gas, which causes certain thermodynamic constraints.

A fuel cell with hydrocarbons would have a slightly better efficiency than the best mobile thermal engines, e.g. of 60%, while the ideal energy per mass ratio is more than double for hydrocarbons in comparison with lithium-air batteries, so even with a better efficiency lithium can never match hydrocarbons in usable energy per mass, not even in lithium-air batteries.

The claim from the parent article is wrong and it is based on an incorrect method for computing the ideal energy per mass ratio for lithium-air batteries.

[gpm]

> A lithium-air battery (in general all metal-air batteries) is likely to have lower efficiencies for a complete cycle than other lithium-based batteries, perhaps not much above 80%, if not even less.

This paper directly contradicts this claim with actual measurements of efficiency.

> The energy efficiency of the first cycle was 92.7%, and it gradually dropped to 87.7% after 1000 cycles.

Which is centered just above the 90% mark the person you are replying to gave.

[grapesodaaaaa]

> The claim from the parent article is wrong and it is based on an incorrect method for computing the ideal energy per mass ratio for lithium-air batteries.

Can you elaborate for laypersons such as myself?

[cyberax]

Basically, Li-Air elements are wasting the energy from the phase change of oxygen. When a Li-Ion battery is discharged, you get the gaseous oxygen and bind it into a solid state molecule.

To do that, you need to expend roughly the same amount of energy that is needed to first liquify and then solidify the oxygen.

In fancy chemistry-speak it's called "entropic loss". You do gain some of that energy back when the battery is charged, as oxygen goes from a well-ordered solid state into the gaseous state. But it's not 100%.

[adrian_b]

The parent article has claimed that lithium-air batteries can have an energy per mass close to gasoline.

That claim is based on dividing the stored energy by the mass of lithium, which is incorrect.

The product of the reaction, i.e. lithium oxide, is stored in the battery, so a lithium-air battery can never be lighter than the lithium oxide.

Because the mass of lithium oxide is what counts, the energy per mass of pure lithium, which is indeed not much less than for gasoline, must be divided by a factor that varies between 2.14 and 5.57, depending on the construction of the lithium-air battery.

The best value of 2.14 is when the discharged battery contains only Li2O. The worst value of 5.57 is when the discharged battery contains only lithium superoxide, LiO2.

In the parent article, they claim that their discharged battery contains mostly Li2O, with only small quantities of peroxide Li2O2 and superoxide LiO2, but the exact amounts of peroxide and superoxide have not been measured.

So when computing correctly the energy per mass ratio, for lithium-air batteries it is limited to a value less than half of that for hydrocarbons. In practice batteries need a lot of materials besides the active reactants, so the achievable energy per mass ratio will be several times lower.

The advantage of hydrocarbons, regardless whether they are used in living cells, thermal engines or fuel cells, is that their reaction products are eliminated into the atmosphere, so their mass does not matter. The energy per mass for carbon atoms in hydrocarbons and for lithium atoms in lithium metal is approximately the same, but with lithium it is impossible to neglect the mass of the oxidant, like with carbon, because the reaction products cannot be dumped outside.

So for any battery except for fuel cells, what counts is the sum of the masses of the reactants, e.g. lithium + oxygen in the best case, or e.g. zinc + manganese in the cheap non-rechargeable batteries. It is wrong to compute the minimum mass of a battery by using only the mass of one of the reactants, like in the parent article, instead of both masses.

[andy_ppp]

Does the engine and drive train weight make these calculations even better for electric vehicles?

[TOMDM]

I feel like a good like to like comparison would be the density of battery + motors compared to the density of fuel tank, engine and drive train.

[adrian_b]

Even more like to like would be to compare the battery with a fuel tank + a turbo-generator.

For any kind of battery, there will be a power threshold over which a fuel tank + a turbo-generator will be smaller and lighter.

So a useful comparison would determine those power thresholds.

[adrian_b]

That theoretical maximum for a lithium-air battery seems much too high, so it is likely to be computed in the wrong way, in order to provide an optimistic but false value.

The mass that must be used for computing the theoretical maximum is that of Li2O, not the mass of lithium. Per atom of lithium, the mass of Li2O is 2.14 times greater, so it is likely that the number quoted by you must be divided by 2.14.

Indeed, computing very approximately 1 electron x the value of the elementary charge x 3 volt x the number of Avogadro (per kmol) / 15 kilogram / 3600 seconds, gives about 5500 Wh/kg, so the value quoted by you is indeed wrong.