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04 July 2018

Energy Density and EVs


One of the simple bits of physics that even I - a definite non-physicist - can understand about electricity versus petrol as a means of powering vehicles is the issue of weight against power. That is heavily slanted in favour of petrol, which is a point the battery industry will at some point have to address fully. We know progress has been made in reducing the overall weight of batteries, and that lithium-ion cells are kilo for kilo substantially more effective than lead-acid, for example, but the reality of current physics is that the energy density of refined crude oil is many, many times better than even the most efficient of today’s batteries. That impacts directly on the range of EVs. 

Half a tonne of batteries

The best range currently comes from Tesla, but in order to achieve that 300-odd miles, the floor of the car is packed with lithium-ion cells. There is a benefit in roadholding, with the centre of gravity so low (because the batteries are at the bottom), but the truth is that the weight of battery the car needs to haul around is far greater than would be the weight of petrol (or diesel) needed to match the range. Somehow reducing that weight is a challenge the industry will need to overcome, because along with the charging issue - both the availability of charging points and the length of time taken to refill the batteries - this is one of the big question marks over EVs. It’s all very well to laud the green credentials of the devices, but if a lot of the power put out by the electric motors is simply used up in dragging around half a tonne of batteries (the weight in a Tesla Model S is 540 kilos), then at least some of the advantage is nullified.

New material

But the good news is that researchers at Lancaster University (in association with Jilin University) may just have stumbled on a material that might help. It’s a new form of carbon, as yet unnamed (referred to as OSPC-1). Currently, the anode in a lithium-ion cell is formed of graphite. What the scientists have found - but let’s remember caution here: this is a very early stage - is that OSPC-1 has two significant advantages over graphite.

First, it seems to be able to store more than twice as many lithium ions (and therefore power) as graphite. Secondly, it can store them - absorb them - twice as quickly, which would result in shorter charging times. That also means it can discharge at a higher rate, enabling use in more energy-hungry applications.

No more fires?

Additionally, the material seems to be longer-lasting than graphite, and - perhaps of particular interest to passengers flying in 787 Dreamliners - less likely to burst in to flames caused by a build-up of fibres (known as dendrites) which can create short-circuits.

I’ve had a shot at understanding the researchers’ paper - which can be found via the University of Lancaster’s website - but I have to confess to struggling with the scientific detail; O-Level physics from a long time ago won’t cut it. But the predictable downside is there, of course. OSPC-1 is considerably more expensive to produce than graphite, so the first applications will probably be in the aerospace sector. 

Significant step forward

But we know that technology gets cheaper over time - if it is one that has practical applications - so it may just be that these researchers have moved the EV revolution on significantly; reducing weight and thus increasing range is one of the key areas of  development, and will help alleviate one of the major current restraints on the adoption of EVs.        

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