The Economist - USA (2020-11-07)

68 TheEconomistNovember 7th 2020

1

W

hen it comesto putting on pace,

some electric vehicles rely not only

on a battery to deliver the necessary watt-

age, but also on a second source of power

called a supercapacitor. The battery serves

as a marathon runner, providing a steady

discharge over a long distance. The super-

capacitor is a sprinter, unleashing a large

amount of energy rapidly.

Speedy discharge is not the only advan-

tage supercapacitors bring. They can be re-

charged more quickly, too. That makes

them particularly useful in regenerative-

braking systems, since they are able to ab-

sorb more of the electricity that is pro-

duced as a vehicle slows down. They can,

though, store only a fraction of the amount

of energy which a battery stuffs away. They

therefore soon run out of puff. Because of

this, engineers have been trying for a while

to hybridise the best bits of a supercapaci-

tor with the most useful features of a bat-

tery, to make a storage device with both

speed and endurance. They are now having

some success. Indeed, nawaTechnologies,

near Aix-en-Provence, France, claims its

supercapacitor-like battery could more

than double the range of an electric car, al-

lowing it to be driven for 1,000km on a sin-

gle charge. This new device could also,

nawasays, be recharged to 80% of its ca-

pacity in as little as five minutes.

The science bit

Capacitors and batteries work in different

ways, so combining them is tricky. A capac-

itor stores energy physically, in the form of

static electricity. This is easily and rapidly

discharged, so capacitors have good power

density (the rate at which they transfer en-

ergy, per unit of weight). A decent modern

supercapacitor has a power density of sev-

eral kilowatts per kilogram.

Batteries store their energy chemically,

in the form of reactive substances in their

two electrodes. These electrodes are held

physically apart, but are connected by a

material called an electrolyte through

which charged atoms, known as ions, can

pass from one to the other, in order to per-

mit a reaction to proceed. That, though,

happens only when the ion flow is bal-

anced by a flow of electrons through an ex-

ternal circuit between the electrodes. This

electron flow is the electric current which

is the reason for the battery’s existence.

Controlled in this way, chemical reac-

tions take time, so batteries have low pow-

er density. A lithium-ion (Li-ion) battery of

the sort used in electric cars might thus

muster only a tenth of a kilowatt per kilo-

gram. But chemicals can hold a lot of ener-

gy, so batteries have high energy density

(the amount of energy they can contain,

again per unit weight). A Li-ion battery can

store 200-300 watt-hours per kilogram

(wh/kg). Supercapacitors generally man-

age less than 10wh/kg.

Capacitors, by contrast—whether basic

or “super”—consist of a pair of electrically

conductive plates placed either side of a

separator material. When a voltage is ap-

plied to these plates, a positive charge

builds up on the surface of one and a corre-

Superbatteries

What do you get when you cross a hare

with a tortoise?

Mixing supercapacitors and batteries may give electric cars what they need to

overcome customer resistance

Science & technology

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