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Modern rechargeable batteries for electronic
gadgets generally use lithium compounds as the positive electrode and
have revolutionized the electronics industry. They can be made very
compact but can still deliver the required voltage to run everything
from cell phones to digital cameras and notebook computers. And, not
forgetting those ubiquitous mp3 players.
As gadgetry becomes sophisticated so consumer
demands on battery life have risen. Moreover, more powerful lithium
batteries are beginning to be used in power tools and may soon be seen
in electric vehicles, applications that are much more draining than
those for which conventional lithium batteries are used.
Now, Kuthanapillil Shaju and Peter Bruce of the
University of St Andrews, Scotland, explain how lithium batteries use
so-called intercalation materials as their anode. These materials are
composed of a solid network of lithium atoms together with other
metals, such as cobalt, nickel, or manganese, meshed together with
oxygen atoms. When you charge a lithium battery, the charging current
pulls the positive lithium ions out of this network. Then, when you
use the battery, it discharges as these lithium ions migrate back into
the electrode, pulling electrons as they go, and so generating a
current.
The challenge is to make new electrode materials
that deliver high power (fast discharge) and high energy storage.
Shaju and Bruce hoped they could solve these problems by developing a
new way of synthesizing a particular lithium intercalation compound
(Li(Co1/3 Ni1/3 Mn1/3)O2).
As a bonus, they hoped to be able to simplify the complicated
manufacturing process.
The St Andrews team devised a new synthetic
approach to the compound that involves simply mixing the necessary
precursor compounds - organic salts of the individual metals - with a
solvent in a single step. This is in sharp contrast to the
conventional multi-step process used for making the compound. Using
this technique, they were able to make highly uniform lithium oxide
intercalation materials in which nickel, cobalt, and manganese ions
are embedded at regular intervals in the solid, which also contains
pores for the electrolyte.
The highly porous nature of the new material is
crucial to its electrical properties. The pores allow the electrolyte
to make intimate contact with the electrode surface resulting in high
rates of discharge and high energy storage. The St Andrews team has
tested their new lithium electrode material by incorporating it into a
prototype battery and found that it gives the battery far superior
power and charge retention. Increasing the rate by 1000%, so that the
battery can be discharged in just six minutes, reduces the discharge
capacity by only 12%. The test results suggest that this approach to
rechargeable batteries could be used to make even higher power
batteries for vehicles and power tools. Most importantly though, the
new lithium materials could mean an end to mp3 player power loss on
that long-haul flight. (Assuming you remembered to charge it up in the
first place.)
There's an added bonus in that replacing a
proportion of the cobalt used in the traditional lithium-cobalt-oxide
electrodes with manganese improves safety by reducing the risk of
overheating. |
