Download 12 October 2000

12 October 2000
Nature 407, 681 - 682 (2000) © Macmillan Publishers Ltd.
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Electrochemistry: Making a potential difference
GEORGE E. BLOMGREN
George E. Blomgren is at Blomgren Consulting Services Ltd, 1554 Clarence Avenue, Lakewood, Ohio 44107, USA.
e-mail: [email protected]
To reduce our use of fossil fuels we will need cheap and safe batteries to run
electric vehicles and store energy — magnesium batteries may be the answer.
On page 724 of this issue1, Aurbach et al. describe a prototype for a rechargeable
magnesium battery, which may eventually replace the standard batteries used in largescale energy conversion. Such a system has been a long-sought goal in electrochemistry
because it could provide a cheap and environmentally benign alternative to existing
options, and may help to reduce the dependence of modern societies on fossil fuels.
Batteries are at the heart of the portable devices of the information age. In the past
decade the lithium-ion battery has become the main source of energy for these devices,
replacing older nickel–cadmium and nickel– metal-hydride varieties. The past year has
also seen the introduction of lithium-ion polymer batteries — in portable telephones and
handheld personal digital assistants, they are replacing nickel and standard lithium-ion
batteries.
Despite these rapid developments in portable technology, better and cheaper versions
have not yet replaced the standard large batteries used for 'heavy load' applications, such
as stationary energy storage. The lack of suitable alternatives has compromised the
progress of replacement technologies for hydrocarbon fuels. For example, growth in the
use of solar power has revealed a need for better batteries to store energy for use after
the Sun goes down. The development of electric vehicles has likewise been held up (
Fig. 1). An ordinary lead–acid battery is made from cheap and abundant, but heavy and
highly toxic, materials, whereas newer nickel batteries contain toxic cadmium or scarce
rare-earth-hydride materials. Even the more expensive lithium systems are reactive
when exposed to air. So they all have serious flaws.
Figure 1 No longer just a concept car? Full legend
High resolution image and legend (76k)
The subject of Aurbach and colleagues' study1 — magnesium — is a light and plentiful
element found in brines and minerals worldwide. It is environmentally benign and safe
to handle. For these reasons, many electrochemists have investigated magnesium as an
anode material for batteries, and several companies have produced primary (not
rechargeable) batteries, mainly for military applications. The problem in developing
rechargeable batteries with magnesium anodes is finding the right electrolyte.
All batteries have an anode (negative electrode), a cathode (positive electrode) and an
ion-conducting electrolyte. The electrolyte forms a continuous phase between the two
electrodes to allow ions to pass and complete the electric circuit. Electrons flow through
the external circuit to do the electrical work. It is important that most of the bulk volume
of each electrode is chemically active and in contact with the electrolyte. The problem
with most electrolytes used with magnesium anodes is that they allow a surface film to
grow on the anode, stopping the electrochemical reaction.
For a battery to be rechargeable it must be able to completely reverse the direction of
the electrochemical reactions at each electrode, from charge to discharge and vice versa.
During discharge a magnesium electrode gives up electrons to the external circuit, and
the resulting magnesium ions pass into the electrolyte. During charge, the magnesium
ions are plated back as metal onto the negative electrode, which also takes up electrons.
For the anode, these two processes are expressed succinctly by the electrochemical
equation: Mg Mg2+ + 2e-, where the reaction goes in the forward direction during
discharge and the backward direction during charge, and e- is an electron. Similarly, the
reactions at the cathode may be represented as MgyMX + zMg2+ Mgy+ zMX + 2e-,
where MgyMX represents a magnesium-containing compound. The challenge has been
finding a suitable cathode material that can reversibly bind magnesium ions and remain
stable.
Electrolyte solutions, such as Grignard reagents, have long been known to deposit
magnesium reversibly2. Grignard reagents have the formula RMgY, where R is an alkyl
or aryl group and Y is chlorine or bromine in ether solution; they are commonly used
for alkylation reactions in organic synthesis. But they are strong electron donors, so they
reduce and destabilize the cathode. Part of the success of Aurbach and co-workers'
study was finding an electrolyte solution that can reversibly deposit magnesium without
destroying the cathode.
The authors used electrolytes based on magnesium–organohalo aluminate salts (actually
two types: Mg(AlCl3R)2 and Mg(AlCl 2RR')2, where R and R' are alkyl groups) with a
solvent of cyclic ether (tetrahydrofuran) or a polyether of the glyme family such as
diglyme. Ether solvents like these have been widely studied in lithium systems and are
known to be stable. But the electrolyte salts are new to battery research. Aurbach et al.
chose to use the aluminate form to improve cathode stability and magnesium deposition.
This type of electrolyte will form a battery with a voltage as high as 2.5 V, much better
than aqueous electrolytes (which are thermodynamically stable to just over 1 V) and
high enough for commercial interest. The voltage range is not as good as with nonaqueous electrolytes used in lithium batteries, but those electrolytes would not
reversibly deposit magnesium.
The second part of the discovery was to find a suitable material for the cathode. The
authors found that MgyMo3S 4 would reversibly accept and release magnesium ions with
a cell voltage of about 1.5 V. Nickel battery cells operate at 1.2 V, whereas lead–acid
cells operate at about 2 V, so the cell is competitive with these batteries. Early results
from tests of this material in electrolyte cells have shown an acceptable lifetime, with a
competitive energy per unit weight at reasonable rates of charge and discharge. Based
on material costs, it is anticipated that the system might fall between aqueous batteries
and lithium-ion batteries in terms of price.
As with any new technology, many tests must be done to fully characterize the system
and to optimize the battery's behaviour, and these will no doubt be carried out by battery
manufacturers with an interest in the system. In the end, of course, it must eventually
prove itself in the marketplace. But our need to find viable alternatives to hydrocarbon
fuels means there is every incentive to make full and fair evaluations of technical
advances such as this one.
References
1. Aurbach, D. et al. Nature 407, 724-727 (2000). Links
2. Connor, J., Reid, W. & Wood, G. J. Electrochem. Soc. 104, 38-41 (1957).