Page 433 - Handbook of Battery Materials

405

14
Lithium Alloy Anodes

Robert A. Huggins

14.1
Introduction

The interest in ever-higher energy content has caused the development of cells with
relatively high voltages to receive much attention in the lithium battery research
community in recent years. This has led to the exploration of a number of positive
electrode materials that operate at potentials of about 4 V, or even more, positive of
the potential of elemental lithium.
However, this is only part of the story, for the voltage of a cell is determined
by the difference between the potentials of the negative and positive electrodes.
The highest voltages are obtained by the use of elemental lithium in the negative
electrode. The use of negative electrode reactants with lithium activities of less
than unity results in electrodes with more positive potentials, thus reducing the
cell voltage.
The voltage is only one of the important parameters of batteries, and other
considerations also are often important in practical systems. One that has received
increasing attention in recent years is the question of safety. Batteries that store
large amounts of energy can be very dangerous if that energy is suddenly released,
and there have been a number of accidents involving lithium batteries. It is now
recognized that these safety problems generally relate to phenomena at the negative
electrode. Local heating to high temperatures, especially above the melting point
of lithium when elemental lithium is used, can lead to serious disasters.
In addition, the cycling behavior of lithium cells is often limited by negative
electrode problems. These may include gradually increasing impedance, which
is observed as decreasing output voltage. In some cases there is a macroscopic
shape change. If elemental lithium is used (below its melting point), there may
be dendrite growth, or a tendency for filamentary or whisker formation. This may
lead to disconnection and electrical isolation of active material, resulting in loss
of capacity. It may also result in potentially dangerous electrical shorting between
electrodes.
Whereas there had been a significant amount of work on the properties of
lithium alloys in the research community for a number of years, this alternative

Handbook of Battery Materials, Second Edition. Edited by Claus Daniel and J¨ urgen O. Besenhard.
 2011 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2011 by Wiley-VCH Verlag GmbH & Co. KGaA.