Page 434 - Handbook of Battery Materials

406 14 Lithium Alloy Anodes

did not receive much attention in the commercial world until about 1990, when
Sony began producing batteries with lithium–carbon negative electrodes. Since
then, there has been a large amount of work on the preparation, structure, and
properties of various carbons in lithium cells.
Another aspect is now beginning to receive attention, also on the basis of
commercial development rather than arising directly from activities in the public
research community. This is the development by Fuji Photo Film Co. of the use of
materials based upon tin oxide as negative electrodes. As will be discussed later,
this involves the formation of alloys by the in-situ conversion of the oxide.

14.2
Problems with the Rechargeability of Elemental Electrodes

In the case of an electrochemical cell with a negative electrode consisting of an
elemental metal, the process of recharging is apparently very simple, for it merely
involves the electrodeposition of the metal. There are problems, however.
One of these is the ‘shape change’ phenomenon, in which the location of the
electrodeposit is not the same as that of the discharge (deplating) process. Thus,
upon cycling, the electrode metal is preferentially transferred to new locations. For
the most part, this is a problem of current distribution and hydrodynamics rather
than being a materials issue, therefore it will not be discussed further here.
A second type of problem relates to the inherent instability of a flat interface upon
electrodeposition [1]. This is analogous to the problems of the interface evolution
during electropolishing and the morphology development during the growth of an
oxide layer upon a solid solution alloy, problems that were discussed by Wagner
[2, 3] some time ago.
Another analogous situation is present during the crystallization of the solute
phase from liquid metal solutions. This leads to the production of protuberances
on the growth interface; these gradually become exaggerated, and then develop
into dendrites. A general characteristic of dendrites is a tree-and-branches type of
morphology, which often has very distinct geometric and crystallographic charac-
teristics due to the orientation dependence of surface energy or growth velocity. The
current distribution near the front of a protrusion develops a three-dimensional
(3-D) character, leading to faster growth than that at the main electrode’s sur-
face, where the mass transport is essentially one-dimensional (1-D). In relatively
low-concentration solutions, this leads to a runaway type of process, so that the
dendrites consume most of the solute and grow farther and farther ahead of the
main, or bulk, interface.
A third type of problem, which is often mistakenly confused with dendrite
formation, is due to the presence of a reaction–product layer upon the growth
interface if the electrode and electrolyte are not stable in the presence of each other.
This leads to filamentary or hairy growth, and the deposit often appears to have a
spongy character. During a subsequent discharge step, the filaments often become