Page 412 - Handbook of Battery Materials

384 13 Rechargeable Lithium Anodes

deposition points is the pits on the lithium anode caused by discharge.
Crystalline defects and the grain boundaries in lithium may also initiate
deposition.
3) As lithium does not deposit uniformly for the reason mentioned above,
mechanical stress is created in the lithium electrode under the protective film.
4) The stress causes lithium-atom transport, which means that deformation of
the lithium occurs to release the stress in it. The lithium transport is not free
but is conditioned by a force created by lithium surface tension (including the
surface tension caused by the protective film) at a curved surface, and it may
also be affected by crystalline defects and grain boundaries.
5) The protective film is broken in certain places on the lithium surface by the
stress. Fiber-like lithium grows, like an extrusion of lithium, through these
broken holes in the film. If the deposition current is small enough and the
stress is therefore small, the protective film will probably not break. In this
case, the deposited lithium may be particle-like or amorphous.
6) After the fiber-like lithium has grown, lithium is still deposited on the lithium
substrate, that is, not at the tip of the fiber-like lithium. If the deposition
continues for a long time, the lithium electrode becomes covered with long,
fiber-like lithium. In this situation, lithium-ion transport in the electrolyte
to the lithium electrode surface is hindered by the fiber-like lithium. Then,
lithium begins to be deposited on the tip and on kinks of the fiber-like lithium,
where there are crystalline defects. The morphology of the deposited lithium
is particle-like or amorphous. As there are many kinks, the current density
of the lithium deposition becomes very low. This low current density may
create particle-like, rather than fiber-like, lithium. Thus the morphology of the
lithium as a whole becomes mushroom-like [31].
The dissolution process of plated lithium may be the reverse of the plating process
(Figure 13.2b). At first, the particle-like lithium on the kinks is dissolved. Then, the
fiber-like lithium at the base is dissolved. During this process, fiber-like lithium
is sometimes cut from the lithium substrate and becomes dead lithium. There
is a large amount of dead lithium when the diameter of the fiber-like lithium is
small under conditions of high-rate and/or low-temperature deposition, because
the whiskers are easily cut.
A microelectrode has been used by Uchida et al. to study lithium deposition in
order to minimize the effect of solution resistance [41]. They used a Pt electrode
(10–30 µm in diameter) to measure the lithium-ion diffusion coefficient in 1 mol
2
L −1 LiClO 4 /PC electrolyte. The diffusion coefficient was 4.7 × 10 −6 cm s −1 at
◦
25 C.
The lithium morphology at the beginning of the deposition was measured
by in-situ atomic force microscopy (AFM) [42]. When lithium was deposited at
0.6 C cm-2, small particles 200–1000 nm in size were deposited on the thin lines
and grain boundaries in LiClO 4 –PC. Lump-like growth was observed in LiAsF 6 –PC
along the line.