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382 13 Rechargeable Lithium Anodes

with LiAsF 6 –THF-based electrolytes. The cell (II) experiment provides a more
predictable result for the cycle life of the Li/TiS 2 full cell it because minimizes the
effect of trace impurities.

13.4.2
Reasons for the Decrease in Lithium Cycling Efficiency
The reasons why lithium cycling efficiency is not 100% are generally considered to
be as follows;
1) Lithium is consumed by reaction with the electrolyte, which forms a protective
film [6]. During the deposition and stripping of lithium, the surface shape
changes and a fresh lithium surface is formed with a new protection film on
it; lithium is consumed in the process.
2) Lithium is isolated in a protective film [8]. During the deposition of lithium,
the protective film may be heated locally by ion transport in the film itself.
As a result of this local heating, part of the protective film (SEI) becomes an
electronic conductor, and therefore lithium metal is deposited in the film. If
local heating does not occur during stripping, the isolated lithium becomes
electrochemically inactive.
3) Deposited lithium is isolated from the base anode [30, 31]. When a cell is
charged, lithium is deposited on the lithium substrate of the anode. Sometimes,
the plated lithium is not flat but fiber-like. When the cell is discharged, the
lithium anode dissolves, and sometimes the fiber-like lithium is cut and
becomes isolated from the anode substrate [31]. This isolated lithium is called
‘dead lithium,’ and it is electochemically inactive but chemically active. During
cycling, this dead lithium accumulates on the anode.

We believe that item 3 above is the main reason for the low cycling efficiency. The
thermal stability of lithium-metal cells decreases with cycling [30], and the dead
lithium may be the cause of this reduction. This indicates that the cycling efficiency
is strongly affected by the morphology of the lithium surface.

13.5
Morphology of Deposited Lithium

There have been many reports on the morphology of the lithium that is electro-
chemically deposited in various kinds of organic electrolyte [32–39].
Figure 13.1 shows a typical lithium deposition morphology. Here, the
lithium is deposited on stainless steel at 3 mA cm −2 for 1 h with 1.5 mol L −1
LiAsF 6 –EC/2MeTHF (1 : 1, v/v).
Koshina et al. have reported that there are three kinds of morphology [40]:
dendritic, granular, and mossy. Mossy lithium is formed when the deposition
current is small and the salt concentration is high. This mossy lithium provides a
high cycling efficiency.