Page 413 - Handbook of Battery Materials

13.7 Improvement in the Cycling Efficiency of a Lithium Anode 385

An electrochemical quartz crystal microbalance (EQCM) or quartz crystal mi-
crobalance (QCM) can be used to estimate the surface roughness of deposited
lithium [43].

13.6
The Amount of Dead Lithium and Cell Performance

From our experimental results [44], the FOM at a low discharge rate is consid-
erably smaller than that at a high discharge rate. The influence of the discharge
rate on the specific surface area of a lithium anode was examined [44] using
−1
2
the Brunauer–Emmett–Teller (BET) equation. The surface area (26 m g ) for
−2
2
−1
low-rate discharge cycles (0.2mA cm ) is double that (13 m g ) for high-rate
−2
discharge cycles (3.0mA cm ). In addition, the surface area increases with an in-
crease in cycle number. The surface area after the sixth discharge at a low discharge
2
−1
rate was 30 times greater than that before cycling (1 m g ). The main reason for
the increase in the lithium surface area is considered to be the accumulation of
dead lithium on the anode surface.
There are four possible ways of explaining [45] why a higher current discharge
creates a smaller amount of dead lithium.
1) When the discharge current is high, delocalized pits (small in size but large
in number) are formed on a native lithium anode. As lithium is deposited on
these pits, the local charge current density becomes low when the discharge
current is high, producing thicker, fiber-like lithium that is not easily cut to
form dead lithium.
2) When the discharge current is large, delocalized pits formed in the anode are
shallow, so the deposited lithium whiskers can easily emerge from the pits,
and stack pressure can be applied to them, as mentioned in Section 13.7.3.
3) Isolated lithium near the anode becomes a local cell because of stray current.
As the stray current is high when the cell discharge current is high, lithium
recombination occurs easily at a high discharge current [46].
4) When the discharge current is high, transport of lithium ions becomes difficult
and stripping occurs from the particle-like lithium on the tip and on the kinks
of the fiber-like lithium. In this case, the fiber-like lithium rarely breaks and
the efficiency increases.

13.7
Improvement in the Cycling Efficiency of a Lithium Anode

There have been many attempts to improve the cycling efficiency of lithium
anodes. We describe some of them below, discussing electrolytes, electrolyte
additives, the stack pressure on the electrode, composite anodes, and alternatives
to the lithium-metal anode anode.