Page 419 - Handbook of Battery Materials

13.7 Improvement in the Cycling Efficiency of a Lithium Anode 391

charge–discharge current (I ps ). When n increased there was an increase in lithium
cycling efficiency at a low I ps and a decrease in the reduction potential of the
◦
glymes. When the conductivities, including those at low temperature (below 0 C),
and charge–discharge cycling at a high current are taken into account, diglyme or
triglyme is superior to the other glymes.

13.7.2.3 Reactive Additives Used to Make a Better Protective Film
The effect of ‘precursors’ was examined by Brummer et al. [65] with the aim
+
of producing a thin and Li -ion conductive film which was impermeable to
solvent molecules. As precursors, they tested CS 2 , PSCl 3 ,PSBr 3 ,POBr 3 , PNBr 2 ,
POCl 3 ,CH 3 SO 2 ,MoOCl 4 ,VOCl 2 ,CO 2 ,N 2 O, and SO 2 with 1 mol L −1 LiClO 4 –PC.
−1
The concentration of additive ranged from 0.01 to 0.36 mol L . A maximum
efficiency of 85.1% was obtained by the addition 0.01 mol L −1 POCl 3 , whereas
the base electrolyte without any additive showed an average efficiency of 40%.
However, without additives, 1 mol L −1 LiAsF 6 –PC has an efficiency of 85.2% and
the addition of POCl 3 to LiAsF 6 provides 75.8% efficiency. These results indicate
that the use of LiAsF 6 provides a better film for cycling Li than those formed by
the precursors. However, we believe that it is still worth attempting to find new
precursors.
Also, the influence of adding O 2 ,N 2 ,Ar,or CO 2 to LiAsF 6 –THF on lithium
cycling efficiency has been examined [66]. Lithium was cycled on an Ni electrode
−2
with Q p = 1.125 C cm −2 and cycling currents of 5 mA cm . Oxygen and N 2
helped to maintain the cycle life relative to Ar, while CO 2 and ungasified electrolyte
did not. The addition of O 2 showed the highest lithium cycling efficiency, which
resulted from the formation of an Li 2 O film. However, the lithium cycling efficiency
rapidly degraded beyond the 10th cycle. On the basis of these results, Dominey
et al. [67] examined the effect of adding KOH to ether-based electrolytes such as
THF, 2MeTHF, or 1,3-dioxolane. They showed that the presence of the hydroxide
modifies the surface film formation. The anode-related heat output was reduced
three to fourfold in cyclic-ether electrolytes containing approximately 100 ppm of
OH . The Li/TiS 2 cell with THF to which 2MeF, KOH, and 12-crown-4 had been
−
added has been reported to show excellent cycling efficiency. Further improvement
in the lithium deposition morphology is still needed, however. LiAsF 6 –2MeTHF
has a good cycling efficiency. Abraham et al. [68] showed that the high cycling
efficiency is caused by 2MeF, which is naturally contained in 2MeTHF as in
impurity.
Quinoneimine dyes, aromatic nitro compounds, and triphenylmethane com-
pounds have been studied [69]. These compounds are highly reactive with lithium.
If the lithium cell includes these compounds as the cathode, it will exhibit cell
voltages of 2–3 V. The cycling efficiency was improved by adding quinoneimine
dyes. However, this effect depends on the charge capacity and the duration of
charge–discharge cycling. The effect of hexamethylphosphoric triamide (HMPA)
has also been examined. HMPA has an extremely high solvation power for cations
whose donor number (DN) is 38 [70]. A unique characteristic of HMPA is that
it produces solvated electrons in contact with alkali metals when there is a large