Page 415 - Handbook of Battery Materials

13.7 Improvement in the Cycling Efficiency of a Lithium Anode 387

tendency (a partial internal short) near the end of its cycle life [52]. However, the
addition of 2MeTHF to EC/PC causes this ‘soft shorting’ to decrease dramatically.
From a safety aspect of utilizing lithium-metal anodes, various fluorinated sol-
vents have been studied as the components of electrolyte solutions for lithium-metal
cells. A typical example of these solvents is methyl difluoroacetate (MFA). MFA
exhibited better thermal stability and lithium cycling efficiencies among various
fluorinated carboxylic acid esters [53].
Another influence that electrolyte materials have on the cycle life of a practical
lithium cell is due to the evolution of gas as a result of solvent reduction by lithium.
For example, EC and PC give rise to [54] evolution of ethylene and propylene gas,
respectively. In a practical sealed-structure cell, the existence of gas causes irregular
lithium deposition. This is because the gas acts as an electronic insulator and
lithium is not deposited on an anode surface where gas has been absorbed. As a
result, the lithium cycling efficiency is reduced and shunting occurs.

13.7.2
Electrolyte Additives

There have been many studies with the goal of improving lithium cycling efficiency
by the use of electrolyte additives. These additives can be classified into three types:

1) stable additives which cover the lithium to limit any chemical reaction between
the electrolyte and lithium,
2) additives which modify the state of solvation of lithium ions, and
3) reactive additives used to make a better protective film.

Some of the studies on additives based on this classification will now be described.
13.7.2.1 Stable Additives Limiting Chemical Reaction between the Electrolyte and
Lithium
Besenhard et al. [55] studied ways to protect lithium anodes from corrosion by
adding saturated hydrocarbons to electrolytes. They considered saturated hydro-
carbons to be chemically stable, and thus able to delay the irreversible reduction
of organic electrolytes by lithium. They found that the deposited lithium was
particle-shaped when cis- or trans-decalin was added to LiC1O 4 –PC electrolyte.
Although there was no change in the cycling efficiency, the long-term storage
characteristics improved.
Naoi and co-workers [56], with a QCM, studied lithium deposition and dissolution
processes in the presence of polymer surfactants in an attempt to obtain the
uniform current distribution at the electrode surface and hence smooth surface
morphology of the deposited lithium. The polymer surfactants they used were
polyethyleneglycol dimethyl ether (molecular weight ≈ 446), or a copolymer of
dimethylsilicone (circa 25 wt%) and propylene oxide (circa 75 wt%) (molecular
weight ≈ 3000) in LiC1O 4 –EC/DMC (3 : 2, v/v).
Yoshio and co-workers [57, 58] tried using aromatic compounds of benzene,
toluene, or 4,4-dipyridyl as additives and found them to be effective.