Page 423 - Handbook of Battery Materials

13.7 Improvement in the Cycling Efficiency of a Lithium Anode 395

phenylethylene carbonate (PhEC) [88] were investigated. VC, VEC, and PhEC were
effective in suppressing the excessive reductive decomposition of GBL.
In order to increase energy density of the lithium cells, high-voltage cells have
been studied. 4 V lithium-ion cells have been commercially applied for electronic
portable equipment such as cellular phones and notebook-type personal computers.
However, lithium cells are finding new uses, such as for electric vehicles and power
storage batteries. These need higher-voltage cells (e.g., 5–6 V) as well as a higher
charge–discharge capacity and a higher energy density. To increase the cell voltage,
not only is the development of new electrode materials important but also the
development of new electrolytes having higher anodic stability than conventional
electrolytes using solvents such as EC, PC, DEC, DMC, and EMC. There have been
many studies to develop new electrolytes for high-voltage lithium cells [89–93].
For example, fluorinated carbonate solvents exhibit higher anodic stability, relative
permittivity, and viscosity [89, 90]. Nitrile solvents show high anodic stability and
low viscosity [91, 92]. Ionic liquids are also known to show higher anodic stability,
noncombustibility, and high ionic conductivity. Especially, aliphatic ammonium
bis(trifluoromethanesulfone)imide shows superior anodic stability [5]. However,
charge–discharge properties, cathodic stability, and compatibility of these solvents
with a lithium anode have not been fully investigated.
Sulfones are investigated for high voltage cells because of their anodic stability
[94–96]. Sulfolane (SL) is a common solvent known to show high anodic stability,
high relative permittivity, and low toxicity. However, SL is solid at room temper-
ature, and its viscosity is too high at liquid phase. Ethyl acetate (EA) is also very
common as an organic solvent. It has good anodic stability and low viscosity, but its
◦
relative permittivity is only 6.02 at 25 C [97]. This is too low to dissociate supporting
salts sufficiently. Then, sulfone–ester mixed solvent electrolytes were examined
for 5 V-class high-voltage rechargeable lithium cells [98]. As the base-electrolyte,
SL–EA mixed solvent containing LiBF 4 solute was investigated. LiBF 4 is used here
because the tolerance of LiBF 4 toward oxidation is reported to be higher than that
of LiPF 6 [99]. Charge–discharge cycling efficiency of a lithium anode in SL–EA
electrolyte was poor, due to its poor tolerance for reduction. To improve lithium
charge–discharge cycling efficiency in SL–EA electrolytes, the following three trials
were carried out; (i) improvement of the cathodic stability of electrolyte solutions
by change in polarization through modification of solvent structure; isopropyl
methyl sulfone and methyl isobutyrate were investigated as alternatives to SL and
EA, respectively, (ii) suppression of the reaction between lithium and electrolyte
solutions by addition to SL–EA electrolytes of low-reactivity surfactants derived
from cycloalkanes (decalin and adamantane) or triethylene glycol derivatives
(triglyme, 1,8-bis(tert-butyldimethylsilyloxy)-3,6-dioxaoctane and triethylene glycol
di(methanesulfonate)), and (iii) change in surface film by addition of surface
film formation agent VC to SL–EA electrolytes. These trials made lithium cycling
behavior better. Of these additives, the addition of VC was the most effective for
improvement of lithium cycling efficiency. A stable surface film is formed on the
lithium anode by adding VC, and the resistance between anode/electrolyte inter-
faces showed a constant value with an increase in cycle number. In the electrolyte