Page 559 - Handbook of Battery Materials

17.2 Components of the Liquid Electrolyte 533

aprotective layer(SEI)ongraphite anodes [101],inhibiting exfoliationevenwhenthe
solvent content of PC exceeds 30%, up to neat PC. The SEI is very stable and resists
many charge/discharge cycles, resulting in optimal conditions for cell operation
over a long period [101]. However, LiBOB is moisture sensitive (see Section 17.2.5)
and shows reduced solubility in well-established blends of solvents. By FTIR
−1
spectroscopy, hydrolysis products (3300 cm ) and residuals from synthesis (∼1658
−1
and 775 cm ) can be detected [532]; both are scarcely soluble in EC-EMC blends.
LiBOB as additive was tested, as well. In LiPF 6 -containing electrolyte solutions,
better thermal stability and enhanced cycling performance of lithium-ion cells
were detected [102]. Interestingly, this investigation resulted in a further new
salt, because at elevated temperatures the oxalate ligand sequesters the generated
PF 5 , and this results in the anion tetrafluoro(oxalato)phosphate (PF 4 (Ox) ) [103].
−
However, up to now no performance data of the neat salt LiPF 4 Ox are available.
For some recent investigations on conductivity studies of LiBOB c. f. Ref. [527].

17.2.2.7 Lithium Difluoro(oxalato)borate and Lithium bis(fluorosulfonyl)imide
Lithium difluoro(oxalato)borate (LiDFOB) and lithium bis(fluorosulfonyl)imide
(LiFSI) are two promising candidates for lithium ion batteries. From previous
studies it was obvious that increasing asymmetry of anions would increase both
solubility and conductivity of dissolved salts. Therefore, this group synthesized
lithium difluoro(oxalato)borate (LiDFOB) [517, 518]. For its electrochemical char-
acterizations, see Refs. [181, 530].
LiDFOB is a member of the group of salts with two different ligands, here
fluorine and oxalate. Other members of this group include a lithium borate with an
manolato and an oxalato ligand [256], an benzenediolato and an oxalato ligand [106],
and an 3-fluoro-1,2-benzenediolato and an oxalato ligand [531]. The use LiDFOB as
a more convenient salt than LiBOB in secondary lithium-ion batteries is discussed
by Zhang [99, 104]. It seemingly combines the merits of LiBF 4 and LiBOB, the
drawbacks of the two salts being mutually exclusive [181, 530]. LiDFOB shows
a higher solubility in carbonate solvents than LiBOB, and it shows satisfactory
◦
conductivity over a wide temperature range. Above 10 C the conductivity of
◦
LiDFOB is insignificantly smaller than that of LiBOB; however, below −30 Cit
shows conductivity similar to that of LiBF 4 . Moreover, the rate capability is very
◦
good over a temperature range from −40 to 70 C. The passivation film of the
cathode current collector (Al) is as good as it is with LiBOB. Furthermore, the
copper anode current collector is protected until 0 V vs Li/Li , permitting metallic
+
lithium to be reversibly deposited. Last but not least, LiDFOB forms a higher
conductive and less temperature-dependent SEI than LiBOB, which explains
the better cycling performance of LiDFOB containing electrolytes, especially at
low temperatures. The salt LiFSI also shows several promising properties when
compared to lithium hexafluorophosphate, including its stability with lithium and
lithiated carbon. Several properties of LFSI including its conductivity in solvents
were recently reported by Han et al. [516]. In a blend of ethylene carbonate/ethyl
methyl carbonate (3:7, v/v), LiFSI shows the highest conductivity of all usual salts,
even higher than LiPF 6 .