Page 558 - Handbook of Battery Materials

532 17 Liquid Nonaqueous Electrolytes

Another drawback of LiPF 6 is its sensitivity to moisture. It produces HF with
water and thus prevents the use of Mn-spinels as cathode materials, so great
effort has to be undertaken to achieve high purity grades of used solvents and a
moisture-free fabrication environment.

17.2.2.4 Lithium Tetrafluoroborate
Electrolyte solutions containing lithium tetrafluoroborate (LiBF 4 )showtwo times
lower conductivity than LiAsF 6 and LiPF 6 (e.g., 4.9mS·cm −1 in 1.0 M EC/DMC
33/67 [87]). This moderate conductivity results from a smaller dissociation constant
−
when compared to LiAsF 6 or LiPF 6 ,albeitBF 4 exhibits the highest ionic mobility
among those salts as determined in PC and γ-butyrolactone (GBL) [88, 89].
Moreover, in the same way as LiPF 6 , this salt tends to degrade the solvents and
hence lowers the lithium cycling efficiency [69].
In contrast to LiPF 6 , which is very moisture sensitive and thermally unstable,
LiBF 4 is thermally stable and hydrolyzes slowly (see Section 17.2.5), but its low
conductivity has inhibited further research. So far it at least occupies a niche,
because its low-temperature performance equals that of other salts [90–92] with
the merits mentioned above and forms a better passivation film at the aluminum
conductor, which allows higher potentials [90].

17.2.2.5 Lithium Fluoroalkylphosphates
Lithium fluoroalkylphosphate (LiFAP) was proposed by Merck KGaA, Darmstadt,
Germany, to replace LiPF 6 as the standard in lithium-ion batteries [80]. It should
avoid the drawbacks of LiPF 6 such as moisture sensitivity and degradation at
elevated temperatures. LiFAP shows a slightly lower conductivity than LiPF 6
(8.6mS·cm −1 in 0.8 M EC/DMC at ambient temperature), but compared to LiPF 6
it shows better electrochemical and thermal stability [80, 93]. A better performance
can be detected in solutions with additives (e.g., 5% vinylene carbonate (VC) and
1% bisalicylato borate, respectively). In these solutions (EC/DEC/DMC 2/1/2) the
◦
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thermal stability increases from 132 C(LiPF 6 ) to 165 C (LiFAP and 5% VC) [93].
Moreover, LiFAP is less sensitive to hydrolysis than LiPF 6 . Addition of 1000 ppm
water to a LiFAP solution increases the HF content imperceptibly over a period of 80
◦
h. Furthermore, it was also shown, especially at elevated temperatures (up to 60 C),
that LiFAP electrolytes offer high cycle stability and low capacity fading [85, 94–96].
17.2.2.6 Lithium Bis(oxalato)borate
Lithium bis(oxalato)borate (LiBOB) shows only moderate solubility up to about
1.0 M in some organic solvents (such as blends of PC and EC). Its conductivity
is about 8–9 mS·cm −1 in appropriate solvents [97] (in DME even 14.9mS·cm −1
at ambient temperature [98]). A major advantage is its thermal stability (up to
◦
300 C [99]) and the passivation film on aluminum, formed by the first cycle. This
passivation film protects the aluminum current collector even at higher potentials
than LiPF 6 does, without breakdown up to 5.75 V [97, 100]. Furthermore, LiBOB
has slightly better cycling stability at ambient temperature, which is considerably
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increased at temperatures up to 70 C [97]. Another advantage is that LiBOB forms