Page 587 - Handbook of Battery Materials

17.4 Bulk Properties 561

The electrochemical stability range covers potentials from the anodic limit E Ox to
the cathodic limit E Red . Electrochemical investigations on solutes can be performed
in this range, also called the voltage window in analogy to spectroscopy. Generally the
electrochemical stability range is established by CV or linear sweep voltammetry.
−1
It depends on experimental conditions, the scan rate v(mV·s ), the arbitrarily
−2
chosen current density i 0 (mA·cm ) for the onset of the electrochemical process,
and the working electrode (WE). Unfortunately no generally accepted conditions
−1
for CV experiments exist; scan rates range from <1 to 100 mV·s , and onset
current densities vary from 0.01 to 3 mA·cm −2 (see Ref. [52] and the literature cited
therein). CV measurements are performed with a three-electrode configuration.
Therefore a WE is needed, most often a metal like Pt, Au, Ag, Al, or glassy carbon
(GC) or active materials, where the electrochemical process is investigated. A REF,
positioned close to WE, defines the potential and makes it indispensable for the
measurements. The counter electrode (CE), for example, a Pt sheet or lithium
metal, closes the electric circuit.
However, in most cases merely a pseudo-reference with lithium metal is used.
Other options are alloy electrodes with Sn or Au [239, 240] or the nonaqueous
+
Ag/Ag -cryptand electrode by Izutsu et al. [241, 242]. This group recently investi-
gated [243] such REFs, which are very interesting, especially for miniaturization.
Electrochemical deposition of Li from a 0.470 mol · L −1 solution LiBOB in PC
onto an Sn-wire (0.5mmØ,20 h at +680 mV vs Li/Li )produces Li/Li 2 Sn 5 REFs.
+
Investigations show that the potential in 0.470 mol·L −1 solution LiBOB in PC
5
was stable at (0.741 ± 0.0005 V) for >2.5 × 10 s before corrosion shifted the
potential.
To compare redox potentials of aqueous and nonaqueous systems, a variety
of internal references were investigated. In 1984, Gritzner and Kuta recom-
mended two systems for nonaqueous electrolytes that are also accepted by
IUPAC [244]. The solvent-independent organometallic redox couples are fer-
0
rocene/ferrocenium (Fc/Fc )(E = 0.158 V vs saturated calomel electrode (SCE))
+
0
and bis(biphenyl)chromium(0)/(1) (BCr/BCr )(E =−0.82 V vs SCE) [245]. Very
+
stable electrode redox potentials E 1/2 vs Ag/Ag -cryptand electrode of 0.478
+
Vfor Fc/Fc + and −0.616 V for BCr/BCr + in EMIm tetrafluoroborate were
measured [246].
Furthermore, many investigations of nonaqueous electrolytes have even been
performed with a saturated calomel electrode despite obvious problems such
as contamination by water. In addition, unknown liquid junction potentials
and insufficient knowledge of electrode reactions must be taken into account
in addition to differing experimental conditions for the interpretation of
such data.
However, even if electrolytes have sufficiently large voltage windows, their
components may be not stable (at least kinetically) with lithium metal; for example,
acetonitrile shows very large voltage windows with various salts, but it polymerizes
at deposited lithium if this reaction is not suppressed by additives such as
SO 2 , which forms a protective ionically conductive layer on the lithium surface.
Comparison of stability limits of low-temperature molten salts without added