Page 541 - Handbook of Battery Materials

514 16 The Anode/Electrolyte Interface

impedance spectra of the lithium electrode in LiPF 6 electrolyte were analyzed by
Takami and Ohsaki [123]. The two semicircles in the impedance spectra have been
interpreted as resulting from two kinds of passivating film. The formation of the
−
first film is considered to be closely related to decomposition of the PF 6 anion,
which should lead to a thick outer passivation film, probably consisting of LiF
[20]. It was found that the most effective solvent for decreasing the film resistance
is an EC–2Me-THF mixed solvent. This may result from enrichment of the SEI
with Li 2 CO 3 . It was found [20] that the SEI formed in LiPF 6 /γ -BL electrolyte
is much thinner than those formed in LiAsF 6 ,LiClO 4 , and LiBF 4 /γ -BL-based
electrolytes. The SEI thickness was found to be less than a few tens of ˚ angstroms
in LiPF 6 /γ -BL, while for other electrolytes it exceeds 200 ˚ A. Moreover, the film
formed in the LiPF 6 -containing electrolyte was very uniform and sufficiently
compact. The thickness of the lithium surface layer in a lithium perchlorate/PC
solution, calculated from the apparent resistance according to the CSL interface
model, was found to increase exponentially with storage time from 100 to 1000 ˚ A
[27]. The values are in good agreement with those deduced from ellipsometric
measurements [31].
The order of the interfacial resistance of the SEI on lithium covered by native
−1
film in 1 mol L LiX/PC solutions was determined by Aurbach and Zaban [18,
23] from Nyquist plots. For the different salts, the order of R SEI was: LiPF 6
LiBF 4 > LiSO 3 CF 3 LiAsF 6 > LiN(SO 2 CF 3 ) 2 > LiBr, LiClO 4 [18]. The values for
2
LiPF 6 /PC and LiN(SO 2 CF 3 ) 2 /PC were about 800 and 23 cm , respectively. The
resistivity of the film was found to be directly proportional to the salt concentration,
and the presence of CO 2 in solutions considerably reduced the interfacial resistance.
In PC-based electrolytes, inorganic ions like Mg ,Zn ,In , and Ga 3+ form
2+
2+
3+
thin layers of lithium alloys at the electrode surface during cathodic deposition of
lithium, and the resulting thin films suppress the dendritic deposition of lithium
that causes the lowering of the coulombicefficiencies inthe charge–discharge cycles
[34, 38–40]. The Li–Sn electrode shows the greatest increase in interfacial resistance
with immersion time and has a double-layer capacitance, C dl , between 0.03 and
2
0.08 mF [38]. The most stable and lowest interfacial resistance (80–100 cm )was
observed with the Li-3% (w/w) Al alloy electrode. The SEI resistance decreased in the
∼
order: no additive > Lil > SnI 2 > AlI 3 = AlI 3 -2MeF. For systems containing AlI 3 ,
in particular, the film resistance was low (5 ), almost constant, and independent
of the cycle number.
The interfacial phenomena in LiX/PE systems were studied extensively by
Scrosati and co-workers [3, 47, 128]. They found that the high-frequency semicircle
in the impedance spectrum of LiClO 4 /P(EO) 8 electrolyte (EO), which is attributed
to the interfacial resistance, is often irregularly shaped and seems to contain an
additional arc. The authors suggested that this impedance response is based on
more than one relaxation phenomenon. The resistance of the passivation film was
found to increase continuously upon storage, reaching a value 3 orders of magni-
5
tude higher than the initial resistance (10 ). In some cases, film growth leads to
the blocking of lithium ion transport and to the almost complete inactivity of the
polymer cell. The progressive decay of capacitance from 0.65 to 0.5 µF in the initial