Page 513 - Handbook of Battery Materials

486 16 The Anode/Electrolyte Interface

[29] investigated the formation of SEI in mixtures of carbonate- and ether-based
solvents, such as PC/THF, PC/2Me-THF, DME/PC, EC/THF, EC/dioxolane, and
EC/PC, containing LiClO 4 or LiAsF 6 . They found that lithium electrodes treated in
LiAsF 6 /PC/THF solutions are covered by a surface film containing carbon, oxygen,
and fluorine. Both PC and THF contribute to the buildup of surface films, but the
F and As peaks in pure THF were much higher. This indicates that the addition
of reactive PC to the ether decreases salt reduction by competing with it, and the
film becomes more organic in nature, containing less LiF. In the case of EC/PC or
EC/ether mixtures, the reduction of EC by lithium seems to be the dominant pro-
cess, followed by the formation of lithium alkylcarbonates (derivatives of ethylene
glycol) [29]. It was suggested [32] that in mixed organic solvent systems, the solvent
+
having higher donicity tends to coordinate preferentially with Li ion and conse-
quently to react at the Li-electrode/electrolyte interface. Matsuda and co-workers
[33, 34] showed that some cyclic compounds containing heteroatoms and conju-
gated double bonds, such as 2-methylthiophene (2MeTp), 2-methylfuran (2MeF),
and aromatic compounds like benzene are very effective in electrolyte solutions
for rechargeable lithium batteries. On the basis of AC-impedance measurements it
was estimated that the reaction between lithium and 2MeTp or 2MeF would result
in a thick SEI of uniform composition [34]. Tobishima et al. [35] showed that the
addition of 2Me-THF improves the cycle life of the Li/EC-PC/V 2 O 5 –P 2 O 5 cell. The
interaction between the cathode and electrolyte leads to the formation of a film
containing vanadium on the lithium anode surface. Mori et al.[36],using aQCMB,
demonstrated the smooth surface morphology and almost constant thickness of the
lithium film in EC/dimethyl carbonate (DMC) solutions in the presence of surfac-
tants like polyethylene glycol dimethyl ether (PEGDME), and a mixture of dimethyl
siloxane and propylene oxide (PO). The morphological properties of the surface
layers formed on the lithium electrode (covered by native film) in sulfolane-based
electrolytes (SFLs) have been investigated [37]. It was found that the surface layers
are essentially homogeneous and consist mainly of waxy degradation material in
which some long white microcrystals are present.
Matsuda and co-workers [38–40] proposed the addition of some inorganic ions,
2+
2+
3+
3+
such as Mg ,Zn ,In ,Ga ,A1 , and Sn , to PC-based electrolyte in order to
2+
3+
improve cycle life. They observed the formation of thin layers of Li/M alloys on the
electrode surface during the cathodic deposition of lithium on charge–discharge
cycling. The resulting films suppress the dendritic deposition of lithium [39, 40].
The Li/Al layer exhibited low and stable resistance in the electrolyte, but the
resistance of the Li/Sn layer was relatively high and unstable.
16.2.2.3 Polymer (PE), Composite Polymer (CPE), and Gelled Electrolytes
It seems clear that in PEs, especially in the gel types, lithium-passivation phenom-
ena are similar to those commonly occurring in liquid electrolytes. The crucial role
played by the nature and composition of the PE in controlling electron transfer
has been described by several authors [3–6, 41–43]. It is postulated that at least
two separate competitive reactions occur simultaneously to form the passive layer.
The first is the reaction of lithium with contaminants. The second reaction is that