Page 514 - Handbook of Battery Materials

16.2 SEI Formation, Chemical Composition, and Morphology 487

of lithium metal with salt. The passivation film appears to consist mainly of Li 2 O
and/or LiOH [44]. Chabagno [45] observed that polyethylene oxide (PEO) and LiF
will not form a complex. LiF is therefore insoluble and will remain at the inter-
face between the lithium electrode and the PE. In one publication [46] PEGDME
(molecular weight MW = 400) was chosen as a model system for the investigation
of the process of passivation of lithium in contact with PEs. The authors showed
that SEI formation was apparently complete in just 2–3 min. The increase in the
SEI resistance (R SEI ) over hours and days is apparently due to the relaxation of the
initially formed passivation films or to the continuation of the reaction at a much
slower rate. Results obtained with PEGDME electrolytes containing different salts
−
showed that the formation of LiF as a result of the reduction of anions like AsF 6
−
or CF 3 SO 3 plays a key role in the lithium passivation mechanism [46].
Finely divided ceramic powders, which have a high affinity for water and
other impurities, were initially added in order to improve the mechanical and
electrical properties of LiX–PEO PEs [47–49]. However, today there is considerable
experimental evidence for higher stability of lithium/composite polymer electrolyte
(CPE) interfaces as compared with pure PEs [50–53]. On the basis of standard free
energies of reactions of lithium with ceramic fillers, such as CaO, MgO, A1 2 O 3 ,
and SiO 2 , it was concluded [54] that lithium passivation is unlikely to occur when
lithium is in contact with either CaO or MgO. However, passivation is possible
in the case of A1 2 O 3 and SiO 2 . It was shown that the interfacial stability can
be significantly enhanced by decreasing the ceramic particle size to the scale of
nanometers [54, 55]. The mechanism of the processes leading to improved stability
is not well understood, and some explanations include scavenging effects and
screening of the electrode with the ceramic phase [54].
The morphology of lithium deposits from 1 to 3 mol L −1 LiClO 4 –EC/PC-ethylene
oxide (EO)/PO copolymer electrolytes was investigated [56]. It was found that, as the
weight ratio of host polymer to liquid electrolyte increased, fewer lithium dendrites
were formed, with no dendrites found in electrolytes containing more than 30%
w/w host polymer. The authors emphasized that good contact between the polymer
and lithium is also of great importance for the suppression of dendrites. Direct
in-situ observation of lithium dendritic growth in Li imide P(EO) 20 PE [57] shows
that dendrites grow at a rate close to that of anionic drift.

16.2.3
Reactivity of e − with Electrolyte Components – a Tool for the Selection
sol
of Electrolyte Materials

As mentioned in Section 16.2.1, solvated electrons may take part in the early stage
of SEI formation and during break-and-repair healing processes during lithium
plating and stripping. It is most important that the formation and the healing of the
SEI, especially on graphite, in the first intercalation step be a very fast reaction, and
that the SEI-building materials have extremely low solubility. The best SEI materials
seem to be LiF, Li 2 CO 3 , and Li 2 O. They are insoluble in most of the lithium battery
organic-based electrolytes, and LiF and Li 2 O are thermodynamically stable with