Page 510 - Handbook of Battery Materials

16.2 SEI Formation, Chemical Composition, and Morphology 483

16.2.2
Chemical Composition and Morphology of the SEI
16.2.2.1 Ether-Based Liquid Electrolytes

16.2.2.1.1 Fresh Lithium Surface Little work has been done on bare lithium
metal that is well defined and free of surface film [13–22]. Odziemkowski and
Irish [13] showed that for carefully purified LiAsF 6 tetrahydrofuran (THF) and
2-methyltetrahydrofuran (2Me-THF) electrolytes the exchange-current density and
corrosion potential on the lithium surface immediately after cutting in situ are
primarily determined by two reactions: anodic dissolution of lithium and cathodic
−
reduction of the AsF anion by bare lithium metal. The SEI was formed in less
6
−
−
than 1 ms. As pointed out by Holding et al. [15], the reduction of AsF ,PF ,BF ,
−
6 6 6
−
and C1O , is thermodynamically favored. All of the electrolytes investigated [13,
4
14] were unstable with respect to bare lithium metal. The most unstable system was
lithium hexafluoroarsenate in THF and 2Me-THF [14]. Odziemkowski and Irish [13]
concluded that in THF, 2Me-THF, and propylene carbonate (PC), electrochemical
reduction of the anion AsF overshadows any possible solvent-reduction reaction.
−
6
This was also pointed out by Campbell et al. [16], who observed that films on
lithium surfaces are formed in most cases by electrochemical reduction of the
electrolyte anions. One of the main products detected in LiAsF 6 /THF electrolyte
by in-situ and ex-situ Raman microscopy [13] was a polytetrahydrofuran (PTHF).
The polymerization reaction is determined by: (i) the ratio of the concentration
of nucleophilic impurities like H 2 O, O 2 , and so on, to the concentration of the
pentavalent Lewis acid AsF 5 , and (ii) the ratio of the volume of the electrolyte to
the active surface area of the lithium electrode. In contrast to THF, 2Me-THF does
not polymerize. However, the brown film formed on lithium was observed in both
electrolytes. It was found that the film decomposes to arsenious oxide when exposed
to air, moisture, and heat, and may not be polymeric [17]. The surface film formed
in imide-based electrolytes was found to be a better electronic conductor and more
permeable to an oxidizer. Thus it is suggested that the SEI in Li imide electrolytes
is less compact than that in LiClO 4 ,LiBF 4 , and LiPF 6 electrolytes regardless of the
type and purity of solvent. It also was noted that the SEI in LiBF 4 /THF electrolyte
was the most water-sensitive [13].
According to the depth profile of lithium passivated in LiASF 6 /dimethoxyethane
(DME), the SEI has a bilayer structure containing lithium methoxide, LiOH, Li 2 O,
and LiF [19]. The oxide–hydroxide layer is close to the lithium surface, and there
are solvent-reduction species in the outer part of the film. The thickness of the
surface film formed on lithium freshly immersed in LiAsF 6 /DME solutions is of
the order of 100 ˚ A.
Kanamura et al. [20] thoroughly studied the chemical composition of surface
films of lithium deposited on a nickel substrate in γ -butyrolactone (γ -BL) and THF
electrolytes containing various salts, such as LiClO 4 , LiASF 6 ,LiBF 4 , and LiPF 6 .
They found, with the use of XPS, that the outer and inner layers of the surface film
covering lithium in LiClO 4 /γ -BL involve LiOH or possibly some Li 2 CO 3 and Li 2 O