Page 533 - Handbook of Battery Materials

506 16 The Anode/Electrolyte Interface

Table 16.4 XPS measurements of HOPG after one cycle in
1.2 mol L –1 LiAsF 6 /EC–DEC electrolyte.

Element Cross-section (%) Basal (%)

C 40.27 63.51
O 23.40 17.16
F 14.46 4.71
Li 19.52 12.10
As 2.35 0.87

[64, 66, 72, 73]. The SEI compositions on these planes are therefore expected to be
different.
Table 16.4 presents the elemental composition of SEI on both basal and
−1
cross-section planes as measured by XPS its after formation in 1.2 mol L
LiAsF 6 /EC : DEC (1 : 2). For light atoms, the XPS detection depth is about 5–10
atomic layers, so Table 16.4 reflects the composition of the SEI which was exposed
to the electrolyte. It is clear that the SEI on the cross section is rich in inorganic
components (Li, F, O, As), while that on the basal plane is rich in carbon com-
pounds. Detailed analysis of the XPS spectra for C, O, As, F, and Li at different
sputtering times, together with atomic accounting, leads [103] to the formation of
an estimated depth profile for the SEI materials (Figure 16.11). As the vacuum in
the XPS instrument is about 10 −10 Torr, it was concluded that the hydrocarbons
found in the SEI are actually polymers (polyolefins). We believe that the hydro-
carbons reported by Takehara to be present in the SEI lithium [24–26] are also
polymers (for the same reason). This conclusion is in agreement with a number
of authors [6, 13, 28, 72, 92] who reported on the formation of polymers on the
surface of lithium or lithium amalgam. It was concluded [103] that at the basal
plane, reduction of the solvents is more pronounced, and this causes the surface of
the SEI to contain about 60% polymers, while at the cross-section the SEI is formed
by the reduction products of the salt anion (AsF 6 − ). The thickness of the SEI on
the cross-section planes is larger than that on the basal plane. This is in agreement
with Besenhard’s [86] conclusion that the film penetrates into the graphite galleries
as a result of co-intercalation of solvent molecules. Additional support for this is
our finding of much carbonate material on the cross-section plane (after 8 min of
sputtering), but none on the basal plane (Figure 16.11). The bulk of the SEI on
both planes consists mainly of LiF (60–80%) and carbonates (more carbonates on
the cross-section planes) (Figure 16.11) [103]. The SEI composition is in agreement
with thermodynamics: at the carbon/SEI interphase we found fully reduced anions
(Figure 16.11) [103], such as Li 2 O, LiF, and As, whereas on the solution side we
found partially reduced materials: semicarbonates, polymers, AsO x , and AsF 3 (as
well as LiF). The cross-section SEI contains very few Li–O–C groups, but on the
basal plane they constitute up to 40% of the surface. The cross-section SEI has a
very strong As XPS peak at 48 eV which is absent from the basal plane (it may be