Page 532 - Handbook of Battery Materials

16.3 SEI Formation on Carbonaceous Electrodes 505

Li 2 SO 3 ,Li 2 S 2 O 5 , and Li 2 S). The predominating surface reactions in LiAsF 6 /MF–EC
electrolytes contaminated withwater [85]result inthe formationof insoluble lithium
alkylcarbonates, Li 2 O, and LiOH (Figure 16.1). EDX analysis of the surface film
formed on mesocarbon-derived carbon fibers in LiBF 4 /EC–PC–DME solution
[113] indicated that the film is composed of C, F, B, and O. SEM measurements
showed that the lithiated carbon fibers cohere as a result of the formation of
a passivating film [113]. An unstable passivating layer on petroleum coke in Li
triflate/EC–PC–DMC, followed by interaction between the electrolyte and the
intercalated lithium , was observed by Jean et al. [114]. The increased stability of
lithium-carbon electrodes in EC-containing electrolytes [86] was related to inorganic
films formed via secondary chemical decomposition of electrochemically formed
EC–graphite-intercalation compounds. Using CV, Inaba et al. [89] found that, for
graphite electrodes, an EC–DEC solvent mixture is preferred over EC–DME with
respect to the formation of a stable passivating film. When graphite electrodes
are charged in PC-based solutions, the solvent decomposes at about 1 V, and
this makes SEI formation difficult. It was shown [76] that LiBF 4 is more reactive
than LiPF 6 toward an Li x C 6 anode. A lithium-ion battery based on LiPF 6 /EC-DEC
(7 : 3) electrolyte [115] underwent more than 300 stable charge–discharge cycles.
2
However an increase of cell resistance from 1.5 to 3.5k cm was observed on
cycling, and this was attributed to the decomposition of electrolyte.
Lithium intercalation into graphite was studied by Morita et al. [116], who used
XRD and electrochemical QCMB techniques. The XRD pattern of the graphite elec-
trode after cathodic polarization in LiClO 4 /EC–DMC solution shows a spectrum
that is more complicated than that for an electrode polarized in EC–PC mixture.
The diffraction angles observed do not correspond exactly to the values expected
from any idealized stage structure of Li x C 6 . Changes in the resonance frequency of
the graphite-coated quartz crystal showed that the cathodic intercalation of lithium
is accompanied by electrochemical decomposition of the electrolyte The mass
+
change per coulomb over the potential range of 0.0–0.2 V vs Li/Li was higher in
EC–DMC than in EC–PC, indicating different surface reactions.
Lithium carbonate and hydrocarbon were identified in XPS spectra of graphite
electrodes after the first cycle in LiPF 6 /EC–DMC electrolyte [109]. Electrochemical
QCMB experiments in LiAsF 6 /EC–DEC solution [102] clearly indicated the for-
mation of a surface film at about 1.5 V vs (Li/Li ). However, the values of mass
+
accumulation per mole of electrons transferred (m.p.e), calculated for the sur-
face species, were smaller than those of the expected surface compounds (mainly
(CH 2 OCO 2 Li) 2 ). This was attributed to the low stability of the SEI and its partial
dissolution.

16.3.5.2 HOPG
HOPG was used as a model electrode to study separately the formation of the
SEI on the basal and cross-section planes [103]. The cross-section planes of HOPG
consist of both zig-zag and armchair planes. Carbon atoms on these two planes
are considered to be much more active than carbon atoms on the basal plane