Page 531 - Handbook of Battery Materials

504 16 The Anode/Electrolyte Interface

The anodic behavior of carbon materials, such as acetylene black, activated car-
bon, and vapor-grown carbon fiber (VGCF), in LiClO 4 /PC solution was studied by
Yamamoto et al. [107]. Irreversible reactions, including gas evolution and disinte-
gration, were mainly observed on that part of the surface occupied by the edge
planes of the graphite. XRD measurements indicated that these reactions were the
decomposition of the electrolyte, leading to the formation of Li 2 CO 3 . The surface
reactions on the carbon-fiber electrodes in LiX/PC–DME solution have been in-
vestigated by XRD, XPS, and differential scanning calorimetry–thermogravimetric
analysis (DSC–TGA) methods [108]. According to this work, the reaction during
the first charge, which includes solvent co-intercalation, can proceed by more
than two mechanisms, with different kinetics. The mechanism was found to de-
pend on the electrolyte composition and discharge-current density. Aurbach and
co-workers [81, 82] carried out an extensive electrochemical and spectroscopic study
of carbon electrodes in lithium-battery systems. The carbons investigated included
carbon black, graphite, and carbon fibers. The solvents MF, PC, EC, THF, DME,
1,3-dioxolane, and their mixtures were used. The salts tested were LiClO 4 , LiAsF 6 ,
and LiBF 4 . It was found that the first charging of carbon with lithium is accompa-
nied by irreversible solvent and salt reduction, and this is followed by coating of
the carbon surface with passivating films. These films are similar in their chemical
structure to those formed on lithium in the same solutions. Thus, PC is reduced on
carbon to ROCO 2 Li, ethers are reduced to alkoxides, and MF to lithium formate.
LiAsF 6 is reduced to LiF and AsF 3 , and further to insoluble Li x AsF y (Figure 16.1).
IR spectra of graphite-EPDM electrodes cycled in LiClO 4 –MF solution seem to
prove the existence of LiClO 3 , LiClO 2 , or LiClO. CO 2 reacts with Li x C 6 to form
Li 2 CO 2 (and probably CO). Because of the high surface area of graphite particles
as compared with the lithium-metal electrode, the role of contaminants, such as
HF in LiPF 6 -and LiBF 4 -based electrolytes, is much less pronounced [109]. Disor-
dered or graphitized carbons with turbostratic structure were shown to be less
sensitive to the solution composition. Aurbach and co-workers [81, 82] emphasized
that the most important aspect of the optimization of lithium-ion batteries is the
modification of the surface chemistry of carbon by the proper electrolyte additives
(e.g., CO 2 , crown ethers) which form better passivating layers and/or prevent
solvent intercalation. The beneficial effect of inorganic additives, such as CO 2 ,
2−
N 2 O, S x , and so on, on the formation of SEI on carbons was also emphasized
by Besenhard et al. [110]. Tibbets et al. [111] showed that oxidative pretreatment
of VGCFs can reduce the capacity of SEI building in LiClO 4 /PC electrolyte by an
order of magnitude. Their experiments confirm the idea that air etching removes
the more active carbon atoms – those capable of decomposing the electrolyte – and
completely alters the fiber morphology.
Ein-Eli et al. [112] showed that the use of SO 2 as an additive to LiAsF 6 /MF or
LiAsF 6 /PC–DEC–DMC solutions offers the advantage of forming fully developed
passive films on graphite at a potential much higher (2.7 V vs Li/Li ) than that
+
+
of electrolyte reduction (<2V vs Li/Li ) or of lithium intercalation (0.3−0V
vs Li/Li ). They claimed that the major surface species are organic lithium
+
alkylcarbonates (ROCO 2 Li) and inorganic lithium salts (Li x AsF y ,Li 2 CO 3 ,Li 2 SO 2 O 4 ,