Page 471 - Handbook of Battery Materials

444 15 Lithiated Carbons

The splitting of the second stage into two, s = II (x = 0.5 in Li x C 6 )and s = II L
(x = 0.33 in Li x C 6 ), is due to different lithium packing densities. It disappears at
◦
◦
temperatures below ∼10 C [98]. At temperatures above 700 CLi x C 6 (0.5 ≤ x ≤ 1)
is transformed into lithium carbide Li 2 C 2 and carbon [94, 110].
Commercial graphites can contain a considerable proportion of rhombohedral
structure units. It has been reported that lithium intercalation mechanisms and
storage capabilities are similar for both rhombohedral and hexagonal graphite
structures [58, 60]. However, the preparation of graphite with a higher proportion of
rhombohedrally structured graphene planes and its use as anode material is claimed
in a patent [111]. Lithiated graphites can also be prepared from a KC 8 precursor,
either (i) chemically by ion exchange reactions [112] or (ii) electrochemically after
de-intercalation of potassium [113, 114].

15.2.2.3 Reversible and Irreversible Specific Charge
Experimental constant current charge–discharge curves for Li + intercalation/
de-intercalation into/out of graphite clearly prove the staging phenomenon
(Figure 15.7). Nevertheless, there are no sharp discontinuities between the
two-phase regions because (i) the packing density of Li x C 6 varies slightly
(a phase width exists), and (ii) various types of overpotentials cause plateau-sloping
in galvanostatic measurements (and peak-broadening in voltammetric measure-
ments). Theoretically, Li + intercalation into carbons is fully reversible. In the
practical charge–discharge curve, however, the charge consumed in the first
cycle significantly exceeds the theoretical specific charge of 372 Ah kg −1 for LiC 6
(Figure 15.7). The subsequent de-intercalation of Li recovers only ∼80–95% of
+
this charge. In the second and subsequent cycles, then, the charge consumption
+
for the Li intercalation half-cycle is lower and the charge recovery is close to 100%.
The excess charge consumed in the first cycle is generally ascribed to SEI
formation and corrosion-like reactions of Li x C 6 [19, 65, 115–117]. Like metallic
lithium and Li-rich Li alloys, lithiated graphites, and more generally lithiated
carbons, are thermodynamically unstable in all known electrolytes, and therefore
the surfaces which are exposed to the electrolyte have to be kinetically protected by
SEI films (see Part III, Chapter 17). Nevertheless, there are significant differences
in the film formation processes between metallic lithium and lithiated carbons.
Simplified, these differences are as follows: Film formation on metallic Li takes
place upon contact with the electrolyte. Various electrolyte components decompose
spontaneously with low selectivity and some of the decomposition products form
the film. When the film grows, the activity of the metallic lithium electrode versus
the electrolyte decreases because of an increasing I R drop in the film. At this stage
the electrolyte reduction processes become more and more selective as the number
of electrolyte components which are still sensitive to reduction versus the (now
partially electronically ‘passivated’) lithium electrode is limited. In contrast, film
formation on carbonaceous hosts takes place as a charge-consuming side reaction
in the first few Li intercalation/deintercalation cycles, especially during the first
+
reduction of the carbon host material. In this case, the electrolyte components which