Page 483 - Handbook of Battery Materials

456 15 Lithiated Carbons

600 100
90
Discharge capacity / Ah·kg -1 400 Capacity 60 Efficiency / %
500
Efficiency
80
70
300
50
40
200
30
20
100
10
0 0
500 1000 1500 2000 2500 3000
Heat treatment temperature / °C
Figure 15.13 First cycle discharge capacities ( )and
charge/discharge efficiencies (---) of a soft (graphitizing)
carbon (Melblon  carbon fibers (MPCF)) at different
heat-treatment temperatures. Reproduced with kind permis-
sion of Petoca, Ltd. (Takamura, T. Tamaki, T. Petoca Ltd.,
personal communication).

◦
Around or above ∼1000 C the graphitizing (soft) carbons develop a structure
with ‘wrinkled’ and ‘buckled’ structure segments (see Figure 15.4). This structure
offers fewer sites for lithium intercalation than graphite [7, 19, 24, 42]. In addition,
crosslinking of carbon sheets in disordered carbons hampers the shift to AA
stacking, which is necessary for the accommodation of a greater amount of lithium
into graphitic sites [253–255]. Correspondingly, rather low specific charges are
observed (x in Li x C 6 is typically between ∼0.5 and ∼0.7) in soft carbons such as
turbostratic carbons [19, 42, 47, 115, 253–255] and more disordered carbons like
cokes [19, 42, 47, 256–266] and certain carbon blacks [260, 267–269]. On the other
hand the charge/discharge efficiency increases with the temperature during heat
treatment (Figure 15.13). A type of soft carbon has been used in the first generation
of Sony’s lithium-ion cell [235].
Figure 15.14 shows the first Li intercalation/de-intercalation cycle of a coke
+
electrode. The potential profile differs from that of graphite, in the sense that the
+
+
reversible intercalation of Li begins at a potential above 1 V vs Li/Li ,and the
curve slopes without distinguishable plateaus. This behavior is a consequence of
the disordered structure providing electronically and geometrically nonequivalent
sites, whereas for a particular intercalation stage in highly crystalline graphite the
sites are basically equivalent [19, 26]. With increasing temperature soft carbons
develop more graphitic structure segments. The sites for lithium storage which were
formerly determined by the disordered structure (see above) change to graphitic
sites, where lithium resides in the van der Waals gaps between ordered graphene
◦
layers. Finally, at ∼3000 C, the structure and the specific charge (Figure 15.13) of
graphite are achieved. The probability of finding disordered and ordered (graphitic)