3.6 Effect of Molten LiCl on the Microstructure of Graphite
31
Fig. 3.8 Microstructure of the graphite–LiCl mixture heated at the rate of 80 °C min −1 to 1250 °C.
a The dominant microstructure consisted of bent layered grains. b Exfoliated nanosheets which
could be found in the microstructure. c TEM image of a graphene sheet. The inset is the selected
area electron diffraction pattern taken from edge of the sheet showing the hexagonal structure of
the (0001) basal plane, reproduced from Ref. [14], copyright 2019, with permission from Elsevier
The second microstructure feature to be considered in the heat-treated powders
can be characterized by corroded graphitic sheets, which are seen in the main panel
of Fig. 3.9a. The upper inset in Fig. 3.9a displays the corrosion pits with more details,
in which a typical polyhedron pit is defined by the arrow. Clearly, in this case, the
pits are deeper and the pit density is larger relative with those seen on the partially
oxidized graphite heated at a similar heating rate (see Fig. 3.8c). Figure 3.9b presents
a TEM image from a typical particle which could be observed in the heat-treated
powders. The inset in Fig. 3.9b is the selected area diffraction pattern recorded on
this particle. The pattern can be indexed to Li 2 C 2 O 4 phase. The formation of these
polyhedral pits is probably due to the reaction between the carbon from the graphite
cathode with lithium oxides dissolved in LiCl to form polyhedral-shaped crystals
containing lithium, carbon and oxygen. The subsequent physical disintegration of
these crystals from the carbon substrate would leave behind holes.
The thermodynamic data for Li 2 C 2 O 4 are not available in the literature, and consequently, the values of the free energy of reactions involving the formation of Li 2 C 2 O 4
cannot be easily calculated. Despite that, one possible reaction might be assumed as
follows:
31
Fig. 3.8 Microstructure of the graphite–LiCl mixture heated at the rate of 80 °C min −1 to 1250 °C.
a The dominant microstructure consisted of bent layered grains. b Exfoliated nanosheets which
could be found in the microstructure. c TEM image of a graphene sheet. The inset is the selected
area electron diffraction pattern taken from edge of the sheet showing the hexagonal structure of
the (0001) basal plane, reproduced from Ref. [14], copyright 2019, with permission from Elsevier
The second microstructure feature to be considered in the heat-treated powders
can be characterized by corroded graphitic sheets, which are seen in the main panel
of Fig. 3.9a. The upper inset in Fig. 3.9a displays the corrosion pits with more details,
in which a typical polyhedron pit is defined by the arrow. Clearly, in this case, the
pits are deeper and the pit density is larger relative with those seen on the partially
oxidized graphite heated at a similar heating rate (see Fig. 3.8c). Figure 3.9b presents
a TEM image from a typical particle which could be observed in the heat-treated
powders. The inset in Fig. 3.9b is the selected area diffraction pattern recorded on
this particle. The pattern can be indexed to Li 2 C 2 O 4 phase. The formation of these
polyhedral pits is probably due to the reaction between the carbon from the graphite
cathode with lithium oxides dissolved in LiCl to form polyhedral-shaped crystals
containing lithium, carbon and oxygen. The subsequent physical disintegration of
these crystals from the carbon substrate would leave behind holes.
The thermodynamic data for Li 2 C 2 O 4 are not available in the literature, and consequently, the values of the free energy of reactions involving the formation of Li 2 C 2 O 4
cannot be easily calculated. Despite that, one possible reaction might be assumed as
follows: