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6 Applications of Carbon Nanostructures Produced in Molten Salts
6.2.2 Graphene-Wrapped Si Nanostructures
It has been demonstrated that graphene nanosheets produced by the cathodic exfoliation of graphite in molten LiCl in an atmosphere of Ar + 4% H 2 can wrap silicon
nanoparticles injected into the melt in order to reduce their surface energy. This characteristic of the molten salt-produced graphene nanosheets can lead to the fabrication
of silicon nanoparticles encapsulated in graphene layers. Figure 6.3 shows a process
by which Si nanoparticles encapsulated in graphene nanosheets can be produced.
Figure 6.3b shows photographs of the graphite reactant and the silicon starting
material as well as the remaining part of the consumed graphite extracted from the
molten salt, and the graphene–Si nanocomposite produced by mixing the graphene
product and Si. Surprisingly, the volume of the graphene–silicon nanocomposite was
considerably less than both the graphene and silicon initial materials. This volume
shrinkage is related to the incorporation of Si nanoparticles in the 3D graphene
nanosheets.
Figure 6.4 exhibits TEM micrographs of the Si raw material and the nanocomposite produced by the molten salt method. It is evident that graphene nanosheets can
wrap Si nanoparticles injected into the melt, and could also connect the individual
Si nanoparticles. The formation of this architecture can be explained as follows: Si
and graphene may be oppositely charged in the molten salt. The stir mixing of these
oppositely charged components can lead to the partial wrapping of silicon nanoparticles by highly flexible graphene nanosheets. Further graphene wrapping of Si is
difficult under the repulsive influence of identically charged graphene sheets. Therefore, Si nanoparticles can be partially covered with single or few-layer graphene as
depicted from Fig. 6.4. This “tight” integration of graphene and silicon nanopowders
can describe the volume shrinkage observed. The electrochemical performance of
the electrodes made out of the nanocomposites fabricated in molten salts was also
evaluated in the potential range 0.01–2.5 V using a 2032-type coin cell with lithium
foil as the counter electrode and LiPF 6 in EC/DMC/FEC (3: 5: 2 v/v) as electrolyte.
For a Si–graphene nanocomposite containing 50wt% Si, the first cycle showed the
discharge and charge capacities of 1764 mAh g
−1 and 1355 mAh g
−1 , respectively.
The Coulombic efficiency at first cycle was 76.8% when tested at a constant current density of 0.5 A g
−1 . The capacity loss was likely to be due to the irreversible
reaction of lithium with electrode materials leading to the formation of a solid electrolyte interface (SEI) layer on the electrode surface, and also to the consumption of
lithium ions trapped in nanoporosity of the electrode. Figure 6.4c shows the lithiation–delithiation specific capacity and the corresponding coulombic efficiency of the
electrode containing 50 wt% Si at 0.5 A g
−1 during 260 cycles. The results obtained
show a highly stable performance with an impressive capacity of 981 mAh g
−1 after
260 charge–discharge cycles, which is about 300% greater than that of commercially
used graphite anode material. The coulombic efficiency significantly increased from
76.8% (at the first cycle) to 99.5% during further cycling; reached ~100% by the
10th cycle. It should be noticed that the active material contained only 50 wt% Si.
The performance of the electrode may therefore be tailored by altering the proportional quantity of Si in the nanocomposite. The results are more significant when it
6 Applications of Carbon Nanostructures Produced in Molten Salts
6.2.2 Graphene-Wrapped Si Nanostructures
It has been demonstrated that graphene nanosheets produced by the cathodic exfoliation of graphite in molten LiCl in an atmosphere of Ar + 4% H 2 can wrap silicon
nanoparticles injected into the melt in order to reduce their surface energy. This characteristic of the molten salt-produced graphene nanosheets can lead to the fabrication
of silicon nanoparticles encapsulated in graphene layers. Figure 6.3 shows a process
by which Si nanoparticles encapsulated in graphene nanosheets can be produced.
Figure 6.3b shows photographs of the graphite reactant and the silicon starting
material as well as the remaining part of the consumed graphite extracted from the
molten salt, and the graphene–Si nanocomposite produced by mixing the graphene
product and Si. Surprisingly, the volume of the graphene–silicon nanocomposite was
considerably less than both the graphene and silicon initial materials. This volume
shrinkage is related to the incorporation of Si nanoparticles in the 3D graphene
nanosheets.
Figure 6.4 exhibits TEM micrographs of the Si raw material and the nanocomposite produced by the molten salt method. It is evident that graphene nanosheets can
wrap Si nanoparticles injected into the melt, and could also connect the individual
Si nanoparticles. The formation of this architecture can be explained as follows: Si
and graphene may be oppositely charged in the molten salt. The stir mixing of these
oppositely charged components can lead to the partial wrapping of silicon nanoparticles by highly flexible graphene nanosheets. Further graphene wrapping of Si is
difficult under the repulsive influence of identically charged graphene sheets. Therefore, Si nanoparticles can be partially covered with single or few-layer graphene as
depicted from Fig. 6.4. This “tight” integration of graphene and silicon nanopowders
can describe the volume shrinkage observed. The electrochemical performance of
the electrodes made out of the nanocomposites fabricated in molten salts was also
evaluated in the potential range 0.01–2.5 V using a 2032-type coin cell with lithium
foil as the counter electrode and LiPF 6 in EC/DMC/FEC (3: 5: 2 v/v) as electrolyte.
For a Si–graphene nanocomposite containing 50wt% Si, the first cycle showed the
discharge and charge capacities of 1764 mAh g
−1 and 1355 mAh g
−1 , respectively.
The Coulombic efficiency at first cycle was 76.8% when tested at a constant current density of 0.5 A g
−1 . The capacity loss was likely to be due to the irreversible
reaction of lithium with electrode materials leading to the formation of a solid electrolyte interface (SEI) layer on the electrode surface, and also to the consumption of
lithium ions trapped in nanoporosity of the electrode. Figure 6.4c shows the lithiation–delithiation specific capacity and the corresponding coulombic efficiency of the
electrode containing 50 wt% Si at 0.5 A g
−1 during 260 cycles. The results obtained
show a highly stable performance with an impressive capacity of 981 mAh g
−1 after
260 charge–discharge cycles, which is about 300% greater than that of commercially
used graphite anode material. The coulombic efficiency significantly increased from
76.8% (at the first cycle) to 99.5% during further cycling; reached ~100% by the
10th cycle. It should be noticed that the active material contained only 50 wt% Si.
The performance of the electrode may therefore be tailored by altering the proportional quantity of Si in the nanocomposite. The results are more significant when it