Page 281 - Polymer-based Nanocomposites for Energy and Environmental Applications

Polymer nanocomposite materials in energy storage: Properties and applications 253

The graphite electrodes are easy to manufacture and have a flat and low working
potential vs lithium; low cost and good stability are additional advantages that make
them popular as anode [154]. But it suffers from several disadvantages. Firstly, the
theoretical reversible energy density is low. In the first cycle, the nonaqueous electro-
lyte decomposes at the interface of the electrolyte and the anode, forming a protective
layer called solid-electrolyte interphase (SEI), which stops further decomposition of
the electrolyte [155]. Although good in one sense, the SEI is also responsible for
inducing irreversibility in intercalation/deintercalation process that leads to the grad-
ual loss of energy density during cyclic charging and discharging process [42].
In addition, the diffusion rate of lithium ion into carbon materials is low, which
results in batteries with low power density [153,154]. Moreover, graphite may pose
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safety issues as the lithiation potential (0.2 vs Li/Li ) is close to the lithium stripping
voltage [153]. Nanomaterials have been suggested as an efficient way to improve the
performance of the anode materials. The nanomaterials can make the charging and
discharging rates faster by offering shorter path for diffusion and increasing the dif-
fusion rate, and miniaturization leads to more surface area per unit volume for contact
with the electrolyte [88,153,154]. For this reason, a wide variety of nanostructured
carbon materials, such as CNTs [156], carbon nanofibers (CNF) [157], ordered meso-
porous carbon [158], graphene oxide in the oxidized and reduced forms [158], and
CNT-graphene hybrid foam [159], have been explored as prospective anode materials
for Li-ion battery.
In concurrent research efforts, different types of inorganic materials have been
explored as the possible replacement of the carbon materials. Silicon and tin are excit-
ing materials because they show capacity for Li ions four times higher than the graph-
ite to form Li 4.4 Si and Li 4.4 Sn, respectively, which allows them a theoretical energy
density ten times higher than the graphite [160]. These materials show very high vol-
ume expansion (>400%) during the intercalation/deintercalation process leading to
stress generation that can lead to material failure, and also, formation of strong SEI
at lower potentials also leads to poor cyclic stability [160]. Spinel structured Li 4 Ti 5 O 12
is another material that has received greater attention because it shows zero volume
expansion during the charging/discharging process and low cost, avoids the formation
of SEI, and shows good energy density and cyclic stability [161]. Apart from these
materials, transition metal oxides such as tin oxide, TiO 2 , and Mn 3 O 4 also have been
explored in the literature [153,154].
As mentioned earlier, the major drawback of noncarbonaceous anode materials is
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the low ionic conductivity (<10–12 S cm ), which limits the capacity of the Li bat-
teries. In the literature, multiples ways have been suggested to overcome this issue
such as tailoring the morphology, making alloy with other atom, mixing with carbon,
and making carbon coating [153]. A prominent approach to upgrade the electronic or
ionic conductivity is via formation of very thin film of conducting polymers with
thickness in few nanometers on the surface of the anode material [21,120,150].
The conducting polymer film offers the following advantages: (1) it improves the
ionic or electronic conductivity by providing a conducting path for the electrons or
ions to travel avoiding interfacial resistance between the nanoparticles and (2) a thin
layer of coating separates the electrolyte from the anode material and strengthens the