Page 270 - Polymer-based Nanocomposites for Energy and Environmental Applications

242 Polymer-based Nanocomposites for Energy and Environmental Applications

Popular anode materials are carbon, silicon, and metal oxides, whereas the cathode
materials are Li salts such as LiCoO 2 , LiFePO 4 , and LiMn 2 O 4 . The standard electro-
lyte is a liquid state and lithium salt such as LiClO 4 , LiPF 6 , LiAsF 6 , LiCF 3 SO 3 , and
LiBF 4 in nonaqueous organic solvents such as ethylene carbonate or polyethylene
carbonate or dimethyl carbonate [37]. The working principle of the lithium ion is
shown in Fig. 9.1. In the charging process, the electrodes are connected to an external
electric supply. The electrons are forced to be transferred from the cathode to the
anode through the external circuit. An electrochemical reaction takes place where
+
the Li ions are also generated and transferred internally to the anode by transfer
through the electrolyte. The reversible reaction goes on until the anode is fully inter-
+
calated with the Li ons (LiC 6 ). During the discharging process, electrons are spon-
+
taneously released from the anode to the cathode, and the Li ions are also move
internally from the anode to the cathode via electrolyte. This process goes on until
all the LiC 6 are fully converted in the carbon. The charging process takes around
6 h in laptop or mobile phones for batteries with carob as anode material.

9.3.1.1 PNCs as electrolyte for the Li ion batteries
Electrolyte is an integral part of the Li-ion batteries. The electrolyte is sandwiched
between the electrodes, cathode, and electrode. The electrolyte is in contact with elec-
trodes and the separator, so it electronically interacts with all the components in the
+
battery. The electrolyte facilitates the transfer of the Li ions from cathode to anode
during charging and from the anode to cathode in the discharging process. The overall
performance such as the energy density, working potential, working temperature, sta-
bility, and cyclic life of the batteries depends on the electrolyte [38]. This has to have
low electronic conductivity so that the electrodes are not short circuit. In addition to
the electrolyte, the electrodes are separated by a separator. That means the electrolyte
has high ionic conductivity and low electronic conductivity. An ideal electrolyte for
the Li-ion batteries should have (1) high ionic conductivity (>10 4 Scm 1 at room
+
temperature) and electronic insulator, (2) large Li ion transference number (close
to unity) by reducing the transportation of anions, (3) high mechanical strength, (4)
electrochemical stability in large voltage range, (5) very good thermal and chemical
stability in the batteries’ environment, and (6) the electrolyte should be environmen-
tally benign [39].
The electrolyte is in contact with the electrodes so the electrolyte should show high
toward chemical corrosion, and therefore, only limited number of materials can be
used as electrolyte. Currently, liquid electrolytes are the most widely used electrolytes
+
for the commercial Li batteries. Lithium salts are dissolved in high dielectric constant
+
organic solvents to achieve high concentration of Li in the electrolyte. The solvent
should have high dielectric constant to not allow the electron transfer through it; it
should have low viscosity and to assist the high ionic conductivity. The liquid elec-
trolyte is mostly lithium salt such as LiClO 4 , LiPF 6 , LiAsF 6 , LiCF 3 SO 3 , and LiBF 4
dissolved in organic solvents (ethylene carbonate, polypropylene carbonate, diethyl
carbonate, dimethyl carbonate, diethoxyethane, dioxolane, and tetrahydrofuran). Sim-
ple ions like Cl, F ,or Br are not chosen because of their inherent characteristics of