Page 581 - Polymer-based Nanocomposites for Energy and Environmental Applications

534 Polymer-based Nanocomposites for Energy and Environmental Applications

V O is the voltage delivered by the battery in operation. It will decrease with repeating
charge/discharge cycles because of the interactions occurring in the cell during
cycling. To quantify the loss of charge capacity, the coulombic efficiency is defined
by

ð
Q i dischargeÞ
η ¼ (19.11)
C
Q i chargeð Þ
where Q i is the charge for the ith charge or discharge of the cell.
To provide high-operating voltage, the cathode should have a low Li chemical
potential, and the anode should have a high one. Currently, these batteries can deliver
a voltage of 3.8 V with an energy density 250 Wh/kg and a cycle life of 1000.
These characteristic performances are expected to be improved by using polymer
composite materials in different components of the cell.

Electrode materials
The electrodes in a cell play different roles in charge and discharge processes. They
are the limiting factors for the capacity (energy density and cyclability) of a battery. In
the following section, we refer to the discharge scheme, and for clarity, we use the
names of electrodes corresponding to this process.
– Cathode materials (positive electrode)

According to the structure of materials, cathode materials can be classified into two
categories. The first one includes transition metal oxides such as LiCoO 2 , LiNiO 2 , and
LiMn 2 O 4 . These materials have compact layer structures with alternative and ordered
+
layers of anion and Li ions. These electrodes have high-energy density, long life, and
high-specific capacity. The second category includes vanadium or manganese oxides
and transition metal phosphates such as V 2 O 5 ,Mn 2 , LiFePO 4 , and LiMnPO 4 .They
have open structures. Compared with vanadium oxides, the phosphate materials have
higher operating voltages permitting higher energy densities.
At the cathode/electrolyte interface, it is usually observed a layer of interphase
formed by the interaction between the electrode and the electrolyte. This layer is
called solid-electrolyte interphase or SEI. The presence of the SEI has been explained
by a three-step formation process [117]: (i) the formation of a native oxide layer on the
surface of the cathode, (ii) the interaction between the oxide layer and the electrolyte,
and (iii) the rearrangement of the chemical species in the layer after the charging. The
presence of the interphase at the cathode induces a high-internal resistance after long-
term cycling and affects the battery performance and its stability.
The inorganic materials for cathodes are largely used in commercial devices
despite several issues such as high cost, toxicity, low conductivity, and low diffusion
coefficient of ions. In addition, they are not environmentally friendly. It has been dem-
onstrated that conjugated polymers such as PANI, PPy, and PT used as cathode mate-
rials can provide interesting performant batteries. The main advantage of such
polymers over their inorganic counterparts is their electronic semiconductor-like