CHAPTER- output of a battery is dependent on the

CHAPTER-
I

INTRODUCTION

We Will Write a Custom Essay Specifically
For You For Only $13.90/page!


order now

1.1.
Introduction to Li- ion batteries:

Energy
demand and supply has always been one of the crucial factors for the evolution
of civilization. Energy in the form of electricity is produced from solar,
wind, nuclear power, burning fossil fuels, etc; however, production of
electricity from renewable sources like, solar and wind need a storage device
for their effective usage during depletion time. In this context, electrochemical
energy storage devices such as batteries play an important role in the
efficient use of renewable energy. Battery is a collective arrangement of
electrical cells that stores and produces electricity by chemical reaction;
storage and release is realized by electrons and ions.1
The
first battery, Voltaic pile was developed by Volta in the year 1800. It
consists of a series of copper and zinc discs separated by card boards
moistened with salt solution. With more than 200 years of development, battery
technology has achieved an era that batteries are safely used for transport of
electricity without heat loss. They can be made in all varieties of sizes and
shapes and useful for various applications.

Batteries
are mainly of two types, viz.

1.
Primary Batteries.  

2.
Secondary batteries.

Primary
batteries are assembled in charged condition and the electrochemical reaction
is mostly irreversible. Examples: Lechlanche, alkaline MnO2, silver oxide,
and zinc/air batteries.2Electrochemical
reaction associated with secondary batteries is reversible. Hence the battery
can be charged/ discharged for a number of cycles and are named as rechargeable
batteries. Examples: lead-acid, nickel-cadmium, nickel-metal hydride and
lithium ion batteries. According to the chemical reaction involved,
rechargeable batteries can further be classified as lead-acid, nickel-metal-hydride,
zinc-air, sodium-sulphur, nickel-cadmium, lithium ion, Li-air batteries etc.

Among
the various rechargeable batteries aforementioned, Rechargeable Li-ion
batteries have gained considerable interest in recent years in terms of highest
specific energy, cell voltage, good capacity retention and negligibly small
self discharge.3

It
is desirable that the energy delivered by a battery during its discharge should
be as high as possible. The energy output of a battery is dependent on the
equivalent weight of active material present in it. Specific capacity i.e.
capacity per gram of active material= 26.8/Equivalent weight Ah.g-1.

Lithium
metal being the third lightest element, lithium based materials with low
molecular weight can effectively produce batteries with high capacity. Further,
lithium ion batteries employs non aqueous electrolyte that offer high operating
voltage (>4V) in comparison to other batteries with aqueous electrolyte
(1-2V). Thus, low weight, compact lithium ion batteries established a strong
market place for portable electronic devices and could find central application
if lithium ion batteries in electric vehicles become reality.4-5

 

1.2.
Basics of Lithium ion batteries

 

Lithium
ion battery consists of three main components, positive and negative electrode
separated by a separator dipped in electrolyte. Negative electrode is normally
an electron donor group which is electropositive in nature like lithium metal.6
Positive electrode is normally an electron acceptor which is strongly
electronegative (e.g. LiMO2 (M= Co, Ni, Mn, etc) compounds). During discharge
process, the negative electrode electrochemically oxidised and releases
electron. This electron moves through outer circuit to the positive electrode
which accepts electron. In Fig. 1.1, the schematic of lithium ion battery
operation is explained using LiCoO2 as positive electrode and carbon
as negative electrode.

 

Positive
electrode:   LiCo3+O2          Charge/Discharge   xLi++
Li1-xCo4+
xCo3+
1-xO2
+
e-

 

Negative
electrode:  C+ xLi+
+
e-   Charge/Discharge   LixC

 

Overall:                    LiCoO2+
C    Charge/Discharge  Li1-xCoO2+
LixC

 

Figure 1.1,
Schematics of Li-ion battery using LiCoO2 as
cathode and graphite as

anode.

During charge,
Li+
moves
from LiCoO2 to carbon through the electrolyte
which causes oxidation of Co3+ to Co4+
and
the reverse happens during discharge; Li+ moves
from carbon to LiCoO2. Role of electrolyte
is to act as a medium for the transfer of ions between the two electrodes. In
general, lithium salt dissolved in organic solvent is used as electrolyte in
lithium ion batteries. Main requirements for the electrolyte are,7
(i)
it should be a good Li-ion conductor and electronic insulator (ii) stability
over the operating voltage window (iii) chemical compatibility with cell
components and electrodes (iv) thermal stability and (v) there should not be
any charge accumulation and concentration polarization. Material that undergoes
chemical reaction producing current during battery operation is known as active
mass or active material.2
In
batteries the electrode itself takes part in chemical reaction apart from being
a charge transfer media. Consequently, the chemistry associated with electrode-electrolyte
interface and bulk of electrode are the main factors that determine the battery
performance. Thus performance of lithium ion battery crucially depends on the
nature of electrode material used.

 

1.3
Parameters of Electrode Quality

Parameters that
are used to validate the quality of electrode material are,

·        
Cell voltage

·        
Conductivity

·        
Specific capacity

·        
Coulombic efficiency

·        
Capacity retention (stability/ cycle
life)

·        
Gravimetric and volumetric energy
density

·        
Power density

·        
Cost, toxicity and safety issues

 

Cell
voltage: Cell voltage is represented by open
circuit voltage (voltage between the two terminals when no external current
flows) or closed circuit voltage (voltage between the two terminals when it is
connected with external circuit).5
Open circuit voltage Voc is calculated from the
chemical potential of the negative (mNLi)
and positive electrode (mPLi)
as,

                          Voc=

 

Where,
F is Faraday constant 96485 JK-1.

Thus
chemical potential of positive electrode should be high and that of negative electrode
should be low in order to achieve high cell voltage. In addition for the electrode
to be thermodynamically stable, redox energies of the electrode should lie within
the band gap of electrolyte material Eg.

 

Conductivity:
Electrode material should be capable of conducting electrons as well as lithium
ions for better battery performance. But many lithium ion insertion materials
are semi conductors by nature. Some of them are even highly insulating
(conductivity