UNIT-II: ENERGY – SOURCES, CONVERSION AND STORAGE
BATTERIES
Galvanic cell: Galvanic cell is a device for converting chemical energy into electrical
energy through a spontaneous redox reaction.
Battery: A battery is an electrochemical device which consists of two or more galvanic
cells connected in series or parallel or both which produces electricity by means of
redox chemical reactions.
Basic components of battery
The basic components in a battery are anode, cathode, electrolyte and separator
1. Anode: The electro-active material at anode is oxidized and liberates electrons to
the external circuit.
The anode electro active material should have:
Ease of oxidation (i. e., low reduction potential)
Capacity to deliver high Columbic output.
Good conductivity
High stability
Ease of fabrication
2. Cathode: The electro-active material at anode is reduced and accepts electrons
from the external circuit.
The cathode electro active materials should have:
High reduction potential
High resistant to the electrolyte
3. Electrolyte: It provides medium for transfer of ions between the anode and
cathode. It is commonly a solution (or slurry) of an acid, alkali or salts having
good ionic conductivity.
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It should be:
Safe to handle
Non-reactive with the electrode
Safe to handle
4. Separator: It is used to separate anode and cathode compartments to prevent
internal short circuiting. It is an electrical insulator.
Eg: Cellulose, cellophane, nafion membranes etc.
Classification of batteries:
1. Primary batteries: These are non-rechargeable batteries.
Eg: Zn-MnO2 battery, Li-MnO2 battery etc.
2. Secondary batteries: These are rechargeable batteries.
Eg: Nickel-cadmium battery, Lithium-ion battery etc
3. Reserve batteries: In these batteries, one of the active components (e.g.
electrolyte) of the battery is separated from the rest of the components. It is
assembled just before the use. Eg: Mg-AgCl and Mg-CuCl batteries; both can be
activated by adding water.
Working of a battery:
Discharging: During discharge, oxidation takes place at the anode and reduction takes
place at the cathode. The reactions are spontaneous. Chemical energy is converted into
electrical energy. It acts as a galvanic cell during discharge.
At anode:
At cathode:
Charging: During charging, reverse reactions take place. The reverse reactions are non-
spontaneous reactions. The battery is connected to an external d.c. power supply.
Electrical energy is converted in to chemical energy. During charging it acts as a
electrolytic cell.
At anode:
At cathode:
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Characteristics of Batteries:
1. Voltage:
The voltage of a battery mainly depends upon the emf of the cells which
constitute the battery system which is given by Nernst equation,
Where, and K is the reaction quotient which is the ratio of the
product of molar concentration of the reaction product molecules to that of reactants.
The maximum voltage can be derived when:
The difference in the electrode potentials of cathode and anode is high.
Polarization and over potential must be minimum.
The internal resistance of the cell must be low.
2. Current:
Current is a measure of the rate at which the battery is discharging.
Higher the rate of spontaneous reaction, higher is the current.
Higher the surface area of the electrodes, higher is the rate of reaction. Current is
measured in A.
3. Capacity:
Capacity is a measure of the amount of electricity that may be obtained from the
battery. It is expressed in Ah (ampere hours).
The charge (C) in Coulombs is given by the Faraday’s relation:
where w is the weight of active material present at one of the electrodes, n = number of
electrons involved in discharge reaction, F = 96500 C/mol, and M its molar mass.
“More is the length of the flat portion of the curve, better is the capacity of the battery”.
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4. Energy density:
It is the ratio of energy available from the battery to its weight (or volume). It
may be expressed in Wh/kg.
If a battery can be discharged at a current I and at an average voltage E for a
period of time t, then the energy density is given by
Energy density = (I x E x t) / w
where w is the weight of the battery.
5. Power density:
It is the power per unit weight (or volume) of the battery.
If a battery can be discharged at a current I and at a voltage E, then the power
density is given by
Power density = (I x E)/w
where w is the weight of the battery. It may be expressed in W/kg.
6. Energy efficiency: The energy efficiency of a rechargeable battery is given by
7. Cycle life: It is the number of discharge–charge cycles possible in a rechargeable
battery before failure occurs. A good battery must have high cycle life.
8. Shelf life: The duration of storage of a battery, at the end of which the battery is able
to give required performance is called shelf life. A good battery must have long shelf
life.
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9. Tolerance to service conditions: The battery must be sufficient for the intended
application. This means that it must be able to produce the right current with the right
voltage. It must have sufficient capacity, energy and power. It should also not exceed
the requirements of the application by too much, since this is likely to result in
unnecessary cost; it must give sufficient performance for the lowest possible price.
Lithium-ion battery:
Principle:
Li-ion batteries are secondary batteries.
During the charge and discharge processes, lithium ions are inserted or extracted
from interstitial space between atomic layers within the active material of the
battery.
The Li-ion is transfers between anode and cathode through an Electrolyte.
Construction:
Anode: Lithiated Graphite carbon
Cathode: Lithium cobalt oxide (LiCoO2)
Separator: Polypropylene membrane
Electrolyte: Lithium salt in an organic solvent (LiPF6 in ethylene carbonate).
Cell representation: Li/Li+,C/LiPF6 in ethylene carbonate/Li-CoO2
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Working:
The traditional batteries are based on galvanic action but Lithium ion secondary
battery depends on an "intercalation" mechanism.
This involves the insertion of lithium ions into the crystalline lattice of the host
electrode without changing its crystal structure.
Cell reactions:
At Anode: During discharging of battery, Lithium atoms present in graphite layer (one
Li atom present in every 6C atoms) are oxidized, liberating electrons and lithium ions.
Electrons flow through the external circuit to cathode and lithium ions flow through the
electrolyte towards cathode.
At Cathode: Cobalt is reduced and lithium atoms and are inserted into the layered
structure of CoO2. The lithium ion is inserted and exerted into the lattice structure of
anode and cathode during charging and discharging.
During discharge current flows through external circuit and light glows. During
charging, no electrons flow in the opposite direction.
Advantages:
• They have high energy density than other rechargeable batteries.
• They are less weight.
• They produce high voltage out about 3.7 V as compared with other batteries.
• They have improved safety, i.e. more resistance to overcharge.
• No liquid electrolyte means they are immune from leaking.
