Dr.VSG & Dr.K.Y. 1 24/04/2015

Battery is a device which converts the chemical energy to electrical energy directly. It consists of an array of one or more voltaic cells (electrochemical cell).

Electrochemical cell is a device in which stored chemical energy (free energy) is converted to electrical energy

Devices in which the chemical reaction is irreversible are called primary cell/ primary battery.

Batteries in which the chemical reaction is reversible are called Secondary/ rechargeable batteries.

Based on the pH of electrolyte used, batteries can be classified as acidic/ alkaline batteries.

Lead acid accumulator is an acidic and rechargeable battery.

Nickel cadmium is an alkaline rechargeable battery.

The common feature of the above two secondary battery is that they act both as electrochemical cell (during discharging) and as an electrolytic cell (during recharging).

The main difference between acidic and alkaline batteries is that there is a significant change in viscosity of electrolyte in the case of acid accumulator, but in Nickel cadmium the electrolyte does not change significantly.

Alkaline battery is an improved form of the dry cell, in which the NH4Cl is replaced with KOH or NaOH. This makes cell last longer mainly because the zinc anode corrodes less rapidly under basic conditions than under acidic conditions.

Batteries

Dr.VSG & Dr.K.Y. 2 24/04/2015

PRIMARY ALKALINE BATTERY

Anode

Zinc

Cathode

Graphite rod + MnO2 paste

Electrolyte

Aqueous KOH containing ZnCl2

Cell representation

Zn /aqueous KOH / MnO2 paste + Graphite rod

Anode reaction

Zn Zn2+ + 2e-

Zn2+ + 2OH (aq) Zn(OH)2(s) + 2e-

Zn(OH)2(s) ZnO (s) + H2O(liq.)

Cathode reaction

2MnO2 (s) + H2O(L) + 2e- Mn2O3 (s)+ 2OH (aq)

Net Cell reaction

Zn(s) + 2MnO2(s) ZnO (s) + Mn2O3 (s)

Constant Voltage

1.5V ( Capacity of AA battery is 700 mAh)

Applications Calculators, watches, camera, flash lights etc.

Advantages: (i) No voltage drop

(ii) longer shelf life.

The only disadvantage is its COST

Dr.VSG & Dr.K.Y. 3 24/04/2015

NICKEL CADMIUM BATTERY

Anode

Cd

Cathode

NiO2

Electrolyte

20 -25 % aqueous KOH

Cell representation

Cd /aqueous KOH / NiO2

Anode reaction

Cd Cd2+ + 2e Cd2+ + 2OH Cd (OH)2

Cathode reaction

NiO2 + 2H2O + 2e Ni (OH)2 + 2OH

Net Cell reaction

Cd + NiO2 + 2H2O Cd (OH)2 + Ni (OH)2

Recharging -ve terminal

2e Cd (OH)2 Cd + 2OH

+ve terminal

Ni (OH)2 + 2OH NiO2 + 2H2O

Net reaction

Cd (OH)2 + Ni (OH)2 Cd + NiO2 + 2H2O

Constant Voltage

1.3 V

Dr.VSG & Dr.K.Y. 4 24/04/2015

Applications: Calculators, digital cameras, laptops, pagers, tape recorders, flash lights, medical devices, electric vehicles, space appliances etc

LEAD ACID ACCUMULATOR

Anode

Pb

Cathode

PbO2

Electrolyte

Dil. H2SO4 (33 to 35% )

Cell representation

Pb / dilute H2SO4 / PbO2

Discharging Anode reaction

Pb Pb2+ + 2e Pb2+ + SO42- PbSO4

Cathode reaction

PbO2 + 4H+ + 2e Pb2+ + 2H2O Pb2+ + SO42- PbSO4

1 NiO2

2 Cd

3- Aqueous KOH

4- Porous membrane

Dr.VSG & Dr.K.Y. 5 24/04/2015

Net Cell reaction

Pb + PbO2 + 4H+ + 2SO42- 2PbSO4 + 2H2O

Recharging -ve terminal

(cathode)

2e- PbSO4 Pb + SO42-

+ve terminal (Anode)

PbSO4 + 2H2O PbO2 + H2SO4 + 2H+

Net reaction

2PbSO4 +2H2O Pb + PbO2 + 2H2SO4

Constant Voltage

2V per cell ( Capacity of car battery is 45Ah)

Dr.VSG & Dr.K.Y. 6 24/04/2015

New battery Exhausted battery (Recharged battery) (Discharged battery)

Applications: Car battery, electrical vehicles, telephone exchanges, Railway trains, mines, laboratories, hospitals, broad casting stations etc.

Lithium Batteries / Swing Battery / Rock Chair Battery

Lithium cells consists of Li anode (E0=0.3V). It is the lightest metal and selection of cathode depends on the required capacity.

Lithium reacts violently with water and with atmospheric nitrogen; hence organic solvents are used as electrolyte in Li cells (ex. THF/ ether). To improve the conductivity, salt of Li is added as a solute. (ex: Lithium per chlorate / lithium tetra fluorophosphates).

Lithium battery is a solid state battery. Anode is lithium and cathode can be TiS2 / MnO2 / V2O5 / MoO2 / Cr3O8 / CoO2 etc. The electrolyte is a solid electrolyte which is a polymer. The polymer permits the passage of ions but not the electrons.

Dr.VSG & Dr.K.Y. 7 24/04/2015

Advantages

Lithium battery is considered to be the cell of the future.

Light weight so easily portable. High energy / power density. ( 1Kg of Li stores 150 Watt ,1Kg NiCd

stores 70 Watts, 6Kg Pb acid stores 150 Watt) Solid electrolyte , hence no leakage Lasts hundreds of charging cycles. Can be made into convenient shapes and sizes.

Limitations

Explosive due to reactivity with water and nitrogen Lasts only 2-3 years from date of manufacture irrespective of usage Sensitive to high temperature

LITHIUM TITANIUM DISULPHIDE BATTERY / SOLID STATE BATTERY

(Refer Class Notes For Diagram)

Anode

Lithium

Cathode

TiS2

Electrolyte

Polymer (permeable only to cations & not electrons)

Ex. Polyacetylene / Polypyrrole / Polyaniline

Cell representation

Li / Polymer / TiS2

Dr.VSG & Dr.K.Y. 8 24/04/2015

Anode reaction

Li Li+ + e

Cathode reaction

TiS2 + e TiS2

Net Cell reaction

discharging Li + TiS2 Li+ + TiS2 LiTiS2

Recharging

Constant Voltage

3 V

Applications

Calculators, transistors, headphones, cordless appliances etc.

LITHIUM SULPHUR BATTERY (LSB) (Fig 1 )

Anode ( negative)

Lithium in molten state (electron donor)

Cathode (positive)

Sulphur in molten state (electron acceptor) The electron from Lithium is conducted to Sulphur by graphite.

Electrolyte

Solid Alumina ( NaAl11O17)

Anode reaction

2 Li 2 Li+ + 2 e

Dr.VSG & Dr.K.Y. 9 24/04/2015

Cathode reaction

S

+ 2 e S2

The S2 formed reacts with elemental Sulphur to form polysulphide ion

S2 + nS [S]( n+1)2

Poly sulphide

Net Cell reaction discharging 2 Li + S

Li2 S(n+1)

Recharging

Constant Voltage

3.7 V

Applications

power tools and electric vehicles.

Fig 1

Graphite electrodes

Dr.VSG & Dr.K.Y. 10 24/04/2015