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Introduction:

The function of Batteries / cells is the conversion of chemical energy into electrical energy.
It is made up of two electrodes (anode & cathode) and electrolyte solution

Definition:

Cell is an arrangement of two electrodes dipped into a solution of electrolyte or electrolytes.
Cathode – Positive terminal – Electrochemical reduction occurs (gain electrons)
Anode – Negative terminal – Electrochemical oxidation occurs (lose electrons)
Electrolytes – Allow Ions to move between electrodes
Terminals – Allows Current to flow out of the battery to �perform work

CELLS

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List of invented cells / Batteries

Alessandro Volta invented the first true battery

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Due to increasing human activity in technology, a number of battery dependent appliances have come into existence.
It is used in wrist watches, electric calling bells, space vehicles and missile firing units.

Importance of Cells / Batteries /

There are two types of cells, 1. Electrolytic cell 2. Electrochemical cell

Types of Cells

1. Electrolytic cell - Electrical energy is used to bring about the chemical reaction.

At anode : Oxidation takes place � (Ni → Ni²⁺ +2e^-)

At cathode : Reduction takes place � ( Ni²⁺ +2e^- → Ni )

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2. Electrochemical / Galvanic cell

Electrical energy is generated due to chemical reactions, which takes place inside the cells
Examples: Daniel cell, Zn / ZnSO₄ // CuSO₄ / Cu

At anode,

Zn → Zn²⁺ +2e^- (Oxidation)

At cathode,

Cu²⁺ + 2e^- → Cu (Reduction)

The overall reaction,

Zn + Cu²⁺ → Zn²⁺ + Cu

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A battery is an arrangement of several electrochemical cells connected in series / parallel to get required amount of electrical energy.
The battery contains several anodes and cathodes.

BATTERIES

Criteria for any cells to be commercial cells

Should be cheap

Light weight and portable
should have long life cycle and high self life.
should be continuous and constant sources of EMF over a long interval of time.
It should be a rechargeable unit.

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Selection of battery depends on the conditions of working, the suitability of a battery depends on the following characteristics:

Type
Voltage
Discharge curve
Capacity
Energy density
Specific energy density
Power density
Temperature dependence
Service life
Physical requirements
Charge/discharge cycle
Cycle life
Cost
Ability to deep discharge
Application requirements

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Cycle life:

It is defined as number of times the discharging and charging operations can be alternated till such time it performs as designed.
It is applicable only to secondary cells. The EMF of cell decrease during discharging.
A good cell should have high cycle life.
some times cycle life would be lower than expected level due to,

The active materials at the electrodes may whither off due to rapid charging conditions.
May be irregular deposition of the products during discharging.
Over charging, the corrosion may occur.

Shelf life:

A good battery should possess a long shelf life.

Self -discharge:

It is defined as the loss of active materials of the cell due to localized action on the electrode even the cell is not in discharge mode.
Longer the life self-discharge, good battery

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Discharging:

The electrons liberated at the anode flow to the cathode through the external wire and take part in the reduction. This process in which spontaneous redox reaction occurs is called discharging.
During discharging , the active materials are converted into inactive materials.
The cell becomes inactive once the active material is consumed.
External energy < Cell energy

Charging:

The cell reaction is reversed if the external current is passed in the reverse direction.
This process of conversion of an inactive material back into active materials in a cell is called charging.
It is a non-spontaneous process.
External energy > Cell energy

Discharging and Charging of a battery

A cell is a battery that is packed that active materials at anode and cathode, redox reaction occur spontaneously.

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The batteries are classified into

Primary batteries
Secondary batteries

1. Primary battery (Non-rechargeable)

The electrode and its reaction cannot be reversed by passing electrical energy externally.
During discharging the chemical compounds are permanently changed and electrical energy is released until the original compounds are completely exhausted.
In such batteries the reaction occurs only once and it is not rechargeable
Lower discharge rate than secondary batteries
Examples: Dry Leclanche cell - Zinc Carbon – Used in flashlights, toys� Heavy Duty Zinc Chloride – Used in radios, recorders� Alkaline – Used in all of the above� Lithium – Used in photoflash � Silver Mercury Oxide – Used in Hearing aid, watches, calculators,� Silver button cell – Small devices like above

Types of Batteries

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2. Secondary battery (Chargeable)

The electrode reactions can be reserved by passing electrical energy externally.
During discharging the chemical compounds which are changed can be reconstituted by the application of an electrical potential between the electrodes – “electrochemical reaction is reversible”
They can be recharged by passing electrical current and used endlessly.
Used when short periods of storage are required
Higher discharge rate than primary batteries.
Thus such cells can be Rechargeable and used many times.
Examples: Lead Acid Battery� Nickel Cadmium Battery� Nickel Metal Hydride Battery� Lithium Ion Battery

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Primary battery - Silver button battery

What is a Button battery?

A Button battery or button cell is a small single cell battery,
Cylindrically shaped about 5 to 25 mm in diameter and 1 to 6 mm high

Structure

Button batteries are formed by compacting metals and metal oxides on either side of an electrolyte-soaked separator.
The unit is then placed in a 2-part metal casing held together by a plastic grommet
The grommet electrically insulates the anode from the cathode.
The metal undergoes oxidation on one side of the separator, �while the metal oxide is reduced to the metal on the other side, �producing a current when a conductive path is provided.
Uses: In small portable electronic devices - wrist watches, pocket calculators, artificial cardiac pacemakers, implantable cardiac defibrillators, hearing aids, toys, etc

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Importance

Button-type silver oxide batteries gives high-energy per unit volume and stable operating voltage.
Also, it is designed to use zero mercury.
Maxell is the first company in Japan to successfully market button-type silver oxide batteries.

Construction and working:

This cell consist of silver oxide as cathode �and zinc metal as the anode.
These electrodes are separated by �semi-permeable membranes and �pottasium hydroxide and sodium hydroxide �is used as an electrolyte
Cell representation

Zn , ZnO / Electrolyte / Ag₂O , Ag

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Cell reactions:

At the anode : Zn + 2OH^- → ZnO + H₂O + 2e^-

At the cathode : Ag₂O + H₂O +2e^-→ 2Ag + 2OH^-

Overall cell reaction : Zn + Ag₂O → ZnO + 2Ag

The cell gives a voltage of 1.3-1.5 V.

Advantages:

During discharge, supplies a stable voltage until the end of the discharge life.
A silver oxide battery’s gives twice the amount of energy capacity as button-type alkaline batteries.
Depending on the composition of the electrolyte, two models are available; a low-drain type (SW type) for analog watches and a high-drain type (W type) for multi-function watches (which incorporate an alarm and a light), medical equipment.
Designed without using mercury and lead and also long lasting, superior leakage - resistant characteristics

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The button-type silver oxide battery uses silver oxide (Ag2O) as its positive active material and zinc (Zn) as its negative active material. Potassium hydroxide (KOH) (W-type) or sodium hydroxide (NaOH) (SW-type) is used as an electrolyte.

Watches
Calculators
Medical Instruments
Measuring Instruments

What is an Implantable Cardioverter Defibrillator (ICD)?
An ICD is a battery-powered device placed under the skin that keeps track of your heart rate. Thin wires connect the ICD to your heart. If an abnormal heart rhythm is detected the device will deliver an electric shock to restore a normal heartbeat if your heart is beating chaotically and much too fast.

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Rechargeable alkaline battery
During charging and discharging, no loss of products
Active materials used in the battery system are,

Anode : Cadmium as a mixture of metal, oxide or hydroxide

Cathode : Nickel oxyhydroxide

Electrolyte : Aquous KOH

Cell representation

Cd , Cd(OH)₂ / KOH(aq) / Ni(OH)₂ , NiO(OH)

Construction

It consists of cadium anode and a metal grid �containing a paste of NiO₂ acting as a cathode
Electrolyte in the cell is KOH it is

Nickel- Cadmium batteries (NICAD)/ Secondary battery

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Working:

During Discharging

When the NICAD battery operates, at the anode

cadmium is oxidised to Cd ²⁺ ions and insoluble Cd(OH)₂ is formed

Cell reaction

At anode: Oxidation takes places at cadmium

Cd → Cd ²⁺ + 2e^-

Cd ²⁺ + 2OH^- → Cd(OH)₂

At cathode: Reduction of nickel oxyhydroxide takes

place in this reaction

2NiO(OH) + 2H₂O + 2e^- → 2Ni(OH)₂ + 2OH^-

Net cell reaction

Cd + 2NiO(OH) + 2H₂O → Cd(OH)₂ + 2Ni(OH)₂

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During Charging

When current is passed in the opposite direction, the electrode reaction gets reversed.
As a result, cadmium gets deposited on the anode and � NiO(OH) gets deposited on the cathode.

At anode:

Cd(OH)₂ → Cd ²⁺ + 2OH^-

Cd ²⁺ + 2e^- → Cd

At cathode:

2Ni(OH)₂ + 2OH^- → 2NiO(OH) + 2H₂O + 2e^-

Net cell reaction

Cd(OH)₂ + 2Ni(OH)₂ → Cd + 2NiO(OH) + 2H₂O

The cell voltage of battery is 1.4 V, which is irrespective of the size of electrodes.

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Applications:

Ni-Cd batteries may be used individually or assembled into battery packs containing two or more cells.
Ni-Cd batteries are used in cordless and wireless telephones, emergency lighting and other applications.
With a low internal resistance, they can supply a high surge current. This makes them a favourable choice for remote controlled model airplanes, boats, cars and camera flash units.

Surge currents are the “currents that raise or fall from the normal rated value in the short duration of time”.

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