Lithium air battery

4.2.7 Lithium air battery

Lithium air battery offers the best energy-to-size ratio compared to mercury, silver oxide and zinc air so far.  Similar in physical appearance as the named cells.

Button cell

Merits

It has a very long shelf life of up to 5 years to reduce capacity to 90% before being de-energized,

It has high e.m.f. of about 3.8 volts

Uses

Backup batteries for low-power computer memories, commonly known as CMOS battery

Portable devices where they are soldered into their circuits to last several years.

Nb. This battery should not be confused with the lithium ion battery used in cell phones which are secondary battery.  This is covered in KCSE Electricity Form 3 course work under storage batteries.

Zinc-air cell

4.2.6 Zinc-air cell

Looks similar to mercury cell but have about twice the capacity. It uses air in its chemical sequences, and are usually supplied sealed; pulling a tab off energizes the cell by letting air in. 

Zinc-air cell

Merits

It has a very long storage or shelf life before being de-energized,

It has a high e.m.f. of about 1.4 volts, and

It is relatively cheap.

Silver oxide cell

4.2.5 Silver oxide cell

Similar in appearance to mercury cell, silver oxide cell provides a very stable and slightly higher e.m.f., 1.5 volts as compared to 1.35 volts of the mercury cell. Silver oxide cell’s cathode is made of silver oxide, with potassium or sodium hydroxide, both strong alkalis, as the electrolyte. The capacity of the silver oxide cell, size for size, is considerably better than that of the mercury cell. 

Silver oxide cell

Because silver is expensive, this cell’s application is thus limited for applications where size and stability of voltage is of supreme importance, such as in watches and hearing aids.

Mercury Cell

4.2.4 Mercury Cell

This cell is also m common use and consists of:

an outer steel Container as the positive electrode,

a cylinder of compressed zinc powder in the centre of the cell as the negative electrode

potassium hydroxide as the electrolyte surrounding the zinc plate, and

manganese dioxide and carbon as the depolarizing mixture.

The cell is constructed so small that it is commonly used in watches, hearing aids, and miniature calculators. Its voltage is 1.35 V.

Mercury cell

The most important features of the mercury cell are:

It can be stored for long periods without losing its charge through local action.

It has a very high energy output for its size

Its drawbacks are:

It is toxic due to high mercury content

It is expensive

This cell has been replaced with a safer silver oxide cell.

Manganese-alkaline cell

4.2.3 Manganese-alkaline cell

The manganese-alkaline cell is commonly available, under several brand-names, as the 'popular long life battery' e.g. trade names Energizer, Duracel etc.

Its construction is more complicated than that of a Leclanché cell as below.

Manganese-alkaline cell

The positive terminal at the top of the cell is connected to a dense layer, near the outside of the cell, consisting of compressed manganese dioxide and graphite. An absorbent separator cylinder is followed (working inwards towards the middle) by a paste of zinc mixed with potassium hydroxide. This is connected to the bottom of the cell by an internal post, riveted or welded to the bottom of the cell.

The e.m.f. of a manganese alkaline cell is 1.5V.

This cell’s capacity, or service life, is many times better than that of Leclanché cell in most applications and secondly, it can be stored for a long time, up to 3 years, without losing its original capacity.

Dry Leclanché and manganese-alkaline cells are manufactured in different sizes for use in torches, radios, remote control devices and other portable electronic gadgets as shown below.

Relative sizes of dry cell

Dry Cell

4.2.2.2 Dry Cell

The dry Leclanché cell or simply dry cell is useful because it is compact and portable. It is constructed with the same materials as the wet cell, except that the electrolyte is in the form of a paste or jelly. The zinc electrode is in the form of a can, as shown below. 

Dry cell

The chemical action is the same as in the wet cell and its e.m.f. is 1.5 V, too.

Leclanché cell

4.2.2 Leclanché cell

The most common type of primary cell in use is the Leclanché cell. In its wet form, it consists of a glass jar containing:

1. a saturated solution of sal ammoniac (ammonium chloride) as the electrolyte,
2. a zinc rod as the negative electrode, and
3. a plate of carbon as the positive electrode.

4.2.2.1 Wet Leclanché Cell

Wet Leclanché cell

A mixture of manganese dioxide and powdered carbon is pressed around the carbon rod and then enclosed in a porous pot which the solution can soak through. The manganese dioxide acts as the depolarizer, while the powdered carbon gives greater conductivity.

The e.m.f. of the Leclanché cell is 1.5 V and the internal resistance of a normal size cell is 1 Ohm. The wet cells are now going obsolete.  They were once a majorly used to supply power to land-line telephone installations in remote areas.

Defects of a simple Electric Cell

4.2.1.1 Defects of a simple Electric Cell

The practical value and performance of simple cells is limited by the following defects:

(a)          Polarization

In the reaction in the simple cell, hydrogen gas is evolved. It collects in bubbles around the positive electrode and-eventually insulate the positive electrode from the solution.  This stops the reaction. This process is known as polarization.

It is minimized by use of a depolarizer.  This is a chemical which reacts with the hydrogen to produce water e.g. manganese dioxide.

(b)          Local Action

Impurities such as iron and lead embedded in the zinc electrode form small local cells. The impurity acts as the positive electrode and zinc as the negative. The formation of these local cells between the impurities and the zinc electrode is referred to as local action. It tends to wear the zinc electrode and the electrolyte; this happens even when the cell is not in use.

Local action may be minimized by using pure zinc, but zinc in its pure state is very expensive. Instead, a cheaper option is used alloying the zinc electrode with mercury. This process is referred to as amalgamation and resultant alloy is called zinc amalgam.

Primary Cell

4.2 Primary Cell

It is not rechargeable.  After it is exhausted or depleted, it is discarded. The reason is that the chemical action that takes place in it is not reversible.

The most common primary cell types are the zinc acid cell, Leclanché cell, Manganese-alkaline cell, mercury cell, silver oxide and lithium-air cell.

4.2.1 The Zinc-acid Cell

The diagram below illustrates a zinc-acid cell also referred to as the simple cell. It consists of:

(a)        zinc as the negative electrode,

(b)        copper as the positive electrode, and

(c)        dilute sulphuric acid as the electrolyte.

Simple cell

The chemical reaction that takes place between zinc and sulphuric acid is:

Zinc + Sulphuric acid  à Zinc sulphate + Hydrogen + Electric energy.

The hydrogen gas collects in bubbles around the copper elec­trode. The e.m.f. of this cell is 1.5V. This cell has many defects and for these, it not viably produced commercially.