Important Secondary Batteries

IMPORTANT SECONDARY BATTERIES 

1. Lead Acid Storage Cell

Storage cell

A lead acid storage cell is a secondary battery, which can operate both as a voltaic cell and as an electrolytic cell. When it acts as a voltaic cell, it supplies electrical energy and becomes "run down". When it is recharged, the cell operates as an electrolytic cell.

Description

Anode: Lead

Cathode: Lead dioxide

Electrolyte: H₂SO₄

Emf: 12 V

Cell diagram: Pb/PbSO₄ // H₂SO₄ (aq) / PbO₂ / Pb

Construction

A lead‒acid storage battery consists of a number of (3 to 6) voltaic cells connected in series to get get 6 to 12 V battery. In each cell, the anode is made of lead. The cathode is made of lead dioxide PbO₂ (or) a grid made of lead, packed with PbO₂. A number of lead plates (anodes) are connected in parallel and a number of PbO₂ plates (cathodes) are also connected in parallel.

Various plates are separated from the adjacent one by insulators like rubber (or) glass fibre. The entire combinations is then immersed in dil. H₂SO₄ (38% by mass) having a density of 1.30 gm/ml.

Working (Discharging)

When the lead‒acid storage battery operates, the following reaction occurs.

At anode: Lead is oxidized to Pb²⁺ ions, which further combines with SO₄^2‒ forms insoluble PbSO₄.

At cathode: PbO₂ is reduced to Pb²⁺ ions, which further combines with SO₄^2‒ forms insoluble PbSO₄.

Overall cell reaction during use (discharging):

From the above cell reactions it is clear that, PbSO₄ is precipitated at both the electrodes and H₂SO₄ is used up. As a result, the concentration of H₂SO₄ decreases and hence the density of H₂SO₄ falls below 1.2 gm/ml. So the battery needs recharging.

Recharging the Battery

The cell can be charged by passing electric current in the opposite direction. The electrode reaction gets reversed. As a result, Pb is deposited on anode and PbO₂ on the cathode. The density of H₂SO₄ also increases.

The net reaction during charging is

Advantages of lead‒acid batteries

(i) It is made easily.

(ii) It produces very high current.

(iii) The self‒discharging rate is low when compared to other rechargeable batteries.

(iv) It also acts effectively at low temperature.

Disadvantages of lead‒acid batteries

(i) Recycling of this battery causes environmental hazards.

(ii) Mechanical strain and normal bumping reduces battery capacity.

Uses

1. Lead storage cell is used to supply current mainly in automobiles such as cars, buses, trucks, etc.,

2. It is also used in gas engine ignition, telephone exchanges, hospitals, power stations, etc.,

2. Nickel‒Cadmium Cell (or) Nicad Battery

This is also a rechargeable battery.

Description

Anode: Cadmium

Cathode: Metal grid + NiO₂ paste

Electrolyte: KOH

Emf: 1.4 V

Cell diagram: Cd | Cd(OH)₂ || KOH (aq) | NiO₂ | Ni

Construction

Nickel‒cadmium cell consists of a cadmium anode and a metal grid containing a paste of NiO₂ acting as a cathode. The electrolyte in this cell is KOH.

Working (Discharging)

When the Nicad battery operates, at the anode cadmium is oxidised to Cd²⁺ ions and insoluble Ca(OH)₂ is formed. It produces about 1.4V.

At anode: Cadmium is oxidised to Cd²⁺ and further it combines with OH^‒ ions to form Cd(OH)₂.

At cathode: NiO₂ is reduced to Ni²⁺ ions which further combine with OH^‒ ions to form Ni(OH)₂.

Overall reaction during use (discharging)

From the above cell reactions it is clear that, there is no formation of gaseous products, the products Ca(OH)₂ and Ni(OH)₂ adhere well to the surfaces. This can be reconverted by recharging the cell.

Recharging the Battery

The recharging process is similar to lead storage battery. When the current is passed in the opposite direction, the electrode reaction gets reversed. As a result, Cd gets deposited on anode and NiO₂ on the cathode.

The net reaction during charging is

Advantage

1. It is smaller and lighter.

2. It has longer life than lead storage cell.

3. Like a dry cell, it can be packed in a sealed container.

Disadvantage

It is more expensive than lead storage battery.

Uses

It is used in calculators, electronic flash units, transistors and cordless appliances.

3. Lithium‒ion battery (LIB) (or) Lithium‒ion cell

Lithium‒ion battery is a secondary battery. As in lithium cell, it does not contain metallic lithium as anode. As the name suggests, the movement of lithium ions are responsible for charging and discharging.

Components

Lithium‒ion cell has the following three components.

• A positive electrode (Layers of lithium‒metal oxide) (cathode)

• A negative electrode (Layers of porous carbon) (anode)

• An electrolyte (Polymer gel) (separator)

Description

Cathode: Lithium‒cobalt oxide (LiCoO₂) Layers

Anode: Graphite (Porous carbon) layers

Electrolyte: Polymer gel (organic solvent)

Construction

The positive electrode is typically made from a layers of chemical compound called lithium‒cobalt oxide (LiCoO₂).

The negative electrode is made from layers of porous carbon (C) (graphite).

Both the electrodes are dipped in a polymer gel electrolyte (organic solvent) and separated by a separator, which is a perforated plastic and allows the Li⁺ ions to pass through.

Working

Charging

During charging, Li⁺ ions flow from the positive electrode (LiCoO₂) to the negative electrode (graphite) through the electrolyte. Electrons also flow from the positive electrode to the negative electrode through the wire. The electrons and Li⁺ ions combine at the negative electrode and deposit there as Li.

LiCoO₂ + C → Li_1‒xCoO₂ + CLi_x

Discharging

During discharging, the Li⁺ ions flow back through the electrolyte from negative electrode to the positive electrode. Electrons flow from the negative electrode to the positive electrode through the wire.

The Li⁺ ions and electrons combine at the positive electrode and deposit there as Li.

Li_1‒xCoO₂ + CLi_x → LiCoO₂ + C

Advantages and Disadvantages of lithium‒ion batteries

Advantages

1. It is light in weight.

2. Rate of self‒discharge is low.

3. Have higher energy density than other rechargeable batteries.

4. Have a greater number of charge and discharge of cycles.

5. Operates at higher voltage (3.7 V)

6. No liquid electrolyte is used.

7. Easy maintenance.

Disadvantages

1. They are expensive.

2. Availability of lithium is difficult.

3. Difficulties in transportation.

4. Improper disposal leads to environment contamination.

5. It is temperature‒sensitive battery and explode at high temperature.

Applications

1. It is used in portable electronic devices like cellular phones, digital cameras, tablets and laptop computers.

2. It is used in biomedical implantable devices such as pace makers.

3. It is currently used in most electric vehicles because of their high energy per unit mass.