Important Secondary Batteries: Lead - Acid Storage Cell, Lithium-ion battery (LIB) (or) Lithium-ion cell

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

A lead-acid storage battery consists of a number of (3 to 6) voltaic cells connected in series to 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.

The cell may be represented as;

Pb / PbSO₄ // H2_SO_{4 (aq)} / PbO₂ / Pb

Working (Discharging)

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

Fig. 8.2. Lead storage cell

At anode: Lead is oxidized to Pb²⁺ ions, which further combines with SO^2-₄ forms insoluble PbSO₄.

At cathode: PbO₂ is reduced to Pb²⁺ ions, which further combines with SO^2-₄  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. 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. 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)

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 Litions 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 - x} CoO₂ + CLi_x

Fig. 8.3 Lithium-ion cell during charging

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 – x} CoO₂ + CLi_x → LiCoO₂ + C

Fig. 8.4 Lithium-ion cell during discharging

Advantages (or) Characteristics of Lithium cell

1. Lithium-ion batteries are high voltage and light weight batteries.

2. It is smaller in size.

3. It produces three time the voltage of Ni-Cd batteries.

4. It has none of the memory effect seen in Ni-Cd batteries.

Uses of Lithium cell

It is used in cell phone, note PC, portable LCD TV, semiconductor driven audio, etc.,