Polarity of Electrodes for Rechargeable Battery

Let's take a look at the following diagram for a sodium-sulfur rechargeable battery.

polarity electrode rechargeable battery diagram

Given that during discharge, sodium is oxidised and sulfur is reduced.

In this video we are interested to deduce the charges of electrode J and K during charging and discharging of the battery.

Would the charges stay the same or be reversed?

1. Discharging

During discharging, a redox reaction takes place in the cell which gives rise to flow of electrons.

polarity electrode rechargeable discharging energy conversion

Hence the energy conversion is chemical energy to electrical energy.

The battery functions as an electrochemical cell during discharging.

During discharging,

polarity electrode rechargeable discharging reaction

Na is oxidised to Na+, electrode J that is in direct contact with sodium will be the anode.

Anode for electrochemical cell is negatively charged.

S is reduced to polysulfide Sx2-, electrode K that is in direct contact with sulfur will be the cathode.

Cathode for electrochemical cell is positively charged.

Hence during discharging, electrode J is negative and electrode K is positive.

2. Charging

During charging, electricity flows through the cell and forces a chemical reaction to convert the products Na+ and Sx2- back to sodium and sulfur.

polarity electrode rechargeable charging energy conversion

Hence the energy conversion is electrical energy to chemical energy.

The battery functions as an electrolytic cell during charging.

During charging,

polarity electrode rechargeable charging reaction

Na+ is reduced to Na, electrode J that is in direct contact with sodium will be the cathode.

Cathode for electrolytic cell is negatively charged.

Sx2- is oxidised to S, electrode K that is in direct contact with sulfur will be the anode.

Anode for electrolytic cell is positively charged.

Hence during charging, electrode J remains negative and electrode K remains positive.

This means that the polarity of electrodes remains unchanged during charging and discharging of a rechargeable battery.

polarity electrode rechargeable compare both

Topic: Electrochemistry, Physical Chemistry, A Level Chemistry, Singapore

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