Recharging Secondary Batteries: Explained for 12th Grade Students

[gracy]

how some batteries can be recharged?i mean i know batteries can be recharged by reversing the direction of current flow but i want to understand the whole mechanism of recharging of cell.how reversing the direction of current flow regenerates the original reactants in secondary cell?i am 12th grade student.

 

[Borek]

Can you write reactions that take place during discharging of a battery? (pick whatever type you like, just select one that can be recharged).

 

[gracy]

Can you write reactions that take place during discharging of a battery? (pick whatever type you like, just select one that can be recharged).

ok i have figured it out.could you please look at this video from 1:28 to 3:58 question is the cell which is described in this video from 1:28 to 3:58 is secondary cell which is rechargeable,why can't the same occur in primary cell which is not rechargeable,how the chemistry of these two cells (secondary cell &primary cell )differ so that these cells get opposite characteristics :i.e in one cell recharging is possible and it is not possible in the other cell.

 

[Borek]

In general it has to do with the irreversibility of the reaction. In primary cells if you put the energy back into the system, it will not go back to the original state. Say, in Leclanche battery, instead of reversing the reaction you will just decompose water present (and produced in the battery reaction) into hydrogen and oxygen - leaving ZnCl₂ and Mn₂O₃ intact, so the battery doesn't recharge.

Thermodynamically - when you try to recharge primary cell it won't get to the original state, but to some local energy minimum. Secondary cells are those that don't have such competing local minimum, so they can be pushed only back to the charged state. Note, that asking "why secondary cells behave this way" is putting the cart before the horse - we call cells that behave this way "secondary", so it is behavior that classifies the cells, not the the classification that tells us how the batteries behave.

 

[gracy]

In general it has to do with the irreversibility of the reaction. In primary cells if you put the energy back into the system, it will not go back to the original state. Say, in Leclanche battery, instead of reversing the reaction you will just decompose water present (and produced in the battery reaction) into hydrogen and oxygen - leaving ZnCl₂ and Mn₂O₃ intact, so the battery doesn't recharge.

Thermodynamically - when you try to recharge primary cell it won't get to the original state, but to some local energy minimum. Secondary cells are those that don't have such competing local minimum, so they can be pushed only back to the charged state. Note, that asking "why secondary cells behave this way" is putting the cart before the horse - we call cells that behave this way "secondary", so it is behavior that classifies the cells, not the the classification that tells us how the batteries behave.

if you don't mind can you please explain concept of local energy minimum.just tell me basics later i will try to figured it out by myself.

 

[Borek]

Actually I think my previous post was a little bit misleading. When recharging the battery we are not necessarily traveling to a minimum. We are moving in a direction that is chemically "the most easy way up".

You do know what a minimum, global minimum and local minimum are in general?

Every chemical system has its energy (actually it would be better to talk about Gibbs free energy, but let's not get into too many details at once). Any change to the system (like discharging a battery) means moving the system on the energy plot (be it curve, or surface, or multidimensional surface). Every system has a tendency of spontaneously moving in the direction of some energy minimum (which one - depends on the shape of the energy function around, think about water flowing down in the mountains - direction of the flow depends on the slope). Discharging a battery mean moving a system (that was initially somewhere on the slope) to its local minimum (in which the system is much more stable than the original battery was; battery was stable because the reagents were artificially separated so that they didn't react spontaneously). When you attempt to recharge the battery (which is not a spontaneous process, so the system can travel up on the energy landscape) you are trying to push it up - but it may prefer to not travel the the starting point, but in some other direction (say, where the slope is less stiff).

 

[gracy]

Actually I think my previous post was a little bit misleading. When recharging the battery we are not necessarily traveling to a minimum. We are moving in a direction that is chemically "the most easy way up".

You do know what a minimum, global minimum and local minimum are in general?

Every chemical system has its energy (actually it would be better to talk about Gibbs free energy, but let's not get into too many details at once). Any change to the system (like discharging a battery) means moving the system on the energy plot (be it curve, or surface, or multidimensional surface). Every system has a tendency of spontaneously moving in the direction of some energy minimum (which one - depends on the shape of the energy function around, think about water flowing down in the mountains - direction of the flow depends on the slope). Discharging a battery mean moving a system (that was initially somewhere on the slope) to its local minimum (in which the system is much more stable than the original battery was; battery was stable because the reagents were artificially separated so that they didn't react spontaneously). When you attempt to recharge the battery (which is not a spontaneous process, so the system can travel up on the energy landscape) you are trying to push it up - but it may prefer to not travel the the starting point, but in some other direction (say, where the slope is less stiff).

it is very complicated concept .but good explanation thanks.

 

[Borek]

Actually it is not complicated, it just requires thinking in some not necessarily intuitive categories. But once you start to understand them, many things become quite obvious.

 

[Pharrahnox]

In my Year 12 Chemistry course, we learned that primary cells differ from secondary cells, in that in primary cells, the products of the discharging reaction are used up in side reactions. This is to shift the equilibrium constant to favour the discharging reaction, so the battery will last longer. So your primary cell cannot be recharged, because for that to happen, the products need to go back into the reactants, and this cannot happen, as the products are used up in the side reactions.

In a secondary cell, this does not occur, so it can be reversed (and therefore recharged), although it means that rechargable batteries have shorter life-spans.

 

[Borek]

primary cells differ from secondary cells, in that in primary cells, the products of the discharging reaction are used up in side reactions

That's only one of possible mechanisms. Imagine a cell in which product is gaseous and leaves the battery - you won't be able to recharge it as well.

In other words - many ways of skinning that cat.