The Lithium Cell,

In many pieces of electronic equipment there is a high capacity lithium cell. You may have wondered what it is. I would like to throw some light on this topic.

Lithium is a metal near the top of the periodic table, in the same group as sodium and potassium. In fact lithium is the first metal in the table and the second element, the first being hydrogen. Lithium is an alkali metal and is very reactive. Potassium is the most reactive of the group, then sodium followed by lithium. All of them will react with water, releasing hydrogen the gas. Sodium will even release hydrogen from ethanol. They all have a high half--cell potential. So they might be used in a cell to produce electricity. As they are of low density the cell would be of very low mass for a given capacity compared to lead, for example. So apart from their undesirable reaction with water they are good material for an electrode. So how could they be used.

The problem can be solved by using a non--aqueous solvent. One possibility is ammonia but this has plenty of hydrogen available. What other solvents have we? Hydrocarbons spring to mind, but the metal, even if it dissolved, would not form ions. So you want a non-aqueous solvent which will solvate ions. These are charged, so the requirement is a solvent that is polarised, like water, and one that does not have available hydrogen. One possibility is propanone otherwise known as acetone. Sodium and potassium are too reactive, so lithium is the choice. But do lithium ions dissolve in acetone? It is quite likely that they do. You have to experiment at this point. Try dissolving anhydrous lithium chloride in acetone. Put it in a copper vessel with lithium metal as the centre electrode and we have a cell. Now usually copper needs an aqueous solution of copper sulphate to make a half cell and the main problem is connecting the lithium half cell to the copper half cell. A salt bridge is the solution for other cells but one needs water on one side and acetone on the other without mixing. Perhaps a membrane of a substance impermeable to water and acetone but that will pass negative ions would do. Lipid layers or charged plastics have this property and if one used copper chloride solution in the copper half cell then the chloride ions could pass the positively charged membrane thus completing the circuit.

I carried out an experiment recently and found that anhydrous lithium chloride is slightly soluble in acetone. I put a spatula of the solid in a boiling tube and added about 20 ml of acetone. After shaking and standing for a few minutes, I took a spot from a glass rod from the acetone in the tube and placed it on a glass slide. When the actone had evaporated a little deposit was left.

I concluded from this that some of the lithium chloride had dissolved. I then took a copper beaker and placed the acetone with the lithium chloride in the beaker. 1 connected the beaker to a meter and then cleaned a small lump of lithium metal. This I connected by crocodile clip and wire to the meter when I dipped the lithium into the solution the meter showed a current of about 1.5 mA. On switching to high impedance it read 2.4 Volts.

So the lithium cell works. The current rapidly got smaller as it flowed and a deposit was seen to form on the lithium electrode, this may have been lithium chloride as the chloride in the solution reacted with the electrode so the system was not practical as it stands. Other cells have this problem and one that comes to mind is the copper/zinc cell in acid. This rapidly polarises as hydrogen forms a thin layer adsorbed to the copper. It can be prevented by adding sodium dichromate to the electrolyte. This must be done before the cell is used as the polarisation does not disapear on addition of the dichromate. This means to me, that a more complicated system is required in the lithium cell.

I do not know how the problem was solved in practice, in the final highly successful lithium cell but these must have been some of the considerations involved in its solution. I hope this is of interest as we often use these cells in our equipment

Chris Strevens © 1975