# Why does a complete discharge destroy a Li-Ion-Battery?

I read that completely discharging a Lithium Ion Battery is a very bad idea because it will lose plenty of capacity. But why?

I know that the reaction (fot the LiFePO4 battery) is: $$\ce{LiFePO4 + C6 <=>[1 e-] FePO4 + LiC6 }$$

So at a complete discharge, we only have our electrodes made of $\ce{LiFePO4}$ and $\ce{C6}$. Why shouldn't this react to the products of the reactions above?

When overcharging the battery I could find a solution, why it destroys the latter. Maybe all the graphite is ionized and so, the electrode is in solution. This would be a very big problem, but the graphite will only ionize if it can react with a cation (like lithium), so a small excess of graphite should prevent this.

This is what I read on the internet:

The discharge voltage is around 2.5 V depending on the cell type; It must not fall below, otherwise, the cell will be destroyed by irreversible chemical reactions. However, many electronic devices already switch off at significantly higher voltages, eg 3.0 V.

What do they mean with the irreversible chemical reactions? Which ones?

• Oxygen is evolved as the cobalt gets more oxidised since at 0.5 Li you get overlap of the cobalt and oxide bands. Oxide ions are oxidised and oxygen is evolved – RobChem Jul 16 '17 at 8:38
• True, but oxygen evolution happens at high potentials, not low ones. – Mark Wolfman Jan 26 at 5:42

Another problem with most secondary (storage) cells, Pb-acid as well as Li, is physical destruction of the structure on over-charging and under-charging. During the charge-discharge cycle, there is some change in volume of the electrodes, which causes them to spall, shedding pieces of the active material. This leads to loss of capacity and sometimes a short-circuit.

Even a fully charged cell deteriorates gradually... but increased discharge increases the growth of precipitates. Li-ion cells self-discharge about 2%/mo, (half-life of ~2 years), so a charged cell is quite stable, with no perceptible growth of whiskers or precipitate for years.

• May I ask why such destruction happens only at total (100%) discharge and not gradually during use of the Pb-acid battery? – Steeven Feb 13 '17 at 15:54
• oxygen loss occurs primarily at high states of charge, at least for layered materials. – Mark Wolfman Jan 26 at 5:32

I know from bitter experience that over discharging a NiCd battery pack ruins it, it will greatly lower the storage capacity. The same thing happens with lithium ion batteries.

What happens during over discharge is that it is possible to reverse charge at least one cell in the battery. This will cause a lot of damage to the cell which is reverse charged. The reverse charging can be regarded in some ways as an extreme version of discharging the cell.

What will happen is that new and unwanted reactions can occur in the cell.

Well it kind of depends on the materials in the battery. One consideration would be the strain or rearrangement of the crystal structure of the cathode material. There's not really room to insert more than 1 Li+ per formula unit into LFP, but if the potential is below 2.5V (or whatever the pristine cell potential is) then there will still be a driving force until 0V to further reduce the iron. Whatever reaction happened in response to that driving force will most likely result in a different crystal structure. Raising the potential doesn't reverse this process, and the new material is probably not going to make a very good electrode. Alternately, the crystal structure could expand so much to accommodate more Li+ that the strain causes the particles to fracture, disconnecting them from the conductive matrix that they're embedded it.