Li-ion Battery Rejuvenation
While there are certainly design and maintenance practices that can help to prevent or slow degradation, there's not really any practical way to repair degradation that has already happened to a given cell.

However, often a battery is made from multiple cells.  And it's often practical to find out which of those are the weakest in terms of reduced capacity and higher internal resistance.  If they're in series, the weakest cell will determine the effective capacity of the whole pack.  So you can replace just the weak cells to rejuvenate the battery as a whole.  Or, since typically the cells are much cheaper than the pack as a whole, just replace all of them.

Be careful, this can be quite hazardous - be sure to research how to do it safely before attempting!