Lifetime of Lithium-ion battery if kept at 80% and never cycled

Question

It is said that lithium-ion batteries should not be kept constantly at max charge and also not stored for a long time empty. Let us say that we connect it to a charger that keeps it charged to about 80% all the time and at optimal temperature, and that we never discharge it (cycle it). What will make the battery degrade in this condition? Will the battery in this case last for decades?

As I understand, the shelf life is given as the time it takes to discharge and then go bad because of being in a discharged state, but in this case we never let it discharge.

Answer 1

What will make the battery degrade in this condition?

Mostly, the degradation of the electrolyte in each cell in the battery. The voltage across the electrodes causes a electrolysis that disassociates the organic salt. (Note: I am not a chemist, so my terminology is probably off.) The higher the cell voltage, and the higher the temperature, the faster the effect.

Also, over time, the SEI layer on the active material thickens, which consumes lithium. That results in less active material and higher resistance between the electrolyte and the active material. The SEI (solid electrolyte interphase) layer is a good thing because it keeps the electrolyte from attacking the active material. But if the SEI layer is too thick, ions can't cross it.

Will the battery in this case last for decades?

It depends on what you mean by "Last". Yes, each cell in the battery will still be a working Li-ion cell after decades. Just not a good one: the resistance will have increased and the capacity will have decreased.