Hi.
Saw somewhere a zener diode in series to a (USB) 5V source and a lithium cell to limit the charge voltage to around 4V. Yes, 4V; not 4.2V. Cannot find the article now.
A plain silicon diode would reduce 0.65V from ~5.1V applying a risky 4.45V. What was that zener solution ? A 1V zener ? Does it exist ?

5V(+)-----------------------Zener-------------------------lithiumcel-----------------------(-)

 

The are many types of specialty Switching-Diodes that will deliver any Voltage-Drop needed.

No Current-Regulation could create a very hazardous condition in any case.
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Not a good idea, not at all. There are many dumb and dangerous ideas floating around the Internet.

You will either; 1) incompletely charge the battery, 2) cause a fire.
There are many solutions to properly charge Lithium cells. Use them!