In addition to a wide initial tolerance, common zeners have a significant tempco and a very soft knee.
In other words, *very sloppy* charging characteristics.
As Klaus mentions, Li ion batteries are sensitive about its charging requirements. In the best of cases, you'll have a not-completely-charged battery. In the worst case, you'll end up with a ruined battery, and in many cases the failure could prove to be catastrophic.
EDIT: After re-reading your original post, I noticed that this is a N-chan Mosfet! As Audioguru mentions, it will never turn on.