The voltage is determined by the chemistry, which unless you seriously deviate from the standard lithium stuff, it's going to be able 4.2 V max.

Aside: When people try to increase it to like 4.3 V with "strengthened electrode material," the lifetime still seriously suffers. I'm typing this comment on a Razer Blade 15 with a currently swollen battery pack, which has been replaced only 10 months ago due to the same swelling issue