Hardware

For 3.95V and other higher voltage cells, cycle life hasn't improved much but capacity does quite a bit

https://www.grepow.com/blog/what-is-a-lithium-ion-polymer-high-voltage-lihv-battery.html

Unfortunately not many articles I can immediately find that specifically discuss what some of the improvements are that allow for higher voltage charging aside from generally better anode, cathode and seperator materials and/or improved electrolyte.