add or adding

[mengta]

Some may argue that bateries containing cadmiujm actually offer a partial solution to the problem by being rechargeable, and thus adding to the lifetime of a battery.

why is there using "adding" rather than "add" in the sentence?

thanks

[Temico]

"Some may argue that batteries containing cadmium actually offer a partial solution to the problem by being rechargeable, and (which is) thus, adding to the lifetime of a battery."

[mengta]

does it still need the "and" if inserted in "which is" ?

thanks