Incorrect spelling


Incorrect spelling, explanation: it is true, that the word excel was created out of excellere, but it changed over time and adapted to English. The remains of excellere are still there in words such as excellence, but we have to remember that as of now, it is excel, not excell that is the base word.

Correct spelling


Correct spelling, explanation: the base word here is the verb excel and the -s suffix is used when we speak in the third person (he/she/it excels). The process here is simple: excel+s=excels, as there is no need to add anything else.

Definition of excels:
Verb, third person, present tense of excel
1. To be very good at something; to be successful at something
Tom does not do so well with math, but he excels in biology.
Not only is he very smart, but he also excels in his career.