I think @jacob.small is correct. It isn’t the data format going out, it is a function within Excel that just “sees” the data as a date / time format and automatically formats it.
Another option would be to Pre-Format your Excel file and tell it the cells / rows / columns that you need are to be string and not date. I have done this in the past for various things as well.
‘1/1 should read as 1/1. The ‘ is a special character in excel which ‘forces’ a string format. Give it a shot and see if it helps.
Another possible solution would be to convert the values to a decimal (Sting.Split node, String.ToNumber node, and a x[0] / x[1] code block), store the number as a decimal, and then convert it to a fraction in excel via formatting. This may get messy if you have odd denominators though.