You have 2 options:
1) Format output[i] in a way that .Net's TimeSpan.Parse likes (using explicit days).
2) Convert output[i] to a TimeSpan and assign it to the cell's value.
See:
http://closedxml.codeplex.com/discussions/566791
http://msdn.microsoft.com/en-us/library/3z48198e%28v=vs.110%29.aspx
1) Format output[i] in a way that .Net's TimeSpan.Parse likes (using explicit days).
2) Convert output[i] to a TimeSpan and assign it to the cell's value.
See:
http://closedxml.codeplex.com/discussions/566791
http://msdn.microsoft.com/en-us/library/3z48198e%28v=vs.110%29.aspx