First of all, it is advised to use writematrix() instead of xlswrite due to high efficiency.
Second, if possible, avoid using a loop for efficiency purposes again. If not avoidable, then 'append' option has to be employed - see this simple example code:
for ii = 1:10
x = ii+1;
writematrix([x,ii], 'LOOP_Data.xlsx', 'writemode', 'append');
end
