You'll need to be more specific about which piece of music you are referring to, as there are a couple of tunes playing during Ichigo using his Bankai for the first time. Please provide a video link or time code or more exact description.
Ichigo first uses his Bankai in episode 58. There are four pieces of music playing in that episode from when he releases his Bankai: Never Meant To Belong just before and as he releases his Bankai, then a softer piece of music which plays when his Bankai is revealed. Storm Center is played as they fight. The episode ends with On The Precipice of Defeat when Ichigo stabs Byakuya.
Episode 59 starts off with a soft piece, some battle music, Nothing Can Be Explained, and the fight ends with On The Precipice of Defeat.