 Lee released his 10th anniversary album 'Synergy' on November 2013, including title track "We Loved Together", which is composed and written by 5 o'clock and features Ha Dong Kyun.The album also includes "Doesn't Make Sense" which was written by Lee Jand composed by both Lee and Jo Jung-chi as well as his previous digital single, "I Am Sorry" which received attention for being the work of Brave Brothers, "Did You Ever Love" and "Let's Stay Together".In July 2015, Lee made an appearance under the pseudonym of 'Tungki' on MBC's King of Mask Singer.