It is not known when Li Sizhi was born, but it is known that he was the oldest son of Li Longji, then the Prince of Linzi under Li Longji's uncle Emperor Zhongzong. His mother Consort Liu, who would eventually receive the imperial consort rank of Huafei, the third highest rank for imperial consorts, later bore two more sons for Emperor Xuanzong, Li Wan the Prince of Rong and Li Sui the Prince of Yi. In 710, after Li Longji's father Li Dan became emperor, Li Longji was created crown prince, and his sons were created princes. Li Sizhi was created the Prince of Xuchang.
During Emperor Xuanzong's reign
In 712, Emperor Ruizong yielded the throne to Li Longji, who took the throne as Emperor Xuanzong. Li Sizhi's title was upgraded to Prince of Tan. In 715, however, he was bypassed for the crown prince position when his younger brotherLi Siqian the Prince of Ying was selected, as Li Siqian's mother Consort Zhao was then Emperor Xuanzong's favorite concubine. In 716, Li Sizhi was made the Protectorate General to Pacify the West and received the title of "Comforter of the Non-Han of the Hedong , Guannei , and Longyou Regions", although there was no evidence that he actually left the capital Chang'an. In 725, Li Sizhi's name was changed to Li Tan, and his title was changed to Prince of Qing. In 727, Li Tan became the military governor of Hexi Circuit as well as the commandant at Liang Prefecture, but, as was the case with 10 other brothers of his who received similar titles, did not actually take up the command. In 736, Li Tan's name was changed to Li Cong. In 737, Li Siqian, whose name had been changed to Li Ying, was forced to commit suicide after false accusations of treason by Emperor Xuanzong's then-favorite concubine Consort Wu, who wanted her son Li Mao the Prince of Shou to be crown prince. However, Emperor Xuanzong subsequently selected another son, Li Yu the Prince of Zhong, crown prince, bypassing Li Cong again in the process. Later when Chancellor Li Linfu tried to depose the new Crown Prince Li Yu, he tried to persuade Emperor Xuanzong to create Li Cong as Li Cong was the eldest, but Emperor Xuanzong silently replied, "Cong, my eldest son, in early years when hunted in the imperial garden, his face was hurt and badly injured." Implying that an injured face was unfit for an emperor, and this might be the reason why Li Cong was always bypassed and never became crown prince. Though Li Linfu still insisted, Emperor Xuanzong never had Li Yu replaced, as Li Yu always behaved well at that time. In 742, Li Cong became military governor of Hedong Circuit.
Posthumous honors
Li Cong died in 752. Emperor Xuanzong, still emperor at that time, posthumously honored him as Crown Prince Jingde, even though he was never crown prince. As he was sonless, he adopted Li Ying's son Li Qiu, and Li Qiu inherited his title of Prince of Qing. In 756, after Li Yu, whose name was by then changed to Li Heng, became emperor, Li Cong was further posthumously honored as Emperor Fengtian. His wife Princess Dou was posthumously honored Empress Gongying, and they were reburied with honors due emperor and empress.
