Lithium iron phosphate (LiFePO₄) is an appealing cathode material for lithium ion batteries. However, the degradation of LiFePO₄ in air presents an unavoidable challenge, due to the vulnerability of divalent Fe against oxygen attack. In this work, we have carried out comprehensive research on the thermal stability and temperature-driven evolution of nanocarbon modified LiFePO₄ in air. The results show that LiFePO₄ retains structural stability up to 250 °C for short periods of exposure to air. At long exposure times, structural evolution occurs at a much lower temperature, 150 °C. The structural evolution proceeds as the temperature increases, and finishes at 400 °C. The final products are monoclinic Li₃Fe₂(PO₄)₃ and α-Fe₂O₃. A quantitative evolution map has been developed through electrochemical cyclic voltammetry and galvanostatic tests. The results show that the largest changes take place between 200 and 250 °C.