Influence of Cut-Off Potential on the Electrochemistry of M0.5TiOPO4(M = Fe, Cu) Synthesized by a New Route

Fe_0.5TiOPO₄ and Cu_0.5TiOPO₄ were identified as electrochemically active materials for lithium-ion batteries. Fe_0.5TiOPO₄ was prepared by a new, direct way of synthesis, which leads to a carbon coating of the particles during synthesis. The material exhibits a specific charge of 680 Ah/kg in the first lithiation and 490 Ah/kg in the first delithiation. Cu_0.5TiOPO₄ exhibits a specific charge of 640 Ah/kg in the first lithiation and 250 Ah/kg in the first delithiation. Both titanium oxyphosphates react with over four lithium ions per formula unit upon lithiation. This is more than the 2 lithium ions expected based on the oxidation state of the two metals in the original structure and also more than the 1.5 lithium ions expected based on the vacant octahedral sites in the unit cell. The lithiation results in three plateaus on the galvanostatic curve but at different potentials for both materials. The reactions of lithium with the material take place at ≈1.8 V and ≈0.2 V vs. Li⁺/Li for both materials, and additionally at ≈0.75 V for Fe_0.5TiOPO₄ and at ≈1.6 V for Cu_0.5TiOPO₄.