Iron-Substituted Lithium Titanium Spinels: Structural and Electrochemical Characterization

Iron-substituted lithium titanium spinels with stoichiometry Li 4/3 - y / 3 Fe y Ti 5/3 - 2 y /3 O 4, were synthesized by a sol gel process. The attention was focused on two compositions, namely Li 1.3 Fe 0.1 Ti 1.6 O 4 and Li 1.25 Fe 0.25 Ti 1.5 O 4 . The first spinel crystallizes in the typical Fd 3̄ m space group while the second adopts the P 4 3 32 symmetry. The two spinels were tested as electrodes in lithium cells. It was observed that these electrodes are capable of inserting up to 1.4−1.6 lithium ion equivalents per formula unit and to cycle reversibly 160−180mAh/g at C/5 in the 2.9−0.2 V potential range. The electrochemical responses are quite different from that of pure Li 4/3 Ti 5/3 O 4, since the lithium insertion (deinsertion) processes evolve through several one-phase and two-phase steps, clearly involving the reduction (oxidation) of both titanium (IV) and iron (III) which occupy different framework positions. The electrochemical discharge mechanism of the two spinel electrodes was investigated by combining electrochemical tests with in situ diffraction measurements carried out using high-energy synchrotron radiation.