(De)lithiation-Induced Phase Transitions of LiMTiO4 Spinels

Electrochemical energy storage using rechargeable Li-ion batteries is one of the most competitive technologies for future stationary and automotive applications. High capacity cathode materials (> 250 mA h g^-1) for Li-ion batteries are of great interest to meet the future demands. A majority of the existing cathode materials deliver capacity based on a single electron or even less than one electron (per transition metal) reaction with limited practical capacity. A feasible strategy to obtain higher capacity is to use cathode materials that enable reversible more than one-electron reaction. In this contribution, we demonstrate that spinel-type lithium transition metal titanates, LiMTiO₄ (M = Fe, Mn), can deliver a reversible discharge capacity of about 250 mA h g^-1. Partial Li⁺ can be extracted out of the spinel host upon charging. Unoccupied crystallographic sites in the spinel host could accommodate guest Li⁺ into the host framework. Thus, an overall up to 1.6 Li⁺ storage capability has been achieved. Detailed structural characterizations were performed to understand the encouraging electrochemical Li⁺ storage performance of the spinel LiMTiO₄.

                Nanosized LiFeTiO₄ and LiMnTiO₄ powders were synthesized by a sol-gel route.¹ LiFeTiO₄ with a native carbon coating was obtained by sintering the xerogels under argon atmosphere, whereas LiMnTiO₄ with a carbon-free bare surface was obtained by air-sintering. Both materials crystallize in disordered cubic spinel phase (Fd-3m space group), as observed from the X-ray diffraction analysis (Fig. 1). In LiFeTiO₄ the occupancy of Li/Fe at tetrahedral 8a sites is about 2:1, whereas in LiMnTiO₄ 8a sites are mainly occupied by Li. The charge-discharge curves and cyclability for both materials are shown in Fig. 2. A discharged capacity of around 250 mA h g^-1 has been achieved at a C/40 rate for LiFeTiO₄/C and at a C/20 rate for LiMnTiO₄. The observed average discharge voltage is 2.5 V and 3.1 V for LiFeTiO₄/C and LiMnTiO₄, respectively. Superior capacity retention was observed for both materials.^2,3

⁵⁷Fe Mössbauer spectroscopy confirmed that the initial Fe³⁺ in LiFeTiO₄ can be partially oxidized to Fe⁴⁺ upon charging to 4.8 V and then the Fe³⁺/Fe⁴⁺ ions can be reduced to Fe²⁺ after discharging to 1.5 V. Upon long-term cycling, Li_xFeTiO₄ (0.4 < x < 2) maintains the spinel phase.² Unlike the sloping charge/discharge profiles for LiFeTiO₄/C, LiMnTiO₄ shows initially a two-plateau charge/discharge profile similar to that of classic spinel LiMn₂O₄. Electrochemical cyclic voltammetry confirmed that the two-plateau regions are associated with Mn²⁺/Mn³⁺ and Mn³⁺/Mn⁴⁺ redox couples. After a few cycles, the charge/discharge profile evolves progressively into a sloping profile. Structural characterization of the cycled LiMnTiO₄ shows a phase transition from spinel to rock-salt and tetragonal Li_1+xMnTiO₄ phases after first discharge. Upon further charging, the rock-salt phase converts reversibly back to the spinel phase. The reversible spinel/rock-salt phase transition occurs with the cubic framework remaining intact. The presence of tetragonal phase indicates that the Mn³⁺ Jahn-Teller distortion is partially involved during lithiation. However, unlike LiMn₂O₄, the cubic/tetragonal two-phase region located at 3.1/2.8 V is related to Mn³⁺ reduction to Mn²⁺ (non Jahn-Teller distortion ions) leading to a final composition of Li₂Mn²⁺Ti⁴⁺O₄. Over further cycling, the content of tetragonal phase (~40%) does not change much. In addition, the lattice of the tetragonal phase shows reversible change during charge/discharge. This distinct feature enables LiMnTiO₄ to be cycled over a broad voltage range (1.5–4.8 V) and makes LiMnTiO₄promising as high-capacity cathode material.

                To summarize, spinel type Li-transition metal titanates exhibit the capability for up to 1.6 Li⁺ storage, durable cycling performance and fast Li⁺kinetics. The abundance and nontoxicity of Fe, Mn and Ti are ideal for the fabrication of electrode materials and this class of materials has the potential to become an alternative for future large-scale applications.

Acknowledgement:This work was financially supported by the BMBF and the “Helmholtz Initiative for Mobile/Stationary Energy Storage Systems”.

References:

1.     R. Chen and S. Indris, European patent: EP 13401030.5, (2013).

2.     R. Chen, M. Knapp, M. Yavuz, R. Witte, R. Heinzmann, S. Ren, V.S.K. Chakravadhanula, H. Hahn and S. Indris, submitted.

3.     R. Chen, M. Knapp, M. Yavuz, R. Heinzmann, D. Wang, S. Ren, V. Trouillet, S. Lebedkin, S. Doyle, H. Hahn and S. Indris, to be submitted.