Orthorhombic Lithium Titanium Phosphate As an Anode Material for Li-Ion Rechargeable Batteries

  The wide voltage window and the reasonable ionic conductivity of the organic electrolytes presently adopted in the Li-ion secondary batteries allow high power output and energy density. However, the thermal instability and comparatively high production costs of these organic electrolytes have become a major hindrance to the significant scale-up of Li-ion batteries. Aqueous electrolytes could considerably reduce the production cost of the lithium-ion batteries and completely eliminate the fire hazard at the expense of reduced energy density.

  On the other hand, the restricted voltage window of the aqueous electrolytes allows only non-conventional sets of anode and cathode electrode materials to meet the proper working voltage of ~1.2 V in the full-cell configuration [1]. This requirement is met by using a non-carbonaceous host such as rhombohedral LiTi₂(PO₄)₃ (RLTP) as an anode material with a redox potential of 2.5 V for the Ti⁴⁺/Ti³⁺ couple against Li. A number of studies on aqueous rechargeable Li-ion batteries using RLTP as an anode have been published along with the development of RLTP. For example, an LiMn₂O₄//LiTi₂(PO₄)₃ cell with 1 M Li₂SO₄ as an aqueous electrolyte showed an initial capacity of 40 mAh g^-1, and LiMn_0.05Ni_0.05Fe_0.9PO₄//LiTi₂(PO₄)₃ with saturated Li₂SO₄ as an aqueous electrolyte showed an initial capacity of 87 mAh g^-1 [2,3]. However, regardless of the nature of the cathode materials, the large initial irreversibility and the gradual capacity fade of RLTP have hindered its application as an anode for aqueous Li-ion batteries. Attempts to improve the electrochemical properties of RLTP by nano-sizing [4] and controlling the oxygen vacancy [5] have recently been reported. Meanwhile, the use of Li-rich lithium titanium phosphate phases instead of current RLTP has not been described yet. We consider that the use of Li-rich phases of Li_1+xTi₂(PO₄)₃ could be expected to reduce the initial irreversibility and improve the cycle life at the expense of somewhat lowered capacity.

  Our preliminary attempts to synthesize Li-rich single-phases of Li_1+xTi₂(PO₄)₃ (0≤x≤2) revealed that this Li-rich phase could easily be incorporated with the well-known rhombohedral LiTi₂(PO₄)₃ contaminated with some other stable phases in the compositional space Li-Ti-P-O, such as LiTiPO₅, Li₄Ti₅O₁₂, LiTi₂(PO₄)₃, and TiO₂, depending on the synthesis temperature and the stoichiometry of the starting materials [6]. Therefore, our primary goal was to synthesize a single lithium-rich phase, which exists in a space group system different from that of RLTP (R-3c). A systematic approach to stabilize isotypic mixed-valent structures was previously pioneered by M. Catti. In that work, it was suggested that mixed-valent superstructures of Li_1+xIn_xTi_2-x(PO₄)₃ could exist when x < 0.5 or x > 1.0 [7], which is in good agreement with our preliminary data. Therefore, the smallest amount of lithium, x=1.5, was chosen to isolate the single phase of mixed-valent orthorhombic Li_1.5Ti₂(PO₄)₃.

  In this preliminary study, we tried to clarify how electrochemical properties could vary in the seemingly similar compounds LiTi₂(PO₄)₃ and Li_1.5Ti₂(PO₄)₃. The electrochemical properties of these materials were evaluated in conventional half cells to measure the interfacial resistance and Li⁺ ionic diffusivity in these structures. In addition, their feasibilities as anodes for aqueous lithium-ion cells were tested in LiFePO₄//Li_xTi₂(PO₄)₃ full-cell configurations using 1M Li₂SO₄ electrolytes.

References

 

[1] R.Ruffo, C.Wessells, R. Huggins, and Y. Cui, Electrochem. Comm., (2009) 11, 247-249.

[2] Y. Cui, Y. Hao, W. Bao, Y. Shi, Q. Zhuang, and Y. Qiang, J. Electrochem Soc., 160 (2013) A53-A59.

[3] X. Liu, T. Saito, T. Doi, S. Okada, and J. Yamaki, J. Power Sources, 189 (2009) 706-710.

[4] H. Roh, H. Kim, K. Roh, and K. Kim, RCS Advances, 4 (2014) 31672-31677.

[5] J. Luo, L. Chen, Y. Zhao, P. He, Y. Xia, J. Power Sources, 194 (2009) 1075-1080.

[6] N. V. Kosova, D. I. Osintsev, N. F. Uvarov, and E. T. Devyatkina, Chemistry for Sustainable Development, 13 (2005) 253-260.

[7] Michele Catti, J. Solid State Chem., 156 (2001) 305-312.