Orthorhombic Lithium Titanium Phosphate As an Anode Material for Li-Ion Rechargeable Batteries
Authors: Kee Yongho , Dimov Nikolay , Inoishi Atsushi , Okada Shigeto 
Published: February 2020

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 LiMn0.05Ni0.05Fe0.9PO4//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 Li1+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 Li1+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 Li1+xInxTi2-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 Li1.5Ti2(PO4)3.
In this preliminary study, we tried to clarify how electrochemical properties could vary in the seemingly similar compounds LiTi2(PO4)3 and Li1.5Ti2(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//LixTi2(PO4)3 full-cell configurations using 1M Li2SO4 electrolytes.

References

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