Electrochemical Properties of TiO2(R) As a High-Potential Negative-Electrode for Lithium Ion Batteries

We have previously reported that a high potential negative electrode (TiO₂(B)) is promising as a negative electrode material for large-scale Li-ion batteries (LIB), which require high capacity (> 300 mAh g^-1), high efficiency, long life, low cost and high safety [1].  However, its morphology, needle-like structure, is a problem for attaining a high volumetric energy density of the electrode.  Ramsdellite type lithium titanium oxide (TiO₂ (R)) also has a high theoretical capacity (335 mAh g^-1) than spinel type lithium titanium oxide (175 mAh g^-1) [2, 3].  It crystalizes without any specific anisotoropy, and hence a high packing density of the electrode and a high energy density will be achieved. In the present study, we prepared TiO₂(R) from Li₂CO₃ and TiO₂(anatase) as raw materials, and investigated the charge/discharge properties as a high potential negative electrode for lithium ion batteries.

Ramsdellite type LiTi₂O₄was synthesized according to the following processes using solid-state reactions.

Li₂CO₃ + 4TiO₂(anatase)→Li₂Ti₄O₉ [750 ^oC, 12 h, air]

Li₂Ti₄O₉ → 2LiTi₂O₄       [1000 ^oC, 18 h, Ar/H₂(10%)]

 TiO₂(R) was obtained from LiTi₂O₄ by ion exchange in 1 M HCl for 3 days. Charge/discharge properties of the TiO₂(R) were investigated at C/30-10C rates between 1.2 and 3.0 V using a two-electrode coin-type cell with a composite working electrode(TiO₂(R): Ketjenblack: PVDF = 8:1:1), a Li metal counter electrode, and 1 M LiPF₆dissolved in a mixture of ethylene carbonate (EC) and dimethyl carbonate(DMC) (1:2, by vol.).

 XRD pattern of the resultant TiO₂ (R) powder is shown in Fig. 1. The pattern indicated that single-phase TiO₂(R) was obtained.  The color of LiTi₂O₄ powder was black, while the TiO₂(R) was white. These should result from the changes in formal oxidation number of Ti from +3.5 to +4. The diameter of TiO₂(R) powder was estimated to be 2.5-10 mm from the SEM image of the TiO₂ (R) powder shown in the inset in Fig. 1. Moreover, the tap density of TiO₂ (R) powder was evaluated to be as high as about 1.33 g cm^-3. Therefore, high-energy density is expected.

Charge and discharge curves of obtained for a TiO₂(R) composite electrode at C/6 are shown in Fig. 2. A couple of plateaux due to the insertion/extraction of lithium ion were observed at around 1.3 V and 2.2 V. This fact suggests that two types of insertion/extraction sites exist in TiO₂ (R).  One is an energetically stable site and the other is relatively unstable.  The TiO₂ (R) electrode showed an initial discharge capacity as high as 205.8 mAh g^-1. In addition, the cycleability was high; 98.4 % of the initial capacity was retained in the 50th cycle.  The electrode density was determined to be 1.24 g cm^-3, and therefore the gravimetric capacity was converted to a volumetric capacity of 255.2 mAh cm^-3.

The rate performance for a TiO₂(R) composite electrode was shown in Fig. 3, together with that for TiO₂(B) as a reference. The initial discharge capacity reached as high as 229.7 mAh g^-1 at C/30. The capacity retention was higher than that of TiO₂(B), particularly at > 3 C rate. The discharge capacity in the 25th cycle (202.7 mAh g^-1), which was obtained after charge/discharge operation at a high rate of 10 C, was almost the same as that in the 7th cycle (204.1 mAh g^-1). These results suggest the high durability of TiO₂(R) electrodes.