Rock-salt-type lithium metal sulphides as novel positive-electrode materials

One way of increasing the energy density of lithium-ion batteries is to use electrode materials that exhibit high capacities owing to multielectron processes. Here, we report two novel materials, Li2TiS3 and Li3NbS4, which were mechanochemically synthesised at room temperature. When used as positive-electrode materials, Li2TiS3 and Li3NbS4 charged and discharged with high capacities of 425 mA h g−1 and 386 mA h g−1, respectively. These capacities correspond to those resulting from 2.5- and 3.5-electron processes. The average discharge voltage was approximately 2.2 V. It should be possible to prepare a number of high-capacity materials on the basis of the concept used to prepare Li2TiS3 and Li3NbS4.

probably owing to the transformation of its polyhedral structure from a trigonal prism to a more stable octahedron [6][7][8] . It was recently reported that the amorphisation of TiS 3 via mechanical milling improves its performance as an electrode material [10][11][12] . Amorphous TiS 3 exhibits a three-dimensional framework, which would be favourable for stabilising its structure against the large volume changes caused by the charge-discharge process. The development of electrode materials with structures that remain stable during multielectron processes is thus strongly desired.
We focused on the lithium predoping of metal sulphide electrodes because (i) it might stabilise the structure and (ii)a lithium-containing positive-electrode material is highly desirable; commercial negative electrodes usually do not contain lithium. In contrast to lithium metal oxides and metal sulphides, lithium-containing metal sulphides have not been widely studied; a few exceptions are Li 2 FeS 2 and LiTi y M 12y S 2 (M 5 V, Cr, or Fe), etc. [13][14][15] . The development of new lithium-containing metal sulphides (Li x MS y ) has been a challenge. However, it should be possible to fabricate new crystallinephase materials with high capacities and good structural reversibility by extracting/inserting Li in lithium-containing metal sulphides.
Here, we report the development of two novel lithium transition metal sulphides, Li 2 TiS 3 and Li 3 NbS 4 , which were fabricated via mechanochemical synthesis. The structures and electrode performances of the sulphides were investigated. We found that Li 2 TiS 3 and Li 3 NbS 4 exhibited reversible charging and discharging with high capacities of 425 and 386 mA h g 21 , respectively; these values correspond to processes involving 2.5 and 3.5 electrons, respectively. Figure 1a shows the X-Ray diffraction (XRD) patterns of TiS 2 , Li 2 S, a mixture of TiS 2 and Li 2 S that was not mechanically milled (0 h), and Li 2 TiS 3 samples prepared by mechanical milling (MM) for 20, 40, 60, 80, and 100 h. Diffraction peaks corresponding to TiS 2 and Li 2 S were not evident after the milling process; however, several new peaks appeared. The XRD patterns of Li 2 TiS 3 after MM for 40, 60, and 100 h were similar. The broad peak observed at 2h 5 10-25u was owing to the KaptonH film. Figure 1b shows the XRD pattern of Li 2 TiS 3 prepared by MM for 40 h and the simulated pattern of Li 2 TiS 3 with a rock-salt-like structure (Fm -3m). It was assumed that its unit cell contains four formula units of [Li 2/3 /Ti 1/3 ] 4a S 4b , and the cell parameter is a 5 5.06 Å . Table 1 shows the simulated powder X-ray data obtained using the software program Powder Cell 16 . The peak positions of the prepared Li 2 TiS 3 sample were consistent with those of the simulated rock-salttype Li 2 TiS 3 . This suggests that the synthesised Li 2 TiS 3 sample had a rock-salt-type structure. Furthermore, the intensity ratios of the XRD peaks of the prepared Li 2 TiS 3 sample were in good agreement with the simulated ones for Li 2 TiS 3 , indicating that the Ti occupancy in the 4a sites was approximately 0.33. Pattern fitting, performed using the program RIETAN-2000 17 , also indicated that the Li and Ti occupancies in the 4a sites were approximately 0.665(4) and 0.335(4), respectively. To the best of our knowledge, this is the first report of a lithium titanium sulphide with a rock-salt-like structure.

Results
The rock-salt-type Li 2 TiS 3 was employed as an electrode active material for lithium secondary batteries. Figure 2a shows the chargedischarge curves for the first 5 cycles of the cells fabricated using Li 2 TiS 3 . The initial charge and discharge capacities were 273 and 425 mA h g 21 , respectively. Figure 2b shows the charge-discharge characteristics as a function of the lithium content. It was found that approximately 1.6 lithium atoms per formula unit could be extracted during the initial charging process and 2.5 lithium atoms could be inserted into the structure during the initial discharging process. The extraction and insertion of 2.5 lithium atoms into the structure of Li 2 TiS 3 was reversible during repeated cycling. Thus, this electrode active material could contain more than 2 lithium atoms in its structure; the structure could be charged and discharged such that it ranged from Li 0.4 TiS 3 to Li 2.9 TiS 3 over voltages of 1.5-3.0 V. The initial charging and discharging curve exhibited significant overlap with the second and third curves for the structure ranging from Li 0.4 TiS 3 to Li 2 TiS 3 , suggesting that the charge-discharge mechanism remained unchanged. Figure 2c shows the cycle performance of the cell fabricated using Li 2 TiS 3 . The discharge profiles did not change drastically over 5 cycles; however, the capacity of the cell faded after the 10 th cycle.  To further investigate the electrochemical reactions occurring during the charge-discharge processes, ex situ XRD measurements were performed for Li x TiS 3 with x 5 2.0, 1.0, 0.4, 1.4, and 2.6 ( Figure 3). The peaks became broad after a 1.0-electron charge, as shown in Figure 3b. Further, the peaks became even broader and decreased in intensity after the charge to Li 0.4 TiS 3 (Figure 3c). Some of these broader, lower-intensity peaks in Figure 3c were identified as being attributable to ZrO 2 , which could have contaminated the test samples during the ball-milling process. Finally, after the discharge to Li 1.4 TiS 3 ( Figure 3d) and Li 2.6 TiS 3 (Figure 3e), the peaks became sharper and increased in intensity. These results indicate that the extraction of lithium atoms from Li 2 TiS 3 results in amorphisation and that reverse reactions occur during the discharge process. It should be noted that the XRD pattern corresponding to the rocksalt-type sample was observed after discharging to Li 2.6 TiS 3 . This suggests that the volume change during charge and discharge can be expected to occur in a three-dimensional manner.
To evaluate the cyclability of Li 2 TiS 3 , an all-solid-state cell was constructed using Li 2 TiS 3 as the electrode material. It is known that a number of electrode materials show high cyclability when used in all-solid-state cells. Figure 4a shows the charge-discharge curves of the all-solid-state cell at 50uC. Figure 4b shows the cycling performance of the all-solid-state cell. The cell exhibited high capacity retention, retaining 97% of its capacity from the 5 th to the 50 th cycle.    Thus, Li 2 TiS 3 exhibits high reversibility when subjected to chargedischarge cycling.
A number of novel lithium metal sulphides can be developed through mechanochemical synthesis on the basis of the concept employed for altering the structure of Li 2 TiS 3 . Here, we show an example of one such material, Li 3 NbS 4 , which has a structure similar to that of Li 2 TiS 3 and was prepared by mechanochemical synthesis from Li 2 S, NbS 2 , and S 8 . Figure 5a shows the XRD pattern of Li 3 NbS 4 . The XRD pattern is similar to that of Li 2 TiS 3 . Therefore, Li 3 NbS 4 should also have a cubic structure, with the cell parameter a 5 5.13 Å . Figure 5b shows the charge-discharge curves of the cell fabricated using Li 3 NbS 4 ; 1 M LiPF 6 in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) was used as the electrolyte. The initial charge-discharge behaviour was similar to that of Li 2 TiS 3 . The reversible capacity was 386 mA h g 21 , which corresponds to 3.5-electron processes. Figure 5c shows the cycling performance of the Li 3 NbS 4 cell. The cell exhibited better cycling performance than that of the Li 2 TiS 3 cell; this was despite the fact that the charging and discharging occurred owing to 3.5-electron processes in the case of the former. The electronic conductivities of compressed pellets of powdered Li 2 TiS 3 and Li 3 NbS 4 were 8 3 10 26 and 2 3 10 23 S cm 21 , respectively. These values are likely related to the electronic conductivities of the compounds.
The elucidation of the charge-discharge mechanism should result in the development of new electrode materials that exhibit better performances. To be able to determine where the excess lithium atoms exist in the sulphides is the most important goal as it will lead to the synthesis of the novel high-capacity electrode materials. Further studies on this topic are currently underway.
The calculated gravimetric energy densities of Li/Li x TiS 3 (0.4 # x # 2.9) and Li/Li 3 NbS 4 (0.4 # x # 3.9) were 850 and 780 W h kg 21 . These values are larger than that for a typical positive electrode of LiCoO 2 , for which the energy density of Li/Li x CoO 2 (0.4 # x # 1) is 660 W h kg 21 . The combination of positive and negative electrodes is an important aspect. In order to use Li 2 TiS 3 and Li 3 NbS 4 as positive electrodes, their initial lithium content should be increased, that is, discharged state materials should be prepared, or lithium-containing negative electrodes such as lithium and lithium-alloy should be used. To conclude, in this study, we prepared the novel materials Li 2 TiS 3 and Li 3 NbS 4 from Li 2 S, TiS 2 , NbS 2 and S 8 via mechanochemical synthesis using ball milling. Li 2 TiS 3 was used as an electrode active material for a lithium secondary battery; it charged and discharged with initial charge and discharge capacities of 273 and 425 mA h g 21 , respectively. The average discharge voltage was approximately 2.2 V. Similarly, Li 3 NbS 4 was also used as an electrode active material and showed a high capacity of 386 mA h g 21 . Reversible charging and discharging were confirmed, despite the processes being multielectron ones.
We have synthesised only two novel lithium metal sulphides through a mechanochemical method. However, it should be possible to fabricate a number of novel lithium metal sulphides using this method.

Methods
A mechanochemical synthesis method was employed to fabricate the lithium titanium sulphides. Mechanochemical synthesis involves the promotion of chemical reactions using mechanical energy. There are many merits of this method, such as the ability to prepare nanoparticles directly using a simple homogeneous reaction at room temperature. The primary advantage of mechanochemical synthesis is the ability to obtain the final product in a metastable state, which is difficult through conventional methods. In particular, amorphous materials and materials with highly symmetrical three-dimensional crystal structures can be obtained. Li 2 TiS 3 was mechanochemically synthesised at room temperature using a planetary ball mill apparatus (P-7, Fritsch GmbH). Lithium sulphide (Li 2 S, 99.9%, Mitsuwa Pure Chemicals) and titanium disulphide (TiS 2 , 99.8%, Wako Pure Chemical Industries) were used as the starting materials. Appropriate amounts of Li 2 S and TiS 2 were weighed and mixed, and the mixture (1.5 g) was placed into a zirconia pot (45 mL) along with 500 zirconia balls (4 mm in diameter). The pot was then placed in an argon-filled glove box. The rotation speed of ball milling was fixed at 510 rpm.
The XRD patterns of the synthesised samples were recorded using an X-ray diffractometer (Rotaflex RU-200B/RINT, Rigaku). Prior to the measurements, the samples were covered with KaptonH film in an argon-filled glove box to prevent exposure to air. The electrochemical cells used to test the samples were also constructed in an argon-filled glove box. The working electrodes were prepared from Li 2 TiS 3 (10 mg), acetylene black (2 mg), and polytetrafluoroethylene (PTFE) powder (0.7 mg). A 1 M solution of LiPF 6 in a 50550 (by volume) mixture of EC and DMC (Tomiyama Pure Chemical Industries Ltd.) was used as the electrolyte. The counter electrode consisted of a Li foil disk (15 mm diameter, 0.2 mm thickness). The electrochemical measurements were performed at 30uC using a charge-discharge unit (TOSCAT-3100, Toyo System) at a current density of 10 mA g 21 between 1.5 and 3.0 V. The all-solid-state cells were constructed as follows. Li 2 TiS 3 , the glass electrolyte 70(0.75Li 2 S?0.25P 2 S 5 )?30LiI, and acetylene black (AB) were mixed in a weight ratio of 60530510 in an agate mortar, and the mixture was used to prepare the positive-electrode material. The glass electrolyte Li 2 S-P 2 S 5 -LiI, which has an ionic conductivity of more than 10 23 S cm 2118-19 , was used as the solid electrolyte. A lithium-indium alloy was used to make the counter electrode. Bilayered pellets consisting of the positive-electrode material (10 mg) and the glass electrolyte (70 mg) were pressed under a pressure of 360 MPa (diameter 5 10 mm). Pieces of indium foil (thickness 5 0.3 mm, diameter 5 9 mm) and lithium foil (thickness 5 0.1 mm, diameter 5 8 mm) were then placed on top of the bilayer pellets by pressing them all together under a pressure of 100 MPa. The pellets were pressed using two stainless steel rods, which were used as current collectors for both the positive electrode and the negative