Titanium and Niobium Polysulfide Electrodes for Lithium/Metal Sulfide Secondary Batteries

Achieving high capacity by making use of multi electron processes has been desired to enhance the energy density of lithium-ion batteries. Recently, layered lithium metal oxides with more than one equivalent of lithium per transition metal have been actively researched and developed as new positive-electrode materials with capacities of more than 200 mAh g⁻¹. However, it is still difficult to achieve reversible capacity of more than 300 mAh g⁻¹. To achieve the high capacity, the redox reaction of anions in addition to that of transition metals is needed to be used; that is, the charge and discharge with processes involving more than two-electrons should be required in materials. Therefore, the challenging is development of new electrode active materials which can show redox reactions with more than two electron processes. Metal sulfides are attractive candidates for positive-electrode materials [1, 2]. Their high capacities originate from charge and discharge processes that involve more than one electron; for example, crystalline TiS₃ charges and discharges with high reversible capacities of greater than 300 mAh g⁻¹, which corresponds to ca.1.5 electron redox reaction.

In this study, amorphous titanium polysulfides a-TiS_x (x = 3, 4) and niobium polysulfides a-NbS_y(y = 3, 4, 5) were prepared by a mechanical milling process and their electrochemical properties were examined using the cells with carbonate-based electrolytes.

Amorphous titanium and niobium polysulfides were mechanochemically synthesized at room temperature using a planetary ball mill apparatus. Sulfur (S₈) and titanium disulfide (TiS₂) or niobioum disulfide (NbS₂) were used as starting materials.

The XRD measurements indicated that the diffraction peaks attributable to S₈ and TiS₂ or NbS₂disappeared by mechanical milling, suggesting that the samples became amorphous.

Fig. 1 shows charge-discharge curves of the cells using crystalline NbS₂ reagent, amorphous NbS₃, NbS₄, and NbS₅. The capacity increases with increasing sulfur/metal ratio and the cell using amorphous NbS₅ shows a high capacity of ca 600 mAh g^-1. The amorphous TiS₄ discharged and charged with a high reversible capacity of ca. 700 mAh g^-1in the 1.5–3.0 voltage range [3]. The cell using the amorphous metal polysulfides charged and discharged despite of the use of a carbonate-based electrolyte. Coulombic efficiency of the composite was higher than that of a sulfur electrode because the dissolution of the polysulfide into electrolytes was suppressed.

Designing amorphous polysulfides is an effective way to develop novel sulfide-based electrode materials with high capacity.

References

[1] M. S. Whittingham, Prog. Solid State Chem. 12 (1978) 41–99.

[2] M.H. Lindic, H. Martinez, A. Benayad, B. Pecquenard, P. Vinatier, A. Levasseur, D. Gonbeau, Solid State Ionics, 176 (2005) 1529–1537.

[3] A. Sakuda, N. Taguchi, T. Takeuchi, H. Kobayashi, H. Sakaebe, K. Tatsumi, Z. Ogumi, Electrochem. Commun. 31 (2013) 71–75.