Antisite occupation induced single anionic redox chemistry and structural stabilization of layered sodium chromium sulfide

Abstract

The intercalation compounds with various electrochemically active or inactive elements in the layered structure have been the subject of increasing interest due to their high capacities, good reversibility, simple structures, and ease of synthesis. However, their reversible intercalation/deintercalation redox chemistries in previous compounds involve a single cationic redox reaction or a cumulative cationic and anionic redox reaction. Here we report an anionic redox chemistry and structural stabilization of layered sodium chromium sulfide. It was discovered that the sulfur in sodium chromium sulfide is electrochemically active, undergoing oxidation/reduction rather than chromium. Significantly, sodium ions can successfully move out and into without changing its lattice parameter c, which is explained in terms of the occurrence of chromium/sodium vacancy antisite during desodiation and sodiation processes. Our present work not only enriches the electrochemistry of layered intercalation compounds, but also extends the scope of investigation on high-capacity electrodes.

Anionic redox reaction in layered NaCr2/3Ti1/3S2 through electron holes formation and dimerization of S–S

“…Accordingly, the questions are whether these concepts could be mutually corroborated by other systems, such as layered chalcogenides, whether the reversible formation and decomposition of S–S dimers or electron holes on S could be evidenced, and what is the underlying nature of anionic redox chemistry in layered chalcogenides. The reversible anionic reduction/oxidation of (S 2 ) 2− + 2e − ↔ 2S 2− was reported previously in non-layered polysulfide electrode materials 23–28 and in layered materials confirmed by X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) techniques 29–31 . However, there has been a lack of core structural evidence on the formation of S–S dimers or electron holes on S in layered chalcogenides.…”

“…1a. The capacity of pure NaCrS 2 comes mainly from the high voltage range above 2.55 V, corresponding to the redox between S 2− and S 2 2− 31 , with a characteristic of large polarization about 0.6 V. As contrast, the capacity of NaTiS 2 comes mainly from low voltage range below 2.55 V, corresponding to the redox of Ti 3+ 34 , with a small polarization about 0.05 V. These features can be observed in the curves of NaCr 1− y Ti y S 2 series, which show small polarization in low voltage range and large polarization in high voltage range. The proportion of capacity from high/low voltage range relates to the ratio of Cr/Ti doped.…”

Exploring the bottlenecks of anionic redox in Li-rich layered sulfides

“…42 Other successful strategies to adjust proper band positioning have consisted in either preparing Ti 3+ -doped Li1.33-y/3Ti 4+ 0.67-2y/3Ti 3+ yS2, or triggering antisite occupation as shown for NaCr 3+ S2, or preparing Li1.33Ti 4+ 0.67S2 and Li1.5Nb 5+ 0.5S2 having disordered rocksalt structures. [46][47][48][49] We herein demonstrate the feasibility to activate the anionic redox activity in Li-rich layered Li1.33-2y/3Ti 4+ 0.67-y/3Fe 2+ yS2 via the use of Fe substitution. This situation is favourable for reversible sulfur redox, since the Fe 2+/3+ redox couple with available electrons (3d 6 ) is expected to be pinned at the top of the S 3p band (Figure 1c).…”

Oxygen Redox Activity in Cathodes: A Common Phenomenon Calling for Density-Based Descriptors

“…Lithium and post-lithium battery cathodes with anionic redox activity during cycling have emerged as heavily investigated material class for future high-performance secondary batteries in recent years. Most of these materials constitute transition metal chalcogenides [1][2][3][4][5] or chalcohalides [6][7] and are primarily of interest due their increased gravimetric capacity compared to the commercially deployed layered transition metal oxides of the O3-type α-NaFeO2 structure (e.g. LiCoO2, LiNixMnyCozO2), which is ascribed to the formal oxidation of anions during charge (and conversely a reduction during discharge) providing additional charge storage capability.…”

“…25 Using spheres with the same volume as obtained from the Bader analysis for each atom in the cell leads to the correct total volume in which the projection is carried out, but the corresponding radii of ~1.2 Å for the transition metal and ~1.5 Experimental investigations on oxygen redox activity reported in the literature rely on techniques like e.g. electron energy loss spectroscopy (EELS) 29 or X-ray absorption spectroscopy (XAS), 4,7 but it should be noted that the interpretation of these results is based on the comparison with reference compounds and they do not provide an unambiguous way to determine atomic charge states in molecules or bulk. Due to the covalent character of bonding in "ionic" transition metal compounds, such benchmarks themselves do not provide pure metal ion reference states.…”