The compound CuCrS2 with a quasi‐layered crystal structure was investigated as room temperature rechargeable sodium‐ion battery electrode. It exhibits excellent performance as anode material with a high reversible capacity of 424 mAh g−1 at 700 mA g−1 after 200 cycles and a capacity retention of 98.6 % compared to the third cycle. Results of ex‐situ X‐Ray diffraction experiments performed at different stages of the discharge process demonstrate that at the beginning of Na uptake, Cu+ cations are reduced to nanosized metallic Cu particles which are expelled from the host lattice. Simultaneously, Na is inserted into the host material leading to the formation of Na0.7Cu0.15CrS2 with significantly expanded interlayer space. Metallic Cu and Na0.7Cu0.15CrS2 coexist at this stage of discharge. Increasing the amount of Na per formula unit leads to successive conversion to X‐ray amorphous Cr, nanocrystalline Na2S and metallic Cu. The formation of highly disordered metallic Cr with domain sizes in the range of few nanometres is revealed by atomic pair distribution function analysis. During the charge process, the nanocrystalline Cu particles are retained and Na0.7Cu0.15CrS2 is at least partially reformed. The finely distributed Cu particles dramatically improve the long‐time stability as evidenced by comparison of the electrochemical behaviour of mere NaCuCrS2.
The pseudo‐layered sulfide NiCr2S4 exhibits outstanding electrochemical performance as anode material in sodium‐ion batteries (SIBs). The Na storage mechanism is investigated by synchrotron‐based X‐ray scattering and absorption techniques as well as by electrochemical measurements. A very high reversible capacity in the 500th cycle of 489 mAh g−1 is observed at 2.0 A g−1 in the potential window 3.0–0.1 V. Full discharge includes irreversible generation of Ni0 and Cr0 nanoparticles embedded in nanocrystalline Na2S yielding shortened diffusion lengths and predominantly surface‐controlled charge storage. During charge, Ni0 and Cr0 are oxidized, Na2S is consumed, and amorphous Ni and Cr sulfides are formed. Limiting the potential window to 3.0–0.3 V an unusual nickel extrusion sodium insertion mechanism occurs: Ni2+ is reduced to nanosized Ni0 domains, expelled from the host lattice, and is replaced by Na+ cations to form O3‐type like NaCrS2. Surprisingly, the discharge and charge processes comprise Na+ shuttling between highly crystalline NiCr2S4 and NaCrS2 enabling a superior long‐term stability for 3000 cycles. The results not only provide valuable insights for the electrochemistry of conversion materials but also extend the scope of layered electrode materials considering the reversible nickel extrusion sodium insertion reaction as new concept for SIBs.
Various electrode materials are considered for sodium-ion batteries (SIBs) and one important prerequisite for developments of SIBs is a detailed understanding about charge storage mechanisms. Herein, we present a rigorous...
We present the convenient synthesis and characterization of the new ternary thiostannate Na 4 SnS 4 (space group I4 1 =acd) by directed removal of crystal water molecules from Na 4 SnS 4 •14 H 2 O. The compound represents a new kinetically stable polymorph of Na 4 SnS 4 , which is transformed into the known, thermodynamically stable form (space group P � 42 1 c) at elevated temperatures. Thermal co-decomposition of mixtures with Na 3 SbS 4 •9 H 2 O generates solid solution products Na 4À x Sn 1À x Sb x S 4 (x = 0.01, 0.10) isostructural to the new polymorph (x = 0). Incorporation of Sb 5 + affects the bonding and local structural situation noticeably evidenced by X-ray diffraction, 119 Sn and 23 Na NMR, and 119 Sn Mössbauer spectroscopy. Electrochemical impedance spectroscopy demonstrates an enormous improvement of the ionic conductivity with increasing Sb content for the solid solution (σ 25°C = 2 × 10 À 3 , 2 × 10 À 2 , and 0.1 mS cm À 1 for x = 0, 0.01, and 0.10), being several orders of magnitude higher than for the known Na 4 SnS 4 polymorph.
A highly unusual solid-state epitaxy-induced phase transformation of Na 4 SnS 4 • 14H 2 O (I) into Na 4 Sn 2 S 6 • 5H 2 O (II) occurs at room temperature. Ab initio molecular dynamics (AIMD) simulations indicate an internal acid-base reaction to form [SnS 3 SH] 3À which condensates to [Sn 2 S 6 ] 4À . The reaction involves a complex sequence of OÀ H bond cleavage, S 2À protonation, SnÀ S bond formation and diffusion of various species while preserving the crystal morphology. In situ Raman and IR spectroscopy evidence the formation of [Sn 2 S 6 ] 4À . DFT calculations allowed assignment of all bands appearing during the transformation. X-ray diffraction and in situ 1 H NMR demonstrate a transformation within several days and yield a reaction turnover of � 0.38 %/h. AIMD and experimental ionic conductivity data closely follow a Vogel-Fulcher-Tammann type T dependence with D(Na) = 6 × 10 À 14 m 2 s À 1 at T = 300 K with values increasing by three orders of magnitude from À 20 to + 25 °C.
The new thiostannate Na 4 Sn 2 S 6 was prepared by directed crystal water removal from the hydrate Na 4 Sn 2 S 6 • 5H 2 O at moderate temperatures. While the structure of the hydrate comprises isolated [Sn 2 S 6 ] 4À anions, that of the anhydrate contains linear chains composed of corner-sharing SnS 4 tetrahedra, a structural motif not known in thiostannate chemistry. This structural rearrangement requires bond-breakage in the [Sn 2 S 6 ] 4À anion, movements of the fragments of the opened [Sn 2 S 6 ] 4À anion and SnÀ SÀ Sn bond formation. Simultaneously, the coordination environment of the Na + cations is significantly altered and the in situ formed NaS 5 polyhedra are joined by corner-and edgesharing to form a six-membered ring. Time-dependent in situ Xray powder diffraction evidences very fast rehydration into Na 4 Sn 2 S 6 • 5H 2 O during storage in air atmosphere, but recovery of the initial crystallinity requires several days. Impedance spectroscopy demonstrates a mediocre room-temperature Na + ion conductivity of 0.31 μS cm À 1 and an activation energy for ionic transport of E a = 0.75 eV.
We present new insights into the electrochemical properties of three metal sulfides MCr2S4 (M = Cr, Ti, Fe) probed as anode materials in sodium-ion batteries for the first time. The electrodes deliver decent reversible capacities and good long-term cycle stability, e.g., 470, 375, and 524 mAh g−1 are obtained after 200 cycles applying 0.5 A g−1 for M = Cr, Ti, and Fe, respectively. The reaction mechanisms are investigated via synchrotron-based X-ray powder diffraction and pair distribution function analyses. The highly crystalline educts are decomposed into Na2S nanoparticles and ultra-small metal particles during initial discharge without formation of intermediate NaCrS2 domains as previously reported for CuCrS2 and NiCr2S4. After a full cycle, the structural integrity of MCr2S4 (M = Cr, Ti, Fe) is not recovered. Thus, the Na storage properties are attributed to redox reactions between nanoscopic to X-ray amorphous conversion products with only local atomic correlations M···S/S···S in the charged and M···M/Na···S in the discharged state.
Graphical Abstract
Wir präsentieren eine sehr bequeme Synthese und die Charakterisierung des neuen Thiostannats Na 4 SnS 4 (I4 1 =acd) durch gezielte thermische Zersetzung von Na 4 SnS 4 •14 H 2 O. Die Verbindung stellt ein neues, kinetisch stabilisiertes Polymorph von Na 4 SnS 4 dar, welches sich beim Erhitzen in die thermodynamisch stabile Form (P � 42 1 c) umwandelt. Durch thermische Ko-Zersetzung von Mischungen mit Na 3 SbS 4 •9 H 2 O werden feste Lösungen Na 4À x Sn 1À x Sb x S 4 (x = 0.01, 0.10) erhalten, die isostrukturell zum neuen Polymorph (x = 0) kristallisieren. Ergebnisse aus Röntgenbeugung, 119 Sn NMR-, 23Na NMR-und 119 Sn-Mößbauerspektroskopie zeigen, dass der Einbau von Sb 5 + die Bindungen und die lokalen Strukturen erheblich beeinflusst. Durch Impedanzspektroskopie wird eine enorme Verbesserung der Ionenleitfähigkeit mit zunehmendem Sb-Gehalt für die feste Lösung ermittelt (σ 25 °C = 2 × 10 À 3 , 2 × 10 À 2 und 0.1 mS cm À 2 1 für x = 0, 0.01 und 0.10), welche um
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