CuFeS2 as a Very Stable High-Capacity Anode Material for Sodium-Ion Batteries: A Multimethod Approach for Elucidation of the Complex Reaction Mechanisms during Discharge and Charge Processes

Pressure-Stabilized High-Entropy (FeCoNiCuRu)S2 Sulfide Anode toward Simultaneously Fast and Durable Lithium/Sodium Ion Storage

Pressure-stabilized high-entropy sulfide (FeCoNiCuRu)S₂ (HES) is proposed as an anode material for fast and long-term stable lithium/sodium storage performance (over 85% retention after 15 000 cycles @10 A g^-1 ). Its superior electrochemical performance is strongly related to the increased electrical conductivity and slow diffusion characteristics of entropy-stabilized HES. The reversible conversion reaction mechanism, investigated by ex-situ XRD, XPS, TEM, and NMR, further confirms the stability of the host matrix of HES after the completion of the whole conversion process. A practical demonstration of assembled lithium/sodium capacitors also confirms the high energy/power density and long-term stability (retention of 92% over 15 000 cycles @5 A g^-1 ) of this material. The findings point to a feasible high-pressure route to realize new high-entropy materials for optimized energy storage performance.

Is there a common reaction pathway for chromium sulfides as anodes in sodium-ion batteries? A case study about sodium storage properties of MCr2S4 (M = Cr, Ti, Fe)

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

Synthetically Produced Isocubanite as an Anode Material for Sodium-Ion Batteries: Understanding the Reaction Mechanism During Sodium Uptake and Release

Bulk isocubanite (CuFe₂S₃) was synthesized via a multistep high-temperature synthesis and was investigated as an anode material for sodium-ion batteries. CuFe₂S₃ exhibits an excellent electrochemical performance with a capacity retention of 422 mA h g^-1 for more than 1000 cycles at a current rate of 0.5 A g^-1 (0.85 C). The complex reaction mechanism of the first cycle was investigated via PXRD and X-ray absorption spectroscopy. At the early stages of Na uptake, CuFe₂S₃ is converted to form crystalline CuFeS₂ and nanocrystalline NaFe_1.5S₂ simultaneously. By increasing the Na content, Cu⁺ is reduced to nanocrystalline Cu, followed by the reduction of Fe²⁺ to amorphous Fe⁰ while reflections of nanocrystalline Na₂S appear. During charging up to -5 Na/f.u., the intermediate NaFe_1.5S₂ appears again, which transforms in the last step of charging to a new unknown phase. This unknown phase together with NaFe_1.5S₂ plays a key role in the mechanism for the following cycles, evidenced by the PXRD investigation of the second cycle. Even after 400 cycles, the occurrence of nanocrystalline phases made it possible to gain insights into the alteration of the mechanism, which shows that Cu_xS phases play an important role in the region of constant specific capacity.