Is there a common reaction pathway for chromium sulfides as anodes in sodium-ion batteries? A case study about sodium storage properties of MCr 2 S 4 ( M = Cr, Ti, Fe)

We present new insights into the electrochemical properties of three metal sulfides MCr 2 S 4 ( 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 Na 2 S nanoparticles and ultra-small metal particles during initial discharge without formation of intermediate NaCrS 2 domains as previously reported for CuCrS 2 and NiCr 2 S 4 . After a full cycle, the structural integrity of MCr 2 S 4 ( 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