Results of CV experiments for the third to fifth cycle of Na/ NiCr 2 S 4 ( see Figure S17 for cycles 1 - 5 , Supporting Information ) , for the third cycle of Na / Cr 3 S 4 and of Na / NiS are shown in Figure 7a . The third to fifth cathodic CV profiles exhibit two pronounced reduction peaks around 0.75 and 0.42 V ( Peaks 1 and 2 ) , whereas two distinct oxidation peaks occur at 1.24 and 1.56 V ( Peaks 3 and 4 ) in the anodic CV curves . For Na / NiS and Na / Cr 3 S 4 , only one major reduction and oxidation peak occur together with less distinct features . The Na / NiS CV agrees with literature data , [ 57,81 ] while no data are available for binary Cr sulfides used in SIBs . The comparison of the CV data recorded for the three different cells does not imply identical Na storage mechanisms , but allows assignment of CV peaks for Na / NiCr 2 S 4 to redox processes involving Ni and Cr as shown in Table 4 .