NaV(SO 4 ) 2 /C, Na 3 V(SO 4 ) 3 /C, and K 2 VO(SO 4 ) 2 /C: three Li-free vanadium sulfate cathode materials for lithium-ion batteries

In this study, three Li-free polyanion vanadium sulfates containing Na/K alkali metals were successfully synthesized by a facile solid-state method at moderate temperature. To improve the electronic conductivity, these three vanadium sulfate samples were mixed with carbon black SuperP by ball milling. These composites were characterized using XRD, IR, Raman, UV–vis diffuse reflectance, and FESEM. Moreover, the NaV(SO 4 ) 2 /C, Na 3 V(SO 4 ) 3 /C, and K 2 VO(SO 4 ) 2 /C composites were investigated as cathode materials for lithium-ion batteries (LIBs) for the first time. There are continuous ion transport channels in all three compounds, which tolerate reversible insertion-extraction of lithium ions. Benefiting from the two-electronic reaction of V 4+ /V 3+ and V 3+ /V 2+ redox pairs, Na 3 V(SO 4 ) 3 /C and K 2 VO(SO 4 ) 2 /C deliver discharge specific capacities of 120 mAh⋅g −1 , while NaV(SO 4 ) 2 /C displays a higher specific capacity of 200 mAh⋅g −1 in the first cycle. However, their rate performance and cyclic stability are poor, which may be attributed to the solubility of sulfate in electrolyte.