Sodium-pillared vanadium oxides as next-gen materials: Does co-inserted water control the cyclic stability of vanadates in an aqueous electrolyte?

Abstract

Layered vanadium oxides are, due to their adaptable interlayer distance and a multivalent state of vanadium, interesting electrode materials for various kinds of metal-ion batteries with both aqueous and nonaqueous electrolytes. In this study, a precursor solution adjusted for hydrothermal synthesis of sodium vanadate Na2V6O16, is modified by addition of carbon nanopowder Vulcan XC72, and this suspension is used for an one-pot hydrothermal synthesis of sodium vanadate/C composite. XRD analysis evidenced that the obtained composite is the mixture of two vanadate phases: Na2V6O16 and NaV6O15, instead of expected single-phase vanadate. SEM and TEM observations of composite revealed that these vanadate phases emerge in a form of both nanowires and nanospheres. The heterogeneity of phases and morphology of the composite is attributed to the directional action of carbon nanoparticles as a nucleation seed. The composite, used in aqueous Li-ion battery with LiFe0.95V0.05PO4/C cathode and LiNO3 electrolyte, displayed improved charge storage (220 mAh g−1 at current rate 150 mA g−1 in the initial discharge), after dc charging, in comparison to NaV6O15 or Na2V6O16 nanowire anodes. Furthermore, versatile insertion capability of this heterostructured composite towards mono (Li+, Na+) and multivalent (Ca2+, Mg2+, Al3+) ions in aqueous electrolytes is demonstrated by Cyclic Voltametry method, where the capacity stability depends on the type of cation, following the order Al3+, Li+, Mg2+, Ca2+. Upon 100 potentiodynamic chaging/discharging cycles, Ca2+ions display much better capacitance retention than Li+ ions (i.e., 64 vs. 13% of the initial value). The coinserted electrolyte constituents, H2O and OH− species, are considered to be responsible for the improved capacity retention, making vanadium ions less strained and less soluble. The pH, the concentration of the electrolyte and the type of anion are found to influence the cyclic behavior of observed vanadate composite in Ca-containing electrolyte. These results open a new directions in vanadate electrochemistry towards more sustainaible and cheaper aqueous batteries.