Operando monitoring the redox activity of sodium vanadium titanium phosphate electrodes in organic and aqueous electrolytes by magnetometry

A sol-gel method was used for the synthesis of composite electrodes consisting of sodium vanadium titanium phosphate (NVTP) and carbon. X-ray diffraction studies confirm that the NVTP possesses the NASICON crystalline structure, and cell tests indicate that the electrodes exhibit good electrochemical performance. Temperature-dependent magnetic susceptibility measurements reveal the presence of the V ³⁺ and Ti ⁴⁺ oxidation states in the pristine Na ₂VTi(PO ₄) ₃. The change in oxidation states during de-/sodiation was monitored continuously by means of operando magnetometry measurements in both organic (1 M NaPF ₆ in EC:PC) and aqueous electrolyte (1 M Na ₂SO ₄). The variation of the magnetic susceptibility demonstrates that in both types of electrolytes, the V ³⁺/V ⁴⁺redox couple is active in the high potential regime, whereas the Ti ³⁺/Ti ⁴⁺ couple operates in the low potential regime, and that no significant side reactions occur. Furthermore, it is shown that the two redox couples are distinctly separated and occur in a consecutive manner. Additionally, the results of our operando study indicate that both redox couples contribute approximately equally to the total capacity of NVTP.