Unraveling the Redox Couples of VIII/VIV Mixed-Valent Na3V2(PO4)2O1.6F1.4 Cathode by Parallel-Mode EPR and In Situ/Ex Situ NMR

We herein present a study on the sodiation/desodiation behavior of Na3V2(PO4)2O1.6F1.4 cathode with I4/mmm tetragonal space group by the combination of dual-mode electron paramagnetic resonance (EPR) and in situ/ex situ solid-state nuclear magnetic resonance (SSNMR) techniques. The complementary parallel/perpendicular mode EPR, in situ 23Na NMR, 23Na/51V MAS NMR, and two-dimensional 23Na MQMAS NMR experiments substantiate that (i) both VIV and VIII are electrochemically active in this material, with the former transforming to VV, whereas the latter transforming to VIV; (ii) VIV continually converts to VV during the whole charge process, whereas VIII mostly reacts at the higher-potential charge plateau region; (iii) the Na+ extraction/intercalation process of Na3V2(PO4)2O1.6F1.4 is a widespread solid-solution mechanism; (iv) electronic spin hopping might occur locally between VIII and VV ions generated during the electrochemical reaction, resulting in the inconsecutive shift in 23Na resonance. This study shows how EPR spectroscopy in combination with SSNMR can be applied to probe electronic structure and long/short-range structure evolutions of vanadium-based cathode materials upon electrochemical process.