Unraveling the Redox Couples of V^III/V^IV Mixed-Valent Na₃V₂(PO₄)₂O_1.6F_1.4 Cathode by Parallel-Mode EPR and in Situ/Ex Situ NMR

We herein present a study on the sodiation/desodiation behavior of Na₃V₂(PO₄)₂O_1.6F_1.4 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 V^IV and V^III are electrochemically active in this material, with the former transforming to V^V, whereas the latter transforming to V^IV (ii) V^IV continually converts to V^V during the whole charge process, whereas V^III mostly reacts at the higher-potential charge plateau region; (iii) the Na⁺ extraction/intercalation process of Na₃V₂(PO₄)₂O_1.6F_1.4 is a widespread solid-solution mechanism; (iv) electronic spin hopping might occur locally between V^III and V^V 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.