Partial cationic exchange boosting sodium storage of NaVP2O7

Abstract

Sodium vanadium pyrophosphate (NaVP2O7 or NVP) with KAlP2O7-type structure exhibits an elevated redox potential (3.9 V vs. Na/Na+) compared to other vanadium-based polyanion cathodes, which enhances its superiority for high-energy sodium-ion batteries (SIBs), but encounters with the low Na+ conduction. Herein, we propose a cation exchange strategy to generate a skeleton-expanded Na1-xKxVP2O7 (x ≈ 0.25, denoted as NKVP) cathode stemming from its pyrophosphate counterpart KVP2O7. Different from previous entire ion-exchange method, the partial retained K skeleton in NKVP expanded the Na-ion conduction bottleneck to facilitate the Na-ions to across, lowered the Na-ion diffusion barriers (0.33 to 0.27 eV), which directly boosted Na-ion diffusion kinetics. As a result, the NKVP exhibited much higher specific capacity than NVP (73.8 vs. 25.2 mAh g−1), and it remained 55.7 % even at 40C (4.1 A g−1), surpassing most of pyrophosphate cathode. This was also verified at high-rate NKVP//NaTi2(PO4)3 and NKVP//hard‑carbon full cells. Moreover, the expanded bottleneck also reinforced the structure stability, enabling the NKVP cycle at 10C over 2200 cycles. This research opens new avenues for the design of high-rate and long-cycling polyanionic materials.