Fast-Charging Failure Mechanism of Na3V2(PO4)3 Cathode and Its Mitigation via Trace Sm3+ Doping

NASICON-type Na3V2(PO4)3 (NVP) is widely regarded as a promising cathode for sodium-ion batteries owing to its robust three-dimensional framework and high operating voltage (~3.4 V vs. Na+/Na). However, NVP suffers severe capacity degradation under fast-charging conditions due to its intrinsically low electronic conductivity, which critically impedes its practical deployment. Herein, we systematically investigate the fast-charging failure mechanism of NVP and propose a trace Sm3+ doping strategy (x = 0.03) to address this limitation. Undoped NVP retains only 13.5% and 56.62% of its initial capacity after 1000 cycles at 5000 mA g−1 and 1307 cycles at 2000 mA g−1, respectively. Post-cycling scanning electron microscopy (SEM) reveals extensive crack formation and particle pulverization, providing direct morphological evidence for structural failure. To overcome this, Sm3+-doped Na3V1.97Sm0.03(PO4)3/C (NVPSM) is synthesized via a sol–gel method. X-ray diffraction (XRD) confirms that the NASICON phase is preserved. Raman spectroscopy reveals an improved graphitization degree (ID/IG = 0.97 vs. 1.02 for NVP), and X-ray photoelectron spectroscopy (XPS) verifies the V3+ oxidation state and the incorporation of Sm3+. Electrochemically, NVPSM achieves capacity retentions of 60.3% after 2300 cycles at 5000 mA g−1 and 83.89% after 1436 cycles at 2000 mA g−1. Electrochemical impedance spectroscopy confirms reduced charge-transfer resistance, and post-cycling SEM shows markedly improved structural integrity. These results demonstrate that trace rare-earth doping effectively mitigates fast-charging-induced structural failure in NVP-based cathodes.