Improving Na3V2(PO4)2F3 half-cell performance with NaBF4-enhanced sodium difluoro(oxalato)borate electrolyte

Abstract

The global shift towards low-carbon energy storage has increased interest in sodium-ion batteries (SIBs) as a safer, cost-effective alternative to lithium-ion batteries. However, the commercial viability has been limited by compatibility issues between high-energy-density cathode materials, such as Na₃V₂(PO₄)₂F₃ (NVPF), and high-voltage electrolytes. Addressing the challenges, H-NaODFB (comprising 93.91% NaODFB and 5.85% NaBF₄) electrolyte significantly improves the electrochemical performance and stability of NVPF cathodes. Na/NVPF half-cells using H-NaODFB electrolyte retained 92.4% capacity after 900 cycles, while Na/Na symmetric cells demonstrated a cycle life exceeding 600 h at 0.5 mA cm⁻². The superior performance is attributed to improved Na⁺ (de)intercalation reversibility, lower interfacial impedance (619.8 vs. 10,650.0 Ω), and faster reaction kinetics compared to NaODFB alone. Advanced time of flight-secondary ion mass spectrometry (TOF-SIMS), X-ray photoelectron spectroscopy (XPS) and aberration corrected transmission electron microscope (AC-TEM), combined with first-principles calculations, revealed that NaBF₄ in the H-NaODFB electrolyte plays a critical role in forming a stable cathode electrolyte interphase (CEI). The CEI consists of an initial inorganic and organic layer, followed by a fluoroborate layer, and finally a stable organic–inorganic polymeric layer, enhancing electrode stability and preventing over-oxidation. These findings provide valuable insights for designing high-performance electrolytes for SIBs.