High-Rate and Long-Cycle Cathode for Sodium-Ion Batteries: Enhanced Electrode Stability and Kinetics via Binder Adjustment

Sodium-ion batteries (SIBs) are heralded as promising candidates for grid-scale energy storage systems due to their low cost and abundant sodium resources. Excellent rate capacity and outstanding cycling stability are always the goals for SIBs. Up to now, nearly all attention has been focused on the control of morphology and structure of electrode materials, but the influence of binders on their performance is neglected, especially in cathode materials. Herein, using Na₃V₂(PO₄)₂O₂F (NVPOF) as a cathode material, the influence of four different binders (sodium alginate, SA; carboxymethylcellulose sodium, CMC; poly(vinylidene fluoride), PVDF; and poly(acrylic latex), LA133) on its electrochemical performance is studied. As a result, when using SA as the binder, the electrochemical performance of the NVPOF electrode is improved significantly, which is mainly because of the high water solubility, rich carboxyl and hydroxyl groups, and high adhesive and cohesive properties of the SA binder, leading to the uniform distribution of active materials NVPOF and carbon black in electrodes, good integrity, low polarization, and superior kinetic properties of the NVPOF electrodes, as demonstrated by scanning electron microscopy, cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic intermittent titration technique. More importantly, when coupled with a hard carbon anode, the fabricated sodium-ion full cells also exhibit excellent rate performance, thus providing a preview of their practical application. This work shows that the battery performance can be improved by matching suitable binder systems, which is believed to have great importance for the further optimization of the electrochemical performance of SIBs.

Temperature-Dependent Electrochemical Properties and Electrode Kinetics of Na3 V2 (PO4 )2 O2 F Cathode for Sodium-Ion Batteries with High Energy Density

Phosphate cathode materials are practical for use in sodium-ion batteries (SIBs) owing to their high stability and long-term cycle life. In this work, the temperature-dependent properties of the phosphate cathode Na₃ V₂ (PO₄ )₂ O₂ F (NVPOF) are studied in a wide temperature range from -25 to 55 °C. Upon cycling at general temperature (above 0 °C), the NVPOF cathode retains an excellent charge/discharge performance, and the rate capability is noteworthy, indicating that NVPOF is a competitive candidate as a temperature-adaptive cathode for SIBs. Upon decreasing the temperature below 0 °C, the cell performance deteriorates, which may be caused by the electrolyte and Na electrode, based on the study of ionic conductivity and electrode kinetics. This work proposes a new breakthrough point for the development of SIBs with high performance over a wide temperature range for advanced power systems.

N/P-Dual-Doped Carbon-Coated Na3V2(PO4)2O2F Microspheres as a High-Performance Cathode Material for Sodium-Ion Batteries

Na₃V₂(PO₄)₂O₂F (NVPOF) is attracting great interest due to its large capacity and high working voltage. However, poor electronic conductivity limits the electrochemical performance of NVPOF. Herein, we fabricate N/P-dual-doped carbon-coated NVPOF microspheres (labeled as NVPOF@P/N/C) via a hydrothermal process followed by heat treatment. This microsphere-structured NVPOF@P/N/C composite has a relatively high tap density of 1.22 g/cm³. TEM and XPS results reveal that the dual-doped carbon layer is tightly coated on the NVPOF surface due to the bridging effect of P and has a good protective effect on NVPOF. Density functional theory (DFT) calculations confirm that a N/P-dual-doped carbon layer is advantageous to achieve higher electronic conductivity and lower migration activation energy than those of the undoped and single N- or P-doped carbon layer. As a cathode material for a sodium-ion battery (SIB), NVPOF@P/N/C exhibits high capacity (128 mAh/g at 0.5 C and 122 mAh/g at 2 C) and ultralong cycle performance (only 0.037% capacity fading rate per cycle in 500 cycles at 2 C). We believe that the NVPOF@P/N/C composite is appealing for high-performance SIBs with large energy density.