Vanadium fluorophosphates: advanced cathode materials for next-generation secondary batteries

Entropy-Driven Enhancement of the Conductivity and Phase Purity of Na4Fe3(PO4)2P2O7 as the Superior Cathode in Sodium-Ion Batteries

Na4Fe3(PO4)2(P2O7) (NFPP) is regarded as a promising cathode material for sodium-ion batteries (SIBs) owing to its low cost, easy manufacture, environmental purity, high structural stability, unique three-dimensional Na-ion diffusion channels, and appropriate working voltage. However, for NFPP, the low conductivity of electrons and ions limits their capacity and power density. The generation of NaFeP2O7 and NaFePO4 inhibits the diffusion of sodium ions and reduces reversible capacity and rate performance during the manufacturing process in synthesis methods. Herein, we report an entropy-driven approach to enhance the electronic conductivity and, concurrently, phase purity of NFPP as the superior cathode in sodium-ion batteries. This approach was realized via Ti ions substituting different ratios of Fe-occupied sites in the NFPP lattice (denoted as NTFPP-X, T is the Ti in the lattice, X is the ratio of Ti-substitution) with the configurational entropic increment of the lattice structures from 0.68 R to 0.79 R. Specifically, 5% Ti-substituted lattice (NTFPP-0.05) inducing entropic augmentation not only improves the electronic conductivity from 7.1 × 10-2 S/m to 8.6 × 10-2 S/m but also generates the pure-phase of NFPP (suppressing the impure phases of the NaFeP2O7 and NaFePO4) of the lattice structure, which is validated by a series of characterizations, including powder X-ray diffraction (XRD), Fourier transform infrared spectra (FT-IR), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT). Benefiting from the Ti replacement in the lattice, the optimal NTFPP-0.05 composite shows a high first discharge capacity (118.5 mAh g-1 at 0.1 C), superior rate performance (70.5 mAh g-1 at 10 C), and excellent long cycling life (1200 cycles at 10 C with capacity retention of 86.9%). This research proposes a new entropy-driven approach to improve the electrochemical performance of NFPP and reports a low-cost, ultrastable, and high-rate cathode material of NTFPP-0.05 for SIBs.

Fe-modified NASICON-type Na3V2(PO4)3 as a cathode material for sodium ion batteries

The sol-gel method is used to synthesize a new compound called Na3Fe0.8V1.2(PO4)3/C (NFVP/C), which has a crystal structure and belongs to the NASICON-type family. The dimensions of NFVP's unit cell are a = 8.717 (1) Å, c = 21.84 (1) Å, and V = 1437.27 (0) Å3. The Na‖NFVP/C battery provides a discharge potential of 3.43 V compared to Na+/Na, an intriguing rate capability of 76.2 mA h g-1 at 40C, and maintains an impressive capacity of 97.8% after 500 cycles at 5C. The excellent efficiency of Na3Fe0.8V1.2(PO4)3/C can be ascribed to its elevated Na+ conductivity and reduced energy barrier for sodium-ion diffusion. The NASICON-type Na3Fe0.8V1.2(PO4)3/C is a promising material for sodium-ion batteries.

Low-Dimensional Vanadium-Based High-Voltage Cathode Materials for Promising Rechargeable Alkali-Ion Batteries

Owing to their rich structural chemistry and unique electrochemical properties, vanadium-based materials, especially the low-dimensional ones, are showing promising applications in energy storage and conversion. In this invited review, low-dimensional vanadium-based materials (including 0D, 1D, and 2D nanostructures of vanadium-containing oxides, polyanions, and mixed-polyanions) and their emerging applications in advanced alkali-metal-ion batteries (e.g., Li-ion, Na-ion, and K-ion batteries) are systematically summarized. Future development trends, challenges, solutions, and perspectives are discussed and proposed. Mechanisms and new insights are also given for the development of advanced vanadium-based materials in high-performance energy storage and conversion.

Simultaneous Mn and Cl doping on Na3V2(PO4)3 with high performance for full sodium-ion batteries

Nowadays, the poor conductivity and unstable structure have become obstacles for the popularization of Na3V2(PO4)3 (NVP). In the current work, a dual-modified Mn0.1Cl0.3-NVP composite doped with Mn and Cl is prepared by a facile sol-gel method. When Mn2+ with a large ionic radius replaces small V3+, it can improve the stability of the NVP crystal structure. In addition, the replacement of V3+ by Mn2+ in a low valence state can generate redundant hole carriers, which is conducive to the rapid transport of electrons. The substitution of PO43- by Cl-, which is more electronegative, can reduce the impedance and facilitate the movement of Na+. Owing to the synergistic effect of Mn and Cl co-substitution, the structural stability of NVP was systematically enhanced, and the electron transfer and ion diffusion were effectively improved. Consequently, the optimized Mn0.1Cl0.3-NVP sample demonstrated superior electrochemical performance and kinetic properties. It exhibited a high reversible capacity of 109.2 mA h g-1 at 0.1C. Even at 15 and 30C, high discharge capacities of 70.3 mA h g-1 and 68.2 mA h g-1 were observed after 2000 cycles with capacity retention above 80%. Moreover, it delivered remarkable capacities of 77.1 and 73.4 mA h g-1 at 100 and 200C with retained capacity values of 50.3 and 47 mA h g-1, respectively, after 2000 cycles. Furthermore, the assembled Mn0.1Cl0.3-NVP//HC full cell delivered a high value of 94.7 mA h g-1 and lit LED bulbs, indicating its excellent application potential.

Promising Cathode Materials for Sodium-Ion Batteries from Lab to Application

Sodium-ion batteries (SIBs) are seen as an emerging force for future large-scale energy storage due to their cost-effective nature and high safety. Compared with lithium-ion batteries (LIBs), the energy density of SIBs is insufficient at present. Thus, the development of high-energy SIBs for realizing large-scale energy storage is extremely vital. The key factor determining the energy density in SIBs is the selection of cathodic materials, and the mainstream cathodic materials nowadays include transition metal oxides, polyanionic compounds, and Prussian blue analogs (PBAs). The cathodic materials would greatly improve after targeted modulations that eliminate their shortcomings and step from the laboratory to practical applications. Before that, some remaining challenges in the application of cathode materials for large-scale energy storage SIBs need to be addressed, which are summarized at the end of this Outlook.