The Development of Vanadyl Phosphate Cathode Materials for Energy Storage Systems: A Review

Diffusion of ferrocene through vanadyl phosphate by density functional theory

Here, we employed the nudged elastic band (NEB) method to simulate the diffusion of ferrocene through vanadyl phosphate (VOPO4), with a focus on understanding the diffusion pathways arising from the complex structure of ferrocene. We systematically evaluated a total of 36 potential diffusion paths, categorizing them into three groups based on their directional orientation: 15 paths between V sites along the [110] direction, 15 paths from V to P sites along the [100] direction, and 6 paths between P sites also along the [110] direction. Our analysis revealed that the energy barriers for diffusion along the [110] direction typically ranged between 0.25 and 0.35 eV, which are notably higher than those observed for pathways along the [100] direction, where the energy barriers ranged from 0.11 to 0.20 eV. To further elucidate the complex deformation of ferrocene during diffusion, we established four key measures to characterize the structural conformation: the angle of the axis of the ferrocene molecule relative to the [010] direction within the (001) plane, the dihedral angle between the two cyclopentadienyl rings, the orientation angle of the -CH bonds with respect to the [001] direction, and the angle between two -CH bonds from the two cyclopentadienyl rings.

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.

Preparation and Electrochemical Performance of Na2-xLixFePO4F/C Composite Cathode Materials with Different Lithium/Sodium Ratios

With their advantages of abundant raw material reserves, safety, and low toxicity and cost, sodium-ion batteries (SIBs) have gained increasing attention in recent years. Thanks to a high theoretical specific capacity (124 mAh g-1), a high operating voltage (about 3.2 V), and a very stable three-dimensional layered structure, sodium ferric fluorophosphate (Na2FePO4F, NFPF) has emerged as a strong candidate to be used as a cathode material for SIBs. However, applications are currently limited due to the low electronic conductivity and slow ion diffusion rate of NFPF, which result in a low actual specific capacity and a high rate performance. In this study, the authors used a high-temperature solid-phase technique to produce Na2-xLixFePO4F/C (0 ≤ x ≤ 2) and evaluated the impact on electrode performance of materials with different Na+ and Li+ contents (values of x). Transmission electron microscopy (TEM) and X-ray diffraction (XRD) were also used to analyze the material's crystal structure and nanostructure. The results show that the material had the best room-temperature performance when x = 0.5. At a charge-discharge rate of 0.1 C, the first discharge-specific capacity of the resulting Na1.5Li0.5FePO4F/C cathode material was 122.9 mAh g-1 (the theoretical capacity was 124 mAh g-1), and after 100 cycles, it remained at 118 mAh g-1, representing a capacity retention rate of 96.2% and a Coulomb efficiency of 98%. The findings of this study demonstrate that combining lithium and sodium ions improves the electrochemical performance of electrode materials.

δ-VOPO4 as a high-voltage cathode material for aqueous zinc-ion batteries

Aqueous zinc-ion batteries (AZIBs) with excellent safety, low-cost and environmental friendliness have great application potential in large-scale energy storage systems and thus have received extensive research interest. Layered oxovanadium phosphate dihydrate (VOPO4·2H2O) is an appealing cathode for AZIBs due to the unique layered framework and desirable discharge plateau, but bottlenecked by low operation voltage and unstable cycling. Herein, we propose delta-oxovanadium phosphate (δ-VOPO4) without conventional pre-embedding of metal elements or organics into the structure and paired it into AZIBs for the first time. Consequently, superior to the layered counterpart, δ-VOPO4 exhibits better performance with a prominent discharge voltage of 1.46 V and a higher specific capacity of 122.6 mA h g-1 at 1C (1C = 330 mA g-1), as well as an impressive capacity retention of 90.88 mA h g-1 after 1000 cycles under 10C. By investigation of structure resolution and theoretical calculation, this work well elucidates the structure-function relationship in vanadyl phosphates, offering more chances for exploration of new cathode materials to construct high performance AZIBs.

Towards Reversible High-Voltage Multi-Electron Reactions in Alkali-Ion Batteries Using Vanadium Phosphate Positive Electrode Materials

Vanadium phosphate positive electrode materials attract great interest in the field of Alkali-ion (Li, Na and K-ion) batteries due to their ability to store several electrons per transition metal. These multi-electron reactions (from V²⁺ to V⁵⁺) combined with the high voltage of corresponding redox couples (e.g., 4.0 V vs. for V³⁺/V⁴⁺ in Na₃V₂(PO₄)₂F₃) could allow the achievement the 1 kWh/kg milestone at the positive electrode level in Alkali-ion batteries. However, a massive divergence in the voltage reported for the V³⁺/V⁴⁺ and V⁴⁺/V⁵⁺ redox couples as a function of crystal structure is noticed. Moreover, vanadium phosphates that operate at high V³⁺/V⁴⁺ voltages are usually unable to reversibly exchange several electrons in a narrow enough voltage range. Here, through the review of redox mechanisms and structural evolutions upon electrochemical operation of selected widely studied materials, we identify the crystallographic origin of this trend: the distribution of PO₄ groups around vanadium octahedra, that allows or prevents the formation of the vanadyl distortion (O^…V⁴⁺=O or O^…V⁵⁺=O). While the vanadyl entity massively lowers the voltage of the V³⁺/V⁴⁺ and V⁴⁺/V⁵⁺ couples, it considerably improves the reversibility of these redox reactions. Therefore, anionic substitutions, mainly O^2- by F^-, have been identified as a strategy allowing for combining the beneficial effect of the vanadyl distortion on the reversibility with the high voltage of vanadium redox couples in fluorine rich environments.

Pseudocapacitance multiporous vanadyl phosphate/graphene thin film electrode for high performance electrochemical capacitors

Supercapacitors are being considered as promising electricity storage devices with green sustainable energy conversion. To efficiently develop and optimize pseudocapacitive material of vanadyl phosphate, herein, multiporous vanadyl phosphate/graphene (denoted as MP-VOPO₄@rGO) is fabricated for the first time with phytic acid as a phosphorus source by extremely simple sol-gel and drop coating methods, and used as the free binder thin film electrode of supercapacitors. The smart combination of honeycomb-like architecture and graphene incorporation results in more active sites and low internal resistance, significantly improving energy storage performance. The effect of introducting polystyrene (denoted as PS) template and rGO on the performance of the nanocomposite is systematically analyzed by comparing the performance of the corresponding thin film electrodes. The MP-VOPO₄@rGO thin film electrode delivers superior pseudocapacitive performance of 672 F g^-1 at 1 A g^-1 as well as a remarkable rate capability of 552 F g^-1 at 5 A g^-1, and it presents a remarkable longterm cycling stability, with a capacitance retention of 83.5% after 5000 cycles. Very interestingly, the results of surface capacitance contribution dominance clearly demonstrates its rapid capacitive response. In addition, based on MP-VOPO₄@rGO thin film as positive and negative electrodes, the corresponding assembled symmetric supercapacitors exihibits outstanding energy density of 26.3 Wh kg^-1 at power density of 249.9 W kg^-1. This investigation can not only provide a versatile strategy to design other thin film electrode materials but also open up a new insight into the development of polyanion phosphate composites for next-generation high performance energy storage systems.