Synthesis, Characterization and Electrochemical Evaluation of Layered Vanadium Phosphates as Cathode Material for Aqueous Rechargeable Zn-ion Batteries

The potential application of rechargeable multivalent ion batteries in portable devices and renewable energy grid integration have gained substantial research interest in aqueous Zn-ion batteries (ZIBs). Compared to Li-based batteries, ZIBs offer lower costs, higher energy density, and safety that make them more attractive for energy storage in grid integration applications. Currently, more research is required to find a suitable cathode material for ZIBs with high capacity and structural stability during charge/discharge cycling. Vanadium phosphate (VOP) compounds as cathode material for ZIBs have been of particular interest, owing to vanadium’s diverse oxidation states. In this present work, two VOP compounds, [H0.6(VO)3(PO4)3(H2O)3].4H2O and VOPO4.2H2O, were synthesized from phosphoric acid and different sources of vanadium via a simple hydrothermal method. Various characterization techniques were carried out, revealing the layered structure of both products and high purity of [H0.6(VO)3(PO4)3(H2O)3].4H2O. Zn/VOP batteries were prepared using Zn metal as counter and reference electrode and 3 M ZnSO4.7H2O as electrolyte. Electrochemical tests were conducted to evaluate the cycling performance of VOPs as cathode material for aqueous Zn-ion batteries. Based on the results, both compounds have shown highly reversible Zn-ion intercalation and deintercalation. VOPO4.2H2O achieved a higher specific capacity of up to 85 mAh/g during discharging, as opposed to 65 mAh/g for the hydrated VOP complex. However, [H0.6(VO)3(PO4)3(H2O)3].4H2O is more stable with higher reproducibility than VOPO4.2H2O during cycling. Nevertheless, more research is still required to enhance the specific capacity and improve the cycling performance of VOP-based cathodes for their prospective use in aqueous ZIBs.

Unveiling the Interlayer Spacing Dependence of Zn-Ion Storage Performance in Layered Vanadium Phosphates

Layered vanadium phosphate (VOPO4•2H2O) is reported as a promising cathode material for rechargeable aqueous Zn2+ batteries (ZIBs) owing to its unique layered framework and high discharge plateau. However, its sluggish Zn2+ diffusion kinetics, the low specific capacity and poor electrochemcial stability remains a major issue in battery application. In this work, a group of phenylamine (PA)-intercalated VOPO4•2H2O with varied interlayer spacing (14.8, 15.6 and 16.5 Å) is synthesized respectively via a solvothermal method for cathode of aqueous ZIBs. The specific capacity is quite dependent on d-spacing in PA-VOPO4•2H2O system followed by an approximate linear tendency, and the maximum interlayer spacing (16.5 Å phase) results in a discharge capacity of 268.2 mAh•g-1 at 0.1 A•g-1 with a high discharge plateau of ~ 1.3 V and an energy density of 328.5 Wh•Kg-1. Both of the experimental data and DFT calculation identify that the optimal 16.5 Å spacing can boost fast Zinc-ion diffusion with an ultrahigh diffusion coefficient of ~ 5.7 × 10-8 cm-2•s-1. The intercalation of PA molecules also significantly increases the hydrophobility in the aqueous electrolyte, resulting in the inhibiting the decomposition / dissolution of VOPO4•2H2O and remarkably improved cycling stability over 2000 cycles at 5.0 A•g-1 with a capacity retention of ~200 mAh•g-1. Our study provides a feasible solution on the sluggish Zn2+ diffusion kinetics and poor cyclstability, and also shows a clear understanding on the interlayer chemistry of layered phosphates towards aqueous Zinc-ion storage.

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 O2− 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.

Principle in Interlayer-Spacing Regulation of Layered Vanadium Phosphates for Superior Zinc-ion Batteries

Layered vanadium phosphate (VOPO4•2H2O) is reported as a promising cathode material for rechargeable aqueous Zn2+ batteries (ZIBs) owing to its unique layered framework and high discharge plateau. However, its sluggish...

Highly crystalline vanadium phosphate catalysts synthesized using poly(acrylic acid-co-maleic acid) as a structure directing agent

Addition of poly(acrylic acid-co-maleic acid) (PAAMA) during the synthesis of vanadium phosphates changes the morphology and improves the catalyst performance.