Tailoring Laminar Li₃VO₄ (LVO) for Superior Cycle Life (>6000 cycles) in Aqueous Zinc-Ion Batteries

Vanadate compounds like Li₃VO₄ (LVO) exhibit vital properties that make them a compelling alternative to traditional MnO and Prussian blue analogues for aqueous zinc-ion batteries (AZIBs), owing to their structural stability, long cycle life, and superior capacity [1]. In this work, we employed a scalable surfactant-free sol-gel method, offering a more efficient synthesis route than conventional solid-state and hydrothermal techniques, to produce LVO with a laminar morphology [2]. To achieve the desired phase of LVO, ammonium metavanadate (Sigma-Aldrich) was dissolved in 2-methoxyethanol to form solution-P, while lithium acetate dihydrate was dissolved separately in the same solvent to create solution-Q. Solution-Q was dropwise added to solution-P with continuous stirring for 48 hours at 60 °C. The resulting gel was then annealed at 800°C for 4 hours to obtain a pure phase of LVO. Powder X-ray diffraction (XRD) confirmed that LVO crystallizes in an orthorhombic structure with the Pmn21 space group [3], and Rietveld refinement provided the lattice constants: a = 5.448 Å, b = 6.327 Å, and c = 4.949 Å. UV-Vis spectroscopy revealed that LVO possesses a broad band gap of 3.95 eV, characteristic of a semiconducting material. High-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) showed sharp diffraction spots, indicating high crystallinity and the presence of a laminar flake-like morphology observed from FESEM. For electrode preparation, the LVO active material was mixed with Super P carbon black, multi-walled carbon nanotubes (MWCNTs), and a PVDF binder in NMP solvent in a 7:2:1 ratio to form a slurry. This slurry was coated onto pre-carbonized carbon cloth (heated to 1300 °C under nitrogen) and dried under vacuum at 80 °C for 24 hours. The electrodes were punched out and assembled into CR2016 cells using an aqueous electrolyte composed of 2 M ZnSO₄ and 0.5 M Zn(CF₃SO₃)₂ in distilled water. Cyclic voltammetry (CV) tests on the LVO||Zn half-cell revealed one oxidation peak at 1.06 V and two reduction peaks at 1.03 V and 0.602 V at a scan rate of 0.1 mV/s. At a higher scan rate of 1 mV/s, a slight increase of 3.7 % in oxidation potential and a reduction potential deviation of -11.1% (V⁵⁺/V⁴⁺) and -1.9 % (V⁴⁺/V³⁺) were observed. A shouldered peak appeared at 1.22 V at 0.5 mV/s, possibly indicating a biphasic mechanism during the vanadium redox process. The LVO||Zn half-cell (mass loading ~1.28 mg/cm²) demonstrated an initial capacity (Biologic BC810 battery cycler/Neware) of 54.71 mAh/g with a coulombic efficiency (CE) of approximately 99.68% and retained a capacity of 36.81 mAh/g (CE ~ 99.87%) after 6000 cycles when cycled at 0.5 A/g between 0.15 V and 1.6 V. At a lower current density of 0.075 A/g, the cell exhibited a discharge capacity of 71.63 mAh/g (CE ~95.96%) and retained 61.09 mAh/g after 200 cycles. When cycled at 0.1 A/g for an additional 1000 cycles, the cell maintained a capacity of 56.27 mAh/g, achieving an overall retention of ~91 %. However, reducing the discharge potential from 0.15 V to 0.1 V caused a significant decline in capacity retention over 5000 cycles, with the cell delivering 186.72 mAh/g in the first cycle and retaining only ~26.6 mAh/g with an average CE of 99.73 %. Additional <i>ex-situ studies</i>, including time-of-flight secondary ion mass spectrometry (ToF-SIMS), are being conducted to investigate the spatial distribution of Zn-ions and various other fragments in solid-electrolyte interphase (SEI) layer for a more comprehensive understanding of the degradation mechanisms.<br/><br/><br/>[1] Cheng, et al, <i>Small</i> 2024, 20, 2305762.<br/><br/>[2] M. Singh et al., pat. (I. Delhi), Provisional Patent Application: 202311051106. US patent application No. 18/786,397.<br/><br/>[3] Song, et al, J. Mater. Sci. Technol., 140, 142-152 (2023).