Comprehensive experiment design for the multication synergistically regulated vanadium oxide cathode for zinc-ion batteries

To solve the drawbacks of cathode materials for zinc-ion batteries (ZIBs), including limited structural stability and slow Zn²⁺ diffusion kinetics, a Na⁺, Mg²⁺, and Al³⁺ co-intercalated hydrated vanadium oxide (NMA-VOH) cathode is designed. Through the multi-ion pre-intercalation, the interlayer spacing of the material is expanded, and oxygen defects are enriched. Meanwhile, a hierarchical structure assembled from nanobelts is achieved for the NMA-VOH. Thus, multi-ions can synergistically regulate the structure of the VOH at the macroscopic and microscopic levels, realizing a “one-stone, three-birds” effect. The structure regulation induces increased specific capacity, enhanced rate performance, and ultra-long cycling stability. The Zn²⁺ reaction mechanism is also studied to explore the enhanced electrochemical performance of the NMA-VOH cathode.

NMA-VOH is synthesized via a facile one-pot hydrothermal reaction. For comparison, single-ion intercalated samples with Na⁺, Mg²⁺, or Al³⁺ ions (N-VOH, M-VOH, and A-VOH, respectively) and pristine VOH are also prepared under identical conditions. The crystal structure, morphology, chemical bonding, and oxygen defects are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FT-IR), Raman spectroscopy, electron paramagnetic resonance (EPR), and X-ray photoelectron spectroscopy (XPS). CR2032 coin cells and quasi-solid-state flexible batteries are fabricated with aqueous and hydrogel electrolytes, respectively. The specific capacity, cycling performance, and rate performance are measured and compared to evaluate the structural superiority. Moreover, in situ EIS, ex situ XRD, ex situ Raman, and ex situ XPS are used to probe the Zn²⁺ reaction mechanism of the NMA-VOH cathode in the ZIBs.

XRD shows that NMA-VOH exhibits a (001) peak at 6.62°, corresponding to an interlayer spacing of 1.32 nm, which is larger than that of N-VOH, M-VOH, A-VOH, and VOH. SEM and TEM images reveal a uniform hierarchical architecture assembled from ultrathin nanobelts, which can shorten the ion diffusion paths and stabilize the structure. EPR confirms the increased oxygen defects in NMA-VOH. As the cathode in ZIBs, NMA-VOH delivers a stable capacity of 384 mAh· g⁻¹ after 400 cycles with a capacity retention of 89.0%, both of which are higher than those of the N-VOH, M-VOH, A-VOH, and VOH cathodes. At a high current of 5.0 A· g⁻¹, NMA-VOH also maintains a high capacity of 192 mAh· g⁻¹ after 20000 cycles, producing a low capacity decay of 0.0017% per cycle. In the quasi-solid-state flexible battery, a capacity of 121 mAh· g⁻¹ is delivered after 200 cycles at 1.0 A·g⁻¹. Moreover, this battery works well under bending at different angles. The ex situ and in situ measurements illustrate the co-intercalation of H⁺ and Zn²⁺ into the NMA-VOH cathode for charge storage. Furthermore, structure reversibility is proved during the electrochemical process.

The co-intercalation of Na⁺, Mg²⁺, and Al³⁺ successfully expands the interlayer spacing, introduces abundant oxygen defects, and constructs a hierarchical architecture, which synergistically facilitates fast Zn²⁺ diffusion. Consequently, NMA-VOH exhibits outstanding specific capacity, rate capability, and ultra-long cycling stability. The quasi-solid-state battery also shows impressive electrochemical performance and promising flexibility, indicating its potential application in wearable devices. This comprehensive experiment provides students with systematic training in materials design, structure characterization, device testing, and mechanism analysis, bridging the gap between theoretical knowledge and scientific innovation. Therefore, this teaching experiment design establishes a multidimensional teaching framework, enabling senior undergraduates with systematic research training and motivating their interest in new energy research.