High-Entropy Strategy to Suppress Volumetric Strain and Enhance Diffusion Rate of Na3V2(PO4)2F3 Cathode for Durable and High-Areal-Capacity Zinc-Ion Battery Pouch Cells
Authors: Xu Jia , Liu Jian 
Published: August 2024

The burgeoning global environmental awareness and the escalating demand for renewable energy storage have intensified the critical need for next-generation battery technologies.1,2 In recent decades, there has been a sustained pursuit of grid-scale energy storage systems that balance practicality and efficiency. In this context, aqueous zinc-ion batteries (AZIBs) have emerged as a promising contender, driven by the distinct advantages of the 2.8-fold higher volumetric capacity (5849 mAh cm-3) of zinc metal over lithium metal (2062 mAh cm-3), and the use of non-toxic, cost-effective aqueous electrolyte. To date, substantial efforts have been dedicated to investigating suitable AZIB cathode materials, including Prussian blue analogs (PBAs), transition-metal (TM) oxides (e.g., MnO2 and V2O5), organic compounds, and polyanion-type compounds.
Recently, there has been a notable surge of interest in polyanion-type compounds. Na3V2(PO4)2F3 (NVPF), demonstrates an operating potential of exceeding 1.60 V and an enhanced energy density of 97.5 Wh kg-1. Regrettably, these advantages are often counterbalanced by undesirable side reactions and sluggish ion kinetics, culminating in incompetent rate capabilities and suboptimal cycling stability.3 High-entropy (HE) materials have emerged as promising candidates for facilitating ion transport and fortifying cathode stability across various energy storage systems due to their exceptional design flexibility and compositional tunability.4,5 Inspired by conventional HE stabilization strategies, this work introduces a carbon-coated Na3V1.9(AlZnMnCrNb)0.1(PO4)2F3 cathode, denoted as HE-NVPF@C. Employing this cathode, coupled with a carbon-film-functionalized Zn anode, referred to as Zn@CF, and acetonitrile(AN)-water hybrid electrolyte, we configured an AZIB with substantial enhancements in long-term cycling stability and high areal capacity. AZIBs based on HE-NVPF@C cathodes presented a high reversible capacity of 77.45 mAh g−1 at 2C and remarkable cycling stability with a capacity loss of a mere 0.0031% per cycle over 6,000 cycles at 20C. In particular, the areal capacity of the HE-NVPF@C cathode reached up to 2.17 mAh cm-2, and HE-NVPF@C also attained a high capacity density of 78.2 mAh g-1 and 61.6 mAh g-1 when working under 50 and -20°C, making it more competitive among the reported cathodes in AZIBs. In addition, the pouch cell provides a long cycling lifespan with 90.8% capacity retention at 5C after 200 cycles. More importantly, the Zn2+ storage mechanism of the high-entropy NASICON-type cathode was systematically studied for the first time. In-situ X-ray diffraction (XRD) and density functional theory (DFT) calculations revealed that NVPF@C with high-entropy doping improved cycling stability due to the mitigated Jahn-Teller distortion, enhanced Zn2+ diffusion rate, and reduced lattice volumetric strain upon Zn2+ extraction and insertion. This work not only lays the door for developing stable AZIBs but also enriches the mechanistic discussion about the improved cycling or structural stability of high-entropy NASICON structures.
Keywords: Zinc-ion battery; High-entropy material; NASICON; Na3V2(PO4)2F3; Pouch cell
Reference
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