Na<sub>3</sub>V<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub>/C in Symmetric Cells: Evaluating\nAnode and Cathode Performance

Renewable energy sources necessitate efficient energy\nstorage,\nhighlighting the critical challenge and importance of electrochemical\nstorage devices, such as sodium-ion batteries. Na<sub>3</sub>V<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub> (NVP), as a typical NASICON structure,\nis a promising electrode material exhibiting fast Na<sup>+</sup> ion\nconductivity due to its 3D framework. In this report, we present the\nsynthesis of NVP/C (Na<sub>3</sub>V<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub>/Carbon composite) via the sol–gel method and investigate\nits electrochemical behavior in both the anodic and cathodic ranges,\nas well as its assessment in a symmetric cell followed by a detailed\npostcycling analysis. Increased overpotential has impacted the capacity\nretention of NVP-C (NVP as a cathode), delivering only 74.61% with\nan initial specific capacity of 120.20 mAh/g, whereas NVP-A (NVP as\nan anode) showed better cyclic stability, exhibiting 142.11 mAh/g\nat the 12th cycle with a capacity retention of 93.03%. NVP-C showed\nspecific capacities of 99.17, 84.74, and 53.20 mAh/g at 25, 0, and\n−5 °C, respectively, while NVP-A exhibited 191.42, 153.33,\nand 91.37 mAh/g under the same conditions, at 0.2C over 60 cycles.\nNVP was studied in a symmetric full cell, exhibiting an average energy\ndensity of 92.94 Wh/kg over 50 cycles. Postcycling investigations,\nincluding ToF-SIMS, XPS, and XANES, revealed that products of parasitic\nreactions, such as Na<sub>2</sub>CO<sub>3</sub>, NaF<sub>2</sub>,\nand Na<sub><i>x</i></sub>PF<sub><i>y</i></sub>O<sub><i>z</i></sub> along with structural distortion on\nthe anode side, significantly impact the lifecycle of NVP. Our findings\nprovide new insights into the performance, which will results in the\nimprovement of NVP symmetric full-cell configurations.