ABSTRACT
Sodium-ion batteries (SIBs) hold great potential for use in large-scale grid storage applications owing to their low energy cost compared to lithium analogs. The symmetrical SIBs employing Na3 V2 (PO4 )3 (NVP) as both the cathode and anode are considered very promising due to negligible volume changes and longer cycle life. However, the structural changes associated with the electrochemical reactions of symmetrical SIBs employing NVP have not been widely studied. Previous studies on symmetrical SIBs employing NVP are believed to undergo one mole of Na+ storage during the electrochemical reaction. However, in this study, it is shown that there are significant differences during the electrochemical reaction of the symmetrical NVP system. The symmetrical sodium-ion cell undergoes ≈2 moles of Na+ reaction (intercalation and deintercalation) instead of 1 mole of Na+ . A simultaneous formation of Na5 V2 (PO4 )3 phase in the anode and NaV2 (PO4 )3 phase in the cathode is revealed by synchrotron-based X-ray diffraction and X-ray absorption spectroscopy. A symmetrical NVP cell can deliver a stable capacity of ≈99 mAh g-1 , (based on the mass of the cathode) by simultaneously utilizing V3+ /V2+ redox in anode and V3+ /V4+ redox in cathode. The current study provides new insights for the development of high-energy symmetrical NIBs for future use.
ABSTRACT
Sodium-ion batteries (SIBs) have become increasingly important as next-generation energy storage systems for application in large-scale energy storage. It is very crucial to develop an eco-friendly and green SIB technique with superior performance for sustainable future use. Replacing the conventional inorganic electrode materials with green and safe organic electrodes will be a promising approach. However, the poor electrochemical kinetics, unstable electrode-electrolyte interface, high solubility of the electrodes in the electrolyte, and large amount of conductive carbon present great challenges for organic SIBs. In this study, the issues of organic electrodes are addressed through atomic-level manipulation of these organic molecules using a series of ultrathin (Å-level) metal oxide coatings (Al2 O3 , ZnO, and TiO2 ). Uniform and precise coatings on the perylene-3,4,9,10-tetracarboxylicacid dianhydride by gas-phase atomic layer deposition technique shows a stable interphase, enhanced electrochemical kinetics (71C, 10 A g-1 ), and excellent stability (89%-500 cycles) compared to conventional organic electrode (70%-200 cycles). Further studies reveal that the chemical stability of the metal oxide coating layer plays a critical role in influencing the redox behavior, and improving kinetics of organic electrodes. This study opens a new avenue for developing high-energy organic SIBs with performance equivalent to inorganic counterparts.
