ABSTRACT
Sodium-ion batteries (SIBs) are potential candidates for large energy storage usage because of the natural abundance and cheap sodium. Nevertheless, improving the energy density and cycling steadiness of SIB cathodes remains a challenge. In this work, F-doping Na3 Al2/3 V4/3 (PO4 )3 (NAVP) microspheres (Na3 Al2/3 V4/3 (PO4 )2.9 F0.3 (NAVPF)) are synthesized via spray drying and investigated as SIB cathodes. XRD and Rietveld refinement reveal expanded lattice parameters for NAVPF compared to the undoped sample, and the successful cation doping into the Na superionic conductor (NASICON) framework improves Na+ diffusion channels. The NAVPF delivers an ultrahigh capacity of 148 mAh g-1 at 100 mA g-1 with 90.8% retention after 200 cycles, enabled by the activation of V2+ /V5+ multielectron reaction. Notably, NAVPF delivers an ultrahigh rate performance, with a discharge capacity of 83.6 mAh g-1 at 5000 mA g-1 . In situ XRD demonstrates solid-solution reactions occurred during charge-discharge of NAVPF without two-phase reactions, indicating enhanced structural stability after F-doped. The full cell with NAVPF cathode and Na+ preintercalated hard carbon anode shows a large discharge capacity of 100 mAh g-1 at 100 mA g-1 with 80.2% retention after 100 cycles. This anion doping strategy creates a promising SIB cathode candidate for future high-energy-density energy storage applications.
ABSTRACT
Configurational entropy-stabilized single-phase high-entropy oxides (HEOs) have been considered revolutionary electrode materials with both reversible lithium storage and high specific capacity that are difficult to fulfill simultaneously by conventional electrodes. However, precise understanding of lithium storage mechanisms in such HEOs remains controversial due to complex multi-cationic oxide systems. Here, distinct reaction dynamics and structural evolutions in rocksalt-type HEOs upon cycling are carefully studied by in situ transmission electron microscopy (TEM) including imaging, electron diffraction, and electron energy loss spectroscopy at atomic scale. The mechanisms of composition-dependent conversion/alloying reaction kinetics along with spatiotemporal variations of valence states upon lithiation are revealed, characterized by disappearance of the original rocksalt phase. Unexpectedly, it is found from the first visualization evidence that the post-lithiation polyphase state can be recovered to the original rocksalt-structured HEOs via reversible and symmetrical delithiation reactions, which is unavailable for monometallic oxide systems. Rigorous electrochemical tests coupled with postmortem ex situ TEM and bulk-level phase analyses further validate the crucial role of structural recovery capability in ensuring the reversible high-capacity Li-storage in HEOs. These findings can provide valuable guidelines to design compositionally engineer HEOs for almighty electrodes of next-generation long-life energy storage devices.
