Oxygen Defect Engineering toward Zero-Strain V2O2.8@Porous Reticular Carbon for Ultrastable Potassium Storage

Potassium-ion batteries (KIBs) are promising candidates for large-scale energy storage devices due to their high energy density and low cost. However, the large potassium-ion radius leads to its sluggish diffusion kinetics during intercalation into the lattice of the electrode material, resulting in electrode pulverization and poor cycle stability. Herein, vanadium trioxide anodes with different oxygen vacancy concentrations (V2O2.9, V2O2.8, and V2O2.7 determined by the neutron diffraction) are developed for KIBs. The V2O2.8 anode is optimal and exhibits excellent potassium storage performance due to the realization of expanded interlayer spacing and efficient ion/electron transport. In situ X-ray diffraction indicates that V2O2.8 is a zero-strain anode with a volumetric strain of 0.28% during the charge/discharge process. Density functional theory calculations show that the impacts of oxygen defects are embodied in reducing the band gap, increasing electron transfer ability, and lowering the diffusion energy barriers for potassium ions. As a result, the electrode of nanosized V2O2.8 embedded in porous reticular carbon (V2O2.8@PRC) delivers high reversible capacity (362 mAh g-1 at 0.05 A g-1), ultralong cycling stability (98.8% capacity retention after 3000 cycles at 2 A g-1), and superior pouch-type full-cell performance (221 mAh g-1 at 0.05 A g-1). This work presents an oxygen defect engineering strategy for ultrastable KIBs.

Defect-Induced Dense Amorphous/Crystalline Heterophase Enables High-Rate and Ultrastable Sodium Storage

Currently, the construction of amorphous/crystalline (A/C) heterophase has become an advanced strategy to modulate electronic and/or ionic behaviors and promote structural stability due to their concerted advantages. However, their different kinetics limit the synergistic effect. Further, their interaction functions and underlying mechanisms remain unclear. Here, a unique engineered defect-rich V₂ O₃ heterophase structure (donated as A/C-V₂ O_{3- x} @C-HMCS) composed of mesoporous oxygen-deficient amorphous _- hollow core (A-V₂ O_{3- x} /HMC) and lattice-distorted crystalline shell (C-V₂ O₃ /S) encapsulated by carbon is rationally designed via a facile approach. Comprehensive density functional theory (DFT) calculations disclose that the lattice distortion enlarges the porous channels for Na⁺ diffusion in the crystalline phase, thereby optimizing its kinetics to be compatible with the oxygen-vacancy-rich amorphous phase. This significantly reduces the high contrast of the kinetic properties between the crystalline and amorphous phases in A/C-V₂ O_{3- x} @C-HMCS and induces the formation of highly dense A/C interfaces with a strong synergistic effect. As a result, the dense heterointerface effectively optimizes the Na⁺ adsorption energy and lowers the diffusion barrier, thus accelerating the overall kinetics of A/C-V₂ O_{3- x} @C-HMCS. In contrast, the perfect heterophase (defects-free) A/C-V₂ O₃ @C-HCS demonstrates sparse A/C interfacial sites with limited synergistic effect and sluggish kinetics. As expected, the A/C-V₂ O_{3- x} @C-HMCS achieves a high rate and ultrastable performance (192 mAh g^-1 over 6000 cycles at 10 A g^-1 ) when employed for the first time as a cathode for sodium-ion batteries (SIBs). This work provides general guidance for realizing dense heterophase cathode design for high-performance SIBs and beyond.