Hierarchical TiO2 microspheres with enlarged lattice spacing for rapid and ultrastable sodium storage

"Grafting" NiSe onto Cu2-xSe with twinborn structure embedded in carbon-based nanofibers to weave freestanding sodium-ion storage electrode

The fabrication of freestanding electrodes for Na⁺ storage is necessary to achieve high energy density. However, the large radius of Na⁺ results in a large volume fluctuation and sluggish reaction kinetics of active materials, particularly at a high active material content, thereby impeding electrochemical performance with undesirable cycling performance or rate capability. In this study, a freestanding electrode based on the "NiSe grafted on Cu_2-xSe" heterostructure with double-carbon protective shells (NiSe/Cu_2-xSe@C@NCNFs) was successfully constructed for Na⁺ storage. In this microstructure, N-doped carbon nanofibers (NCNFs) serve as the stem of the twinborn NiSe/Cu_2-xSe heterostructure with a built-in electric field, where NiSe improves Na⁺ absorption and Cu_2-xSe enhances Na⁺ diffusion. The "graft" design enabled the freestanding NiSe/Cu_2-xSe@C@NCNFs electrode with a high active mass content of 76.1 wt% to exhibit superior electrochemical performance for Na⁺ storage (75 mAh g^-1 at 2 A g^-1) compared to those of Cu_2-xSe@C@NCNFs (26 mAh g^-1 at 2 A g^-1) and NiSe@C@NCNFs (9 mAh g^-1 at 2 A g^-1).

Coupling High Rate Capability and High Capacity in an Intercalation-Type Sodium-Ion Hybrid Capacitor Anode Material of Hydrated Vanadate via Interlayer-Cation Engineering

Layered metal vanadates with intercalation pseudocapacitive behaviors show great promise for applications in sodium-ion hybrid capacitor anode materials due to their large interlayer distances, which benefit the fast Na⁺ solid-state diffusion. However, their charge storage capacity is significantly constrained by the limited available sites that allow the intercalation of Na⁺ ions. In this work, by engineering the interlayer cations, Ni_0.12Zn_0.2V₂O₅·1.07H₂O is designed as a high-performance anode material in sodium-ion hybrid capacitors. The Ni/Zn codoping in the layered vanadate leads to the integration of high rate capability and high specific capacity. Specifically, the spacious interlayer spacing and the pillaring effects of Zn ions together lead to the high rate performance and decent cycling stability, while the redox reactions of the interlayer Ni ions efficiently upgrade the charge storage capacity of this layered material. Accordingly, this work offers a promising avenue to further optimizing the Na⁺ storage performance of layered vanadates via interlayer-cation engineering.