Sodium ion pre-intercalation regulates the electron density and structural stability of vanadium oxide nanowires for Ca–ion batteries

Abstract

Benefiting from the abundant resource, low cost, and high operating potential, calcium–ion batteries (CIBs) have attracted great attention as emerging energy storage devices over lithium-based systems. Nevertheless, the progression of CIBs remains at an early stage and faces significant challenges in finding suitable cathode materials and electrolytes. In this work, we pioneer a novel sodium ion-pre-intercalated layered V₂O₅ nanowire (Na_0.33V₂O₅ nanowire, denoted as NVO NW) as a binder-free and stable cathode for rechargeable CIBs. The pre-intercalated sodium ions function as supportive structures, linking neighboring layers, widening interlayer distance, and partially converting pentavalent vanadium cations into tetravalent states, which can reduce electrostatic interactions, enhance electronic conductivity, promote calcium ion insertion/extraction, and provide excellent structural stability. Meanwhile, a hybrid electrolyte composed of water and tetraethylene glycol dimethyl ether (TEGDME) can boost the swift transport of Ca²⁺ while inhibiting vanadium species dissolution due to its weak solvation effect. Consequently, the NVO NW electrode shows a favorable specific discharge capacity of 196.3 mA h g⁻¹ at 0.05 A g⁻¹, a superior rate capacity of 108.8 mA h g⁻¹ at 3.0 A g⁻¹, and a commendable extended durability over 2000 cycles. Furthermore, in accordance with the computations rooted in density functional theory, pre-intercalated sodium ions can regulate the electron density and evaluate the intrinsic kinetic properties of the NVO NW. Additionally, multiple ex situ characterization techniques further reveal the energy storage mechanism. This work may accelerate the development of CIBs and promote their commercial application.