Multidefect N-Nb2 O5- x @CNTs Incorporated into Capillary Transport Framework for Li+ /Na+ Storage

Abstract

As an ion-embedded material with small strain and low transport energy barrier, the limited ion transport rate and conductivity of niobium pentaoxide (Nb₂ O₅ ) are the main factors limiting its application in lithium/sodium storage systems. In this work, the microsphere composites (N-Nb₂ O_{5- x} @CNTs) are prepared by combining Nb₂ O₅ , rich in nitrogen doping and vacancy defects, with carbon nanotubes (CNTs) penetrating the bulk phase. With the capillary effect, CNTs can enable the rapid electrolyte infiltration into the microspheres, thus shorting the Li⁺ /Na⁺ transport path. In addition, CNTs also hinder the direct contact between the electrolyte and Nb₂ O₅ , and inhibit the irreversible reaction. Meanwhile, nitrogen doping and oxygen vacancy defects reduce the energy barrier of Li⁺ /Na⁺ transport, and improve their transport rate, proved by density functional theory. Highly conductive CNTs and unpaired electrons from defects also ameliorate the insulation property of Nb₂ O₅ . Therefore, N-Nb₂ O_{5- x} @CNTs display good electrochemical performance in both Li/Na half-cell and Li/Na hybrid capacitors. Interestingly, kilogram-scale microsphere composites can be produced in laboratory conditions by using industrial grade raw materials, implying its potential for practical application.

Keywords: capillary effects; density functional theory (DFT); doping; interfaces; vacancies.