Three-Dimensional Topotactic Host Structure-Secured Ultrastable VP-CNO Composite Anodes for Long Lifespan Lithium- and Sodium-Ion Capacitors

Abstract

Performance degradation of lithium/sodium ion capacitors (LICs/SICs) mainly originated from the anode pulverization, particularly the alloying and conversion types, has been spurred research into alternatives with insertion mechanism. Three-dimensional (3D) topotactic host materials remain much unexplored over two dimensional (2D) ones (graphite, etc.). Herein, vanadium monophosphide (VP) is designed as a 3D topotactic host anode. Ex-situ electrochemical characterizations reveal that there are no phase changes during (de) intercalation, which follows the topotactic intercalation mechanism. Computational simulations also confirm the metallic feature and topotactic structure of VP with spacious interstitial position for accommodation of guest species. In order to boost the electrochemical performance, carbon nano onions (CNOs) are coupled with 3D VP. Superior specific capacity and rate capability of VP-CNOs vs. lithium/sodium can be delivered due to the fast ion diffusion nature. An exceptional capacity retention of above 86% maintains after 20,000 cycles benefited from the topotactic intercalation process. Optimized LICs/SICs exhibit high energy/power densities and ultra-stable lifespan of 20,000 cycles, which outperforms most state-of-the-art LICs and SICs, demonstrating the potential of VP-CNOs as insertion anode. This exploration would draw attention regarding to insertion anodes with 3D topotactic host topology and give new insights into the anode selection for LICs/SICs.