Nanohybridization of POM/CNT Anode Materials for Enhanced Cycle Stability and Superior Discharge Capacity in Sodium-Ion Batteries

Abstract

Polyoxometalates (POMs) have emerged as high-energy-density electrodes acting as 'electron/ion sponges' for pseudocapacitive energy storage, attributed to their swift and reversible multi-redox reactions. In sodium-ion batteries (SIBs), POM-based electrodes have given excellent energy density. However, the negligible conductivity of pristine POMs and high electrolyte dissolution can lead to subpar electrochemical performance in poor capacity retention, and rate capability. To address these challenges, we employed a facile ultrasonication strategy to prepare polyoxometalate/carbon-nanotube (POM/CNT) nanohybrids. CNTs were modified with the tetrabutylammonium polyoxotungstate, TBA3[PW12O40].nH2O (TBA-PW12), yielding TBA-POM/CNT nanohybrids. These were synthesized using four CNT sources: single-walled (SW), multi-walled (MW), and their hydroxyl-functionalized analogues (SWOH and MWOH). The nanohybrids were characterized using FT-IR, Raman spectroscopy, powder XRD, TGA, SEM/EDX, STEM, XPS, and BET analysis. Electrochemical evaluation of TBA-PW12/SW and TBA-PW12/MW nanohybrids as an anode for SIB showed superior Na-ion storage, delivering reversible capacities of 69.4 mAh g-1 and 27.5 mAh g-1, respectively, at a current density of 2 A g-1 after 1000 cycles. Under the same conditions, the nanohybrids from functionalized SWOH and MWOH also showed enhanced performance, achieving discharge capacities of 66.2 mAh g-1 and 57.3 mAh g-1, respectively. This impressive electrochemical performance was ascribed to the multiple active sites of TBA-PW12 combined with conductive pathways and surface functionalities of CNTs, which enable rapid electron transfer, high Na-ion conductivity, and efficient ion diffusion. Overall, POM/CNT nanohybridization presents a promising strategy to overcome the intrinsic limitations of pristine POMs, thereby advancing the design of high-performance anodes for SIBs and sustainable energy applications.