Surface Engineering Strategy Using Urea To Improve the Rate Performance of Na2 Ti3 O7 in Na-Ion Batteries.

Sara I R Costa,Alistair J Fielding,Andrew J Naylor,Yong‐Seok Choi,Zdeněk Sofer,Nuria Tapia‐Ruiz,David O Scanlon,John M Griffin

doi:10.1002/chem.202003129

Abstract

Na2Ti3O7 (NTO) is considered a promising anode material for Na‐ion batteries due to its layered structure with an open framework and low and safe average operating voltage of 0.3 V vs. Na+/Na. However, its poor electronic conductivity needs to be addressed to make this material attractive for practical applications among other anode choices. Here, we report a safe, controllable and affordable method using urea that significantly improves the rate performance of NTO by producing surface defects such as oxygen vacancies and hydroxyl groups, and the secondary phase Na2Ti6O13. The enhanced electrochemical performance agrees with the higher Na+ ion diffusion coefficient, higher charge carrier density and reduced bandgap observed in these samples, without the need of nanosizing and/or complex synthetic strategies. A comprehensive study using a combination of diffraction, microscopic, spectroscopic and electrochemical techniques supported by computational studies based on DFT calculations, was carried out to understand the effects of this treatment on the surface, chemistry and electronic and charge storage properties of NTO. This study underscores the benefits of using urea as a strategy for enhancing the charge storage properties of NTO and thus, unfolding the potential of this material in practical energy storage applications.

Highlights

The increasing demand for electrochemical energy storage devices has resulted in rapid development and utilisation of Liion batteries (LIBs) in recent years.[1]
The formation of oxygen vacancies leads to the reduction of Ti4+ to Ti3+ ions in NTO, together with the formation of hydroxyl groups and a secondary phase, Na2Ti6O13
The enhanced electrochemical performance agrees with the higher Na+ ion diffusion coefficient, higher charge carrier density and reduced bandgap observed in these samples, without the need of nanosizing and/or complex synthetic strategies

Summary

Introduction

The increasing demand for electrochemical energy storage devices has resulted in rapid development and utilisation of Liion batteries (LIBs) in recent years.[1]. Electrochemical impedance spectroscopy (EIS) data were collected on an Ivium potentiostat (Alvatek) with an AC amplitude of 10 mV in the frequency range between 0.05 and 105 Hz. Data were acquired during the first discharge process at OCV ( % 2.5 V), 1, 0.4, 0.2 and 0.01 V vs Na + /Na. Ab initio calculations: All the calculations performed in this work used the density functional theory (DFT) method as implemented in the Vienna Ab initio Simulation Package code.[25,26] The projector augmented wave approach[27] was employed to describe the interaction between the core and valence electrons. The VBM level with respect to the vacuum level, that is, ionisation potential (IP), was calculated by comparing the obtained (Evac) and (Ecore,slab) values with VBM (EVBM,bulk) and core levels (Ecore,slab) of bulk structures according to [Eq (3)]: ÁÀ

Results and Discussion

Conclusions

Conflict of interest