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Na+-conductive Na2Ti3O7 modified P2-type Na2/3Ni1/3Mn2/3O2 via a smart in situ coating approach: Suppressing Na +/Vacancy Ordering and P2-O2 Phase Transition Rongbin Dang, Minmin Chen, Qi Li, Kang Wu, Yu Lin Lee, Zhongbo Hu, and Xiaoling Xiao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b17976 • Publication Date (Web): 12 Dec 2018 Downloaded from http://pubs.acs.org on December 14, 2018
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ACS Applied Materials & Interfaces
Na+-Conductive Na2Ti3O7 Modified P2-type Na2/3Ni1/3Mn2/3O2 Via a Smart in Situ Coating Approach: Suppressing Na+/Vacancy Ordering and P2-O2 Phase Transition Rongbin Dang1, Minmin Chen1, Qi Li1, Kang Wu1, Yu Lin Lee 2, Zhongbo Hu1, Xiaoling Xiao*1 AUTHOR ADDRESS 1College
of Materials Science and Opto-electronic Technology, University of Chinese Academy
of Sciences, Beijing 100049, P. R. China. 2
Department of Materials, Imperial College London, Royal School of Mines, Exhibition Road,
London SW7 2AZ, UK. KEYWORDS: sodium-ion batteries, cathode material, Na+ conductor-Na2Ti3O7, in situ coating approach, phase transition, Na+/vacancy ordering
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ABSTRACT Sodium-ion batteries (SIBs) have shown great superiority for grid-scale storage applications due to the low cost, and the abundance of sodium. P2-type Na2/3Ni1/3Mn2/3O2 cathode materials have attracted much attention for their high capacities and operating voltages as well as their simple synthesis processes. However, Na+/vacancy ordering and the P2-O2 phase transition are unavoidable during Na+ insertion/extraction, leading to undesired voltage plateaus and deficient battery performances. We show that this defect can be effectually eliminated by coating a moderate Na+ conductor-Na2Ti3O7-with a smart in situ coating approach and a concomitant of Ti4+ into bulk structure. Based on the combined analysis of ex situ x-ray diffraction (XRD), scanning electron microscopy (SEM), electrochemical performance tests, and electrochemical kinetic analyses, Na2Ti3O7 coating and Ti4+ doping effectively refrain Na+/vacancy ordering and P2-O2 phase transition during cycling. Additionally, the Na2Ti3O7 coating layer suppresses particle exfoliation and accelerates Na+ diffusion at the cathode and electrolyte interface. Hence, Na2Ti3O7 coated Na2/3Ni1/3Mn2/3O2 exhibits excellent cycling stability (almost no capacity decay after 200 cycles at 5C) and outstanding rate capability (31.1% of the initial capacity at a high rate of 5C compared to only 10.4% for the pristine electrode). This coating strategy can provide a new guide for the design of prominent cathode materials for SIBs that are suitable for practical application. 1. INTRODUCTION Recently, sodium-ion batteries (SIBs) have garnered great research interests for large-scale energy storage systems (EES) due to the huge natural abundance and low cost of sodium compared to lithium.1-4 To date, several anode materials of sodium-ion batteries, like amorphous carbon,5 alloys,6 Ti-based oxides,7 have been proposed and they show excellent electrochemical performances. However, the development of cathode materials has been more challenging, and
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ACS Applied Materials & Interfaces
SIBs
still
require
cathode
materials
with
high
reversible
capacities,
rapid
Na+
insertion/extraction, as well as good cycling stabilities. A variety of cathode materials, like layered oxides,8 phosphates,9 and Prussian blue10 have been studied as possible candidates for use in SIBs. Among these cathode materials, NaxTMO2 layered oxides (0
