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Structurally stable Mg-doped P2-Na 2/3 Mn 1−y Mg y O 2 sodium-ion battery cathodes with high rate performance: insights from electrochemical, NMR and diffraction studies

DOI: 10.1039/C6EE01750A DOI Help

Authors: Raphaële J. Clément (Department of Chemistry, University of Cambridge) , Juliette Billaud (School of Chemistry, University of St. Andrews) , A. Robert Armstrong (School of Chemistry, University of St. Andrews) , Gurpreet Singh (CIC ENERGIGUNE) , Teófilo Rojo (CIC ENERGIGUNE) , Peter G. Bruce (University of St. Andrews; University of Oxford) , Clare P. Grey (University of Cambridge)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Energy Environ. Sci. , VOL 9 , PAGES 3240 - 3251

State: Published (Approved)
Published: August 2016

Open Access Open Access

Abstract: Sodium-ion batteries are a more sustainable alternative to the existing lithium-ion technology and could alleviate some of the stress on the global lithium market as a result of the growing electric car and portable electronics industries. Fundamental research focused on understanding the structural and electronic processes occurring on electrochemical cycling is key to devising rechargeable batteries with improved performance. We present an in-depth investigation of the effect of Mg doping on the electrochemical performance and structural stability of Na2/3MnO2 with a P2 layer stacking by comparing three compositions: Na2/3Mn1−yMgyO2 (y = 0.0, 0.05, 0.1). We show that Mg substitution leads to smoother electrochemistry, with fewer distinct electrochemical processes, improved rate performance and better capacity retention. These observations are attributed to the more gradual structural changes upon charge and discharge, as observed with synchrotron, powder X-ray, and neutron diffraction. Mg doping reduces the number of Mn3+ Jahn–Teller centers and delays the high voltage phase transition occurring in P2-Na2/3MnO2. The local structure is investigated using 23Na solid-state nuclear magnetic resonance (ssNMR) spectroscopy. The ssNMR data provide direct evidence for fewer oxygen layer shearing events, leading to a stabilized P2 phase, and an enhanced Na-ion mobility up to 3.8 V vs. Na+/Na upon Mg doping. The 5% Mg-doped phase exhibits one of the best rate performances reported to date for sodium-ion cathodes with a P2 structure, with a reversible capacity of 106 mA h g−1 at the very high discharge rate of 5000 mA g−1. In addition, its structure is highly reversible and stable cycling is obtained between 1.5 and 4.0 V vs. Na+/Na, with a capacity of approximately 140 mA h g−1 retained after 50 cycles at a rate of 1000 mA g−1.

Subject Areas: Materials, Chemistry, Environment


Instruments: I11-High Resolution Powder Diffraction

Documents:
C6EE01750A.pdf