Structurally stable Mg-doped P2-Na2/3Mn1-yMgyO2 sodium-ion battery cathodes with high rate performance: insights from electrochemical, NMR and diffraction studies
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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 mAhg-1 at the very high discharge rate of 5000 mAg-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 mAhg-1 retained after 50 cycles at a rate of 1000 mAg-1.
Clément , R , Billaud , J , Armstrong , R , Singh , G , Rojo , T , Bruce , P G & Grey , C P 2016 , ' 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 ' Energy & Environmental Science , vol 9 , no. 10 , pp. 3240-3251 . DOI: 10.1039/C6EE01750A
Energy & Environmental Science
Copyright The Author(s). Open Access Article. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.
This work was partially supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231, under the Batteries for Advanced Transportation Technologies (BATT) Program subcontract #7057154 (R.J.C). C.P.G. and R.J.C. thank the EU ERC for an Advanced Fellowship for CPG. This work was partially supported by the LINABATT project from the Ministerio de Economía Competitividad under Contract No. ENE2013-44330-R (G.S. and T.R.). P.G.B. is grateful to the EPSRC, including the SUPREGEN programme, for financial support.
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