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Robert Aughterson Institute of Materials Engineering, Australian Nuclear Science and Technology Organisation, Kirrawee DC, NSW 2232, Australia

Wesley M. Dose and Scott W. Donne Discipline of Chemistry, University of Newcastle, Callaghan, NSW 2308, Australia

Helen E. A. Brand Australian Synchrotron, Clayton, Victoria 3168, Australia

Neeraj Sharmaa) School of Chemistry, UNSW Australia, Sydney, NSW 2052, Australia (Received 24 July 2014; accepted 2 October 2014)

Designing materials for application as electrodes in sodium-ion batteries may require the use of unconventional materials to realize acceptable reversible sodium insertion/extraction capabilities. To design new materials simple electrochemical methods need to be coupled with other techniques such as in situ x-ray diffraction (XRD) to correlate the influence of electrochemical performance on a parameter that can be modified, e.g., the crystal structure of the material. Here we use in situ synchrotron XRD data on Gd2TiO5-containing cells to show the minor changes in reflection positions during discharge/charge that illustrates minimal volume expansion and contraction due to insertion/extraction reactions. These small changes correlate to the Gd2TiO5 anode material in both lithium- and sodium-ion batteries showing reversible capacities of ;45 and ;23 mA h/g after 20 cycles, respectively. Analysis of sodium location in the crystal structure shows a preference for sodium in the smaller channels along the c axis direction during the first discharge before moving to the larger channels at the charged state. Therefore, in this work, in situ studies highlight minimal structural changes with respect to volume expansion during electrochemical cycling and illustrate where sodium ions locate within the Gd2TiO5 structure. I. INTRODUCTION

The ever increasing demand for electronic devices for various applications has also given rise to the demand for reliable energy storage devices. Currently, rechargeable lithium-ion batteries are the leading energy storage technology for many portable electronic devices such as laptops and mobile phones as they offer high energy density and relatively long cycle-life.1 However, lithium-ion batteries do not meet the demands of all current and emerging applications.2 A number of challenges remain and future energy storage systems require better safety characteristics, higher power and capacity, and should feature faster recharging times. Another significant challenge for lithium-ion technology is the rarity of lithium and the materials associated with the cathode (e.g., Co) that a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2014.311 J. Mater. Res., Vol. 30, No. 3, Feb 14, 2015

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brings about the relatively high cost of these batteries.3 For example, the estimated global lithium reserves are 15 Mt, which is considered to be insufficient to fulfill future demand for lithium batteries, especially if electric vehicles become more widely adopte