Identification of the critical synthesis parameters for enhanced cycling stability of Na-ion anode material Na2Ti3O7

Na2Ti3O7 is a promising insertion anode material for Na-ion batteries owing to its low reaction voltage (0.3 V vs. Na+/Na) and high specific capacity (178 mAh/g). However, such promising attributes are shaded by the poor capacity retention that has so far been obtained for this material. Here the impact of solid state synthesis parameters on the electrochemical properties is reported and, in particular, on the capacity retention. In agreement with Na-23 solid state NMR and FTIR it has been found that small amounts of insulating Na2CO3, either from unreacted precursor or formed by surface corrosion, strongly contribute to the capacity fading. These were avoided by eluding Na2CO3 as a precursor and preventing hydration of the samples by controlling the atmosphere of the cooling process. Once the synthesis conditions were optimized a Na2CO3 free sample was carbon-coated to enhance the electrochemical properties obtaining the best charge capacity retention reported up to date after 100 cycles (78%) with a measured capacity of 124 mAh/g. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.