Sodium titanates: a promising negative electrode material for sodium-ion batteries Pooja Kumari and Rebecca Boston Department of Materials Science and Engineering, University of Sheffield, Sheffield, UK Sodium-ion battries (NIBs) offer advantages in cost and sustainability vs. Li-ion, because of Na-abundance, low- cost raw materials, and no over-discharge characteristics [1] . NIBs face challenges in anode development and although numerous candidates exist (e.g. NiCo 2 O 4 , Li 4 Ti 5 O 12 ) their electrochemical performance still suffers from poor specific capacity and rapid capacity decay. Na + ions intercalate into hard carbon and provide the first storage capacity of 250 mA h g -1 with good capacity retention [2] , however, using hard carbon as an anode raises safety concerns (dendrite formation/mossy structure) due to a low voltage discharge plateau between 0 and 0.1 V vs. Na/Na + [3] . Of the many materials under consideration for anodes, the sodium titanates in the Na 2 Ti n O 2n+1 (2 ≤ n ≤ 9) family have been highlighted as a promising candidates. Sodium titanates, Na 2 Ti 3 O 7 , can be reversibly intercalates sodium with a plateau centred at 0.3 V vs. Na/Na + (theoretical specific capacity 178 mA h g -1 ) [4] . The main challenge faced is instability upon electrochemical cycling, leading to rapid fade in Na-storage capacity [5] . Owing to high surface area, short diffusion pathways, high electrical and ionic conductivity, nanostructured electrode materials are extremely promising for providing high reversible capacity, power capability and long cycling stability. Herein, we synthesize nanostructured Na 2 Ti 3 O 7 and Na 2 Ti 6 O 13 with controlled morphology using a simple ionic liquid (IL) method. This method is based on a novel phase-transfer route which produces pure functional materials via a dehydrated IL.The structure of the synthesised nanostructures was characterised using powder X-ray diffraction, which confirms the formation of a mixed Na 2 Ti 3 O 7 and Na 2 Ti 6 O 13 phase, with majority Na 2 Ti 3 O 7 phase, in contrast to previous synthesis methods. The initial discharge and charge capacity of nanostructured Na 2 Ti 3 O 7 and Na 2 Ti 6 O 13 were measured as 325.20 mA h g -1 and 149.07 mA h g -1 , respectively, at 10 mA g -1 between 0.01 – 2.5 V. References 1. A. Rudola, C. J. Wright, and J. Barker, "Reviewing the Safe Shipping of Lithium-Ion and Sodium-Ion Cells: A Materials Chemistry Perspective," Energy Material Advances, ( 2021) 1-12, doi: 10.34133/2021/9798460. 2. X. Chen, Y. Zheng, W. Liu, C. Zhang, S. Li, J. Li, “High-performance sodium-ion batteries with a hard carbon anode: transition from the half-cell to full-cell perspective,” Nanoscale , 11 (2019), 22196-22205. 3. J. M. Bray, C. L. Doswell, G. E. Pavlovskaya, L. Chen, B. Kishore, H. Au, H. Alptekin, E. Kendrick, M.-M. Titirici, T. Meersmann, M. M. Britton, “Operando visualisation of battery chemistry in a sodium-ion battery by 23Na magnetic resonance imaging,” Nat. Commun. 11 (2020) 2083. 4. M. M. Doeff, J. Cabana, and M. Shirpour, "Titanate Anodes for Sodium Ion Batteries," Journal of Inorganic and Organometallic Polymers and Materials, 24 (2013) 5-14. 5. J. Xu, C. Ma, M. Balasubramanian et al. , "Understanding Na 2 Ti 3 O 7 as an ultra-low voltage anode material for a Na-ion battery," Chem Commun, 50 (2014) 12564-12567.
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