Improving the sodium storage capacity of tunnel structured NaxFexTi2-xO4 (x = 1, 0.9 & 0.8) anode materials by tuning sodium deficiency

• DOI: 10.1016/j.jpowsour.2017.08.112
• Publication Date: 2017-09-14T00:21:23Z
• Authors: Deu S. Bhange; Ghulam Ali; Ji-Young Kim; Kyung Yoon Chung; Kyung-Wan Nam
• Keywords: Stoichiometry

Abstract Due to their abundance and environmentally benign nature, iron and titanium present as the most attractive potential elements for use in rechargeable sodium-ion batteries (SIBs). Accordingly, two structurally different Fe and Ti based compounds, stoichiometric NaFeTiO 4 and sodium deficient Na x Fe x Ti 2-x O 4 (where x = 0.9, and 0.8), are explored as anode materials for SIBs. Their structure and sodium storage capacity are systematically investigated by using combined structural and electrochemical analysis. Rietveld refinement analysis reveals that the sodium deficiency leads to the structural transformation from a single-tunnel structure (NaFeTiO 4 ) to a zigzag-type double-tunnel structure (Na 0.9 Fe 0.9 Ti 1.1 O 4 and Na 0.8 Fe 0.8 Ti 1.2 O 4 ). The series of sodium deficient compounds bears systematic sodium ion vacancies in their structure up to 20%. Sodium deficiency in the Na x Fe x Ti 2-x O 4 logically provides additional space for accommodating the excess sodium ions as such the Na x Fe x Ti 2-x O 4 compounds with higher level of sodium deficiency show higher specific capacities than the stoichiometric NaFeTiO 4 . All the compounds exhibited very good electrochemical cycling stability, with minimal capacity loss during cycling. The present approach is a model example of improvement in the sodium storage capacity of the anode materials by tuning the chemical composition, and could facilitate the performance improvement of known or new electrode materials for SIBs.