Improving the sodium storage capacity of tunnel structured Na
x
Fe
x
Ti
2-
x
O
4
(x ¼ 1, 0.9 & 0.8) anode materials by tuning sodium deficiency
Deu S. Bhange
a, 1 , 2
, Ghulam Ali
b, 2
, Ji-Young Kim
c
, Kyung Yoon Chung
b
,
Kyung-Wan Nam
a, *
a
Department of Energy and Materials Engineering, Dongguk University-Seoul, 04620 Seoul, Republic of Korea
b
Center for Energy Convergence Research, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea
c
Advanced Analysis Center, Korea Institute of Science and Technology (KIST), Seoul 136-791, Republic of Korea
highlights graphical abstract
Series of Na
x
Fe
x
Ti
2-x
O
4
compounds
(x ¼ 1, 0.9, and 0.8) are synthesized.
Na deficiency leads to the structural
change from single-to double-tunnel
structure.
Na deficiency logically provides an
extra space for housing excess Na
ions.
Sodium storage capacity increases
with increasing Na deficiency.
article info
Article history:
Received 19 May 2017
Received in revised form
9 August 2017
Accepted 30 August 2017
Keywords:
Sodium ion battery
Anode
NaFeTiO
4
Tunnel structure
Synchrotron X-rays
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 ¼ 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 trans-
formation 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 so-
dium 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.
© 2017 Elsevier B.V. All rights reserved.
1. Introduction
Lithium ion batteries (LIBs) are currently used in handy devices
owing to their high energy density, and are set to drive electric
* Corresponding author.
E-mail address: knam@dongguk.edu (K.-W. Nam).
1
Permanent Address: Department of Chemistry, Shivaji University, Kolhapur
416004, India.
2
D.S.B. and G.A. equally contributed to this work.
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Journal of Power Sources
journal homepage: www.elsevier.com/locate/jpowsour
http://dx.doi.org/10.1016/j.jpowsour.2017.08.112
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Journal of Power Sources 366 (2017) 115e122