Investigation of the Na Intercalation Mechanism into Nanosized
V
2
O
5
/C Composite Cathode Material for Na-Ion Batteries
Ghulam Ali,
†,‡
Ji-Hoon Lee,
†
Si Hyoung Oh,
†,‡
Byung Won Cho,
†
Kyung-Wan Nam,
§
and Kyung Yoon Chung*
,†,‡
†
Center for Energy Convergence Research, Korea Institute of Science and Technology, Hwarang-ro 14-gil 5, Seongbuk-gu, Seoul
136-791, Republic of Korea
‡
Korea University of Science and Technology, 217 Gajeong-ro Yuseong-gu, Daejeon 305-333, Republic of Korea
§
Department of Energy and Materials Engineering, Dongguk University-Seoul, Seoul 100-715, Republic of Korea
ABSTRACT: There is a significant interest to develop high-
performance and cost-effective electrode materials for next-
generation sodium ion batteries. Herein, we report a facile
synthesis method for nanosized V
2
O
5
/C composite cathodes
and their electrochemical performance as well as energy
storage mechanism. The composite exhibits a discharge
capacity of 255 mAh g
-1
at a current density of 0.05 C,
which surpasses that of previously reported layered oxide
materials. Furthermore, the electrode shows good rate
capability; discharge capacity of 160 mAh g
-1
at a current
density of 1 C. The reaction mechanism of V
2
O
5
upon sodium
insertion/extraction is investigated using ex situ X-ray
diffraction (XRD) and synchrotron based near edge X-ray absorption fine structure (NEXAFS) spectroscopy. Ex situ XRD
result of the fully discharged state reveals the appearance of NaV
2
O
5
as a major phase with minor Na
2
V
2
O
5
phase. Upon insertion
of sodium into the array of parallel ladders of V
2
O
5
, it was confirmed that lattice parameter of c is increased by 9.09%,
corresponding to the increase in the unit-cell volume of 9.2%. NEXAFS results suggest that the charge compensation during de/
sodiation process accompanied by the reversible changes in the oxidation state of vanadium (V
4+
↔ V
5+
).
KEYWORDS: Na-ion batteries, nanosized V
2
O
5
, NaV
2
O
5
, X-ray diffraction, near-edge X-ray absorption fine structure
■
INTRODUCTION
Electricity produced by different intermittent renewable power
sources such as wind and solar energies often relies on
stationary energy storage systems (ESS). Achieving the high
efficiency and superior safety is essential for large-scale
rechargeable batteries.
1,2
Lithium-ion batteries (LIBs) are
facing the issues of high production cost, limited resources,
and serious safety concerns thus not suitable for the use of ESS.
In recent years, researchers have been investigating sodium-ion
batteries (NIBs) as a potential alternative to the LIBs for the
use of large-scale ESS.
3-5
World-wide abundant resources of
sodium make NIBs a potentially low cost technology.
5-7
Although, sodium is the next smallest and lightest alkali-metal
after lithium in periodic table,
8
NIBs are still inferior to LIBs in
terms of energy and power densities because of the larger size
and lower negative reduction potential of sodium as compared
to its counterpart.
9
To solve these issues, electrode material
should be designed with a large concentration of interstitial site,
where the sodium could be reversibly occupied, as well as large
open tunnels.
10
As one of the strategies, a number of layered
cathode materials (Na
x
MO
2
; x = 0.44-1, M = 3d transition
metals) have been investigated for the reversible sodium
intercalation. Unfortunately, they delivered limited reversible
capacity of ≤160 mAh g
-1
and suffered with poor cycle and rate
performance because of structural instability.
8
Vanadium pentoxide (V
2
O
5
) is considered as promising
active material because of its unique crystal structure with large
interlayer spacing of 4.4 Å.
11,12
To date, layered V
2
O
5
has been
intensively investigated in LIBs and supercapacitors.
13,14
The
lithium insertion mechanism of V
2
O
5
is well understood.
15
Two
lithium ions can be reversibly inserted/extracted from the
layered V
2
O
5
(with a composition of Li
2
V
2
O
5
), resulting in a
high capacity of 294 mAh g
-1
.
11,12
It is also known that layered
V
2
O
5
is electrochemically active when the electrode was applied
in NIBs but exhibiting less noticeable performances.
16
Layered
vanadium oxide xerogel delivers high capacity in NIBs but it
shows fast capacity fade because of large lattice breathing upon
de/sodiation.
17
Despite having a high theoretical capacity, V
2
O
5
is also facing barriers of poor electrical conductivity and low
working potential compared to other cathodes in NIBs.
15
Furthermore, sodium insertion mechanism in layered V
2
O
5
is
Received: December 8, 2015
Accepted: February 18, 2016
Published: February 18, 2016
Research Article
www.acsami.org
© 2016 American Chemical Society 6032 DOI: 10.1021/acsami.5b11954
ACS Appl. Mater. Interfaces 2016, 8, 6032-6039