Environmental Engineering Reference
In-Depth Information
Fig. 18 The voltage profiles
of the Na
2
FeP
2
O
7
electrode
charged/discharged at a C/20
rate [
29
]
The charge/discharge curves show one voltage plateau at 2.5 V based on a single-
phase reaction and another voltage slope in the potential range of 3.0-3.25 V
based on a two-phase reaction (Fig.
18
). The material can deliver a reversible
capacity of 90 mAh g
-1
and has good cycling stability, which make it a promising
cathode candidate for low cost and long-term Na-ion batteries.
Fe-based mixed polyanion cathode material (Na
4
Fe
3
(PO
4
)
2
(P
2
O
7
)) was studied
[
43
]. As shown in Fig.
19
, this material exhibits higher reversible capacity
([100 mAh g
-1
) and potential plateau (~3.0 V) than Na
2
FeP
2
O
7
[
29
]. (PO
4
F)
4-
based Na-Fe compounds were also investigated [
44
-
46
]. Orthorhombic Na
2
Fe-
PO
4
F is composed of [FePO
4
F] layers, which are formed by the joining of bioc-
tahedral Fe
2
O
7
F
2
units with PO
4
tetrahedra. The [FePO
4
F] layers are stacked to
form two-dimensional pathways providing Na-ion migration. The reversible Na-
ion intercalation/deintercalation capacity reached 120 mAh g
-1
, with an average
voltage of 3.0 V and good cycling stability [
44
]. If Fe was partly substituted by
Mn, the electrochemical performance of this material decreased rapidly with
increasing Mn content, due to a strong tendency of structural transition from 2D to
3D [
44
].
3.2.2 Hexacyano-Type Compound
In the above-mentioned cathode materials, Na ions lie in the complex environment
composed of oxides and polyanions. The immigration of Na ions in these close
packed structures is difficult due to the strong interaction between Na
+
and O
2-
ions. The replacement of O
2-
ions by CN
-
ions would greatly weaken the
interaction of the complexants with Na
+
, leading to the reduction of the activation
energy for Na
+
ion migration. Thus, Hexacyano-type compounds have attracted
great attention [
31
,
47
,
48
].
Yang et al. proposed a new family of Na transition metal cyanides, such as
hexacyanoferrate Na
4
Fe(CN)
6
and Prussian blue Na
x
M
y
Fe(CN)
6
as Na-ion hosts.
The as prepared Na
4
Fe(CN)
6
/C nanocomposite displayed a full utilization of its
redox capacity of 87 mAh g
-1
at a high potential of ~3.4 V, an excellent cycling
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