REVIEW
(1 of 24) 1600275
wileyonlinelibrary.com
©
2017 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Polyanion-Type Electrode Materials for Sodium-Ion
Batteries
Qiao Ni, Ying Bai,* Feng Wu, and Chuan Wu*
Dr. Q. Ni, Prof. Y. Bai, Prof. F. Wu, Prof. C. Wu
Beijing Key Laboratory of Environmental
Science and Engineering
School of Materials Science & Engineering
Beijing Institute of Technology
Beijing 100081, P. R. China
E-mail: membrane@bit.edu.cn; chuanwu@bit.edu.cn
Prof. F. Wu, Prof. C. Wu
Collaborative Innovation Center of Electric Vehicles in Beijing
Beijing 100081, P. R. China
DOI: 10.1002/advs.201600275
of electricity and because both solar and
wind energy are very dependent on envi-
ronmental factors such as the weather,
season, and location, they are currently
considered unsuitable for modern grids.
To overcome this problem, large-scale elec-
trochemical energy storage (EES) technol-
ogies based on batteries have been valued
in recent years for their high round-trip
efficiency, flexible power, suitable energy
characteristics to meet different grid
functions, long cycle life, and low main-
tenance.
[1]
Figure 1a shows the stock-flow
diagram of renewable energy generation,
EES, and energy sources needed for dif-
ferent electronic equipment and electric
vehicle transports, all of which affect our
daily lives.
According to data from the U.S. Geo-
logical Survey, the global lithium reserves
in 2014 were approximately 13 million
tons.
[2]
The average annual demand for
lithium carbonate (Li
2
CO
3
) will grow by
16.76% within the next six years; there-
fore, global lithium reserves without
recycling can only last for 28 years. We
can imagine that the demand will become astronomic if
more electric vehicles are introduced because electric vehi-
cles generally use a 60 KWh lithium-ion battery pack. These
data generate fear of a potential Li shortage and further price
increases.
[3]
Electrical energy storage technology is the key
to the development of new energy sources and increased
manufacture of electric vehicles. Nevertheless, batteries are
closely related to the development of large-scale renewable
energy; thus, the resource-depleting and price-rising lithium
resources cannot meet the requirements of increased indus-
trial production.
Recently, much attention has been focused on room-temper-
ature Na-ion batteries due to the cost-effectiveness of sodium
resources as a result of virtually limitless seawater. Although
Na-ion batteries have a similar charge–discharge principle as
Li-ion batteries, the larger cation radius and the heavier atomic
weight combined with the higher standard potential of Na than
that of Li generally result in an inferior reversible capacity and
lower energy density (Figure 1b). However, the alkali metals
of Na and Li lie in the same main group, and thus they have
similar chemical performance, allowing much of the work that
has been carried out for Li-ion batteries to be equally applied to
Na-ion batteries. As depicted in Figure 1c, which summarizes
the development of batteries over the past 200 years, studies of
Sodium-ion batteries, representative members of the post-lithium-battery
club, are very attractive and promising for large-scale energy storage applica-
tions. The increasing technological improvements in sodium-ion batteries
(Na-ion batteries) are being driven by the demand for Na-based electrode
materials that are resource-abundant, cost-effective, and long lasting. Poly-
anion-type compounds are among the most promising electrode materials for
Na-ion batteries due to their stability, safety, and suitable operating voltages.
The most representative polyanion-type electrode materials are Na
3
V
2
(PO
4
)
3
and NaTi
2
(PO
4
)
3
for Na-based cathode and anode materials, respectively.
Both show superior electrochemical properties and attractive prospects in
terms of their development and application in Na-ion batteries. Carbonophos-
phate Na
3
MnCO
3
PO
4
and amorphous FePO
4
have also recently emerged and
are contributing to further developing the research scope of polyanion-type
Na-ion batteries. However, the typical low conductivity and relatively low
capacity performance of such materials still restrict their development. This
paper presents a brief review of the research progress of polyanion-type elec-
trode materials for Na-ion batteries, summarizing recent accomplishments,
highlighting emerging strategies, and discussing the remaining challenges of
such systems.
This is an open access article under the terms of the Creative Commons
Attribution License, which permits use, distribution and reproduction in
any medium, provided the original work is properly cited.
1. Introduction
So far, fossil fuels remain our primary power supply resource.
However, extensive use of fossil fuels is the main cause of
global warming because they emit large amounts of carbon
dioxide. Therefore, the development and utilization of renew-
able energy such as solar and wind energy for power genera-
tion have become urgent. However, because normal operation
of a power grid requires the stable and continuous generation
Adv. Sci. 2017, 1600275
www.advancedscience.com
www.advancedsciencenews.com
REVIEW
1600275 (2 of 24)
wileyonlinelibrary.com
©
2017 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Na-based batteries were carried out even earlier than those of
Li-based batteries. Nevertheless, the rapid expansion of Li-ion
batteries has resulted in minimal research on Na-ion batteries.
Among the many anode and cathode materials available for
Na-ion batteries, such as layered oxides, polyanion-type com-
pounds, metal hexacyanometalates, and organic compounds,
polyanion-type compounds are perceived as one of the most
promising for future Na-ion batteries on account of their
structural stability, safety, and appropriate operating potential.
Taking phosphate as an example, it contains special tetrahe-
dral PO
4
units with strong covalent bonding, which results in
the relative isolation of valence electrons from polyanions.
[4]
This special three-dimensional (3D) stereostructure is quite
favorable to the intercalation and deintercalation behavior
of Na ions because the smaller energy orbit leaps from the
highest occupied molecular orbital (HOMO) to the lowest
unoccupied molecular orbital (LUMO), frequently accom-
panied by multi-electron mechanisms.
[5]
Therefore, under-
standing the unique electronic structure is a good way to
develop practical polyanion-type electrical materials for Na-ion
batteries.
[6]
Here we summarize the recent research progress and
prospective future of polyanion-type electrode materials for
Na-ion batteries. To have a good knowledge of such materials,
special focus is given to the morphology and material modifi-
cations, together with the problems that remain to be solved.
In addition, some strategies are summarized and proposed to
enhance the electrochemical performance of polyanion-type
electrode materials. It is believed that this review will inform
readers of the rationality and prominence of polyanion-
type electrode materials as powerful candidates for Na-ion
batteries.
2. Characteristics of the Structures and Properties
of Polyanion-Type Electrode Materials
Polyanion-type electrode materials can be classified as a type
of compounds that contain a series of tetrahedron anion units
(XO
4
)
n–
or their derivatives (X
m
O
3m
+ 1
)
n–
(X = S, P, Si, As, Mo,
or W) with strong covalent-bonded MO
x
polyhedra (M repre-
sents a transition metal).
[7]
In most of the polyanion-type compounds, (XO
4
)
n–
not
only allows fast ion conduction in an open framework that is
selected for the working alkali ion on discharge, it can also sta-
bilize the operative redox potentials of transition metals. Such a
special framework consisting of two-dimensional (2D) van der
Waals bonding or 3D frameworks offer a significant advantage
in terms of inserting/extracting alkali-metal atoms.
[8]
Compared
to layered oxide compounds, the strong X
O bonding in poly-
anion-type compounds can introduce ionicity in M
O bonding,
and the weaker ionic bonding in M
O increases the distance
between its antibonding orbitals vis-à-vis the Na/Na
+
redox
couple, leading to a higher redox potential. This is called the
“inductive effect” in polyanion-type electrode materials.
[9]
Fur-
thermore, the strong X
O covalent bonds greatly improve the
stability of O in the lattice, thus increasing the safety of such
materials, which make them more suitable for rechargeable
secondary batteries.
Since the first report of an LiFePO
4
cathode material by
Padhi,
[9]
olivine-type-structured or NASICON-structured(Na
Super Ionic Conductor structure) materials have been con-
sidered promising hosts for rechargeable secondary batteries,
in which an MO
6
(M = transition metal) octahedron shares
corners or edges with an XO
4
(X = S, P, Si, As, Mo, or W)
tetrahedron. Such special framework compounds have been
known to undergo topotactic insertion/extraction of mobile
atoms,
[8]
resulting in small volume changes during cycling
and minimal structural rearrangement during alkali metal
ion insertion/extraction in electrode materials. Thanks to their
structural diversity and stability, combined with the strong
inductive effect of polyanions, such electrode materials gener-
ally have suitable operating potential and outstanding cycling
performance.
Qiao Ni received his
Bachelor’s Degree in Applied
Chemistry in 2014 from Jiangxi
Science and Technology
Normal University, China. He
is now a Ph.D. candidate in
School of Materials Science &
Engineering at BIT, under the
supervision of Prof. Chuan Wu.
His research interests focus
on electrode materials for
rechargeable Na-ion batteries.
Ying Bai is currently an
associate professor at Beijing
Institute of Technology (BIT).
Her research interests focus
on electrochemical energy
storage and conversion
technology, including elec-
trode materials and polymer
electrolytes for Li-ion, Na-ion
and Al-ion batteries.
Chuan Wu is a professor at
Beijing Institute of Technology
(BIT). He received his Ph.D.
degree in Applied Chemistry
from BIT in 2002, His
research interests are new
energy materials, and electro-
chemical devices, including
Li-ion batteries and post Li
battery chemistries such as
Na-ion and Al-ion batteries.
www.advancedscience.com
Adv. Sci. 2017, 1600275
www.advancedsciencenews.com
REVIEW
(3 of 24) 1600275
wileyonlinelibrary.com
©
2017 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
3. Recent Advances in Polyanion-Type Electrode
Materials for Na-ion Batteries
3.1. Phosphates
As one of the most typical representatives of polyanion-type
compounds, phosphates have attracted significant attention.
Olivine-type-structured NaMPO
4
(Fe, Mn) and NASICON-struc-
tured Na
x
M
2
(PO
4
)
3
(M = V, Ti) represent the main phosphate
compounds being researched for Na-ion batteries due to their
good electrochemical properties.
3.1.1. NaMPO
4
(M = Fe, Mn)
NaFePO
4
, one of the earliest and most characterized poly-
anion-type electrode materials for Na-ion batteries, can be cat-
egorized into two different types of structures: triphylite-type
and maricite-type (Figure 2). Along the b direction, triphy-
lite-NaFePO
4
has a one-dimensional (1D) Na
+
ion transport
channel, whereas maricite-NaFePO
4
lacks transmission chan-
nels for the diffusion of sodium ions. Thus, maricite-NaFePO
4
has generally been considered an electrochemically inactive
structure.
Adv. Sci. 2017, 1600275
www.advancedscience.com
www.advancedsciencenews.com
Figure 1. a) A simplified model for the relationship between renewable energy generation, grid, commercial secondary batteries and hybrid/electric
vehicle transport. b) The comparison between Na and Li. c) The battery development history of the past 200 years.
REVIEW
1600275 (4 of 24)
wileyonlinelibrary.com
©
2017 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Owing to the lower diffusion coefficient of Na ions and the
higher contact and charge-transfer resistances in NaFePO
4
cath-
odes, the rate performance of C-NaFePO
4
in Na-ion batteries is
much worse than that of C-LiFePO
4
in Li-ion batteries. How-
ever, the cycling stability of C-NaFePO
4
is almost comparable to
that of C-LiFePO
4
, retaining 90% of its capacity even after 100
charge/discharge cycles at rate of 0.1 C.
[11]
Conventional solid-phase reactions at high temperatures are
no longer suitable for synthesizing triphylite-type NaFePO
4
because the thermodynamically stable phase of NaFePO
4
is
not triphylite but maricite.
[12]
Poul was the first to discover that
the guest Li ions in the olivine iron phos-
phate host can be replaced by Na ions.
[13]
Later, triphylite-type NaFePO
4
was frequently
obtained via chemical or electrochemical
displacement methods from triphylite-type
LiFePO
4
in organic solutions.
[11,14,15]
Cao prepared a triphylite-type NaFePO
4
/C
microsphere cathode by a two-step aqueous
electrochemical transition process from
an LiFePO
4
/C precursor,
[15]
LiFePO
4
as the
working electrode, activated carbon as the
counter electrode, and Ag/AgCl as refer-
ence electrode (Figure 3a). The obtained
NaFePO
4
/C cathode showed a high discharge
capacity of 111 mAh g
−1
and excellent cycling
stability, with 90% capacity retention over
240 cycles at 0.1 C. Moreover, the existence of a Na
2/3
FePO
4
intermediate was first observed during the Na
+
intercala-
tion process with conventional electrochemical techniques.
Figure 3b shows the cyclic voltammetry profile. At scan rates
from 0.5 mV s
−1
to a high rate of 2 mV s
−1
, the two well-defined
reductive peaks indicate an identical two-step phase transition
reaction. Recently, maricite-type NaFePO
4
was reinvestigated,
and for the first time, the Na extraction/insertion was proven
to be reversible in the maricite NaFePO
4
electrode, in contrast
to the conventional view that maricite NaFePO
4
is electro-
chemically inactive. Quantum mechanics calculations (the PBE
www.advancedscience.com
Adv. Sci. 2017, 1600275
www.advancedsciencenews.com
Figure 3. a) Synthetic scheme of the aqueous electrochemical displacement process from olivine LiFePO
4
to isostructural NaFePO
4
. b) Cyclic voltam-
mograms of NaFePO
4
/C electrode in 1 mol L
−1
NaPF
6
/EC:DEC (1:1 in vol) solution at various scan rates. Reproduced with permission.
[15]
Copyright
2015, American Chemical Society. c) Comparison of TEM images between pristine maricite NaFePO
4
and partially charged maricite Na
1−x
FePO
4
. This
confirms the two-phase reaction at transformation from maricite NaFePO
4
to a-FePO
4
during the first charge. d) Galvanostatic curves of maricite
NaFePO
4
over 200 cycles at C/20 in a Na cell (inset: discharge curves of maricite NaFePO
4
as a function of the C rate from C/20 to 3 C). Reproduced
with permission.
[16]
Copyright 2014, Royal Society of Chemistry.
Figure 2. Crystal structure of phosphate-based compounds with Fe: a) triphylite-type Na(Li)
FePO
4
; b) maricite-type NaFePO
4
. Reproduced with permission.
[10]
Copyright 2014, American
Chemical Society.
REVIEW
(5 of 24) 1600275
wileyonlinelibrary.com
©
2017 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
functional of density functional theory) and experiments were
combined to identify the electrochemical mechanism respon-
sible for the electrochemical activity of maricite NaFePO
4
.
[16]
The investigation on the Na
+
re-(de)intercalation mechanism
revealed that all Na ions were deintercalated from the nano-
sized maricite NaFePO
4
. X-ray diffraction (XRD) and extended
X-ray absorption fine structure (EXAFS) analyses showed that
after the first deintercalation of Na ions, maricite FePO
4
trans-
formed into a-FePO
4
(Figure 3c), which allowed substantially
smaller barriers for Na to hop from site to site. Amorphous
FePO
4
formed after maricite NaFePO
4
fully desodiated, which
delivered a capacity of 142 mAh g
−1
(92% of the theoretical
value) at the first cycle, and it showed outstanding cyclability,
with a 95% capacity retention of the initial cycle after 200 cycles
(Figure 3d).
Similar to NaFePO
4
, NaMnPO
4
also has two structure
modifications: maricite-type and olivine-type structures. Both
structures consist of layers composed of corner-sharing metal
octahedra bridged through the oxygen atoms in PO
4
3−
groups.
The hydrothermal method was reported to have been used for
the preparation of maricite NaMnPO
4
single crystals at 420 °C
for 6 days, and a solid-state reaction was reported for the syn-
thesis of olivine NaMnPO
4
at 400 °C for 6 h then at 900 °C
for 24 h.
[17]
However, neither of these two structural modifica-
tions have shown favorable electrochemical performance, even
though new synthesis methods such as thermal decomposition
and ion exchange have been reported.
[18]
Recently, based on
an ion-exchange reaction, olivine-type NaMnPO
4
manifested a
reversible capacity of 80–85 mAh g
−1
, corresponding to 0.5 Na
intercalation. Although the electrochemical performance of
NaMnPO
4
seems to be unsatisfactory, it is likely to exhibit more
ion intercalation by optimizing the structure and external elec-
trolyte solutions.
[19]
3.1.2. NASICON-Structured Na
x
M
2
(PO
4
)
3
(M = V, Ti; x = 1,2,3)
NASICON-structured materials were first reported as solid-
state electrolytes by Yao.
[20]
NASICON-structured Na
x
M
2
(PO
4
)
3
(M = V, Ti; x = 1,2,3) is a kind of fast ion conductor with open
3D ion transport channels and high ion diffusion rates. As
variable valence metal ions in the NASICON structure, this
special framework was first reported by Goodenough and his
co-workers, who researched the potential variation of V
4+
/V
3+
,
V
3+
/V
2+
, Fe
3+
/Fe
2+
, Nb
5+
/Nb
4+
, and Nb
4+
/Nb
3+
in the process of
lithium insertion/extraction.
[8,21]
3.1.3. NASICON-Structured Na
3
V
2
(PO
4
)
3
Na
3
V
2
(PO
4
)
3
as a fast Na
+
-transportable NASICON framework
has attracted much attention as a promising cathode material
for Na-ion batteries.
[22–24]
Chen and his group first reported
the fabrication of carbon-coated Na
3
V
2
(PO
4
)
3
as a novel elec-
trode material for Na-ion batteries by a one-step solid-state
reaction.
[22]
It showed a flat voltage plateau at 3.4 V vs. Na
+
/
Na in a non-aqueous Na-ion battery. Its initial charge and dis-
charge capacities were 98.6 and 93 mAh g
−1
, respectively, which
demonstrated that carbon coating can significantly improve
the sodium storage performance. In order to determine the
mechanism of sodium insertion/extraction into/out of the
Na
3
V
2
(PO
4
)
3
lattice, both ex-situ X-ray photoelectron spectros-
copy (XPS) (Figure 4a) and in-situ XRD (Figure 4b) were carried
out.
[25,26]
The results indicated that the mechanism of sodium
insertion/extraction can be ascribed to a kind of typical two-
phase reaction at 3.4 V. The results also showed that all peaks
from Na
3
V
2
(PO
4
)
3
were maintained and that the intensities of
their peaks gradually decreased, indicating a typical two-phase
reaction between Na
3
V
2
(PO
4
)
3
and NaV
2
(PO
4
)
3
. In an effort
to understand the 3D characteristics of the internal ion trans-
portation paths of Na
3
V
2
(PO
4
)
3
, first-principles calculations
combined with experiments were conducted by evaluating the
activation energies towards Na
3
V
2
(PO
4
)
3
. It was proven that two
pathways along the x and y directions and one possible curved
route for ion migration were favored with 3D transport char-
acteristics (Figure 4c and d), providing ample evidence for the
theoretical capacity of 117 mAh g
−1
.
[27]
Despite many advantages associated with Na
3
V
2
(PO
4
)
3
, such
as high stability and relatively high voltage, low conductivity is
still the key drawback to its commercial application. In the past
few years, researchers have investigated many different ways to
overcome this problem, mainly concentrating on optimizing
the synthetic strategies,
[23,28,29]
surface-conducting modifica-
tions,
[22,30–32]
element doping, and so forth.
[33–36]
To optimize the morphology and further improve the elec-
tronic conductivity and structural stability of Na
3
V
2
(PO
4
)
3
,
various synthetic strategies such as traditional solid-state reac-
tions, sol–gel processing, the electrospinning method, and
hydrothermal and solvothermal processing routes have all
been attempted. Recently, a solvothermal processing method
named “facile self-sacrificed route” for synthesizing a 3D NVP
nanofiber framework was reported.
[23]
For the first time, an
outside–in morphological evolution mechanism was proposed
based on time-dependent experiments. The controllably con-
structed NVP cathode material showed outstanding cycling
stability and rate performance in both a sodium half-cell and
a full battery. Through electrospinning, a 1D nanostructured
Na
3
V
2
(PO
4
)
3
material was synthesized.
[29]
The Na
3
V
2
(PO
4
)
3
nanoparticles were uniformly encapsulated in 1D carbon
nanofibers, which greatly shortened the ion diffusion path and
increased the elelctrode/electrolyte contact area.
Surface modification has been widely used to improve the
electronic conductivity of Na
3
V
2
(PO
4
)
3
. Carbon coatings are
particularly attractive because of their high conductivity, even
using carbon concentrations as low as 0.5–10 wt%.
[32]
Further-
more, their low cost, simplicity of introduction during or after
the synthesis of Na
3
V
2
(PO
4
)
3
, and chemical stability all pro-
mote the wide application of such coatings in surface modifica-
tion. Various carbon sources and different carbon frameworks
have been proposed to form the carbon coating layer. Since
the excellent cycling stability and superior rate capability of
Na
3
V
2
(PO
4
)
3
was first reported for Na-ion batteries using cati-
onic surfactants as carbon resource,
[25]
different kinds of carbon
coating methods have emerged. In an attempt to achieve both
high rate capability and stable cyclability, another effective
strategy for surface modification is to embed Na
3
V
2
(PO
4
)
3
par-
ticles in highly conductive and interconnected carbon frame-
works. However, coated carbon formed from pyrolysis of an
Adv. Sci. 2017, 1600275
www.advancedscience.com
www.advancedsciencenews.com