Enhancing cycling performance of FeF3 cathode by introducing a lightweight high conductive adsorbable interlayer

Article abstract of DOI:10.1016/j.jallcom.2017.06.266

Iron fluorides, as a kind of high specific capacity conversion-type cathode materials for lithium rechargeable batteries, are attracting an increasing number of researchers. However, their practical applications are hindered by the poor electrical conductivity and the volume effect during cycling. In this work, a lightweight porous hollow carbon nanofiber (PHCNF) interlayer is proposed to coat on the original FeF₃ cathode to solve these problems. This interlayer is synthesized through a facile carbonizing-activating process using polypyrrole (PPy) as raw material, and plays a difunctional role in trapping the escaped FeF₃ particles and improving the electrical conductivity of electrode. By introducing the high conductive coating layer, an extremely high specific capacity of 217 mAh g^?1 for 40 cycles in the 2–4.5 V region is achieved, which is close to the theoretical specific capacity of 237 mAh g^?1 for FeF₃. Also, a superior power capability is retained delivering a reversible specific capacity of 193 mAh g^?1 at 200 mA g^?1 and 101 mAh g^?1 even at 1000 mA g^?1.

Full text of DOI:10.1016/j.jallcom.2017.06.266

Journal of Alloys and Compounds 723 (2017) 317e326
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
Enhancing cycling performance of FeF cathode by introducing a
3
lightweight high conductive adsorbable interlayer
a
a
b
a
a, *
Xiangyang Zhou , Hongxu Sun , Haochen Zhou , Zhanglin Xu , Juan Yang
a
School of Metallurgy and Environment, Central South University, Changsha, 410083, China
Department of Aerospace, Tsinghua University, Beijing, 100084, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 15 February 2017
Received in revised form
Iron fluorides, as a kind of high specific capacity conversion-type cathode materials for lithium
rechargeable batteries, are attracting an increasing number of researchers. However, their practical ap-
plications are hindered by the poor electrical conductivity and the volume effect during cycling. In this
work, a lightweight porous hollow carbon nanofiber (PHCNF) interlayer is proposed to coat on the
18 June 2017
Accepted 24 June 2017
Available online 27 June 2017
original FeF
activating process using polypyrrole (PPy) as raw material, and plays a difunctional role in trapping the
escaped FeF particles and improving the electrical conductivity of electrode. By introducing the high
conductive coating layer, an extremely high specific capacity of 217 mAh g for 40 cycles in the 2e4.5 V
3
cathode to solve these problems. This interlayer is synthesized through a facile carbonizing-
3
Keywords:
Iron fluoride
Porous hollow carbon nanofiber
Interlayer
Electrode pulverization
Cathodes
ꢀ1
ꢀ
1
region is achieved, which is close to the theoretical specific capacity of 237 mAh g for FeF
3
. Also, a
ꢀ
1
superior power capability is retained delivering a reversible specific capacity of 193 mAh g
at
ꢀ1
ꢀ1
ꢀ1
200 mA g and 101 mAh g even at 1000 mA g .
© 2017 Elsevier B.V. All rights reserved.
1
. Introduction
fluorides such as NiF
2
with an average discharge charge voltage of
ꢀ
1
2
.96 V and a theoretical specific capacity of 554 mAh g [16], CoF
2
ꢀ1
ꢀ1
Recently, with the vigorous development of portable electronic
devices, lithium rechargeable batteries are now considered as one
of the dominant power sources for a variety of electric storage
applications due to their high storage capacity, and long cycle life
(2.85 V, 553 mAh g ) [17], CuF
(2.66 V, 571 mAh g ) [20,21], FeF
2
(3.55 V, 528 mAh g ) [18,19], FeF
2
ꢀ1
ꢀ1
3
(2.74 V, 712 mAh g ) [22,23],
etc have been investigated as cathode materials for Li-ion batteries,
owing to their high operating voltages and high theoretical specific
[1e3]. However, current state-of-the-art Li-ion batteries could not
3
capacities. A representative example is iron trifluoride (FeF ), the
satisfy the fast-growing demand for higher specific capacity, power
density and safety. To further improve the performance of lithium
ion batteries, the research for new type electrode material, espe-
cially cathode materials, for Li-ion batteries has been highlighted.
The theoretical capacity of traditional cathode materials (such as
electrochemical reaction mechanism of which was proposed by
Badway [24e26], which can be described as a two-step reaction.
þ
With the initial insertion of Li , the intercalated “LiFeF
3
” is formed.
A first voltage plateau is around 3.4 V, and then the second plateau
of around 2.1 V appears with the conversion reaction producing Fe
and LiF. These two step reactions involve three-electron transfer, so
ꢀ1
LiFePO
intercalation reactions based on single or less electron reaction
4,5]. Accordingly, an effective strategy for high capacity electrodes
is to utilize conversion reaction with multi-electron transfer
6e15]. during the conversion reactions, phase transitions occur
4 2
, LiCoO ) is often low (140e170 mAh g ), because of the
3
it is generally acknowledged that FeF has a high theoretical specific
ꢀ1
[
capacity of 237 mAh g (one-electron transfer) testing between
2.0 and 4.5 V, this voltage range is similar to some other high ca-
pacity conversion-type cathode materials, such as vanadium oxides
[27,28]. Furthermore, a larger theoretical specific capacity of
[
with the general equation as follows: MX
where M is transition metal, X is anion.
m
þ nLi 4 M þ nLiXm/n
,
ꢀ
1
712 mAh g (three-electron transfer) can be achieved testing be-
tween 1.5 V and 4.5 V region [29e32]. Moreover, the peculiarities of
low toxicity, low cost and better cyclability than other metal fluo-
Among kinds of the conversion-type compounds, metal
3
rides, make FeF a very promising candidate of the cathode mate-
rials for Li-ion batteries.
*
Corresponding author.
E-mail address: jyang_csu@163.com (J. Yang).
Nevertheless, several drawbacks, such as high ionicity and poor
http://dx.doi.org/10.1016/j.jallcom.2017.06.266
925-8388/© 2017 Elsevier B.V. All rights reserved.
0

318
X. Zhou et al. / Journal of Alloys and Compounds 723 (2017) 317e326
structural stability, prevent FeF
cathode material. The high ionicity of FeF
thus FeF , like most metal fluorides, invariably exhibits insulating
3
from its practical application as a
problems of volume effect and electrode pulverization, from which
most conversion-type electrodes are suffering [36].
3
induces a large band gap,
3
The application of the carbon nanofiber interlayer enables the
behavior, giving rise to large hysteresis voltage, poor cycling and
rate capability. What is worse, the pulverization of electrodes,
resulting from the large volume effect accompany with the struc-
tural changes during battery operation, severely degrades the
cyclability of the conversion-type FeF cathode [20]. To address the
3
above-mentioned issues, various measures were taken, which
could be mainly summed up in three aspects: i) Rational mor-
pure FeF
3
cathode to retain an extremely high reversible specific
ꢀ
1
ꢀ1
capacity of 217 mAh g after 40 cycles at 20 mA g , which is close
to the theoretical specific capacity of 237 mAh g of FeF . The
3
ꢀ1
ꢀ1
specific energy is also retained at more than 612 Wh kg under
these conditions. To the best of our knowledge, this is the highest
specific capacity and energy among the ever published results
under the same test conditions. Interestingly, the composite cath-
ode also deliver an excellent rate performance, reversible capacities
3
phologies designing and nanocrystallization of FeF . Rational
ꢀ ꢀ1 ꢀ1
1
designed morphologies could alleviate the volume effect and
shorten the ion transport path, while nanoscaled crystallite
dimension could significantly improve the reaction activity of the
of 193 mAh g , 174 mAh g and 101 mAh g are achieved at
ꢀ1
ꢀ1
ꢀ1
200 mA g , 400 mA g and 1000 mA g , respectively. It should be
noted that the outstanding performance of the novel design of
electrode configuration may well prove the possibility of
conversion-type compounds being used as commercial electrode
materials for Li rechargeable batteries.
FeF
dimensionally ordered macroporous FeF
delivered a reversible capacity of 190 mAh g for 30 cycles at
3
particles. For example, Ma et al. prepared
a three-
3
hybrid structure which
ꢀ1
ꢀ
1
2
0 mA g
3
[22]. ii) Fabrication of FeF based composite with
conductive carbonaceous materials [23,33]. The building of the
carbonaceous conductive framework could effectively improve the
cyclability and rate performance of the composite materials. Kim
2. Experimental
2.1. Synthesis of iron fluoride (FeF3)
et al. fabricated FeF
reversible capacity of nearly 200 mA g for 30 cycles at 20 mA g
23]; Liu et al. synthesized uniform iron fluoride nanocrystals on
3
nanoflowers on CNT branches, sustained a
ꢀ
1
ꢀ1
FeF
Firstly, 70 mL FeCl
Teflon-lined autoclave, then, 30 mL HF solution (40%) was added
into FeCl ethanol solution dropwise, after stirring, the bright yel-
3
powders were synthesized by a facile liquid-phase method.
[
3
(0.02 M) ethanol solution was prepared in
ꢀ
1
reduced graphene oxide sheets, a capacity of 205 mA g for 30
ꢀ1
cycles at 45 mA g was achieved [33]. iii) Element doping. Smaller
band gap can improve the electronic conductivity of FeF itself and
alleviate the sluggish kinetics of FeF , in addition, doping element
can effect microcrystal growth. For example, Bai et al. prepared Fe(1-
x)Ti by a hydrothermal method, which retained the specific ca-
3
3
low solution gradually became colorless. After another stirring for
3
0.5 h, the Teflon-lined autoclave was sealed and heated in an oven
ꢁ
at 60 C for 10 h. The generated pink precipitate was collected and
x
F
3
washed with absolute ethanol for several times, following by dry-
ꢀ
1
ꢀ1
ꢁ
pacity of 174 mAh g at 23.7 mA g [34]. Overall, a majority of
ing at 80 C for 8 h to get FeF
3
$3H
2
O powders as precursor. After
light green powders were
ꢁ
research focus on the fabrication of FeF
cyclability and rate performance of FeF
3
/C composite to modify the
, and some good results are
another drying at 120 C for 48 h, the FeF
obtained.
3
3
achieved. But the current resulting reversible capacities are not
satisfactorily high, and the cycle performance still need to be
improved, which may be partially responsible for the pulverization
2.2. Synthesis of the PHCNF
phenomenon of FeF
in the case of low carbon content.
3
based composite cathodes, since it still exists
The porous hollow carbon nanofiber was prepared from poly-
pyrrole nanofiber. The polypyrrole nanofiber was synthesized by
our previous work [37,38]. Briefly, 7.3 g CTAB and 13.7 g ammonium
persulfate was dissolved in 120 mL HCl solution (1 M), respectively.
After stirring in ice bath for 0.5 h, a white reactive template was
achieved. Then, 8.3 mL pyrrole monomer was added slowly, the
Considering that the electrode pulverization is difficult to avoid
for conversion-type cathode materials. A macroscopic electrode
structure was designed by introducing an interlayer inserted be-
tween electrode and separator by our group [35], and an excellent
performance was achieved which retained an ultrahigh capacity of
ꢁ
black precipitate was obtained after 24 h stirring at 0e5 C. After
ꢀ1
ꢀ1
6
00 mAh g at 100 mA g for 60 cycles. Unfortunately, the pro-
collecting and washing for several times with deionized water and
ꢁ
portion of interlayer in electrode remains to be further reduced. In
this work, a lightweight conductive interlayer between active ma-
terial and separator is designed to apply to conversion-type cath-
absolute ethanol, the black precipitate was during at 80 C in an
oven for 12 h. Then, the as-synthesized polypyrrole nanofiber was
ꢁ
heated at 700 C for 2 h under a N
2
atmosphere for carbonization.
ode materials, simply by coating the surface of FeF
3
cathode with a
The as-obtained carbon nanofiber and KOH were mixed with a
porous hollow carbon nanofiber (PHCNF) film directly. The coating
film is thick in order to ensure a high proportion of active material,
which accounted for only 27.9 wt% of the whole cathode (cathode
layer and interlayer). A schematic illustration of the designed cell
configuration and the preparation process of PHCNF are shown in
Scheme 1. The PHCNF can be synthesized through a facile
carbonizing-activating process using PPy as raw material. The as-
synthesized PHCNF possesses a three dimensional cross-linked
network structure with a large specific surface area, which comes
from the porous hollow structure of every carbon fiber. The hollow
structure can reduce the weight of the interlayer, and the highly
porous structure can provide abundant accommodate space for the
mass ratio of 3:1 in ethanol and deionized water mixed solution.
ꢁ
After drying, the mixture was heated at 700 C for 1 h in N
2
at-
mosphere for activation, and then the product was washed to
ensure that the chloride ions was removed completely. Finally, the
porous hollow carbon nanofiber (PHCNF) doped with nitrogen
ꢁ
were obtained after drying in an oven overnight at 80 C.
2.3. Materials characterization
The phases of the synthesized materials were characterized by
transmission electron microscopy (TEM, JEM-2100F, Japan), scan-
ning electron microscope (SEM, Nova NanoSEM230, USA), and X-
ray diffraction (XRD, Rigaku-TTRIII, Japan). Elemental analysis was
made by an energy dispersive spectrometer (EDS). The pore
diameter distribution, total pore volume and specific surface area
were identified through a Surface Area and Porosity Analyzer (ASAP
2020HD88, USA). Digital photos were taken with a digital camera
3
escaped FeF nanoparticles also benefit the electrolyte permeation.
The cross-linked conductive carbon network plays a key role in
stabilizing the electrode structure and reducing the internal resis-
tance of the battery simultaneously. Thus the “double-layer” elec-
trode configuration provides an effective way to resolve the

X. Zhou et al. / Journal of Alloys and Compounds 723 (2017) 317e326
319
Scheme 1. A schematic of electrode configuration and the preparation process of porous hollow carbon nanofiber (PHCNF).
(
CASIO EX-ZR400, Japan). Surface elemental analysis was carried
out by using an X-ray photoelectron spectrometer (XPS, K-Alpha
063, USA).
3 2
and FeF $0.33H O [39e41]. Water molecules act as structural sta-
bilizers during the electrochemical reaction, but limit the theoret-
ical capacity partly and are likely to bring about the side reaction
1
with LiPF
anhydrous FeF
the water of crystallization from hydrated ones through the high
temperature heat treatment. In this study, anhydrous FeF were
synthesized through a facile liquid-phase method and subsequent
mild heat treatment. The X-ray diffraction (XRD) pattern of the as-
6
in the electrolyte. Usually, it is difficult to obtain the
3
, because Fe is often detected while removing
2 3
O
2
.4. Electrode preparation and electrochemical measurements
3
The iron fluoride cathode was made by mixing 20% acetylene
3
black, 10% polyvinylidene fluoride (PVDF) and 70% pure FeF in N-
methyl-2-pyrrolidinone (NMP). The well mixed slurry was coated
on aluminum foil and dried at 120 C overnight in a vacuum oven.
As for the preparation of the “double-layer” cathode, the slurry of
carbon nanofiber was produced by mixing 95% PHCNF and 5%
polyvinylidene fluoride (PVDF), followed by coating onto as-dried
iron fluoride cathode with a scraper blade and drying at 120
for another 12 h. The electrodes were cut into pellets of 1.0 cm in
diameter, and the coin cells were assembled with Li metal foil as the
anode in an argonfilled glove box (Super 1220/750, Shanghai
Mikrouna Co. Ltd.). A Celgard 2400 polypropylene membrane was
ꢁ
synthesized FeF
shown, all XRD peaks of the as-synthesized powders are identified
as FeF (JCPDS 33-0647), crystallizing in a ReO -type structure with
3
powders is clearly represented in Fig. 1a. As is
3
3
a space group of R-3c, no other impurity signal is detected in XRD
pattern. The widened peaks are mainly associated with the removal
of crystal water under heat treatment, which leads to an amor-
phous characteristic of the corresponding XRD peaks.
ꢁ
C
The morphology of FeF
shaped morphology of FeF
length of 10e50 m. Interestingly, it can be found that a few
spindle-shaped FeF particles agglomerate into a radiated aggre-
gate. As shown from the TEM image of FeF in Fig. 1d. The as-
synthesized FeF is a kind of typical micro-nano structure.
Although the grain size of the FeF particles observed from SEM
3
is shown in Fig. 1bed, a regular spindle-
3
particles is observed with an average
m
6
served as the separator. LiPF (1 M) in ethylene carbonate (EC) and
diethyl carbonate (DEC) with a volume ratio of 1: 1 as the elec-
trolyte. The cells were tested in voltage ranges of 4.5e2.0 V and
4
3
3
þ
3
.5e1.5
0e1000 mA g
V
(~vs. Li/Li ) at different current densities of
ꢀ
1
3
2
on a LAND CT-2001A (Wuhan, China). Cyclic
image is relatively huge, the spindle-shaped particles are composed
of primary nanoparticles with diameter of only about 20 nm. The
primary nanoparticles can improve the reaction activity of FeF ,
3
and the gaps between them are expected to increase the contact
area with the electrolyte.
voltammograms (CV) were measured by an electrochemical
workstation (PARSTAT 4000, USA) at a scanning rate of 0.1 mV s
in voltage ranges of 4.5e2.0 V and 4.5e1.5 V. Also, electrochemical
impedance spectra (EIS) were obtained by PARSTAT 4000 electro-
chemical workstation from 100 kHz to 10 mHz with an automatic
scanning mode.
ꢀ
1
3
However, for pure FeF electrode, the structural collapse of the
FeF particles is difficult to avoid in subsequent cycling. The high
3
conductive cross-linked carbon network interlayer composing of
PHCNF can solve this issue well. Typical SEM and TEM images of
PHCNF are shown in Fig. 2a - b, the as-obtained PHCNF shows
clearly homogeneous morphology of cross-linked hollow nano-
fibers with external diameter of about 100 nm and internal diam-
eter of about 30e40 nm (they can be seen clearly in Fig. S1a), the
lightweight hollow structure of PHCNF make itself more applicable
for being an interlayer material. The hollow structure of PHCNF
forms in the preparation process of polypyrrole (PPy) nanofibers
3
. Results and discussion
Anhydrous FeF
3
possesses a hexagonal lattice belonging to the
type structure
R-3c space group, which is described to have a ReO
with the lattice parameter values of a (5.20 Å), b (5.20 Å), c (13.32 Å)
and V (311.99 Å ), and this structure is larger than other similar
type structure, such as TiOF
hydrated structures, and kinds of them have been studied as
cathode materials by researchers, such as FeF $3H O, FeF $0.5H O
2
3
3
2 2 3
and Nb OF [6]. FeF has a number of
3
2
3

320
X. Zhou et al. / Journal of Alloys and Compounds 723 (2017) 317e326
3
Fig. 1. X-ray diffraction (XRD) pattern (a), SEM images (b, c) and TEM image (d) of as-synthesized FeF powders.
[
42]. After carbonization, the hollow carbon nanofibers are ach-
membered ring, quaternary N, and other N, which are referred to
as N - 5, N - 6, N - Q, and N - X. The four peaks can be identified at
400.36, 398.74, 401.45, and 403.20 eV, respectively. Thus the in-situ
N-doped PHCNF is expected to be able to have certain adsorption
ability, with its large specific surface area and good electrical con-
ductivity [38,46].
ieved. In order to promote the adsorption capacity of the hollow
carbon nanofibers, the subsequent activation of KOH makes the
polypyrrole pyrolysis carbon nanofibers a unique porous one-
dimensional nanostructure. The porous characteristics of PHCNF
can be observed from the typical HRTEM image of PHCNF in
Fig. S1b. Nitrogen adsorption desorption isotherms and pore size
distribution patterns of PHCNF are shown in Fig. 2c, the nitrogen
adsorption desorption isotherms depicts a typical type IV iso-
therms. The hysteresis loop indicates the existence of a micro-
mesoporous hybrid structure [20]. The pore size distribution
pattern, fitting from the corresponding absorption curve by BJH
mode, suggests that most pores are smaller than 1.1 nm in diam-
eter, and the average pore diameter is estimated to be 2.05 nm. The
BET surface area of PHCNF is calculated to be 1404.38 m g with a
total pore volume of 0.72 cm
and porous structure are expected to strengthen the adsorption
capacity and the electrolyte permeation of PHCNF.
It is reported that the presence of heteroatom at the carbon
surface has a strong interatomic attraction, which can not only
enhance electrical conductivity but also improve the adsorption
ability of the carbons [43,44]. Fig. 2d shows the X-ray photoelectron
spectroscopy spectra of PHCNF. It can be observed that three peaks
centering at about 400.0, 285.0 and 532.0 eV corresponding to N1s,
C1s and O1s, respectively, in the XPS survey spectrum of PHCNF
3
Photographs illustrating the preparation process of FeF cathode
with PHNCF coating film are shown in Fig. 3a - b, PHCNF is uni-
formly coated onto the as-dried iron fluoride cathode with a
scraper blade, a smooth surface is observed. Fig. 3c shows the cross-
sectional image of electrode with PHCNF coating film, the thickness
of the PHCNF film (PHCNF
(FeF
þ carbon black þ PVDF) are about 3.5
respectively. The weight of the PHCNF film and the FeF
þ
PVDF) and the FeF
m and 26
layer is
3
layer
3
m
mm,
3
2
ꢀ1
about 0.40 and 1.03 mg, which are determined by weighing the
electrode before and after the coating process. That is, the PHCNF
coating film accounted for about 27.9 wt% of the whole cathode (the
3
ꢀ1
. The large specific surface area
g
PHCNF film and the FeF
the mass of active material is calculated by the mass of FeF
3
layer). During the charge/discharge test,
in FeF
3
3
layer to be 0.72 mg. Fig. 3d shows the front-view image of PHCNF
coating film, the porous and cross-linked network can be observed
obviously.
3
Electrochemical performances of FeF cathode materials with
PHCNF coating film interlayer were investigated. It is widely
considered that FeF
237 mAh g (2.0e4.5 V) and 712 mAh g (1.5e4.5 V) by the
following reactions [29e32]:
3
has high theoretical specific capacities of
ꢀ1
ꢀ1
[45]. The tests show the proportion of N in PHCNF is 2.24 wt %, the
high-resolution N 1s spectrum of PHCNF is presented in Fig. 2d. The
N 1s spectrum of PHCNF indicates that four different peaks, pyrrolic
N located in a five-membered ring, pyridinic N located in a six-
FeF
3
þ Liþ þ eꢀ ¼ LiFeF
3
(4.5 V ꢀ 2 V)
(1)

X. Zhou et al. / Journal of Alloys and Compounds 723 (2017) 317e326
321
Fig. 2. Typical SEM (a) and TEM (b) images of PHCNF; Nitrogen adsorption desorption isotherms and pore size distribution patterns of PHCNF (c); XPS spectrum for PHCNF with
high-resolution spectrum for the N1s peaks (d).
LiFeF
3
þ 2Liþ þ 2eꢀ ¼ Fe þ 3LiF (2 V ꢀ 1.5 V)
(2)
Fig. S2c (corresponding voltage-capacity curves from the 1st to 3rd
cycle can be seen in Fig. S2d), which do not measure up to the
theoretical specific capacity of 712 mAh g , this is likely due to that
ꢀ1
Normally, Eq. (1) is ascribed to an intercalation reaction, FeF
3
maintain its original structure with the Li ion insertion during the
reaction, and Equ. (2) represent a conversion reaction, which in-
volves a structural change while cycling with the formation of zero-
valent iron and LiF [19]. It is worth mentioning that LiF has the same
partial FeF , with the poor electrical conductivity, was not involved
in the electrode reaction. The pulverization of the FeF3 cathode
3
materials took place in the subsequent cycles, the discharge ca-
ꢀ1
pacities are down to about 452 and 412 mAh g , and after 20 cy-
ꢀ1
insulating properties with FeF
the complete three-electron energy storage reaction. Fig. 4a - b
show the cyclic voltammograms of FeF cathode with PHCNF
coating film and pure FeF cathode without PHCNF coating film
3
, resulting in an obstacle for using
cles, the discharge capacity of 277 mAh g is retained. The test
results for the rate performance of the cells measured at
ꢀ
1
3
20e400 mA g are demonstrated in Fig. S2e, a reversible capacity
ꢀ
1
ꢀ1
,
3
of about 196 and 134 mAh g are achieved at 200 and 400 mA g
respectively. After 35 cycles, the discharge capacity returns to
measured between 2.0 V and 4.5 V. As shown in Fig. 4a, there are
two peaks in each reaction, and the cathodic and the anodic peak
appear near 2.8 V and 3.2 V. In contrast, the cathodic and the anodic
peaks appear near 2.1 V and 3.5 V in Fig. 4b. Their voltage differ-
ences are around 0.4 V and 1.4 V for the two samples, respectively,
which indicates that the PHCNF coating film strengthen the kinetics
of the electrochemical reaction obviously, and the large voltage
hysteresis can be suppressed effectively. As shown in Figs. S2aeb,
the similar results from CV tests can be observed while measuring
in the 1.5e4.5 V region, further proving that the PHCNF coating film
ꢀ
1
ꢀ1
301 mAh g at 20 mA g . The relatively good test results of FeF3
cathodes with the PHCNF coating film compared with pure FeF3
cathodes benefits from the difunctional interlayer. Nevertheless, in
the 1.5e4.5 V region, the voltage hysteresis is still serious and the
capacity fades obviously in subsequent cycles, which result in
serious energy loss.
Therefore, the same galvanostatic discharge-charge test was
performed in the 2e4.5 V region. As shown in Fig. 4c, during the
ꢀ
1
fore several cycles, a specific capacity of about 254 mAh g was
ꢀ
1
improve the reaction activity of the FeF
3
cathode obviously.
achieved at a current rate of 20 mA g . This higher specific capacity
ꢀ1
The cyclability and rate performance of the batteries were tested
in various conditions to obtain a comprehensive evaluation for the
role of PHCNF interlayer in improving electrochemical perfor-
mance. Galvanostatic discharge-charge profiles were first per-
than the theoretical specific capacity of 237 mAh g is likely due to
that partial FeF participated in the reaction of Eq. (2). In the sub-
3
sequent cycles, the interlayer enabled the FeF cathode to retain an
3
ꢀ1
extremely high reversible specific capacity of about 217 mAh g
ꢀ
1
formed in the 1.5e4.5 V region at 20 mA g . The first charge and
for 40 cycles, which is close to the theoretical specific capacity of
ꢀ1
ꢀ1
discharge capacities are about 594 and 536 mAh g as shown in
237 mAh g . Fig. 4c also shows the corresponding specific energy

322
X. Zhou et al. / Journal of Alloys and Compounds 723 (2017) 317e326
Fig. 3. Photographs of the coating process: as-prepared raw cathode (a) and double-layer cathode coated with PHCNF film (b); a typical cross-sectional SEM image of fresh cathode
with PHCNF coating film (c) and a typical front-view SEM image of PHCNF coating film (d).
of FeF
3
cathode materials with interlayer of PHCNF coating film, the
charge-transfer resistance (Rsfþct), and an inclined line at low fre-
quency relating to the ion transfer process [28,47]. An equivalent
electrical circuit is used to fit the experimental impedance spectra
ꢀ
1
specific energy retained more than 612 Wh kg , which is much
higher than conventional cathode materials. Remarkably, the
excellent specific capacity and energy are unprecedented, which
are higher than the reported capacity under the same test condi-
tions [22,23]. The parameters of electrochemical performance
comparison with other studies are listed in Table S1. Fig. 4d shows
the corresponding charge and discharge voltage-capacity curves
as shown in Fig. 5c. Impedance parameters of FeF
3
cathode with
PHCNF coating film and pure FeF cathode without PHCNF coating
3
film measured before cycling and after 30 cycles at different voltage
are listed in Table 1. Impedance spectra of electrodes after 30 cycles
are tested with a relaxation time of 30 min, and impedance spectra
of lithium inserted electrodes are tested after another half cycle,
when the voltage discharge to 2.0 V. Before cycling, the value of
ꢀ1
from the 5th to 40th cycle at 20 mA g , the coincident curves
certified the outstanding cyclability of FeF with the interlayer.
Notably, the interlayer obviously improved the rate performance of
FeF cathode as well. As shown in Fig. 4eef, the rate performance
was measured at 20e1000 mA g , under the high current rate
3
R
sfþct for FeF
value increases to 51.3
due to the structural change of the active material. Rsfþct testing at
2.0 V is lower than at 4.5 V, indicating that lithium-inserted FeF
has a better electronic conductivity. Also, it clearly shows that the
values of Rsfþct for FeF cathode with interlayer are much lower
than that of pure FeF under the same condition, which can prove
3
cathode with interlayer is calculated to 28.8
U. This
3
U
after 30 cycles at 4.5 V, which is mainly
ꢀ
1
ꢀ1
ꢀ1
condition, it can deliver close to 193 mAh g at 200 mA g
Meanwhile, superior capacities of 174, 123 and 101 mAh g were
obtained at 400, 700 and 1000 mA g , respectively. Fig. 4f shows
the corresponding discharge curves at different current densities.
Obviously, the difunctional interlayer can obviously enhance elec-
.
3
ꢀ
1
ꢀ1
3
3
that the one-dimensional N-doped PHCNF conductive interlayer
trochemical activity of FeF
3
cathode materials, which is widely
significantly enhance the charge transfer rate of the electrode. After
considered as an insulating unsuitable material for cathode.
To further investigate the interfacial charge transfer in the
30 cycles, the constant phase element capacitance values (CPEsfþct
at 2.0 V and 4.5 V of “double-layer” electrode are lower than that of
pure FeF electrode, also indicating an excellent charge transfer
capacity of “double-layer” electrode. Therefore, the EIS data show
that the strong interaction between the PHCNF and FeF improves
its own electrical conductivity and the “double-layer” electrode
presents a superior charge transfer process enabling pure FeF
)
“
double-layer” electrode, electrochemical impedance spectra (EIS)
were carried out at room temperature. Fig. 5a shows the typical
Nyquist plots of FeF cathode with interlayer before cycling and
after cycling for 30 cycles, and pure FeF cathode in Fig. 5b for
3
3
3
3
comparison. It can be found that all Nyquist plots for these two
types of electrodes consist of a single depressed semicircle in the
high-to-medium frequency region relating to surface-film and
3
cathode to deliver extremely high specific capacity and excellent
rate capability.

X. Zhou et al. / Journal of Alloys and Compounds 723 (2017) 317e326
323
Fig. 4. Cyclic voltammograms of FeF
Electrochemical performances of FeF
corresponding specific energy (c); voltage-capacity curves at 20 mA g for 35 cycles from the 5th to 40th cycle (d); Specific capacity at different current rates from 20 mA g to
3
cathode with PHCNF coating film (a) and pure FeF
3
cathode without PHCNF coating film (b) measured at a voltage range of 2.0e4.5 V;
ꢀ
1
3
cathode materials with PHCNF coating film measured in the 2.0e4.5 V region: cycling performance at a current rate of 20 mA g and
ꢀ
1
ꢀ1
ꢀ
1
1
000 mA g (e); corresponding discharge profiles at different rates (f).
During the redox reaction of active materials, there is a large
volume effect accompanying the structural changes, the bulk FeF
pulverizes into small particles, resulting in poor electrical conduc-
tivity and cyclability of FeF . In order to investigate the function of
the N-doped PHCNF coating film, Fig. 6a - c show the front-view
images of the cathode with PHCNF coating film before cycling, af-
ter 45 and 100 cycles. Obvious changes occur on the surface of the
electrode, a large number of nano-sized particles embedded on the
surface of PHCNF with the increase of cycles as shown in Fig. 6bec,
it strongly indicates that the serious pulverization of electrodes
exist indeed. After coated by PHCNF, by feat of the adsorption
film cycled for 100 cycles. C, Fe and F signals are detected in the
selected area. It implies that FeF particles were evenly distributed
on the surface of PHCNF after cycles. The SEM images, coupled with
the EDS and elemental mapping results indicate that the interlayer
of PHCNF coating film can effectively relieve the decay of the
3
3
3
cyclability resulted from the pulverization of the FeF
materials. The function of the interlayer can be ascribed into two
points: On one hand, the trapped FeF particles, adsorbed by the
3
cathode
3
PHCNF, can be reactivated and continue to participate in reversible
electrode reaction, on the other hand, the interconnected conduc-
tive network of PHCNF provides rapid electron transfer path to both
the active materials left on the electrode and the trapped active
materials particles.
3
ability of the N-doped PHCNF, the escaped FeF particles from the
electrode are adsorbed in PHCNF surface. To reveal the element
composition of the adsorbed particles, an energy dispersive spec-
trometer (EDS) analysis was performed on the surface of the PHCNF
coating film. Fe and F signals are detected in the EDS result shown
in Fig. 6d. Fig. 6e shows the elemental mapping of PHCNF coating
4. Conclusions
In summary, an approach to enhance cycling performance of

324
X. Zhou et al. / Journal of Alloys and Compounds 723 (2017) 317e326
3 3
Fig. 5. EIS data of FeF cathode with PHCNF coating film (a) and pure FeF cathode without PHCNF coating film (b) measured before cycling and after 30 cycles at different voltage
respectively; Equivalent electrical circuit used to fit the experimental impedance spectra (c).
Table 1
Impedance parameters of FeF
voltage.
3
3
cathode with PHCNF coating film and pure FeF cathode without PHCNF coating film measured before cycling and after 30 cycles at different
Sample
State
R
0
/U
R
sfþct
/U
CPEsfþct/mF
“
double-layer” electrode
Before cycling (OCV 3.0 V)
5.4
4.3
6.5
7.4
4.8
5.4
28.8
51.3
38.6
79.5
166.4
94.6
20.9
7.8
8.6
18.4
11.8
14.5
3
3
0 cycles (4.5 V)
0 cycles (2.0 V)
Pure FeF
3
electrode
Before cycling (OCV 3.0 V)
3
3
0 cycles (4.5 V)
0 cycles (2.0 V)
Fig. 6. Typical front-view SEM images of the cathode with PHCNF coating film: before cycling (a) and post-cycled characterization after 45 cycles (b) and 100 cycles (c) with its EDX
spectrum (d); Elemental mapping of the PHCNF coating film cycled for 100 cycles (e).
pure FeF
3
cathode has been proposed simply by coating a light-
difunctional interlayer composing of high conductive cross-linked
carbon network not only provides an effective way to resolve the
weight N-doped porous hollow carbon nanofiber interlayer. The

X. Zhou et al. / Journal of Alloys and Compounds 723 (2017) 317e326
325
problem of electrodes pulverization, but also improves the elec-
trical conductivity of the FeF cathode. The N-doped PHCNF inter-
layer enables the FeF cathode achieve a superior electrochemical
performance of enhanced specific capacity of about 217 mAh g at
P.J. Chupas, C.P. Grey, Comprehensive study of the CuF
mechanism in
5169e15184.
19] Y.H. Cui, M.Z. Xue, Y.N. Zhou, S.M. Peng, X.L. Wang, Z.W. Fu, The investigation
on electrochemical reaction mechanism of CuF thin film with lithium, Elec-
2
trochim. Acta 56 (2011) 2328e2335.
20] H. Song, H. Cui, C. Wang, Extremely high-rate capacity and stable cycling of a
2
conversion reaction
lithium ion battery, J. Phys. Chem. C 118 (2014)
a
3
1
3
[
[
[
ꢀ1
ꢀ
1
2
0 mA g and rate performance delivering a high capacity of 193
ꢀ1
ꢀ1
ꢀ1
and 101 mAh g at 200 mA g and 1000 mA g in the 2e4.5 V
region. It is expected that the presented PHCNF interlayer in this
paper may trigger more exploration in this direction to enhance the
electrochemical performance of the conversion-type electrode
materials for high-power Li rechargeable batteries.
highly ordered nanostructured carboneFeF
A 3 (2015) 22377e22384.
2
battery cathode, J. Mater. Chem.
21] M.A. Reddy, B. Breitung, V.S.K. Chakravadhanula, C. Wall, M. Engel, C. Kübel,
A.K. Powell, H. Hahn, M. Fichtner, CF derived carbon-FeF nanocomposites for
reversible lithium storage, Adv. Energy Mater. 3 (2013) 308e313.
x
2
[22] D. Ma, Z. Cao, H. Wang, X. Huang, L. Wang, X. Zhang, Three-dimensionally
ordered macroporous FeF3 and its in situ homogenous polymerization coating
for high energy and power density lithium ion batteries, Energy Environ. Sci. 5
Acknowledgements
(2012) 8538e8542.
[
[
[
23] S.W. Kim, D.H. Seo, H. Gwon, J. Kim, K. Kang, Fabrication of FeF nanoflowers
3
on CNT branches and their application to high power lithium rechargeable
batteries, Adv. Mater. 22 (2010) 5260e5264.
24] F. Badway, N. Pereira, F. Cosandey, G.G. Amatucci, Carbon-metal fluoride
This work was supported by the National Nature Science
Foundation of China (no. 51204209 and 51274240), the Project of
Innovation-driven Plan in Central South University and the
Fundamental Research Funds for the Central Universities of Central
South University (no. 2017zzts122).
nanocomposites structure and electrochemistry of FeF :C, J. Electrochem. Soc.
3
150 (2003) A1209eA1218.
25] F. Badway, F. Cosandey, N. Pereira, G.G. Amatucci, Carbon metal fluoride
nanocomposites high-capacity reversible metal fluoride conversion materials
as rechargeable positive electrodes for Li batteries, J. Electrochem. Soc. 150
(
2003) A1318eA1327.
Appendix A. Supplementary data
[
26] F. Cosandey, J.F. Alsharab, F. Badway, G.G. Amatucci, P. Stadelmann, EELS
spectroscopy of iron fluorides and FeFx/C nanocomposite electrodes used in
Li-ion batteries, Microsc. Microanal. Off. J. Microsc. Soc. Am. Microbeam Anal.
Soc. Microsc. Soc. Can. 13 (2007) 87e95.
27] A. Sakunthala, M.V. Reddy, S. Selvasekarapandian, B.V.R. Chowdari, P.C. Selvin,
Preparation, characterization, and electrochemical performance of lithium
trivanadate rods by a surfactant-assisted polymer precursor method for
lithium batteries, J. Phys. Chem. C 114 (2010) 8099e8107.
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jallcom.2017.06.266.
[
References
[
[
[
1] B. Dunn, H. Kamath, J.M. Tarascon, Electrical energy storage for the grid: a
battery of choices, Science 334 (2011) 928e935.
2] G. Liu, Development of a general sustainability indicator for renewable energy
systems: a review, Renew. Sustain. Energy Rev. 31 (2014) 611e621.
3] Z.Q. Liu, A.X. Xu, Y.Y. Lv, X.X. Wang, Promoting the development of distributed
concentrated solar thermal technology in China, Renew. Sustain. Energy Rev.
[28] A. Sakunthala, M.V. Reddy, S. Selvasekarapandian, B.V.R. Chowdari, P.C. Selvin,
5
Energy storage studies of bare and doped vanadium pentoxide, (V1.95 0.05)O ,
M
M ¼ Nb, Ta, for lithium ion batteries, Energy & Environ. Sci. 4 (2011)
1712e1725.
[29] F. Wang, R. Robert, N.A. Chernova, N. Pereira, F. Omenya, F. Badway, X. Hua,
M. Ruotolo, R. Zhang, L. Wu, Conversion reaction mechanisms in lithium ion
batteries: study of the binary metal fluoride electrodes, J. Am. Chem. Soc. 133
(2011) 18828e18836.
[30] N. Yamakawa, M. Jiang, B. Key, C.P. Grey, Identifying the local structures
formed during lithiation of the conversion material, iron fluoride, in a Li ion
battery: a solid-state NMR, X-ray diffraction, and pair distribution function
analysis study, J. Am. Chem. Soc. 131 (2009) 10525e10536.
[31] R. Prakash, C. Wall, A.K. Mishra, C. Kübel, M. Ghafari, H. Hahn, M. Fichtner,
Modified synthesis of [Fe/LiF/C] nanocomposite, and its application as con-
version cathode material in lithium batteries, J. Power Sources 196 (2011)
5936e5944.
16 (2012) 1174e1179.
[
[
[
4] J. Hu, W. Li, Y. Duan, S. Cui, X. Song, Y. Liu, J. Zheng, Y. Lin, F. Pan, Single-
particle performances and properties of LiFePO
teries, Adv. Energy Mater. 7 (2017) 1601894.
4
nanocrystals for Li-Ion bat-
5] K.S. Tan, M.V. Reddy, G.V.S. Rao, B.V.R. Chowdari, High-performance LiCoO
molten salt (LiNO3:LiCl) synthesis for Li-ion batteries, J. Power Sources 147
2005) 241e248.
6] M.V. Reddy, S. Madhavi, G.V.S. Rao, B.V.R. Chowdari, Metal oxyfluorides TiOF
and NbO F as anodes for Li-ion batteries, J. Power Sources 162 (2006)
312e1321.
2
by
(
2
2
1
[
[
7] M.V. Reddy, G.V.S. Rao, B.V.R. Chowdari, Metal oxides and oxysalts as anode
materials for Li ion batteries, Chem. Rev. 113 (2013) 5364e5457.
8] A.S. Hameed, M.V. Reddy, N. Sarkar, B.V.R. Chowdari, J.J. Vittal, Synthesis and
electrochemical investigation of novel phosphite based layered cathodes for
Li-Ion batteries, Rsc Adv. 5 (2015) 60630e60637.
[32] L. Li, F. Meng, S. Jin, High-capacity lithium-ion battery conversion cathodes
based on iron fluoride nanowires and insights into the conversion mecha-
nism, Nano Lett. 12 (2012) 6030e6037.
[33] J. Liu, Y. Wan, W. Liu, Z. Ma, S. Ji, J. Wang, Y. Zhou, P. Hodgson, Y. Li, Mild and
cost-effective synthesis of iron fluoride-graphene nanocomposites for high-
rate Li-ion battery cathodes, J. Mater. Chem. 1 (2013) 1969e1975.
[34] Y. Bai, X. Zhou, Z. Jia, C. Wu, L. Yang, M. Chen, H. Zhao, F. Wu, G. Liu, Un-
derstanding the combined effects of microcrystal growth and band gap
reduction for Fe(1ꢀx)Ti F nanocomposites as cathode materials for lithium-
x 3
ion batteries, Nano Energy 17 (2015) 140e151.
[35] J. Yang, Z. Xu, H. Sun, X. Zhou, A three-dimensional interlayer composed of
graphene and porous carbon for Long-life, High capacity Lithium-Iron Fluo-
ride Battery, Electrochim. Acta 220 (2016) 75e82.
[9] M. Prabu, M.V. Reddy, S. Selvasekarapandian, G.V.S. Rao, B.V.R. Chowdari,
Synthesis, impedance and electrochemical studies of lithium iron fluo-
rophosphate, LiFePO F cathode, Electrochim. Acta 85 (2012) 572e578.
4
[
10] M. Nagarathinam, K. Saravanan, E.J.H. Phua, M.V. Reddy, B.V.R. Chowdari,
J.J. Vittal, Redox-active metal-centered oxalato phosphate open framework
cathode materials for lithium ion batteries, Angew. Chem. Int. Ed. Engl. 124
(
2012) 5866e5870.
[
11] M.V. Reddy, G.V.S. Rao, B.V.R. Chowdari, Long-term cycling studies on 4 V-
cathode, lithium vanadium fluorophosphate, J. Power Sources 195 (2010)
[36] Z. Xiao, Z. Yang, L. Wang, H. Nie, M. Zhong, Q. Lai, X. Xu, L. Zhang, S. Huang,
5768e5774.
2
Lithium-Sulfur batteries: a lightweight TiO /graphene interlayer, applied as a
highly effective polysulfide absorbent for fast, long-life lithium-sulfur batte-
ries (Adv. Mater. 18/2015), Adv. Mater. 27 (2015) 2890.
[
12] S. Wei, X. Wang, M. Jiang, R. Zhang, Y. Shen, H. Hu, The FeF
3
$0.33H
2
O/C
nanocomposite with open mesoporous structure as high-capacity cathode
material for lithium/sodium ion batteries, J. Alloys Compd. 689 (2016)
[37] L. Qie, W.M. Chen, Z.H. Wang, Q.G. Shao, X. Li, L.X. Yuan, X.L. Hu, W.X. Zhang,
Y.H. Huang, Nitrogen-doped porous carbon nanofiber webs as anodes for
lithium ion batteries with a superhigh capacity and rate capability, Adv.
Mater. 24 (2012) 2047e2050.
[38] J. Yang, J. Xie, X. Zhou, Y. Zou, J. Tang, S. Wang, F. Chen, L. Wang, Function-
alized N-Doped porous carbon nanofiber webs for a lithiumesulfur battery
with high capacity and rate performance, J. Phys. Chem. C 118 (2014)
1800e1807.
[39] Y.-L. Shi, N. Wu, M.-F. Shen, Y.-L. Cui, L. Jiang, Y.-H. Qiang, Q.-C. Zhuang,
945e951.
[
13] D.P. Opra, S.V. Gnedenkov, A.A. Sokolov, A.B. Podgorbunsky, N.M. Laptash,
S.L. Sinebryukhov, Fluorine substituted molybdenum oxide as cathode ma-
terial for Li-ion battery, Mater. Lett. 160 (2015) 175e178.
[
[
14] T. Erkert, Nanomaterials for lithium-ion batteries, Russ. Chem. Rev. 84 (2015)
8
26e852.
15] D. Sua, Y. Zhaoa, Y. Dong, C. Dinga, M. Ninga, J. Zhanga, J. Lia, H. Jina, Enhanced
composites of V nanowires decorating on graphene layers as ideal cathode
materials for lithium-ion batteries, J. Alloys Compd. 695 (2016) 2974e2980.
16] Y.L. Shi, M.F. Shen, S.D. Xu, X.Y. Qiu, L. Jiang, Y.H. Qiang, Q.C. Zhuang, S.G. Sun,
Electrochemical impedance spectroscopic study of the electronic and ionic
2
O
5
Electrochemical behavior of iron(III) fluoride trihydrate as
lithium-ion batteries, Chemelectrochem 1 (2014) 645e654.
a cathode in
[
[40] G. Ali, S.H. Oh, S.Y. Kim, J.Y. Kim, B.W. Cho, K.Y. Chung, An open-framework
transport properties of NiF
399e3415.
2
/C composites, Int. J. Electrochem. Sci. 6 (2011)
iron fluoride and reduced graphene oxide nanocomposite as a high-capacity
cathode material for Na-ion batteries, J. Mater. Chem. A 3 (2015)
10258e10266.
3
[
[
17] Z.W. Fu, C.L. Li, W.Y. Liu, J. Ma, W. Ying, Q.Z. Qin, Z.W. Fu, C.L. Li, W.Y. Liu, J. Ma,
Electrochemical reaction of lithium with cobalt fluoride thin film electrode,
J. Electrochem. Soc. 152 (2005) E50eE55.
[41] B. Li, Z. Cheng, N. Zhang, K. Sun, Self-supported, binder-free 3D hierarchical
iron fluoride flower-like array as high power cathode material for lithium
batteries, Nano Energy 4 (2014) 7e13.
18] X. Hua, R. Robert, L.S. Du, K.M. Wiaderek, M. Leskes, K.W. Chapman,

326
X. Zhou et al. / Journal of Alloys and Compounds 723 (2017) 317e326
[
[
[
42] Y. Shen, M. Wan, Tubular polypyrrole synthesized by in situ doping poly-
merization in the presence of organic function acids as dopants, J. Polym. Sci.
Part A Polym. Chem. 37 (1999) 1443e1449.
43] G. Wu, C. Dai, D. Wang, D. Li, N. Li, Nitrogen-doped magnetic onion-like
carbon as support for Pt particles in a hybrid cathode catalyst for fuel cells,
J. Mater. Chem. 20 (2010) 3059e3068.
44] F. Su, C.K. Poh, J.S. Chen, G. Xu, D. Wang, Q. Li, J. Lin, X.W. Lou, Nitrogen-
containing microporous carbon nanospheres with improved capacitive
properties, Energy & Environ. Sci. 43 (2011) S229eS236.
[45] Y.S. Su, A. Manthiram, A new approach to improve cycle performance of
rechargeable lithium-sulfur batteries by inserting a free-standing MWCNT
interlayer, Chem. Commun. 48 (2012) 8817e8819.
[46] X. Zhou, J. Tang, J. Yang, J. Xie, B. Huang, Seaweed-like porous carbon from the
decomposition of polypyrrole nanowires for application in lithium ion bat-
teries, J. Mater. Chem. A 1 (2013) 5037e5044.
[47] M.V. Reddy, G.V.S. Rao, B.V.R. Chowdari, Nano-(VSbSn)O : a high capacity, high
rate anode material for Li-ion batteries, J. Mater. Chem. 21 (2011)
10003e10011.