batteries
Article
The Carbon Additive Effect on Electrochemical
Performance of LiFe0.5Mn0.5PO4/C Composites by
a Simple Solid-State Method for Lithium Ion Batteries
Chun-Chen Yang 1,2, *, Yen-Wei Hung 1 and Shingjiang Jessie Lue 3
1
2
3
*
Department of Chemical Engineering, Ming Chi University of Technology, New Taipei City 243, Taiwan;
mi-nered@hotmail.com
Battery Research Center of Green Energy, Ming Chi University of Technology, New Taipei City 243, Taiwan
Department of Chemical and Materials Engineering and Green Technology Research Center,
Chang Gung University, Kwei-shan, Taoyuan 333, Taiwan; jessie@mail.cgu.edu.tw
Correspondence: ccyang@mail.mcut.edu.tw; Tel.: +886-2-2908-9899
Academic Editor: Maciej Swierczynski
Received: 28 April 2016; Accepted: 7 June 2016; Published: 15 June 2016
Abstract: This work reported a solid-state method to prepare LiFe0.5 Mn0.5 PO4 /C (LFMP/C)
composite cathode materials by using LiH2 PO4 , MnO2 , Fe2 O3 , citric acid (C6 H8 O7 ), and sucrose
(C12 H22 O11 ). The citric acid was used as a complex agent and C12 H22 O11 was used as a carbon
source. Two novel hollow carbon sphere (HCS) and nanoporous graphene (NP-GNS) additives
were added into the LFMP/C composite to enhance electrochemical performance. The HCS and
NP-GNS were prepared via a simple hydrothermal process. The characteristic properties of the
composite cathode materials were examined by micro-Raman spectroscopy, X-ray diffraction (XRD),
scanning electron microscopy (SEM), elemental analysis (EA), and alternating current (AC) impedance
methods. The coin cell was used to investigate the electrochemical performance at various rates.
It was found that the specific discharge capacities of LFMP/C + 2% NP-GNS + 2% HCS composite
cathode materials were 161.18, 154.71, 148.82, and 120.00 mAh¨ g´1 at 0.1C, 0.2C, 1C, and 10C rates,
respectively. Moreover, they all showed the coulombic efficiency ca. 97%–98%. The advantage of
the one-pot solid-state method can be easily scaled up for mass-production, as compared with the
sol-gel method or hydrothermal method. Apparently, the LFMP/C composite with HCS and NP-GNS
conductors can be a good candidate for high-power Li-ion battery applications.
Keywords: LiFe0.5 Mn0.5 PO4 (LFMP); Fe2 O3 ; MnO2 ; specific capacity; cathode materials;
solid-state method
1. Introduction
Li-ion batteries are appropriate for use in cell phones, laptop computers, digital cameras,
renewable energy storage, and smart grid applications because of their relatively high energy density,
low cost, and high rate capabilities. Li-ion batteries used in hybrid-electric vehicle or electric vehicle
applications must be charged and discharged rapidly; therefore, the electrodes must perform at
a high rate and maintain high discharge capacity and reliable cycle-life stability. Compared with the
LiFePO4 cathode material [1], the LiMnPO4 cathode material exhibits a higher working potential at
4.1 V (versus Li/Li+ ) and is compatible with conventional liquid carbonate-based electrolytes. The
energy density of the LiMnPO4 (697 Wh¨ kg´1 ) [2] is higher than that of the LiFePO4 (586 Wh¨ kg´1 );
however, the electrical conductivity of the LiMnPO4 material (less than 10´10 S¨ cm´1 ) is much lower
than that of the LiFePO4 material (at 1.8 ˆ 10´8 S¨ cm´1 ). The electrochemical performance can be
improved by coating the samples’ surface with carbon, or by doping with Fe atoms or nano-sized
cathode materials. Olive phosphate materials combined with mixed transition-metal ions, such as
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LiFe1´x Mnx PO4 /C, have recently attracted considerable attention [3–14]. Zou et al. [15] prepared
LiFe0.2 Mn0.8 PO4 /C cathode materials by a solid-state method and sucrose used as the carbon source.
The as-prepared cathode materials achieved capacities of 150 mAh g´1 and 110 mAh g´1 at 1/20C
and 1C, respectively. Liu et al. [16] recently synthesized a LiMn1´x Fex PO4 /C (x = 0.1, 0.2, 0.3) cathode
material by a ball-milling process with MnPO4 ¨ H2 O precursors, LiOH, NH4 H2 PO4 , and 30 wt%
glucose. The as-prepared LiMn0.7 Fe0.3 PO4 /C cathode material had a capacity of 140 mAh¨ g´1 and
high rate performance. Mi et al. [17] prepared mesoporous LiFe0.6 Mn0.4 PO4 /C materials—comprising
of different amounts of muti-wall carbon nanotubes (MWCNTs) by using a two-step carbon coating
and spray drying process. The LiFe0.6 Mn0.4 PO4 /C materials with 2 wt% MWCNTs showed excellent
performance by delivering a discharge capacity of 163.3 mAh¨ g´1 at 0.1C and yielding an ultra-high
rate capacity of 64.23 mAh¨ g´1 at 50C. Hu et al. [18] prepared Fe-doped LiMn1´x Fex PO4 (x = 0.5)
nanomaterials by using a solvothermal method. LiMn0.5 Fe0.5 PO4 notably delivered a 100% capacity
retention with a discharge capacity of 147 mAh¨ g´1 at 1C rate. Zhong et al. [19] most recently
synthesized a carbon-coated LiFe0.5 Mn0.5 PO4 /C (LFMP/C) material by a rheological phase reaction
method with stearic acid as the carbon source (i.e., a solid-state method). The LFMP/C material
exhibited the highest electrochemical performance compared with the LiMn0.2 Fe0.8 PO4 /C and
LiMn0.8 Fe0.2 PO4 /C materials. The LFMP/C material delivered discharge capacities of 138, 99, 80, 72,
67, and 55 mAh¨ g´1 at 0.1C, 1C, 5C, 10C, 15C, and 20C rates, respectively. The LFMP/C material
achieved long and stable cycling performance with a capacity of 103 mAh¨ g´1 at 1C rate during
a 100-cycle test.
In this work, nanostructured carbon materials show a variety of fascinating and admirable
properties for Li-ion batteries, such as high surface area, low diffusion distance, and high electron
and ionic conductivity [20,21]. Therefore, nano-sized carbon materials show extremely promising to
improve the discharge capacity, the power capability, and the long cycling stability. We studied the
carbon conductor effect on performances of LFMP/C composite. Both hollow carbon sphere (HCS)
and nanoporous graphene (NP-GNS) were synthesized by a hydrothermal process. The characteristic
properties of the LFMP/C composite cathode materials were examined by micro-Raman spectroscopy,
X-ray diffraction (XRD), scanning electron microscopy (SEM), elemental analysis (EA), and alternating
current (AC) impedance method.
2. Experimental
2.1. Preparation of Hollow Carbon Sphere and Nanoporous Graphene Carbon Conductors
The HCS was also synthesized by a simple, one-pot, green, hydrothermal process. In a typical
procedure [22], 40 g glucose (Aldrich, St. Louis, MO, USA) and 4 g sodium dodecyl sulfate (SDS,
Aldrich) were first dissolved into in 460 mL distilled water. Then, the resultant solution was stirred
for three days at 50 ˝ C. The obtained mixture solution was transferred into a Teflon-lined autoclave
of 600 mL capacity with a stainless steel shell, followed by a hydrothermal treatment at 190 ˝ C for
10 h. After that, the autoclave was allowed to reach room temperature. The as-prepared carbon sphere
materials was filtered and washed with distilled water and ethanol several times, and then dried at
80 ˝ C for overnight. The HCS materials were obtained by sintering the as-prepared powders at 900 ˝ C
for 4 h in N2 atmosphere.
The nanoporous graphene oxide (GO) was fabricated by a solvothermal process [23]. Both the
as-prepared GO and ferrocene were used as the precursors. The concentration of GO solution was
around 4 mg¨ mL´1 . The ratio of GO to ferrocene was kept at 1:20. The solvothermal process was
carried out at 180 ˝ C for 10 h. The mixture solid product was separated through filtration, washed
with ethanol for several times to remove the residual ferrocene, and then dried. The ferrocene/GO
mixed composite was annealed at 800 ˝ C for 4 h in a 95% Ar/5% H2 atmosphere; at the same time,
the ferrocene was decomposed into iron/Austenite nanosphere. The iron/Austenite nanosphere was
further removed with 12 M HCl solution, then nano-porous graphene sheet was obtained through
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filtration and annealed in Ar at 800 ˝ C for 1 h. The NP-GNS was generated through a simple thermal
reduction process.
2.2. Preparation of LiFe0.5 Mn0.5 PO4 /C Materials
The LFMP/C cathode materials were prepared using a wet-balled method. Appropriate amounts
of LiH2 PO4 , Fe2 O3 , MnO2 , carbon black (BP2000), sucrose, and citric acid (Aldrich, 99%) as starting
materials were dissolved in acetone. The molar ratio of Li:Fe:Mn:P was 1:0.5:0.5:1. Approximately
10 wt% sucrose and 5 wt% citric acid were used as the carbon source and complex agent, respectively.
The sucrose and citric acid were mixed with the acetone and starting raw materials and then wet
ball-milled (Planetary Ball-Milling Apparatus, ball-to-powder weight ratio = 10:1, Primium Line,
Fritsch, Idar-Oberstein, Germany) at 400 rpm for approximately 10–15 h. The ball-milling process
was performed in an inert atmosphere that was controlled to avoid the Fe2+ and Mn2+ species from
oxidation. The ball-milled mixture was dried and subsequently heated at 650, 675, and 700 ˝ C for 15 h
in an argon atmosphere. The carbon content was approximately 5%–9%. The LFMP/C + 2% graphene
(denoted as NP-GNS) +2% carbon sphere (denoted as CS) composites were prepared by adding
nano-sized graphene (16 nm thickness, 50 m2 ¨ g´1 ) and carbon sphere (prepared by a hydrothermal
process). The obtained LFMP/C composite precursor was further calcined at 650, 675, and 700 ˝ C for
15 h in an argon atmosphere. The residual carbon content was approximately 5%–9%.
2.3. Material Characterization
The crystal structures of the as-prepared LFMP/C samples were examined using an XRD
spectrometer (D2 Phaser, Bruker, Karlsruhe, Germany). The surface morphology and the residual
carbon morphology were examined using SEM (Hitachi, Toyo, Chiba, Japan) and high-resolution
transmission electron microscopy (HR-TEM, JEOL 2010F, Toyo), respectively. The micro-Raman spectra
were recorded using a confocal micro-Renishaw fitted with a 632 nm He–Ne excitation laser (Renishaw
inVia, London, UK). The residual carbon content in the sample was examined using an elemental
analyzer (FLASH 2000, Thermo Fisher Scientific, Waltham, MA, USA).
2.4. Electrochemical Performance of LiFe0.5 Mn0.5 PO4 /C
The electrochemical performances of the LFMP/Li and LFMP/C + 2% NP-GNS + 2% HCS
half-cells were measured using a two-electrode system (i.e., CR-2032 coin cells assembled in an Ar-filled
glove box). The LFMP/C composite electrodes were prepared by mixing active LFMP/C materials
with Super P and poly(vinyl fluoride) (PVDF) binder at a weight ratio of 80:10:10 [24]. The resulting
sample was coated on an Al foil (Aldrich) and dried in a vacuum oven at 80 ˝ C for 12 h. Li foils
(Aldrich) were used as the counter and reference electrodes, and a microporous polyethylene (PE)
film (Celgard 2400, Charlotte, NC, USA) was used as the separator. The active material loading on Al
current collector is around 2–3 mg¨ cm´2 , and the thickness of the dried LFMP/C electrode is about
30 µm. The total thickness of the electrode is 50 µm; the Al current collector is 20 µm. The electrolyte
was 1 M of LiPF6 in a mixture of EC and DEC (1:1 in v/v, Merck, Kenilworth, NJ, USA). The LFMP/Li
half-cells were charged at a constant current (CC) and a potential range of 2.0–4.5 V (versus Li/Li+ ) at
0.1C-rate, using a battery tester (Arbin BT2000, College Station, TX, USA). However, the LFMP/Li
cells were charged using a CC–CV protocol, and discharged using a CC profile, at a potential range of
2.0–4.5 V (versus Li/Li+ ) at various C-rates (i.e., 0.2–10C-rates) by using a battery tester (Arbin BT2000).
The second CV charge step of 4.5 V was terminated when the charge current was less than 0.1C.
The AC spectra of LFMP/Li half-cells were measured at room temperature an Autolab PGSTAT-30
electrochemical system with GPES 4.8 Package Software (Eco Chemie, B.V., Utrecht, The Netherlands).
3. Results and Discussion
Figure 1a shows the typical SEM image of HCS via a hydrothermal carbonation of glucose.
The size of HCS is around 1–2 µm; they have hollow and semi-hollow structure. Figure 1b shows the
XRD pattern of the HCS sample. Two broad peaks are located at 24˝ and 43˝ that correspond to the
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(002) and (101) planes, indicating the graphitic state of HCS sample. Figure 1c shows the SEM image
(002) sample.
and (101)They
planes,
indicating
thecurved
graphitic
state of HCSwith
sample.
Figure 1c shows
the SEM
of NP-GNS
show
a highly
morphology
a nanoporous
structure.
image of NP-GNS sample. They show a highly curved morphology with a nanoporous structure.
(c)
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(002) and (101) planes, indicating the graphitic state of HCS sample. Figure 1c shows the SEM
image of NP-GNS sample. They show a highly curved morphology with a nanoporous structure.
(c)
Figure 1. (a) Scanning electron microscopy (SEM) image for hollow carbon sphere (HCS); (b) X-ray
Figure 1.
(a) Scanning
electron
(SEM)
image
for hollowgraphene
carbon(NP-GNS)
sphere (HCS);
diffraction
(XRD) pattern
formicroscopy
HCS; and (c) SEM
image
for nanoporous
at 3k×. (b) X-ray
diffraction (XRD) pattern for HCS; and (c) SEM image for nanoporous graphene (NP-GNS) at 3kˆ.
Figure 2 shows the XRD patterns of the LFMP/C and LFMP/C + 2% NP-GNS + 2% HCS samples
prepared using the wet ball-milling method at 650–700 °C. All of the Bragg diffraction peaks of the
Figure
2 shows the XRD patterns of the LFMP/C and LFMP/C + 2% NP-GNS + 2% HCS samples
compounds were well-indexed based on a single-phase olivine-type structure indexed with the
˝ C. All of the Bragg diffraction peaks of
prepared
using
wet
ball-milling
method
at peaks,
650–700
Figurethe
1. (a)
Scanning
(SEM)
imagesuch
for hollow
sphere
(b) X-raywere
orthorhombic
Pnma
spaceelectron
group.microscopy
No
impurity
as Li3carbon
PO4 and
Mn2(HCS);
P2O7 phases,
(XRD)
pattern
for HCS;
and on
(c) SEM
for nanoporous
graphene
(NP-GNS) atindexed
3k×.
the compounds
well-indexed
based
a single-phase
olivine-type
structure
with the
found.diffraction
By were
comparison,
the lattice
parameters
ofimage
the LFP/C,
LFMP/C,
and SP-LFMP/C
composite
samples
are
listed
in
Table
1.
The
carbon
peaks
cannot
be
observed
because
of
the
small
amount
of
orthorhombic Pnma space group. No impurity peaks, such as Li3 PO4 and Mn2 P2 O7 phases, were
Figure carbon
2 showscontent
the XRD
patterns
of thewith
LFMP/C
and LFMP/C
+ 2% NP-GNS
+ 2%
samples
2+ ions
residual
(C%
ca. 5%–9%)
amorphous
configuration.
Fe2+ and
MnHCS
were
found. the
By
comparison,
the
lattice
parameters
of650–700
the
LFP/C,
LFMP/C,
and
SP-LFMP/C
composite
prepared
using
the
wet
ball-milling
method
at
°C.
All
of
the
Bragg
diffraction
peaks
the
2+
located at the tetrahedral 4c sites in LiFe1−yMnyPO4 (y = 0.5). The Mn ion (with an ionic radius ofof0.97
samplescompounds
are listed in
Table
1.
The
carbon
peaks
cannot
be
observed
because
of
the
small
amount
of
were
well-indexed
based
on
a
single-phase
olivine-type
structure
indexed
with
the
2+
Å) has a larger ionic size than the Fe ion (with an ionic radius of 0.92 Å). It was
found that
the
2+
2+
the residual
carbon
content
(C%group.
ca.
5%–9%)
with
configuration.
Fe2P2OInand
Mn were
ions were
orthorhombic
Pnmamaterials
space
No as
impurity
peaks,
such
as
Li3PO4 and
Mn
7 phases,
2+
LFMP/C
composite
increased
the
Mnamorphous
-doping
concentration
increased.
addition,
the
2+ ion
found.
By
comparison,
the lattice
parameters
of the
LFP/C,
and
SP-LFMP/C
located unit
at
the
4cLFMP/C
sites
incomposite
LiFe
= 0.5).LFMP/C,
The Mnwith
ancomposite
ionic radius of
y PO
celltetrahedral
volume
of the
materials
increased
linearly
the (with
Mn2+-substitution.
1´y Mn
4 (y
samples
are
Table
1. The
peaks
cannot
observed
of Å).
the small
ofthat the
shown
in listed
Figure
thethan
peaks
ofcarbon
the
LFMP/C
sample
(i.e.,
301,
121,because
and
410)
shifted
to amount
the
lower
0.97 Å) As
has
a larger
ionicin1,
size
the
Fe2+
ion
(with
anbe
ionic
radius
of 0.92
It was
found
2+ and Mn2+ ions were
the
residual
carbon
content
(C%
ca.
5%–9%)
with
amorphous
configuration.
Fe
2+ -doping
angle
directionmaterials
compared increased
with those as
of the
the Mn
LFP/C
sample [20].
The primary
crystal sizes
the
LFMP/C
composite
concentration
increased.
In of
addition,
the
located at
thethe
tetrahedral
sitesNP-GNS
in LiFe1−y+Mn
4 (y = 0.5). The Mn2+ ion (with an ionic radius of 0.97
LFMP/C
and
LFMP/C 4c
+ 2%
2%yPO
HCS
composite materials estimated using Scherrer’s
2+
unit cellÅ)
volume
of
the
LFMP/C
composite
materials
increased
linearly
with
the
Mn
-substitution.
has awere
larger
Fe2+ ion
(with ancarbon
ionic is
radius
of 0.92
Å). It wasanalysis
found (EA)
that the
equation
ca.ionic
50–65size
nm.than
Thethe
content
of residual
studied
by elemental
in
2+-doping
As shown
in Figure
1, thematerials
peaks of
the LFMP/C
sample
(i.e.,
301, 121, increased.
and 410) In
shifted
to the
the lower
LFMP/C
composite
increased
as
the
Mn
concentration
addition,
the composites lists in Table 2. The residual carbon content of the LFMP/C materials with 5 wt%
unit cell20
volume
of the
LFMP/C
increased
linearly
with
the are
Mn2+
-substitution.
angle direction
compared
with
those
ofBP2000
the materials
LFP/C
sample
[20].
The
primary
crystal
sizes of the
sucrose,
wt% citric
acid,
and
5 composite
wt%
at different
sintering
temperatures
around
7.4%–
As
shown
in
Figure
1,
the
peaks
of
the
LFMP/C
sample
(i.e.,
301,
121,
and
410)
shifted
to
the
lower
LFMP/C
and
the
LFMP/C
+
2%
NP-GNS
+
2%
HCS
composite
materials
estimated
using
Scherrer’s
4.8%; in contrast, the residual carbon of the LFMP/C + 2% NP-GNS + 2% HCS composite materials
angleadding
direction
compared
with
those
of thecarbon
LFP/Cconductor
sample [20].
The primary
crystal the
sizes
of the
with
extra
2%
NP-GNS
+
2
wt%
HCS
is
around
9.31%.
Clearly,
residual
equation were ca. 50–65 nm. The content of residual carbon is studied by elemental analysis (EA)
LFMP/C
and the
LFMP/Cwhen
+ 2%the
NP-GNS
+ 2%
HCS composite
materials It
estimated
using
carbon
content
decreases
sintering
temperature
was increased.
was alsomaterials
foundScherrer’s
thatwith
the 5 wt%
in the composites
lists
in
Table
2.The
The
residual
carboncarbon
content
of theby
LFMP/C
equation
were
ca.
50–65
nm.
content
of
residual
is
studied
elemental
analysis
(EA) in
electrical conductivity of the LFMP/C + 2% NP-GNS + 2% HCS composite materials can be improved
sucrose, the
20 wt%
citric acid, and 5 wt% BP2000 at different sintering temperatures are around 7.4%–4.8%;
to ca.composites
10−3 S cm−1. lists in Table 2. The residual carbon content of the LFMP/C materials with 5 wt%
in contrast,
the 20
residual
carbon
of the
LFMP/C
+ different
2% NP-GNS
+ 2%
HCS composite
materials
sucrose,
wt% citric
acid, and
5 wt%
BP2000 at
sintering
temperatures
are around
7.4%– with
4.8%; 2%
in contrast,
the+residual
of(a)the
LFMP/C
+ 2%isNP-GNS
2% HCS
composite
materials carbon
adding extra
NP-GNS
2 wt% carbon
HCS carbon
conductor
around +9.31%.
Clearly,
the residual
LFMP/C-650
C
(b) LFMP/C-675 C
with addingwhen
extra 2%
+ 2 temperature
wt% HCS
carbon conductor is around 9.31%. Clearly, the residual
(c) LFMP/C-700 Cwas increased. It was also found that the electrical
content decreases
theNP-GNS
sintering
(d) LFMP/C+2%NP-GNS+2%HCS-650 C
6000
carbon content decreases when the sintering
temperature was increased. It was also found that the
conductivity of the LFMP/C + 2% NP-GNS + 2% HCS composite materials can be improved to
electrical
conductivity of the LFMP/C
+ 2% NP-GNS + 2% HCS composite materials can be improved
5000
´
3
´
1
ca. 10 to
S¨cm
ca. 10−3. S cm−1.
o
o
o
o
4000
Counts
o
(a) LFMP/C-650 C
o
(b) LFMP/C-675 C
o
(c) LFMP/C-700 C
o
(d) LFMP/C+2%NP-GNS+2%HCS-650 C
3000
6000
2000
(d)
(c)
(b)
5000
1000
(a)
4000
0
Counts
(d)
3000
10
20
30
40
50
60
(c)
70
2 Theta
2000
(b)
Figure 2. XRD 1000
patterns of all LiFe0.5Mn0.5PO4/C (LFMP/C) sample.
(a)
0
10
20
30
40
50
60
70
2 Theta
Figure 2. XRD patterns of all LiFe0.5Mn0.5PO4/C (LFMP/C) sample.
Figure 2. XRD patterns of all LiFe0.5 Mn0.5 PO4 /C (LFMP/C) sample.
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Table 1. The lattice parameters of all LFMP/C samples from XRD analysis.
Sample
LFMP/C-650
a (nm)
b (nm)
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2016, 2, 18
c (nm)
V (nm3 )
0.47216
1.03957
0.60575
0.29733
LFMP/C + 2% NP-GNS + 2% HCS-650
LFMP/C-675
0.47138
1.03826
0.60499
0.29609
0.47184
1.03926
0.60545
0.29690
LFMP/C-700
0.47177
1.03904
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0.60542
0.29686
Table 1. The lattice parameters of all LFMP/C samples from XRD analysis.
Sample
LFMP/C
+ 2% of
NP-GNS
+ 2% HCS-650
Table
2. The LFMP/C-650
residual carbon
contents
all LFMP/C
samplesLFMP/C-675
by elementalLFMP/C-700
analysis (EA).
a (nm)
0.47216
b (nm)
1.03957
Sample
c (nm)
0.60575
3)
LFMP/C-650
V (nm
0.29733
0.47138
1.03826
Sample#1/mg Sample#2/mg
0.60499
1.880.29609
2.33
0.47184
1.03926
C#1/wt%
0.60545
7.34
0.29690
0.47177
1.03904
C#2/wt%
0.60542
7.40
0.29686
Cavg /wt%
LFMP/C-675
1.97
2.4
5.19
5.31
LFMP/C-700
1.71of all LFMP/C 1.94
4.76 analysis4.85
Table 2. The residual carbon contents
samples by elemental
(EA).
LFMP/C + 2% NP-GNS + 2% HCS-650
2.11
1.88
9.31
9.31
Sample
Sample#1/mg Sample#2/mg
LFMP/C-650
1.88
2.33
LFMP/C-675
2.4
Figure 3 shows
the SEM images of1.97
the LFMP/C-650,
LFMP/C-700
1.71
1.94
LFMP/C + 2% NP-GNS + 2% HCS-650 composites prepared
LFMP/C + 2% NP-GNS + 2% HCS-650
2.11
1.88
7.37
5.25
4.81
9.31
C#1/wt% C#2/wt% Cavg/wt%
7.34
7.40
7.37
5.19
5.31 LFMP/C-700,
5.25
LFMP/C-670,
and
4.76
4.85
4.81
via the wet ball-milled method.
9.31
9.31
9.31
The secondary particle sizes of the LFMP/C composite ranged from 2 µm to 10 µm with a spherical
morphology.
The3surface
is SEM
highly
porous
a macro/nano
three-dimensional
hierarchical
Figure
shows the
images
of thewith
LFMP/C-650,
LFMP/C-670,
LFMP/C-700, and(3D)
LFMP/C
+
2% NP-GNS
+ 2%
HCS-650
ball-milled
method.
The secondary
structure.
Figure 3d
also
showscomposites
the SEMprepared
images via
of the
thewet
LFMP/C
+ 2%
NP-GNS
+ 2% HCS-650
particle
sizes
of the LFMP/C
composite
from 2 composite
μm to 10 μm
withtwo
a spherical
morphology.
composites.
The
LFMP/C
+ 2% NP-GNS
+ ranged
2% HCS-650
with
conductive
carbons added
The
surface
is
highly
porous
with
a
macro/nano
three-dimensional
(3D)
hierarchical
structure.
shows a good conductive network because of the uniform coverage of the carbon on the surface of
Figure 3d also shows the SEM images of the LFMP/C + 2% NP-GNS + 2% HCS-650 composites. The
LFMP/C material. In other words, the double carbons greatly improved the coating uniformity on the
LFMP/C + 2% NP-GNS + 2% HCS-650 composite with two conductive carbons added shows a good
LFMP/C
composite
material
(see later
transmission
microscopy
micro-Raman
conductive
network
because
of thefor
uniform
coverageelectron
of the carbon
on the (TEM)
surface and
of LFMP/C
results).material.
More importantly,
thethe
LFMP/C
composite
showed
slightly
smaller
particlesonsize
In other words,
double carbons
greatly
improved
the coating
uniformity
the around
material
later for
transmission
electron
microscopy
andthat the
50 nm; LFMP/C
it is due composite
to the high
content(see
of carbon
being
added (C%
= 9%).
It is well (TEM)
accepted
results).
More
the LFMP/C
composite
slightly
smaller
carbon micro-Raman
precursors can
inhibit
theimportantly,
LFMP crystalline
growth
[24]. showed
Therefore,
it was
wellparticles
accepted that
size around 50 nm; it is due to the high content of carbon being added
(C% = 9%). It is well accepted
+
the smaller particle size of LFPM/C will have much shorter Li ion diffusion length. It will greatly
that the carbon precursors can inhibit the LFMP crystalline growth [24]. Therefore, it was well
improve
the electrochemical performance, in particular at high rate. Figure 4 shows the HR-TEM
accepted that the smaller particle size of LFPM/C will have much shorter Li+ ion diffusion length. It
˝ C. It was
images will
of the
LFMP/C
and
+ 2% performance,
NP-GNS + 2%
HCS samples
at 650–700
greatly
improve
theLFMP/C
electrochemical
in particular
at highsintered
rate. Figure
4 shows the
found that
a uniform
coating
onLFMP/C
LFMP/C
temperatures
650, 675, and
HR-TEM
images carbon
of the LFMP/C
and
+ 2%samples
NP-GNS sintered
+ 2% HCSat
samples
sintered atof650–700
°C. It was
that aof
uniform
carbon coating
onand
LFMP/C
samples
sintered atwas
temperatures
of In
650,contrast,
700 ˝ C with
the found
thickness
approximately
10, 3,
2 nm,
respectively,
observed.
675,
and
700
°C
with
the
thickness
of
approximately
10,
3,
2
nm,
respectively,
was
observed.
In
Figure 4h presents the TEM image for the LFMP/C + 2% NP-GNS + 2% HCS composite sintered
at
contrast, Figure 4h presents the TEM image for the LFMP/C + 2% NP-GNS + 2% HCS composite
˝
650 C. It was shown the much thick carbon layer is around 4.7 nm.
sintered at 650 °C. It was shown the much thick carbon layer is around 4.7 nm.
Figure 3. SEM images of LFMP/C sample in 5 k×: (a) LFMP/C-650; (b) LFMP/C-675; (c) LFMP/C-700;
Figure 3.
SEM
images +of2%
LFMP/C
in 5 kˆ: (a) LFMP/C-650; (b) LFMP/C-675; (c) LFMP/C-700;
and
(d) LFMP/C
NP-GNS sample
+ 2% HCS-650.
and (d) LFMP/C + 2% NP-GNS + 2% HCS-650.
Batteries 2016, 2, 18
Batteries 2016, 2, 18
6 of 12
6 of 12
Figure 4. High-resolution transmission electron microscopy (HR-TEM) images of LFMP/C sample:
Figure
4. LFMP/C-650;
High-resolution
microscopy
images
LFMP/C
sample:
(a,b)
(c,d)transmission
LFMP/C-675; electron
(e,f) LFMP/C-700;
and (HR-TEM)
(g,h) LFMP/C
+ 2%ofNP-GNS
+ 2%
(a,b) HCS-650.
LFMP/C-650; (c,d) LFMP/C-675; (e,f) LFMP/C-700; and (g,h) LFMP/C + 2% NP-GNS +
2% HCS-650.
Figure 5 shows the micro-Raman spectra of the LFMP/C samples and the LFMP/C + 2%
NP-GNS + 2% HCS composite, indicating three major Raman vibration peaks at 951 cm−1, 1328 cm−1,
Figure 5 shows the micro-Raman spectra of the LFMP/C samples and the
and 1585 cm−1. Several Raman peaks at approximately between 947–950 cm−1 and 596–446 cm−1 were
LFMP/C + 2% NP-GNS + 2% HCS composite, indicating three major Raman
vibration peaks at
identified
as the´1vibration of the ´P–O
bond, and the peak located at 638 cm−1 was identified as the ´1
´
1
1
951 cm
, 1328
, and
1585 cm . Several Raman peaks at approximately between 947–950 cm
vibration
ofcm
the FeO
x groups. Two Raman peaks of the LFMP/C samples and the LFMP/C + 2%
´1 were identified as the vibration of the P–O bond, and the peak located at 638 cm´1
and 596–446
cm
NP-GNS + 2% HCS composite were located at approximately 1328 cm−1 and 1585 cm−1; these peaks
was identified
as the
of carbon
the FeO
Two
Raman
peaks
the LFMP/C
samples
were attributed
tovibration
the residual
source.
Raman
peaks
at 1326
cm−1of(D-band)
and 1588
cm−1 and
x groups.
the LFMP/C
+ 2%observed
NP-GNS
2%
HCS composite
were
located
at approximately
(G-band) was
in +
the
LFMP/C
+ 2% NP-GNS
+ 2%
HCS composite.
Broadening1328
the Dcm
(A´1g1 and
´
1
andpeaks
G (E2gwere
symmetry)
bands
a strong D-band
a localized
in-plane
sp2cm´1
1585 symmetry)
cm ; these
attributed
towith
the residual
carbon indicated
source. Raman
peaks
at 1326
3
´1 (G-band)
graphitic
domain
and a disordered
sp amorphous
carbon, respectively.
The+intensity
of
(D-band)
andcrystal
1588 cm
was observed
in the LFMP/C
+ 2% NP-GNS
2% HCSratio
composite.
the
D-band
to
the
G-band
(i.e.,
R
=
I
D/IG) was used to estimate the carbon quality of the LFMP/C
Broadening the D (A1g symmetry) and G (E2g symmetry) bands with a strong D-band indicated
samples. The R-value of the LFMP/C samples at various sintered temperatures of 650–700 °C (R =
a localized in-plane sp2 graphitic crystal domain and a disordered sp3 amorphous carbon, respectively.
1.11–1.12) was slightly lower than that of the LFMP/C + 2% NP-GNS + 2% HCS composite (R = 1.15).
The intensity ratio of the D-band to the G-band (i.e., R = ID /IG ) was used to estimate the carbon quality
The discharge capacities and rate capabilities of the LFMP/C samples are strongly related to the
of the
LFMP/C
R-value [22–24].
of the LFMP/C
samples
various
of
intensity
ratiosamples.
of the D- The
and G-bands
Table 3 shows
the Rat
values
of allsintered
LFMP/Ctemperatures
samples by
˝ C (R = 1.11–1.12) was slightly lower than that of the LFMP/C + 2% NP-GNS + 2% HCS
650–700
micro-Raman analysis in detail.
composite (R = 1.15). The discharge capacities and rate capabilities of the LFMP/C samples are
strongly related to the intensity ratio of the D- and G-bands [22–24]. Table 3 shows the R values of all
LFMP/C samples by micro-Raman analysis in detail.
Batteries 2016, 2, 18
7 of 12
Batteries 2016, 2, 18
7 of 12
(a)
(b)
(c)
(d)
30000
LF M P /C -650 o C
o
LF M P /C -675 C
o
LF M P /C -700 C
LF M P /C +2% N P -G N S +2% H C S -650 o C
25000
Batteries 2016, 2, 18
(b)
Intensity/ a.u.
20000
7 of 12
(d)
(a)
(b)
(c)
(d)
15000
30000
10000
(a)
LF M P /C -650 o C
o
LF M P /C -675 C
o
(c)
LF M P /C -700 C
LF M P /C +2% N P -G N S +2% H C S -650 o C
25000
5000
(b)
Intensity/ a.u.
20000
(d)
0
(a)
15000
(c)
500
10000
1000
1500
R am an shift/ cm
2000
-1
5000
Figure 5. Micro-Raman curve of all LFMP/C samples.
Figure 5. Micro-Raman curve of all LFMP/C samples.
0
Table 3. Micro-Raman spectroscopic analysis of LFMP/C samples.
Table 3. Micro-Raman spectroscopic
analysis1500
of LFMP/C
samples.
500
1000
2000
3−
IG
PO4
R = ID/IG (ID + IG)/PO43−
Samples
ID
-1
R am an shift/ cm
LFMP/C-650
19,627 17,656 3761
1.112
9.91
3´
SamplesLFMP/C-675
ID 24,021
R = ID /IG 5.78(ID + IG )/PO4 3´
PO4samples.
Gall LFMP/C
Figure 5. Micro-Raman
curve Iof
20,940
7773
1.147
LFMP/C-700
14,166 4286
LFMP/C-650
19,62715,962 17,656
3761 1.124 1.112 7.03
9.91
Table 3. Micro-Raman
spectroscopic
analysis of
LFMP/C
samples.
LFMP/C
+ 2% NP-GNS
+ 2% HCS-650
22,399 20,940
19,508
8270
LFMP/C-675
24,021
7773 1.148
1.147 5.07
5.78
LFMP/C-700 Samples
15,962 ID 14,166
7.03
IG
PO4286
43−
R = ID/IG 1.124
(ID + IG)/PO43−
Figure
6 shows the
initial
charge/discharge
profiles
LFMP/C
samples,
LFMP/C
+ 2% NP-GNS
+LFMP/C-650
2%
HCS-650
22,39919,627
19,508
8270
5.07
17,656of three
3761
1.112 1.148
9.91 i.e., LFMP-650,
LFMP/C-675, and LFMP/C-700,
+ 2%20,940
NP-GNS
+ 2% HCS
LFMP/C-675and the LFMP/C
24,021
7773
1.147 composite
5.78at 0.1C rate and
15,962 14,166
4286
1.124 plateaus7.03
25 °C, indicating that LFMP/C-700
the LFMP/Li half-cell displayed
two flat
discharge
at ca. 4.00–4.03 V
Figure
6 shows
the
initial
charge/discharge
profiles
three
LFMP/C
samples,
i.e.,2+ LFMP-650,
LFMP/C
2%
NP-GNS
HCS-650 22,399
19,508of
8270
5.07 Fe3+/Fe
and ca. 3.50–3.55
V+ (versus
Li/Li++2%
), respectively,
associated
with
two 1.148
Mn3+/Mn2+ and
redox
LFMP/C-675,
and
LFMP/C-700,
and
the
LFMP/C
+
2%
NP-GNS
+
2%
HCS
composite
at
0.1C rate and
pairs. The LFMP/C, based on a wet ball-milled method, half-cells based on the as-synthesized
Figure that
6 shows
the
initial charge/discharge
profiles
of three
LFMP/C −1samples,
i.e., at
LFMP-650,
25 ˝ C,materials
indicating
the
LFMP/Li
half-cell
displayed
two
flat
discharge
plateaus
ca. 4.00–4.03
V
yielded an
initial
discharge
capacity
of 148, 139,
and
144
mAh·g at
0.1C, respectively.
By
LFMP/C-675, and LFMP/C-700,+and the LFMP/C + 2% NP-GNS + 2% HCS composite
at 0.1C
rate and3+
3+
2+
2+
and ca.
3.50–3.55
V (versus
Li/Li
), LFMP/C
respectively,
associated
with
two
Mn /Mn
Fe /Fe
contrast,
as shown
in Figure
6, the
+ 2% NP-GNS
+ 2%
HCS
composite
showedand
an initial
25 °C, indicating that the LFMP/Li half-cell
displayed two flat discharge plateaus at ca. 4.00–4.03 V
−1 at 0.1C, indicating that it is superior in performance to
capacity
of around
161.2
mAh·g
redoxdischarge
pairs.
The
LFMP/C,
based
on
a
wet
ball-milled
method,
half-cells
based
on
the
as-synthesized
and ca. 3.50–3.55 V (versus Li/Li+), respectively, associated with two Mn3+/Mn2+ and Fe3+/Fe2+ redox
´1 Figure
the LFMP/C
composite
with
a suitable
amount of
of carbon
additives,
ca. mAh¨
4 wt% here.
reveals
materials
yielded
an initial
discharge
capacity
139,
and 144
0.1C,6respectively.
pairs.
The LFMP/C,
based
on a wet
ball-milled 148,
method,
half-cells
based ong the at
as-synthesized
the
typical
charge/discharge
curves
of
the
LFMP/C
+
2%
NP-GNS
+
2%
HCS
composite
at
various
−1
By contrast,
as shown
6, the LFMP/C
+ 2%
HCS composite
showedBy
an initial
materials
yieldedinanFigure
initial discharge
capacity+of2%
148,NP-GNS
139, and 144
mAh·g
at 0.1C, respectively.
discharge
i.e., atin0.2–10C.
´1 + 2% NP-GNS + 2% HCS composite showed an initial
contrast,rates,
as shown
Figure 6, the LFMP/C
discharge capacity of around 161.2 mAh¨ g at 0.1C, indicating that it is superior in performance to
−1 at 0.1C, indicating that it is superior in performance to
discharge
capacity of
around
161.2 mAh·g
the LFMP/C
composite
with
a suitable
amount
of carbon additives, ca. 4 wt% here. Figure 6 reveals
(a ) L F M P /C -6 5 0
the LFMP/C composite with a suitable) amount
of carbon additives, ca. 4 wt% here. Figure 6 reveals
L F LFMP/C
M P /C -6 7 5
the typical charge/discharge curves of (b
the
+ 2% NP-GNS + 2% HCS composite at various
(c ) the
LFM P
/C -7 0 0
the typical charge/discharge curves of
LFMP/C
+ 2% NP-GNS + 2% HCS composite at various
(d ) L F M P /C + 2 % N P -G N S + 2 % H C S -6 5 0
discharge
rates,
i.e.,
at
0.2–10C.
discharge rates, i.e., at 0.2–10C.
4500
(a )
(b )
(c )
(d )
Voltage (mV)
4000
L F M P /C -6 5 0
L F M P /C -6 7 5
L F M P /C -7 0 0
L F M P /C + 2 % N P -G N S + 2 % H C S -6 5 0
3 540500 0
4000
Voltage (mV)
3000
3500
(b )
2500
3000
(a )
(c )
2000
0
20
40
60
80
100
120
140
160
(d )
180
(b))
C a p a c ity (m A h /g
2500
(a )
(d )
Figure 6. The initial sample charge/discharge curves of LFMP/C
samples
at 0.1C/0.1C at 25 °C.
(c )
2000
0
20
40
60
80
100
120
140
160
180
C a p a c ity (m A h /g )
Figure 6. The initial sample charge/discharge curves of LFMP/C samples at 0.1C/0.1C at 25 °C.
Figure 6. The initial sample charge/discharge curves of LFMP/C samples at 0.1C/0.1C at 25 ˝ C.
Batteries 2016, 2, 18
8 of 12
The LFMP/C + 2% NP-GNS + 2% HCS composite exhibited discharge capacities of
154.7, 150.3, 148.8, 136.7, 130.3, and 120.0 mAh¨ g´1 at charge/discharge rates of 0.2C/0.2C,
0.2C/0.5C,
0.2C/1C, 0.2C/3C, 0.2C/5C, and 0.2C/10C, respectively. These results indicate
that
Batteries 2016, 2, 18
8 of 12
the LFMP/C + 2% NP-GNS + 2% HCS composite exhibited excellent high-rate capability and reliable
Batteries 2016, 2, 18
8 of 12
The LFMP/C
+ 2% NP-GNS
+ 2%[17],
HCSthe
composite
exhibited
discharge
of 154.7,
cycle-life stability.
In a previous
study
discharge
capacities
of acapacities
LiFe0.6 Mn
0.4 PO150.3,
4 /C sample
−1
148.8,
136.7,
130.3,
and
120.0
mAh·g
at
charge/discharge
rates
of
0.2C/0.2C,
0.2C/0.5C,
0.2C/1C,
LFMP/C
+ 2% NP-GNS
+ 2% HCSusing
composite
exhibited
discharge
capacities ofca.
154.7,
150.3,
containing 2The
wt%
MWCNTs
and prepared
a spray
drying
method—were
163.3
mAh¨ g´1
0.2C/3C,
0.2C/5C,
and
0.2C/10C,
respectively.
These
results
indicate
that
the
LFMP/C
+
2%
NP-GNS
−1
´
1
148.8,
136.7,
and 120.0 mAh·g
at charge/discharge
of 0.2C/0.2C,
0.2C/0.5C,
0.2C/1C,
and 148.7
mAh¨
g 130.3,
at 0.1C-rate
and 1C-rate,
respectively.rates
These
results are
comparable
with our
+ 2% HCS
composite
exhibited
excellent high-rate
capability
and reliable
cycle-life +stability.
In a
0.2C/3C,
0.2C/5C,
and
0.2C/10C,
respectively.
These
results
indicate
that
the
LFMP/C
2%
NP-GNS
´
1
´
1
results, previous
which are ca. 161
g and capacities
149 mAh¨ofg a LiFe
at 0.1C
and 1C rate, respectively. However, in
[17],mAh¨
the
discharge
0.6Mn0.4PO4/C sample containing 2 wt%
+ 2% HCSstudy
composite
exhibited
excellent high-rate
capability
and reliable cycle-life stability. In a
−1 using
−1 at phase
Zhong etMWCNTs
al. [19], the
discharge
capacities
of
a LFMP/C
material
prepared
by
a rheological
and
prepared
using
a
spray
drying
method—were
ca.
148.7
mAh·g
previous study [17], the discharge capacities of a LiFe0.6Mn0.4163.3
PO4/CmAh·g
sampleand
containing
2 wt%
´
1 and 100 mAh¨
´1 atare
0.1C-rate
and
1C-rate,
respectively.
These
results
comparable
with
our
results,
which
are
ca.
method—were
138
mAh¨
g
g
0.1
and
1C-rate,
respectively;
these
findings
MWCNTs and prepared using a spray drying method—were ca. 163.3 mAh·g−1 and 148.7 mAh·g−1161
at show
−1 and 149 mAh·g−1 at 0.1C and 1C rate, respectively. However, in Zhong et al. [19], the
mAh·g
that our0.1C-rate
sampleand
results
are respectively.
clearly superior
theirare
results.
1C-rate,
These to
results
comparable with our results, which are ca. 161
discharge
capacities
a LFMP/C
material
prepared
by using a rheological
−1 at 0.1C
mAh·g
and
149 mAh·g
andthe
1C
rate, capability
respectively.
However, phase
in Zhong
al. [19], 138
thehalf-cell
Figure
7−1−1presents
a of
comparison
of
rate
performance
ofmethod—were
theetLFMP/Li
−1 at 0.1 and 1C-rate, respectively; these findings show that our sample
mAh·g
and
100
mAh·g
discharge
capacities
of a LFMP/C
material
prepared
by using
a rheological
phase+method—were
138
based on
the
LFMP/C
samples
(without
carbon
additives)
and
the
LFMP/C
2%
NP-GNS
+ 2% HCS
results −1are
clearly
results.
mAh·g
and
100 superior
mAh·g−1 to
at their
0.1 and
1C-rate, respectively; these findings show that our sample
composite comprised
of extra
2 wt% ofNP-GNS
+ 2 wt%performance
HCS carbon
at 0.2–10C
Figure
7 presents
a comparison
the rate capability
of theconductors,
LFMP/Li half-cell
results
are clearly
superior
to their results.
rates. We
observed
that
the
LFMP/C
+
2%
NP-GNS
+
2%
HCS
composite
exhibited
higher
rate
based
on
the
LFMP/C
samples
(without
carbon
additives)
and
the
LFMP/C
+
2%
NP-GNS
+
2%
HCS
Figure 7 presents a comparison of the rate capability performance of the LFMP/Li half-cell
composite
comprised
of
extra
2
wt%
NP-GNS
+
2
wt%
HCS
carbon
conductors,
at
0.2–10C
rates.
We
performance
than
that ofsamples
the LFMP/C
sample.
As shown
Figure
8,NP-GNS
the discharge
based on
the LFMP/C
(without carbon
additives)
and the in
LFMP/C
+ 2%
+ 2% HCSspecific
observed
the LFMP/C
+ 2%
NP-GNS
2%
HCS
exhibited
higher
rate˝performance
capacities
of thethat
LFMP/C
2%
NP-GNS
+ +2%
composite
sintered
atat650
C decreased
composite
comprised
of+extra
2 wt%
NP-GNS
+ 2HCS
wt%composite
HCS carbon
conductors,
0.2–10C
rates. We from
than ´that
of the
LFMP/C
sample.
As shown
Figure 8, the discharge
specific
capacities
of the
observed
LFMP/C
2%the
NP-GNS
+ 2%inHCS
exhibited
higher
rate
154 mAh¨
g 1 tothat
120themAh¨
g´1+ as
discharge
rate composite
was increased
from
0.2C
toperformance
10C.
By contrast,
−1 to 120
LFMP/C
+
2%
NP-GNS
+
2%
HCS
composite
sintered
at
650
°C
decreased
from
154
mAh·g
than
that
of
the
LFMP/C
sample.
As
shown
in
Figure
8,
the
discharge
specific
capacities
of
the discharge-specific
capacities
theincreased
LFMP/Cfrom
samples
sintered
the same
temperaturethe
of 650 ˝ C
mAh·g−1 +as2%
theNP-GNS
discharge
rateof
was
0.2C
By at
contrast,
the154
discharge-specific
LFMP/C
+ 12%
HCS
composite´sintered
at to
65010C.
°C decreased
from
mAh·g−1 to 120
´
1
decreased
from
141themAh¨
g samples
to 113 mAh¨ gat the
. We observed
thatofthe
capability
performance
capacities
LFMP/C
650rate
°C
from
141
−1 asofthe
mAh·g
discharge
rate was sintered
increased fromsame
0.2C temperature
to 10C. By contrast,
thedecreased
discharge-specific
−1
−1
of the LFMP/C
+of113
2%
NP-GNS
+ 2%
HCS
composite
with
a two-type
of carbon
mAh·g to
observed
thatatthe
capability
performance
the conductors,
LFMP/C
2%namely,
capacities
the mAh·g
LFMP/C. We
samples
sintered
therate
same
temperature
of 650
°Cofdecreased
from+ 141
NP-GNS
+ 2%
HCS
composite
with3D
a that
two-type
of carbon
namely,
two-dimensional
−1 to
−1. We observed
two-dimensional
(2D)-graphene
and
carbon
sphere,
isconductors,
greatly
improved,
as
compared
mAh·g
113
mAh·g
the rate
capability
performance
of the
LFMP/C
+ 2% to the
(2D)-graphene
and
carbon
sphere,
greatly improved,
as comparednamely,
to the LFMP/C
material
NP-GNS
+ 2%
HCS3D
composite
withconductor.
a istwo-type
of carbon conductors,
two-dimensional
LFMP/C
material
without
any
carbon
without any carbon
conductor.
(2D)-graphene
and 3D
carbon sphere, is greatly improved, as compared to the LFMP/C material
without any carbon conductor.
160
116400
(d )
(a )
-1
Capacity/
mAh g mAh g-1
Capacity/
114200
(c )
112000
(c )
1 0800
(d )
(a))
(b
(b )
8600
o
6400
4200
2 00
0
(a ) L F M P /C -0 .2 C -1 0 C -6 5 0 C
(b ) L F M P /C -0 .2 C -1 0 C -6 7 5 o C
(c ) L F M P /C -0 .2 C -1 0 C -7 0 0 oo C
(a ) L F M P /C -0 .2 C -1 0 C -6 5 0 C
(d ) L F M P /C + 2 % N P -G N S + 2
o %HCS
o -6 7 5 C
(b ) LaFt M
P /C
-00.2CC-6-1500C
C
0 .2
C -1
(c ) L F M P /C -0 .2 C -1 0 C -7 0 0 o C
(d ) L F M P /C + 2 % N P -G N S + 2 % H C S
0a.5
tC
0 .2 C -1 01CC-6 5 0 o C 3 C
0 .2 C
5C
10C
1 -3 c y c le 1 -3 c y c le 1 -3 c y c le 1 -3 c y c le 1 -3 c y c le 1 -3 c y c le
0 .5 C
0 .2 C
1C
3C
5C
10C
-3 c yLFMP/C
c le 1 -3 c y c samples
1 -3capability
c y c le 1 -3 c y c le
1 -3 c y c le
le 1 -3 c y c le at 0.2–10C.
Figure 7. The rate
curves
of1all
Figure 7. The rate capability curves of all LFMP/C samples at 0.2–10C.
Figure 7. The rate capability curves of all LFMP/C samples at 0.2–10C.
(f) (e) (d) (c) (b)
(a)
4500
(f) (e) (d) (c) (b)
(a)
Voltage/Voltage/
mV
mV
4500
4000
4000
3500
3500
3000
3000
2500
2500
2000
0
(a) 0.2C/0.2C
(b) 0.2C/0.5C
(c) 0.2C/1C
(a)
(d) 0.2C/0.2C
0.2C/3C
(b)
(e) 0.2C/0.5C
0.2C/5C
(c)
0.2C/1C
(f) 0.2C/10C
(d) 0.2C/3C
(e) 0.2C/5C
(f) 0.2C/10C
20
40
60
80
2000
100
120
140
160
180
140
160
180
-1
0
20
40
Capacity/ m Ahg
60
80
100 120
Capacity/ +
m 2%
Ahg NP-GNS + 2% HCS composite sintered at
Figure 8. The charge/discharge curves of the LFMP/C
650 °C at 0.2–10C rates.
Figure 8. The charge/discharge curves of the LFMP/C + 2% NP-GNS + 2% HCS composite sintered at
Figure 8.
charge/discharge
curves of the LFMP/C + 2% NP-GNS + 2% HCS composite sintered at
650The
°C at
0.2–10C rates.
-1
650 ˝ C at 0.2–10C rates.
Batteries 2016, 2, 18
9 of 12
Batteries 2016, 2, 18
9 of 12
Figure 9A shows the cycle-life performance of the LFMP/C-650, LFMP/C-675, and LFMP/C-700
samples, and
the9A
LFMP/C
+ 2%
NP-GNS
+ 2% HCS
composite
at charge/discharge
of 0.1C/0.1C
Figure
shows the
cycle-life
performance
of the
LFMP/C-650,
LFMP/C-675, andrate
LFMP/C-700
samples,
and
the
LFMP/C
+
2%
NP-GNS
+
2%
HCS
composite
at
charge/discharge
rate
of
0.1C/0.1C
for 30 cycles for comparison. It was found that the discharge capacity of the LFMP/C-650 sample was
for 30 cycles
for comparison.
found thatduring
the discharge
the LFMP/C-650
sampledischarge
was
maintained
at 141–139
mAh¨ g´It1was
at 0.1C-rate
the 30capacity
cyclingoftest,
and the average
−1
maintained
at 141–139 mAh·g
at 0.1C-rate
during the 30 excellent
cycling test,
andstability
the average
capacity
was approximately
141 mAh¨
g´1 , demonstrating
cycle
withdischarge
no apparent
capacity was approximately 141 mAh·g−1, demonstrating excellent cycle stability with no apparent
capacity after the 30-cycle test. We observed that the average current efficiency of the LFMP/C-650
capacity after the 30-cycle test. We observed that the average current efficiency of the LFMP/C-650
sample at 0.1C was approximately 99.32% during the 30-cycle test. Moreover, the discharge capacity
sample at 0.1C was approximately 99.32% during the 30-cycle test.´Moreover,
the discharge capacity
1 at 0.1C-rate during the 30 cycles.
of theofLFMP/C-675
sample
145–137
mAh¨
g 0.1C-rate
−1 at
the LFMP/C-675
samplewas
wasmaintained
maintained atat145–137
mAh·g
during the 30 cycles. The
´1
The average
capacitywas
wasapproximately
approximately
137
mAh¨
a fading
rate
of ´0.57%/cycle
−1 g
average discharge
discharge capacity
137
mAh·g
withwith
a fading
rate of
−0.57%/cycle
at
at 0.1C
rate
during
the
cycling
test.
In
addition,
it
was
found
that
the
average
coulombic
efficiency
0.1C rate during the cycling test. In addition, it was found that the average coulombic efficiency
of
of thethe
LFMP/C-675
at0.1C
0.1Cwas
was
approximately
99.3%
during
the 30-cycle
test. Furthermore,
LFMP/C-675 sample
sample at
approximately
99.3%
during
the 30-cycle
test. Furthermore,
the
−1 at
dischargecapacity
capacity of
of the
the LFMP/C-700
LFMP/C-700 sample
at at
148–139
mAh·g
the discharge
samplewas
wasmaintained
maintained
148–139
mAh¨
g´10.1C-rate
at 0.1C-rate
´1 with
during
thecycles.
30 cycles.
average
dischargecapacity
capacitywas
was approximately
approximately 140
a fading
during
the 30
TheThe
average
discharge
140mAh·g
mAh¨−1gwith
a fading
rate
of
−0.63%/cycle
at
0.1C
rate
during
the
cycling
test.
In
addition,
it
was
revealed
that
the
average
rate of ´0.63%/cycle at 0.1C rate during the cycling test. In addition, it was revealed that the average
current efficiency of the LFMP/C-700 sample at 0.1C was approximately 99.3% during the 30-cycle
current efficiency of the LFMP/C-700 sample at 0.1C was approximately 99.3% during the 30-cycle test.
o
(a )
(b )
(c )
(d )
L F M P /C - 6 5 0 C
o
L F M P /C - 6 7 5 C
o
L F M P /C -7 0 0 C
o
L F M P /C + 2 % G N S + 2 % C S - 6 5 0 C
(d )
L F M P /C -6 5 0 oC
L F M P /C -6 7 5 oC
L F M P /C -7 0 0 oC
L F M P /C + 2 % N P -G N S + 2 % H C S -6 5 0 oC
150
(d )
(a )
(a )
125
(b )
(c )
(b )
100
75
Voltage/ mV
-1
(c )
Capacity / mAh g
180
170
160
150
140
130
120
110
100
90
80
70
60
50
40
30
20
10
0
(a )
(b )
(c )
(d )
Voltage/ mV
Capacity / mAh g
-1
test.
50
0
C a p a c it y / m A h g
2000
25
100
4000
0
-1
100
C a p a c it y / m A h g - 1
0
0
1
2
3
4
5
6
C y c le N u m b e r
(A)
7
8
9
10
0
10
20
30
40
50
60
70
80
90
100
C y c le N u m b e r
(B)
Figure 9. The cycle-life performance of all LFMP/C samples at: (A) 0.1C/0.1C rate; and (B) 1C/1C rate.
Figure 9. The cycle-life performance of all LFMP/C samples at: (A) 0.1C/0.1C rate; and (B) 1C/1C rate.
As expected, the discharge capacity of the LFMP/C + 2% NP-GNS + 2% HCS composite was
achieved
at 161–155
mAh·g−1 atcapacity
0.1C-rateofduring
the 30 cycles.
average +discharge
was was
As expected,
the discharge
the LFMP/C
+ 2%The
NP-GNS
2% HCScapacity
composite
−1
´
1
approximately
158
mAh·g
with
a
fading
rate
of
−0.23%/cycle
at
0.1C
rate
during
the
cycling
test.
In was
achieved at 161–155 mAh¨ g at 0.1C-rate during the 30 cycles. The average discharge capacity
addition, it was found that
the
average
current
efficiency
of
the
LFMP/C
+
2%
NP-GNS
+
2%
HCS
approximately 158 mAh¨ g´1 with a fading rate of ´0.23%/cycle at 0.1C rate during the cycling test.
composite at 0.1C was approximately 98.55% during the 30-cycle test. These results indicated that all
In addition, it was found that the average current efficiency of the LFMP/C + 2% NP-GNS + 2% HCS
LFMP/C sample and the LFMP/C + 2% NP-GNS + 2% HCS composite exhibit excellent and stable
composite
at 0.1C was approximately 98.55% during the 30-cycle test. These results indicated that all
electrochemical cycling properties. Figure 9B shows the cycle-life performance of the LFMP/C-650,
LFMP/C
sample and
LFMP/Csamples,
+ 2% NP-GNS
2% HCS
composite
excellent
andat
stable
LFMP/C-675,
and the
LFMP/C-700
and the +
LFMP/C
+ 2%
NP-GNS exhibit
+ 2% HCS
composite
electrochemical
cycling
properties.
shows
cycle-life performance
the LFMP/C-650,
charge/discharge
rate
of 1C/1C Figure
for 100 9B
cycles
forthe
comparison.
The discharge of
capacity
of the
−1 at 1C/1C
LFMP/C-675,
and
LFMP/C-700
samples,
and the
LFMP/C
+ 2%
2% HCS
composite
LFMP/C-650
sample
was maintained
at 122–114
mAh·g
rateNP-GNS
during the+100-cycle
test,
and
−1
the average discharge
wasfor
approximately
mAh·g , demonstrating
high cycle
stability
at charge/discharge
ratecapacity
of 1C/1C
100 cycles 118.2
for comparison.
The discharge
capacity
of the
with no apparent
capacity
decay after
the 100-cycle
It also
observed
that the
current
LFMP/C-650
sample was
maintained
at 122–114
mAh¨ test.
g´1 at
1C/1C
rate during
theaverage
100-cycle
test, and
efficiency
of the LFMP/C-650
sample
was approximately
over 99%.
the discharge
capacity
the average
discharge
capacity was
approximately
118.2 mAh¨
g´1 ,Moreover,
demonstrating
high cycle
stability
of the LFMP/C-675 sample was kept at 130–107 mAh·g−1 at a 1C discharge rate during the 100-cycle
with no apparent capacity decay after the 100-cycle test. It also observed that the average current
test. Additionally, the average discharge capacity of the LFMP/C-700 was around 115.7 mAh·g−1
efficiency of the LFMP/C-650 sample was approximately over 99%. Moreover, the discharge capacity
during the 100-cycle test and the average current efficiency
of this sample was approximately 97%–
of the98%.
LFMP/C-675
sample
was kept
at 130–107
g´1 +at2%
a 1C
discharge
the 100-cycle
Particularly,
the discharge
capacity
of themAh¨
LFMP/C
NP-GNS
+ 2%rate
HCSduring
composite
was
test. Additionally,
the
average
discharge
capacity
of
the
LFMP/C-700
was
around
115.7
mAh¨ g´1
−1
achieved at 141–130 mAh·g at a 1C discharge rate and the average discharge capacity was around
−1 during
during
themAh·g
100-cycle
test and
average
current
of this
approximately
138
the the
100-cycle
test.
It wasefficiency
found that
the sample
average was
current
efficiency of97%–98%.
this
Particularly,
capacity
the LFMP/C
2% NP-GNS
+ 2% HCS
was achieved
LFMP/Cthe
+ discharge
2% NP-GNS
+ 2% ofHCS
composite+ was
approximately
99%.composite
These results
clearly at
demonstrate
2% NP-GNS
+ 2% HCS
composite
exhibits
excellent
141–130
mAh¨ g´1that
at athe
1C LFMP/C
discharge+ rate
and the average
discharge
capacity
was
aroundand
138 stable
mAh¨ g´1
during the 100-cycle test. It was found that the average current efficiency of this LFMP/C + 2%
NP-GNS + 2% HCS composite was approximately 99%. These results clearly demonstrate that the
Batteries 2016, 2, 18
10 of 12
LFMP/C
+ 2% NP-GNS + 2% HCS composite exhibits excellent and stable electrochemical
cycling
Batteries 2016, 2, 18
10 of 12
properties. This may be due to the composite with good electrical conducting channels being built on
electrochemical
cycling
properties.
This may
to sphere
the composite
with
good
electrical (the
the electrode
material
by using
2D graphene
and be
3D due
carbon
additives.
The
polarization
conducting
channels being
on the +electrode
material
byHCS
usingcomposite
2D graphene
3D carbon
charge
transfer resistance)
of thebuilt
LFMP/C
2% NP-GNS
+ 2%
wasand
markedly
reduced.
sphere
additives.
The polarization
(the was
charge
transfer to
resistance)
of thethe
LFMP/C
+ 2% NP-GNS
+ 2%
The
AC
impedance
spectroscopy
applied
investigate
interface
properties
of the
HCS composite was markedly reduced.
LFMP/C samples. The AC spectrum of the LFMP/C samples and the LFMP/C + 2% NP-GNS +
The AC impedance spectroscopy was applied to investigate the interface properties of the
2% HCS composite is illustrated in Figure 10. The equivalent circuit for LFMP/C is displayed in inset
LFMP/C samples. The AC spectrum of the LFMP/C samples and the LFMP/C + 2% NP-GNS + 2%
of Figure
10. Each AC plot comprised of one semicircle at a higher frequency followed by a linear
HCS composite is illustrated in Figure 10. The equivalent circuit for LFMP/C is displayed in inset of
portion
at
lower
The lower
region
the straight
linefollowed
was considered
Figurea 10.
Eachfrequency.
AC plot comprised
of frequency
one semicircle
at aofhigher
frequency
by a linearas the
Warburg
impedance,
which
was
used
for
long-range
lithium-ion
diffusion
in
the
bulk
portion at a lower frequency. The lower frequency region of the straight line was considered phase.
as the Rb
indicates
the bulk
resistance
at the
Rct referslithium-ion
to the charge
transfer
at theRactive
Warburg
impedance,
which
waselectrolyte,
used for long-range
diffusion
in resistance
the bulk phase.
b
indicates
the bulk
resistance
at the electrolyte,
Rct refers to capacitance
the charge transfer
resistance
at the
active
material
interface,
and
CPE represents
the double-layer
and surface
film
capacitances.
interface,
CPEof
represents
the Rdouble-layer
capacitance and surface film capacitances.
Tablematerial
4 summarizes
theand
values
the Rb and
ct for all LFMP/C samples. The Rb and Rct values of the
Table
4
summarizes
the
values
of
the
R
b and Rct for all LFMP/C samples. The Rb and Rct values of the
LFMP/C-650 sample were approximately 5.46 Ω and 156 Ω, respectively. The Rb and Rct values of the
LFMP/C-650 sample were approximately 5.46 Ω and 156 Ω, respectively. The Rb and Rct values of
LFMP/C-675 sample were approximately 4.36 Ω and 234 Ω, respectively. Moreover, the Rb and Rct
the LFMP/C-675 sample were approximately 4.36 Ω and 234 Ω, respectively. Moreover, the Rb and
values of the LFMP/C-700 sample were approximately 6.06 Ω and 193.6 Ω, respectively. By contrast,
Rct values of the LFMP/C-700 sample were approximately 6.06 Ω and 193.6 Ω, respectively. By
the LFMP/C
+ 2%
NP-GNS
2% HCS+ composite
show 5.3
Ω and
Ω forΩthe
Rct Rvalues.
b and
contrast, the
LFMP/C
+ 2%+NP-GNS
2% HCS composite
show
5.3 Ω63.95
and 63.95
forRthe
Rb and
ct
In fact,
the
LFMP/C
+
2%
NP-GNS
+
2%
HCS
composite
showed
the
lowest
charge
transfer
resistance,
values. In fact, the LFMP/C + 2% NP-GNS + 2% HCS composite showed the lowest charge transfer
as compared
other LFMP/C
samples.
prepared
the LFMP/C
composite
material
with 2D
resistance,with
as compared
with other
LFMP/CWe
samples.
We prepared
the LFMP/C
composite
material
and 3D
they can
buildcan
upbuild
the electrical
conducting
channels
within
LFMP
withcarbon
2D andadditives,
3D carbon and
additives,
and they
up the electrical
conducting
channels
within
LFMP
electrode,
which
facilitate
the
electron
transport
and
also
greatly
reduce
the
electrode
electrode, which facilitate the electron transport and also greatly reduce the electrode polarization.
polarization.
conclusion,
theNP-GNS
LFMP/C ++ 2%
+ 2% HCS
composite
can be
a
In conclusion,
theInLFMP/C
+ 2%
2%NP-GNS
HCS composite
can
be prepared
byprepared
a simplebyone-pot
simple
one-pot
solid-state
ball-milled
process.
The
LFMP/C
+
2%
NP-GNS
+
2%
HCS
composite
solid-state ball-milled process. The LFMP/C + 2% NP-GNS + 2% HCS composite showed excellent
showed excellent electrochemical performance; this is due to the reduction of the charge transfer
electrochemical performance; this is due to the reduction of the charge transfer resistance.
resistance.
o
(a) LFMP/C-650 C
o
(b) LFMP/C-675 C
o
(c) LFMP/C-700 C
(d) LFMP/C+2%NP-GNS+2%HCS-650 o C
160
(a)
140
(b)
(d)
120
100
-Z''/ ohm
(c)
80
60
40
20
Rb
0
CPE
R ct Z w
0
100
200
300
400
Z'/ ohm
Figure 10. The Nyquist plots of all LFMP/C samples at open circuit potential (OCP); the inset for
Figure
10. The Nyquist plots of all LFMP/C samples at open circuit potential (OCP); the inset for
equivalent circuit.
equivalent circuit.
Table 4. The parameters of alternating current (AC) impedance analysis.
Table 4. The parameters of alternating current (AC) impedance analysis.
Sample
LFMP/C-650
Sample
LFMP/C-675
LFMP/C-700
LFMP/C-650
LFMP/C LFMP/C-675
+ 2% NP-GNS + 2% HCS-650
LFMP/C-700
LFMP/C + 2% NP-GNS + 2% HCS-650
Rb (Ω)
5.46
R
b (Ω)
4.36
6.06
5.46
5.30
4.36
6.06
5.30
Rct (Ω)
156.24
Rct (Ω)
234.36
193.66
156.24
63.95
234.36
193.66
63.95
Batteries 2016, 2, 18
11 of 12
4. Conclusions
In this work, a LFMP/C material was first prepared using a wet ball-milling solid-state
method. A LFMP/C + 2% NP-GNS + 2% HCS composite with mixed carbon conductors was
then prepared and examined. The properties of the resulting samples were examined using XRD,
SEM, TEM, micro-Raman spectroscopy, and the AC impedance spectroscopy, and galvanostatic
charge-discharge methods. The LFMP/C + 2% NP-GNS + 2% HCS composite exhibited discharge
capacities of 154.7, 150.3, 148.8, 136.7, 130.3, and 120.0 mAh¨ g´1 at charge/discharge rates of 0.2C/0.2C,
0.2C/0.5C, 0.2C/1C, 0.2C/3C, 0.2C/5C, and 0.2C/10C, respectively. These results indicate that the
LFMP/C + 2% NP-GNS + 2% HCS composite exhibited excellent high-rate capability and reliable
cycle-life stability. This work demonstrated that the excellent performance of the LFMP/C can
be synthesized by a one-pot solid-state method when the suitable amount of carbon conducting
additives (optimum around 4 wt%) was added. Those carbon conductors play a key role in improving
electrochemical performance. This study shows that the LFMP/C composite material is a good
candidate for application in high-power Li-ion batteries.
Acknowledgments: Financial support from the Ministry of Science and Technology, Taiwan (Project No: MOST
103-2632-E-131 -001) is gratefully acknowledged.
Author Contributions: Chun-Chen Yang and Shinhjiang Jessie Lue wrote the paper, Yen-Wei Hung designed
the experiment and did the electrochemical performance analysis. All authors examined and approved the
final manuscript.
Conflicts of Interest: The authors declare no conflict of interest.
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1.
2.
3.
4.
5.
6.
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8.
9.
10.
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12.
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