materials
Article
Deep Eutectic Solvent Synthesis of LiMnPO4/C
Nanorods as a Cathode Material for Lithium
Ion Batteries
Zhi Wu 1 , Rong-Rong Huang 1 , Hang Yu 1 , Yong-Chun Xie 1 , Xiao-Yan Lv 3 , Jing Su 1,2 ,
Yun-Fei Long 1,2 and Yan-Xuan Wen 1,2, *
1
2
3
*
School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, China;
[email protected] (Z.W.); [email protected] (R.-R.H.); [email protected] (H.Y.);
[email protected] (Y.-C.X.); [email protected] (J.S.); [email protected] (Y.-F.L.)
Guangxi Colleges and Universities Key Laboratory of Novel Energy Materials and Related Technology,
Nanning 530004, China
The New Rural Development Research Institute, Guangxi University, Nanning 530004, China;
[email protected]
Correspondence: [email protected]; Tel./Fax: +86-771-3233718
Academic Editors: Bingqing Wei and Jian-Gan Wang
Received: 22 November 2016; Accepted: 3 February 2017; Published: 6 February 2017
Abstract: Olivine-type LiMnPO4 /C nanorods were successfully synthesized in a chloride/ethylene
glycol-based deep eutectic solvent (DES) at 130 ◦ C for 4 h under atmospheric pressure.
As-synthesized samples were characterized by X-ray diffraction (XRD), scanning electron microscopy
(SEM), transmission electron microscopy (TEM), Raman spectroscopy, Fourier transform infrared
spectroscopy (FTIR) and electrochemical tests. The prepared LiMnPO4 /C nanorods were coated
with a thin carbon layer (approximately 3 nm thick) on the surface and had a length of 100–150 nm
and a diameter of 40–55 nm. The prepared rod-like LiMnPO4 /C delivered a discharge capacity of
128 mAh·g−1 with a capacity retention ratio of approximately 93% after 100 cycles at 1 C. Even at
5 C, it still had a discharge capacity of 106 mAh·g−1 , thus exhibiting good rate performance and
cycle stability. These results demonstrate that the chloride/ethylene glycol-based deep eutectic
solvents (DES) can act as a new crystal-face inhibitor to adjust the oriented growth and morphology
of LiMnPO4 . Furthermore, deep eutectic solvents provide a new approach in which to control the size
and morphology of the particles, which has a wide application in the synthesis of electrode materials
with special morphology.
Keywords: lithium ion batteries; cathode materials; LiMnPO4 ; deep eutectic solvents
1. Introduction
Lithium ion batteries (LIBs) have been considered as the most promising power sources for
modern electronic devices due to their long cycle life, high energy density and good safety [1–3].
The cathode material is an important part of LIBs and plays a critical role in determining their
performance [4,5]. Thus, many types of cathode materials have been investigated to improve the
performance of LIBs. Since the pioneering work of Goodenough and co-workers [6], LiMPO4 (M = Fe,
Ni, Mn, Co) with an olivine-type structure has received intensive attention over the past decades [7–9].
Among the known olivine-type materials, LiMnPO4 has been investigated as a promising cathode
material for the next generation of LIBs due to its optimal redox potential (4.1 V vs. Li+ /Li), giving
an energy density that is approximately 20% larger than that of LiFePO4 and is compatible with
most liquid electrolytes presently used in LIBs [4,6,10–12]. However, LiMnPO4 suffers from slow
lithium ion diffusion (10−15 cm·s−1 ) and poor electronic conductivity (10−10 cm·s−1 ), due to the
Materials 2017, 10, 134; doi:10.3390/ma10020134
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Materials 2017, 10, 134
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holes localized on the Mn3+ sites and the interfacial strain that exists between the LiMnPO4 and
MnPO4 phases during charge/discharge processes [13–15]. In order to overcome these drawbacks,
3+ sites and the interfacial strain that exists between the
heavy
polarized holes
on the(1)
Mn
three approaches
havelocalized
been adopted:
reducing
the particle size and controlling the morphology
LiMnPO
and
MnPO
phases during
processes4 [19–22];
[13–15]. and
In order
to overcome
these
[16–18]; 4(2)
coating
a 4conductive
layercharge/discharge
on the surface of LiMnPO
(3) doping
with cations
drawbacks,
three approaches have been adopted: (1) reducing the particle size and controlling the
[23–26].
morphology
[16–18];
(2) coating
a conductive
layernanoparticles
on the surfaceand
of LiMnPO
andbeneficial
(3) doping
4 [19–22];are
According
to previous
literature
[10,27–29],
nanostructures
in
with
cations
[23–26].
increasing capacity, as they can effectively shorten the diffusion length of the Li-ion and electron.
According
to approaches
previous literature
[10,27–29],
and LiMnPO
nanostructures
are beneficial in
Therefore,
many
have been
carriednanoparticles
out to prepare
4/C with nano-size and
increasing
capacity,
as
they
can
effectively
shorten
the
diffusion
length
of
the
Li-ion
and
electron.
Therefore,
nanostructure [30–32]. In these adopted approaches, hydrothermal/solvothermal methods
have
many
approaches
haveattention
been carried
out totoprepare
LiMnPO4 /Csynthesis
with nano-size
and nanostructure
[30–32].
attracted
particular
owing
the controllable
of LiMnPO
4/C with a special
In
these adopted
approaches, hydrothermal/solvothermal
methods havesuch
attracted
particular[16,21,36];
attention
morphology
[2,20,21,33–35].
Therefore, various special morphologies
as nanoplates
owing
to
the
controllable
synthesis
of
LiMnPO
/C
with
a
special
morphology
[2,20,21,33–35].
Therefore,
4
nanorods [10,33,35]; nanosheets [18,37]; flower-like
nanostructures [34,38]; hemoglobin [39]; and
various
special
morphologies
such
as
nanoplates
[16,21,36];
nanorods [10,33,35];
[18,37];
wedges [40] were prepared using the hydrothermal/solvothermal
methodnanosheets
to enhance
the
flower-like
nanostructures
[34,38];
hemoglobin
[39];
and
wedges
[40]
were
prepared
using
the
electrochemical performance of LiMnPO4. LiMnPO4 nanoplates were synthesized using an ethylene
hydrothermal/solvothermal
methodapproach
to enhanceand
thedelivered
electrochemical
performance
of LiMnPO
–1 at 0.5
4 . LiMnPO
4
glycol (EG)-assisted solvothermal
a discharge
capacity
of 92 mAh·g
nanoplates
were
synthesized
using
an
ethylene
glycol
(EG)-assisted
solvothermal
approach
and
delivered
C [32]. LiMnPO4 nanorods were prepared via a facile solvothermal approach, and exhibited a high
acapacity
discharge
capacity
of–192
g−1 at
C [32]. LiMnPO
were prepared viamethods
a facile
4 nanorods
of 110
mAh·g
at mAh
10 C ·[35].
In 0.5
summary,
the use of
hydrothermal/solvothermal
−
1
solvothermal
approach,
and exhibited
a high capacityand
of 110size
mAh·of
g LiMnPO
at 10 C [35].
In summary,
the use
made it easy
to control
the morphology
4 particles.
However,
of
hydrothermal/solvothermal
methods
made
it
easy
to
control
the
morphology
and
size
of
LiMnPO
4
hydrothermal/solvothermal methods require the cumbersome use of autoclaves and high
particles.
However,
hydrothermal/solvothermal
methods
require
the
cumbersome
use
of
autoclaves
temperature, which limits their practical application. Recently, the ionothermal method based on
and
temperature,
limits their
Recently,
thehydrothermal/solvothermal
ionothermal method based
ionichigh
liquids
(ILs) haswhich
been adopted
to practical
overcomeapplication.
the drawbacks
of the
on
ionic
liquids
(ILs)
has
been
adopted
to
overcome
the
drawbacks
of
the
hydrothermal/solvothermal
methods, as ILs have low vapor pressure and excellent solvating properties and thus the use of
methods,
as ILs
low vapor
pressureFor
andinstance,
excellent Barpanda
solvating properties
thus the usenanostructure
of autoclaves
autoclaves
canhave
be avoided
[41–43].
et al. [44]and
synthesized
can
be
avoided
[41–43].
For
instance,
Barpanda
et
al.
[44]
synthesized
nanostructure
LiMnPO
4 via
LiMnPO4 via an ionothermal route, and the prepared sample exhibited a reversible capacity
of an
95
−1 at 1/20 C with
ionothermal
route,
and
the
prepared
sample
exhibited
a
reversible
capacity
of
95
mAh
·
g
–1
mAh·g at 1/20 C with good cycling stability. However, the application of the ionothermal method
good
stability.
However,
application of
the as
ionothermal
method failed to
become
popular
failedcycling
to become
popular
due the
to limitations
such
poor biodegradability,
toxicity
and
high due
cost
to
limitations
such
as
poor
biodegradability,
toxicity
and
high
cost
[45,46].
[45,46].
Since
Since the
the term
term deep
deep eutectic
eutectic solvents
solvents (DESs)
(DESs) was
was first
first coined
coined by
by Abbott
Abbott et
et al.
al. [47,48],
[47,48], they
they have
have
been
investigated
as
a
sustainable
media
in
which
to
overcome
the
limitations
of
ILs
[46].
Compared
been investigated as a sustainable media in which to overcome the limitations of ILs [46]. Compared
with
withtraditional
traditional ILs,
ILs, DESs
DESs can
can be
be readily
readily formed
formed by
by mixing
mixing quaternary
quaternary ammonium
ammonium or
or phosphonium
phosphonium
salt
with aa metal
metalsalt
saltororhydrogen
hydrogen
bond
donor
(HBD),
as amide,
acid
or alcohol
under
salt with
bond
donor
(HBD),
suchsuch
as amide,
acid or
alcohol
under simple
simple
operational
conditions
[49,50].
Taking
choline
chloride/alcohols-based
DESs
as
an
example,
operational conditions [49,50]. Taking choline chloride/alcohols-based DESs as an example, the
the
formation
mechanism
of choline
chloride
and
hydrogen
bond
DESs
presentedininFigure
Figure1 1[51].
[51].
formation
mechanism
of choline
chloride
and
hydrogen
bond
inin
DESs
is is
presented
Figure 1. Schematic illustration of the formation mechanism of choline chloride and hydrogen bond
Figure 1. Schematic illustration of the formation mechanism of choline chloride and hydrogen bond in
in deep eutectic solvents (DESs).
deep eutectic solvents (DESs).
DESs share most of the same characteristics as ILs, such as negligible vapor pressure, tenability
share most of the
same characteristics
as ILs,
such as negligible
tenability
and DESs
wide electrochemical
potential
windows [47,48].
However,
DESs havevapor
somepressure,
unique advantages
and
wide
electrochemical
potential
windows
[47,48].
However,
DESs
have
some
unique
advantages
such as nontoxicity and low cost, and they are biodegradable; and easily are prepared [46]. The
such
as nontoxicity
and DESs
low cost,
and
they
are biodegradable;
easily are than
prepared
[46].
freezing
point of these
even
can
be reduced
to a lowerand
temperature
that of
ILsThe
duefreezing
to their
point
of
these
DESs
even
can
be
reduced
to
a
lower
temperature
than
that
of
ILs
due
to
low
low lattice energy [52]. Thus, DESs have widespread applications in the fields of chemistry, their
including
lattice
energy
Thus,
DESs have
widespread
applications
in adsorption
the fields of[56],
chemistry,
synthesis
[46],[52].
metal
deposition
[53,54],
nanomaterials
[55], gas
analysisincluding
[57] and
Materials 2017, 10, 134
3 of 16
synthesis [46], metal deposition [53,54], nanomaterials [55], gas adsorption [56], analysis [57] and
electrochemistry [58]. For example, sulfonamide-based deep eutectic electrolytes display significantly
higher transport numbers than organic carbonates electrolyte solutions such as LiPF6 [59]. Similarly,
Lesch et al. [60] reported that the LiTFSI/urea based deep eutectic electrolyte with high urea
concentrations can significantly increase ionic diffusivities and lithium transference numbers. Zinc has
been successfully electrodeposited in choline chloride/urea and choline chloride/ethylene glycol deep
eutectic solvents, respectively [61], with the morphology of zinc coatings obviously changed in those
different choline chloride-based DESs due to their different physical properties. These results indicate
that DESs are a potential media that can be used in the synthesis of numerous materials. However,
the synthesis of nanostructure LiMnPO4 in DESs has not yet been reported.
In this paper, we prepared LiMnPO4 /C nanorods in a chloride/ethylene glycol-based deep
eutectic solvent (DES) under atmospheric pressure and investigated the electrochemical properties of
the prepared sample. The results of this work show that DES can be used as a new structure-directing
agent to prepare LiMnPO4 with a special rod-like morphology.
2. Experimental Methodology
2.1. Sample Synthesis
LiMnPO4 /C nanorods were prepared using H3 PO4 (85% P2 O5 ), MnSO4 ·H2 O and LiOH·H2 O
as the raw materials and chloride/ethylene glycol-based DES as the solvent and reaction medium.
All chemicals were analytical grade reagents from Sinopharm Chemical Reagent Co. Ltd. (Beijing, China)
without further purification. During the synthesis processes, the mole ratio of Li:Mn:P was 3:1:1.
The DES was prepared by mixing ethylene glycol and choline chloride with a mole ratio of 2:1 at
80 ◦ C for 30 min to form a colorless liquid [46]. First, a MnSO4 saturated solution and H3 PO4 were
dissolved in DES. Next, a LiOH saturated solution was added dropwise into the DES (containing
MnSO4 and H3 PO4 ) to form a yellowish-brown precursor suspension, where the concentration of
Mn2+ was controlled at 0.2 mol·L−1 . The yellowish-brown suspension was then heated at 130 ◦ C for
4 h with the heating rate of 3 ◦ C·min−1 under atmospheric pressure to obtain a whitish suspension.
After centrifuging the whitish suspension with ethanol and deionized water several times, whitish
LiMnPO4 (LMP) powders were obtained. In order to improve the conductivity and crystallization of
the sample, the collected LMP powders were dispersed in a sucrose solution with a weight ratio of
7.5:2.5 for LMP: sucrose, and the obtained LMP/sucrose suspension was dried with a spray dryer to
obtain a dried LMP/sucrose mixture powder. The conditions of the spray drying are as follows: air inlet
temperature was 200 ◦ C, air inlet volume was 3 m3 ·min−1 , feed rate was 1.2 L·h−1 , the diameter of
nozzle was 0.5 mm, and the solid content is 50 g·L−1 . The LMP/sucrose mixture powder was heated at
300 ◦ C for 1 h and then sintered at 600 ◦ C for 5 h with the heating rate of 5 ◦ C·min−1 under N2 flow in
a tube furnace. Finally, LiMnPO4 /C (LMP/C) was obtained upon cooling to room temperature.
In order to study the effects of chloride and ethylene glycol, a second LiMnPO4 /C sample was
synthesized using only ethylene glycol as the solvent with other synthetic conditions kept unchanged.
2.2. Characterization
X-ray diffraction (XRD) measurements were tested on a X’Pert PRO equipment and used a Cu
Kα radiation source (λ = 0.154060 nm) with a scan range of 10◦ to 80◦ and a scan step of 0.0065◦
and 10 wt % standard silicon powder was added to correct the test instrument error. The power
morphology was observed using scanning electron microscopy (SEM) in a Hitachi S-4800 instrument
(Hitachi, Ltd., Tokyo, Japan) with an accelerating voltage of 3.0 KV and working distance of 4.5 mm;
and transmission electron microscope (TEM) in a Tecnai G2 F20 apparatus (FEI Corporation, Hillsboro,
OR, USA). The carbon content of the final product was obtained with a Thermo Scientific Flash
2000 elemental analyzer (Thermo Scientific Corporation, Warminster, MA, USA) with a carrier gas
velocity of 140 mL·min−1 ; oven temperature of 65 ◦ C; oxygen injection time of 5 s and a running time
Materials 2017, 10, 134
4 of 16
of 720 s. Raman spectroscopy was undertaken by a Renishaw (Renishaw, London, UK) in Via Reflex
with a 785 nm wavelength laser. The Fourier Transform Infrared Spectrum (FT-IR) was tested using a
Thermo Nicolet IS50 spectrometer (Thermo Scientific Corporation, Warminster, MA, USA).
2.3. Electrochemical Measurements
The prepared LiMnPO4 /C, acetylene black and a poly (vinylidene fluoride) (PVDF, HSV900,
(MTI Corporation, Shenzhen, China) binder sample were mixed with a weight ratio of 80:15:5 in
N-methyl-2-pyrrolidone (Xilong Chemical Co., Ltd., Guangzhou, China). The slurry was painted onto
an aluminum foil current collector with a thickness of 100 µm and dried overnight in a vacuum at
120 ◦ C. The working electrodes were obtained after punching with a diameter of 14 mm, and the
surface density of the active material was approximately 1.5 mg·cm−2 .
The charge/discharge behaviors of the prepared LiMnPO4 /C were tested using CR2032 coin-type
cells assembled in an argon-filled glove box. Lithium foil was used as the counter electrode,
Celegard® 2300 as the separator and 1 mol·L–1 LiPF6 /(EC + DEC 1:1 by volume) as the electrolyte.
Charge/discharge experiments were carried out on a CT-3008 battery testing system (Neware
Technology Limited, Shenzhen, China) at different C rates between 2.5 and 4.5 V (vs. Li/Li+ )
at room temperature (25 ◦ C), where a 1 C rate was 170 mAh·g−1 . Current densities and specific
capacities were calculated based on the mass of the LiMnPO4 /C of the electrode. A three-electrode
cell (Lithium Battery Cell 990-00344, Gamry Instruments, Warminster, PA, USA) was assembled and
used for cycle voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements.
CV tests were performed at scan rates ranging from 0.1–0.4 mV·s−1 between 2.5–4.5 V under
25 ◦ C. EIS measurements were carried out using a Gamry PCI4/750 electrochemical workstation
(Gamry Instruments, Warminster, PA, USA) with an AC voltage of 5 mV amplitude and a frequency
range from 100 kHz to 0.001 Hz. During the CV and EIS experiments, the electrode containing the
prepared LiMnPO4 /C was used as the working electrode and Li was used as both the counter and
reference electrode.
3. Results and Discussion
3.1. Material Identification
XRD and FTIR were used to analyze the crystalline structure and chemical composition of the
precursor obtained from the yellowish-brown precursor suspension, LMP and LMP/C. The diffraction
peaks of the precursor shown in Figure 2a may be ascribed to Mn(H2 PO4 )2 (PDF# 41-0001) and
Mn2 P4 O12 ·H2 O (PDF# 50-0384). It is obvious that a lack of LiMnPO4 diffraction peaks and other
lithium compounds can be observed in Figure 2a, suggesting that LiMnPO4 cannot be formed by
mixing the raw materials in DES at room temperature. The XRD patterns of LMP and LMP/C are
given in Figure 2b. The Si peak in Figure 2b is in accordance with the standard silicon powder used to
correct for instrumentation error during the XRD tests. The peaks shown in Figure 2b can be perfectly
indexed to the orthorhombic structure LiMnPO4 with a Pnmb space group (PDF# 74-0375), indicating
that LiMnPO4 can be obtained by heating the yellowish-brown precursor suspension at 130 ◦ C for 4 h
in DES. The reaction temperature and time used for DES synthesis are much lower than those used
in the hydrothermal/solvothermal [27,37–39,62] and ionothermal processes [63,64]. The strong and
sharp diffraction peaks in Figure 2b demonstrate that both the LiMnPO4 and LiMnPO4 /C samples
were well crystallized.
Materials 2017, 10, 134
Materials 2017, 10, 134
5 of 16
5 of 15
(b)
(400 / 023)
(160 / 331)
(131)
(211)
(140)
(012 / 221 / 041)
(112)
(220)
(020)
(120)
(101)
(200 / 031)
Si PDF#26-1481
(011)
Mn(H2PO4) PDF#41-001
2
(311)
(222 / 321)
LiMnPO4 PDF#74-0375
(111)
(a)
LMP/C
Precursor
LMP
LiMnPO4 PDF#74-0375
Mn2P4012H2O PDF#50-0384
10
20
30
40
50
60
70
2/degree
80
10
20
30
40 50 60
2 (degree)
70
80
Figure 2. X-ray diffraction (XRD) pattern of (a) precursor; and (b) LiMnPO4 (LMP) and LiMnPO4/C
Figure 2. X-ray diffraction (XRD) pattern of (a) precursor; and (b) LiMnPO4 (LMP) and LiMnPO4 /C (LMP/C).
(LMP/C).
The lattice
lattice parameters
parameters of
of LMP
LMP and
and LMP/C
LMP/Care
arelisted
listedin
inTable
Table 1.
1. The
Thevalues
values of
of the
the cell
cell volume
volume (V)
(V)
The
and
the
lengths
(a,
b
and
c)
of
both
samples
are
close
to
those
in
previous
studies
[65,66].
The
values
of
and the lengths (a, b and c) of both samples are close to those in previous studies [65,66]. The values
thethe
lattice
parameters
of LMP
areare
close
to those
of LMP/C,
indicating
thatthat
the the
annealing
process
has
of
lattice
parameters
of LMP
close
to those
of LMP/C,
indicating
annealing
process
little
influence
on
the
lattice
parameters.
Moreover,
the
carbon
content
of
LiMnPO
/C
determined
by
4
has little influence on the lattice parameters. Moreover, the carbon content of LiMnPO
4/C determined
thethe
CHNS/O
elemental
analyzer
is 9.0
wtwt
%,%,
and
nono
detectable
reflections
corresponding
to carbon
are
by
CHNS/O
elemental
analyzer
is 9.0
and
detectable
reflections
corresponding
to carbon
visible
in
Figure
2b
due
to
its
amorphous
structure.
are visible in Figure 2b due to its amorphous structure.
Table
T
able 1.
1. Lattice
Lattice parameters
parameters of
of the
the prepared
prepared LMP
LMP and
and LMP/C.
LMP/C.
Sample
LMP
LMP/C
Samplea (Å)a (Å)
b (Å)b (Å) c (Å)
LMP10.4437
10.4437
10.4439
LMP/C
10.4439
6.09804.7424
6.0980
6.10214.7430
6.1021
V (Å3c)(Å)
4.7424
302.0
4.7430
302.3
V (Å3)
302.0
302.3
Figure 3 presents the FTIR spectra of the precursor, LMP and LMP/C. According to previous
Figure 3 presents the FTIR spectra of the precursor, LMP and LMP/C. According to previous
literature [22,67,68], the internal vibrations of LMP/C consisted mainly of four parts [66]: (1) the bands
literature [22,67,68], the internal vibrations of LMP/C consisted mainly of four parts [66]: (1) the
between 1000 cm−1 and −1150
cm−1,which can be attributed to the anti-symmetric stretching P–O
bands between 1000 cm 1 and 1150 cm−1 ,which can be attributed to the anti-symmetric stretching
3−
vibrations of the PO4 anion
mode (v3); (2) the band around 980 cm−1, which is related to the
P–O vibrations of the PO4 3− anion mode (v3 ); (2) the band around 980 cm−1 , which is related to the
symmetric PO43−
stretching P–O vibration mode (v1); (3) the bands ranging from 650 cm−1 to 550 cm−1
symmetric PO4 3− stretching P–O vibration mode (v1 ); (3) the bands ranging from 650 cm−1 to 550 cm−1
can be ascribed to the anti-symmetric bending O–P–O mode (v4); and (4) the band at 457 cm−−11 can be
can be ascribed to the anti-symmetric bending O–P–O mode (v4 ); and (4) the band at 457 cm can be
associated with the symmetric bending mode (v2). In addition, the band at 420 cm−−11 may arise from
associated with the symmetric bending mode (v2 ). In addition, the band at 420 cm may arise from
vibrations of the Mn–O groups [41,67]. The FTIR spectra seen in Figure 3, shows that the precursor
vibrations of the Mn–O groups [41,67]. The FTIR spectra seen in Figure 3, shows that the precursor
−1, which is associated with O–P–O groups; (2) the
contains three type of bands: (1) the band at 576 cm−
contains three type of bands:−1(1) the band−1at 576 cm 1 , which is associated with O–P–O groups; (2) the
bands ranging from 980 cm−1to 1135 cm −, 1which are related to the P–O groups; and (3) the band at
bands ranging from 980 cm to 1135 cm , which are related to the P–O groups; and (3) the band at
950 cm−1−1can be ascribed to H–O groups. However, no obvious Li–O characteristic band was observed
950 cm can be ascribed to H–O groups. However, no obvious Li–O characteristic band was observed
in the FT-IR spectra of the precursor. The reason for this may be due to the fact that the insoluble
in the FT-IR spectra of the precursor. The reason for this may be due to the fact that the insoluble solids
solids were MnH2P3O16 and Mn2P4O12·H2O and that lithium exists in the form of soluble lithium
were MnH2 P3 O16 and Mn2 P4 O12 ·H2 O and that lithium exists in the form of soluble lithium compounds
compounds when the raw materials are mixed in DES at room temperature. After undergoing
when the raw materials are mixed in DES at room temperature. After undergoing centrifugation
centrifugation several times, the soluble lithium compounds were left in the solution so that the
several times, the soluble lithium compounds were left in the solution so that the precursor solid
precursor solid did not contain any lithium compounds. As seen in Figure 3, the LMP and LMP/C
did not contain any lithium compounds. As seen in Figure 3, the LMP and LMP/C have similar
have similar FT-IR spectra. However, the intensity of bands of LMP/C was obviously stronger than
FT-IR spectra. However, the intensity of bands of LMP/C was obviously stronger than that of LMP,
that of LMP, indicating that the crystallinity of LiMnPO4 can be further improved during heatindicating that the crystallinity of LiMnPO4 can be further improved during heat-treatment processes
treatment processes at high temperature [22].
at high temperature [22].
Materials 2017,
2017, 10,
10, 134
134
Materials
of 16
15
66 of
Materials 2017, 10, 134
6 of 15
LMP/C
420
LMP/C
LMP
1135
980
LMP
1110
1135
1074
980
1110
Precursor
1074
Precursor
576
632
576
632
553
553
506
457
420
457
506
120011001000 900 800 700 600 500 400
-1
120011001000Wavenumber
900 800 700
(cm600
) 500 400
-1
Wavenumber
(cm spectra
)
Figure 3. Fourier transform infrared spectroscopy
(FTIR)
of precursor LMP and LMP/C.
Figure 3. Fourier transform infrared spectroscopy (FTIR) spectra of precursor LMP and LMP/C.
Figure 3. Fourier transform infrared spectroscopy (FTIR) spectra of precursor LMP and LMP/C.
SEM
and TEM were conducted to identify the change of morphology and size during the
synthesis
and TEM
Figure
4a,bofindicate
that theand
precursor
powders
SEM processes.
and TEM The
wereSEM
conducted
to images
identifyinthe
change
morphology
size during
the
SEM
and
TEM
were
conducted
to
identify
the
change
of
morphology
and
size
during
the
synthesis
had
no
special
morphology.
From
Figure
4c,d,
it
can
be
clearly
seen
that
the
LiMnPO
4
particles
synthesis processes. The SEM and TEM images in Figure 4a,b indicate that the precursor powders
processes.
The
SEM
TEM
in Figure
4a,bit
indicate
the
precursor
powders
had 4no
special
prepared
in
DES
at and
130 °C
forimages
4 h exhibited
a rod
structure
with
a length
of around
80–110
nm
and a
had
no special
morphology.
From
Figure
4c,d,
can be that
clearly
seen that
the LiMnPO
particles
morphology.
From
Figure
4c,d,
it
can
be
clearly
seen
that
the
LiMnPO
particles
prepared
in
DES
4
diameter
of
around
30–50
nm,
which
was
similar
to
the
nanorods
prepared
by
the
EG-assisted
prepared in DES at 130 °C for 4 h exhibited a rod structure with a length of around 80–110 nm andat
a
◦ C for 4 h exhibited a rod structure with a length of around 80–110 nm and a diameter of around
130
solvothermal
approach
[27,35,69].
As seen
in Figure
as-prepared
4/C by
maintained
the roddiameter
of around
30–50
nm, which
was
similar4e,f,
to the
nanorodsLiMnPO
prepared
the EG-assisted
30–50
nm,(approximately
which
was similar
to the
nanorods
prepared
by nm
the EG-assisted
solvothermal
m. the
As seen
like form
100–150
nm
in length
and4e,f,
40–55
in diameter),
suggesting
that
rodsolvothermal
approach
[27,35,69].
As
seen
in Figure
as-prepared
LiMnPO
4/C maintained
rodin
Figure
4e,f,
as-prepared
LiMnPO
/C
maintained
the
rod-like
form
(approximately
100–150
nm
4
like
structure
was
sufficiently
stable
and
would
not
be
destroyed
during
the
heat
treatment
processes
like form (approximately 100–150 nm in length and 40–55 nm in diameter), suggesting that the rodin
length
and
40–55
nm
in
diameter),
suggesting
that
the
rod-like
structure
was
sufficiently
stable
[33].structure
Furthermore,
the size of stable
the LiMnPO
4/C particles
increasedduring
slightly
after
can
like
was sufficiently
and would
not be destroyed
the
heatannealing,
treatment which
processes
and
would
not
be destroyed
heat
treatment
processes
[33].
Furthermore,
the which
size out
of can
the
be ascribed
to the
further
ofthe
crystals
during the
annealing
process
[35].
As pointed
by
[33].
Furthermore,
the
size growth
of during
the LiMnPO
4/C particles
increased
slightly
after
annealing,
LiMnPO
/C
particles
increased
slightly
after
annealing,
which
can
be
ascribed
to
the
further
growth
of
4
Hong
et
al.
[35],
the
LiMnPO
4
/C
nanorods
prepared
in
our
work
could
shorten
lithium
ions
and
be ascribed to the further growth of crystals during the annealing process [35]. As pointed out by
crystals
during
the
annealing
process
[35].
As
pointed
out
by
Hong
et
al.
[35],
the
LiMnPO
/C
nanorods
4 electrolyte,
electron
diffusion
pathways
provide aprepared
large interface
the shorten
electrodelithium
and
Hong
et al.
[35], the
LiMnPOand
4/C nanorods
in our between
work could
ions and
prepared
in
our
work
could
shorten
lithium
ions
and
electron
diffusion
pathways
and
provide
a large
which
would
improve
its
performance
rate.
electron diffusion pathways and provide a large interface between the electrode and electrolyte,
interface
between
the
electrode
and
electrolyte,
which
would
improve
its
performance
rate.
HR-TEM
was
used
to
further
observe
the
crystal
structure
and
the
carbon
coating
layer
on the
which would improve its performance rate.
HR-TEM
was
used
to
further
observe
the
crystal
structure
and
the
carbon
coating
layer
on
the
surface
of the synthesized
4/C. As shown
in Figure
5a, the
spacing
of 0.37
HR-TEM
was used to LiMnPO
further observe
the crystal
structure
andclear
the lattice
carbonfringe
coating
layer on
the
surface
of the
thesynthesized
synthesized
LiMnPO
/C.
in
Figure
5a,
clear
lattice
fringe
spacing
of
4LiMnPO
nm is related
to
the (101) planes
of4/C.
4shown
(PDF#
74-0375),
that
the
crystallinity
the
surface
of
LiMnPO
AsAs
shown
in Figure
5a, indicating
the the
clear
lattice
fringe
spacing
ofof0.37
0.37
nm
is
related
to
the
(101)
planes
of
LiMnPO
(PDF#
74-0375),
indicating
that
the
crystallinity
of
the
4
LiMnPO
4 was
the HR-TEM
image also
clearly that
showed
that the primary
nm
is related
toperfect.
the (101)Additionally,
planes of LiMnPO
4 (PDF#
74-0375),
indicating
the crystallinity
of the
LiMnPO
Additionally,
HR-TEM
image
also
clearly
showed
thatThis
the primary
LiMnPO
4
LiMnPO444 was
particles
were
coated by athe
thin
carbon
layer
(approximately
3 nm).
thin the
carbon
layer
LiMnPO
wasperfect.
perfect.
Additionally,
the
HR-TEM
image
also clearly
showed
that
primary
particles
were
coated
by
a
thin
carbon
layer
(approximately
3
nm).
This
thin
carbon
layer
favors
the
favors
the
inhibition
of
the
growth
of
crystal
and
reduces
the
agglomeration
of
the
particles
[21,33,70],
LiMnPO4 particles were coated by a thin carbon layer (approximately 3 nm). This thin carbon layer
inhibition
of
the
growth
of by
crystal
reduces
thereduces
agglomeration
of the
particles
[21,33,70],
which
have
which the
have
been
proved
the and
results
in Figure
4e,f. Inthe
addition,
the
LiMnPO
4/C
crystallites
were
favors
inhibition
of the
growth
of crystal
and
agglomeration
of the
particles
[21,33,70],
been
proved
by the
results
Figure
4e,f.
In
addition,
the
LiMnPO
crystallites
were
connected
4 /CLiMnPO
connected
with
the
thin
carbon
layer
to form
excellent
conducting
network,
which
could
which
havedirectly
been
proved
byin
the
results
in
Figure
4e,f.an
In
addition,
the
4/C crystallites
were
directly
with
the
thin
carbon
layer
to
form
an
excellent
conducting
network,
which
could
effectively
effectively
enhance
the
electronic
conductivity
of
LiMnPO
4
/C
and
further
improve
its
capability
rate
connected directly with the thin carbon layer to form an excellent conducting network, which could
enhance
the
electronic
conductivity
of
LiMnPO
/C
and
further
improve
its
capability
rate
[12,69].
4 LiMnPO4/C and further improve its capability rate
[12,69]. enhance the electronic conductivity of
effectively
[12,69].
Figure 4. Cont.
Materials 2017, 10, 134
Materials 2017, 10, 134
Materials 2017, 10, 134
7 of 16
7 of 15
7 of 15
Figure 4. SEM and TEM images of the prepared samples: (a,b) precursor; (c,d) LMP; and (e,f) LMP/C.
Figure 4. SEM and TEM images of the prepared samples: (a,b) precursor; (c,d) LMP; and (e,f) LMP/C.
Figure 4. SEM and TEM images of the prepared samples: (a,b) precursor; (c,d) LMP; and (e,f) LMP/C.
G(sp2)
G(sp2)
2
D(sp )
D(sp2)
Intensity
(a.u.)
Intensity
(a.u.)
(b)
(b)
sp3
sp3
600
600
sp3
sp3
900 1200 1500 1800 2100
-1
900Raman
1200shift
1500
(cm1800
) 2100
-1
Raman shift (cm )
Figure 5. (a) HR-TEM image; and (b) Raman spectra of the prepared LiMnPO4/C.
Figure 5. (a) HR-TEM image; and (b) Raman spectra of the prepared LiMnPO4/C.
Figure 5. (a) HR-TEM image; and (b) Raman spectra of the prepared LiMnPO4 /C.
The structural and physical properties of the carbon layer were further characterized by Raman
The structural
and physical
of theLiMnPO
carbon layer
were further
characterized
by Raman
spectroscopy.
The Raman
curve properties
of the prepared
4/C shown
in Figure
5b can be fitted
with
The
structural
and
physical
properties
of
the
carbon
layer
were
further
characterized
by
Raman
spectroscopy.
Raman
curve
of the
prepared
LiMnPO
/C −1shown
in Figure
5bprevious
can be fitted
with
four
Gaussian The
bands
located
at 1173,
1344,
1496, and
1590 4cm
. According
to the
literature
spectroscopy.
The
Raman
curve
of
the prepared
LiMnPO
shown
in Figureto
5bthe
canprevious
be
fitted literature
with four
−1. According
4 /Ccm
four
Gaussian
bands
located
at
1173,
1344,
1496,
and
1590
−1
−1
[71,72], the band at 1590 cm (G band) is the graphitic band
and the band at 1344 cm (D band) is the
−1 . According to the previous literature [71,72],
Gaussian
bands
located
atcm
1173,
1496,
and
1590 cmband
−1 (G1344,
[71,72], the
bandThe
at 1590
band)
is the
graphitic
and theaband
at 1344coating
cm−1 (Dofband)
is the
disorder
band.
existence
of
the
G
band
and
D
band
indicates
successful
carbon
on
−1 (G band) is the graphitic band and the band at 1344 cm−1 (D band) is the
the
band
at
1590
cm
disorder
band.
The
existence
the G band
and Dthe
band
indicates
successful
coating
of carbon
on
the
surface
of the
LiMnPO
4/C of
particles
[22]. Both
G band
and Da band
belong
to sp2-type
carbon
disorder
band.
The
existence
of
the G band
and
Dthe
band
indicates
a successful
coating
carbon
on
2of
the
surface
of
the
LiMnPO
4
/C
particles
[22].
Both
G
band
and
D
band
belong
to
sp
-type
carbon
−1
−1
3
vibrations, while the other peaks at around 1173 cm and 1476 cm are associated with
sp -type
the
surface of
the LiMnPO
particles
[22]. Both
G
band1476
and cm
D band
to sp2with
-typespcarbon
−1 and
−1 arebelong
3-type
4 /C
vibrations,
while
the
peaks
at of
around
1173 the
cm
associated
carbon
vibrations.
Theother
intensity
ratio
the G band
and
D
band
(I
D/IG) can
be
used
to
characterize
the
−1 and 1476 cm−1 are associated with sp3 -type
vibrations,
while
the
other
peaks
at
around
1173
cm
carbon vibrations.
Theofintensity
ratio
of the G[22].
bandInand
band the
(ID/Ivalue
G) canof
bethe
used
characterize
the
graphitization
degree
the carbon
materials
thisDstudy,
ID/Ito
G for
the prepared
carbon
vibrations.
Theof
intensity
ratiomaterials
of the G band
and
D study,
band (Ithe
used
to
characterize
the
D /Ivalue
G ) canofbe
graphitization
degree
the
carbon
[22].
In
this
the
I
D
/I
G
for
the
prepared
LiMnPO4/C nanorods was 0.85, which is similar to the value reported by Qin et al. [32] and lower
graphitization
degree ofwas
the carbon
materials
[22]. In
this
the
value by
of the IDet
/IG for
theand
prepared
LiMnPO
nanorods
which
similar
the study,
value
reported
[32]
lower
than
that4/C
reported
by Fan et0.85,
al. [22].
Theislower
ID/Ito
G value
indicates
that the Qin
carbonal.
layer
obtained
in
LiMnPO
/C
nanorods
was
0.85,
which
is
similar
to
the
value
reported
by
Qin
et
al.
[32]
and
lower
4reported by Fan et al. [22]. The lower ID/IG value indicates that the carbon layer obtained in
than
that
our work had a higher degree of graphitization, which is beneficial to electron diffusion and electron
than
that reported
by Fan
et al.of
[22].
The lower IDwhich
/IG value
indicatestothat
the carbon
layerand
obtained
in
our work
had[22].
a higher
degree
graphitization,
is beneficial
electron
diffusion
electron
conductivity
our
work had[22].
a higher degree of graphitization, which is beneficial to electron diffusion and electron
conductivity
The formation processes of the LiMnPO4/C nanorods can be schematically illustrated by Figure 6.
conductivity
[22].
The formation
of and
the LiMnPO
4/C nanorods
canplays
be schematically
illustrated
Figure 6.
According
to crystalprocesses
nucleation
growth theory
[73], DES
an important
role in thebyformation
According
to
crystal
nucleation
and
growth
theory
[73],
DES
plays
an
important
role
in
the
formation
3−
2+
of LiMnPO4/C nanorods. Initially, PO4 anions react with Mn ions to generate insoluble manganese
of LiMnPO4/C
nanorods.
43− Mn(H
anions2PO
react
Mn2+when
ions toLiOH
generate
insoluble
manganese
phosphates
(mainly
Mn2PInitially,
4O12·H2OPO
and
4)2 with
particles)
solution
is added
to DES
phosphates
(mainly
Mn
2
P
4
O
12
·H
2
O
and
Mn(H
2
PO
4
)
2
particles)
when
LiOH
solution
is
added
to DES
at room temperature.
at room temperature.
Materials 2017, 10, 134
8 of 16
The formation processes of the LiMnPO4 /C nanorods can be schematically illustrated by Figure 6.
According to crystal nucleation and growth theory [73], DES plays an important role in the formation of
LiMnPO4 /C nanorods. Initially, PO4 3− anions react with Mn2+ ions to generate insoluble manganese
phosphates (mainly Mn2 P4 O12 ·H2 O and Mn(H2 PO4 )2 particles) when LiOH solution is added to DES
at room2017,
temperature.
Materials
10, 134
8 of 15
Figure 6.
6. Schematic
Schematic illustration
illustration of
of the
the formation
formation of
of LiMnPO
LiMnPO4/C
nanorods.
Figure
4 /C nanorods.
When heated in DES, Li++ ions react directly with Mn(H2PO4)2 to form LiMnPO4 crystal nuclei,
When heated in- DES, Li- ions react directly with Mn(H2 PO4 )2 to form LiMnPO4 crystal nuclei,
and Li++, Mn2+,2+
H2PO4 and OH react with Mn2P4O12·H2O to generate LiMnPO4 crystal nuclei. The main
and Li , Mn , H PO4 - and OH- react with Mn2 P4 O12 ·H2 O to generate LiMnPO4 crystal nuclei.
reaction equations 2are described
below:
The main reaction equations are described below:
2LiOH + 2H3PO4 + MnSO4 = Mn(H2PO4)2 + Li2SO4 + 2H2O
(1)
2LiOH + 2H3 PO4 + MnSO4 = Mn(H2 PO4 )2 + Li2 SO4 + 2H2 O
(1)
4LiOH + 4H3PO4 + 2MnSO4 = Mn2P4O12 + Li2SO4 + 8H2O
(2)
4LiOH
+ 2MnSO
+ Li SO + 8H2 O
3 PO24PO
4 = Mn
2 PLiH
4 O12
2LiOH++4H
Mn(H
4)2 = LiMnPO
4 +
2PO4 +2 2H24O
2LiOH
+ Mn(H2 PO
2H42 O
4 )LiH
2 = LiMnPO
4 + LiH24PO
4 +2SO
10LiOH + Mn
2P4O12 + 3MnSO
4 +
2PO4 = 5LiMnPO
+ 3Li
+ 6H2O
(3)(2)
(4)(3)
+ Mn
3LiDES
+ 6H2adsorb
O
(4)
2 P4 O12 + 3MnSO
4 + LiH
2 PO
4 = 5LiMnPO
2 SO4could
During 10LiOH
the further
crystallization
process,
we
speculated
that4 +
the
on the
surface
of the
LiMnPO
4 as a structure-directing agent to induce the formation of
During
thenewly
furtherformed
crystallization
process,
we speculated that the DES could adsorb on the surface
nanorods
[50].
In
addition,
the
large
viscosity
of DES could
particle
and inhibit
the crystal
of the newly formed LiMnPO4 as a structure-directing
agent limit
to induce
the size
formation
of nanorods
[50].
continuous
growth
by
capping
the
crystal
faces
during
the
reaction
process
[46].
Hence,
DES
can
In addition, the large viscosity of DES could limit particle size and inhibit the crystal continuous
provide
more
LiMnPO
nuclei and
a lower
growth
rate,
resulting
theprovide
formation
of
growth by
capping
the4 crystal
faces
duringcrystal-oriented
the reaction process
[46].
Hence,
DESincan
more
LiMnPO
4 particles with a rod-like morphology. The specific effects of choline chloride and ethylene
LiMnPO4 nuclei and a lower crystal-oriented growth rate, resulting in the formation of LiMnPO4
glycol
onwith
the synthesis
LiMnPO4 material
will be
further
discussed
in theand
following
particles
a rod-likeof
morphology.
The specific
effects
of choline
chloride
ethylenesection.
glycol on the
synthesis of LiMnPO4 material will be further discussed in the following section.
3.2. Electrochemical Characterization
3.2. Electrochemical
Figure 7 showsCharacterization
the charge-discharge behaviors of the prepared LiMnPO4/C nanorods. As seen
in Figure
7a,
the
charge-discharge
curves ofbehaviors
the 1st, 20th,
40th,
60th, 80th
and 100th
cycles had obvious
Figure 7 shows the charge-discharge
of the
prepared
LiMnPO
4 /C nanorods. As seen
charge/discharge
around 4.25
V and
4.05
which
is 60th,
in accordance
lithium
in Figure 7a, the plateaus
charge-discharge
curves
of the
1st,V,20th,
40th,
80th and with
100ththe
cycles
had
extraction
and
insertion
processes,
respectively
[35].
The
good
overlap
of
the
charge-discharge
curves
obvious charge/discharge plateaus around 4.25 V and 4.05 V, which is in accordance with the
at
different
cycles shows
that theprocesses,
Li+ extraction
and insertion
processes
are reversible
[35]. The initial
lithium
extraction
and insertion
respectively
[35]. The
good overlap
of the charge-discharge
–1
+
charge/discharge
capacity
139.0
and 127.9
, respectively,
the initial
curves at differentspecific
cycles shows
thatwas
the Li
extraction
andmAh·g
insertion
processes areand
reversible
[35].
−
1
coulombic
efficiency
was
approximately
92.0%,
which
is
mainly
attributed
to
unavoidable
The initial charge/discharge specific capacity was 139.0 and 127.9 mAh·g , respectively, and the
passivation
phenomena
of the
liquid
electrolyte at
high potential;
and sideattributed
reactions to
between
active
initial coulombic
efficiency
was
approximately
92.0%,
which is mainly
unavoidable
materials
and
electrolytesof[16,35,66,74,75].
As seen
Figure
7b, theand
prepared
samplebetween
gave an initial
passivation
phenomena
the liquid electrolyte
at in
high
potential;
side reactions
active
–1 and maintained over 92.6% of the initial capacity after 100 cycles
discharge
capacity
of
128
mAh·g
materials and electrolytes [16,35,66,74,75]. As seen in Figure 7b, the prepared sample gave an initial
at
1 C, exhibiting
improved
cycling
could
be dueafter
to the
discharge
capacitygood
of 128cycle
mAhstability.
·g−1 and The
maintained
over
92.6% stability
of the initial
capacity
100nanorodcycles at
like
morphology,
high
crystallization
and
uniform
carbon
coating.
1 C, exhibiting good cycle stability. The improved cycling stability could be due to the nanorod-like
morphology, high crystallization and uniform carbon coating.
Cell Potenital (V)
Specific capacity (mAh g-1)
4.5 (c)
4.2
3.9
3.6
3.3
3.0
2.7
2.4
0
0.2C
5C
2C
1C
0.5C
30 60 90 120 150 180 210
Specific capacity (mAh g-1)
Specific Capacity (mAh×g-1)
4.8
100
(a)
80
4.5
60
1
4.2
20
40
3.9
1
100
20
3.6
80
40
60
3.3
3.0
2.7
2.4
0 20 40 60 80 100 120 140 160
140 (b)
120
100
80
60
40
20
0
0
20
Specific Capacity (mAh g-1)
Cell Potential (V)
coulombic efficiency was approximately 92.0%, which is mainly attributed to unavoidable
passivation phenomena of the liquid electrolyte at high potential; and side reactions between active
materials and electrolytes [16,35,66,74,75]. As seen in Figure 7b, the prepared sample gave an initial
discharge capacity of 128 mAh·g–1 and maintained over 92.6% of the initial capacity after 100 cycles
2017, 10, 134good cycle stability. The improved cycling stability could be due to the nanorod9 of 16
atMaterials
1 C, exhibiting
like morphology, high crystallization and uniform carbon coating.
150 (d)
0.2C
140
130
120
110
100
90
80
0
10
40
60
80
Cycle number (n)
100
0.2C
0.5C
1C
2C
5C
20 30 40 50
Cycle number (n)
60
Figure
4 /C:
(a) charge-discharge curves at 1
Figure 7.
7. Electrochemical
Electrochemicalproperties
propertiesfor
forthe
theprepared
preparedLiMnPO
LiMnPO
4 /C: (a) charge-discharge curves at
C1rate;
(b)(b)
cycle
performance
at at
1 C1 C
rate;
(c)(c)
charge-discharge
curves
atatdifferent
C rate;
cycle
performance
rate;
charge-discharge
curves
differentrates;
rates;and
and(d)
(d)rate
rate
performances
performancesatatdifferent
differentrates.
rates.
As shown in Figure 7c, the voltage platform gap between the charge and discharge increased
with the increase of the charge-discharge rate, which can be ascribed to the increase of electrode
polarization [31]. The charge capacity given in Figure 7c was greater than the theoretical capacity
(170 mAh·g−1 ), which can be ascribed to the side reaction of the electrolyte at high potential [16,66].
As seen in Figure 7d, the prepared LiMnPO4 /C nanorods delivered a discharge capacity of 144, 136,
129 120 and 106 mAh·g−1 at 0.2, 0.5, 1, 2 and 5 C, respectively. The rate performance of LiMnPO4 /C
was much better than that of the nanorod-like LiMnPO4 /C reported in References [76,77], and close
to that provided by Hong et al. [35]. The improved performance rate can be attributed to the unique
nano-sized rod structure and the carbon layer.
A comparative experiment was conducted to explore the role of ethylene glycol and choline
chloride in the DESs synthesis of LiMnPO4 . Figure 8 compares the cycling performance of the
LiMnPO4 /C prepared in ethylene glycol/choline chloride and ethylene glycol solvent, respectively.
As shown in Figure 8, the LiMnPO4 /C prepared in ethylene glycol solvent gave a specific discharge
capacity of 117 mAh·g−1 with a capacity retention ratio of 88% after 50 cycles at 1 C, which is close to
that of LiMnPO4 /C prepared in the ethylene glycol solvent [18,30,33], while the LiMnPO4 /C prepared
in ethylene glycol/choline chloride delivered 128 mAh·g−1 with a capacity retention ratio of 95% after
50 cycles at 1 C. The performance of LiMnPO4 /C prepared in ethylene glycol/choline chloride was
much better than that in ethylene glycol, indicating that choline chloride plays a more important role
during the synthesis of LiMnPO4 in the DES. However, the interaction of choline chloride and ethylene
glycol in the DES is under further investigation.
Materials 2017, 10, 134
10 of 16
Materials 2017, 10, 134
10 of 15
Specific Capacity/ mAh×g
-1
140
130
120
110
100
90
Chloride/ethylene glycol-based DES
Ethylene glycol
80
70
60
0
5
10
15
20
25
30
35
40
45
50
Cycle number (n)
Figure 8. The 1 C rate cycling performance of the LiMnPO4/C prepared in DES and EG, respectively.
Figure 8. The 1 C rate cycling performance of the LiMnPO4 /C prepared in DES and EG, respectively.
CV and EIS were carried out to further understand the electrode reaction of the prepared
CV 4and
EIS were
carried
out behaviors
to furtherwith
understand
therange
electrode
the−1prepared
LiMnPO
/C. Figure
9 shows
the CV
a scan rates
fromreaction
0.1 to 0.4ofmV·s
. As seen
−1 . As seen in
LiMnPO
/C.
Figure
9
shows
the
CV
behaviors
with
a
scan
rates
range
from
0.1
to
0.4
mV
·
s
4
in Figure 9a, the CV curves present obvious redox peaks corresponding to the extraction and
insertion
Figure
9a, the
CVduring
curvesthe
present
obvious redoxprocesses
peaks corresponding
to the
insertionthe
of
of
lithium
ions
charge/discharge
[33,35]. When
theextraction
scan rateand
increased,
lithium ions
during
thetocharge/discharge
the
scanmoved
rate increased,
thepotential
cathodic
cathodic
peak
moved
the low potentialprocesses
direction[33,35].
and theWhen
anodic
peak
to the high
peak
moved
to
the
low
potential
direction
and
the
anodic
peak
moved
to
the
high
potential
direction
direction due to the increase in electrochemical polarization [35,70]. The plot of the anodic peak
due to the
increase(ipin
electrochemical
polarization
[35,70].
plot
of the
peak current
densities
current
densities
) compared
with the
square root
of theThe
scan
rates
(v1/2anodic
) is presented
in Figure
9b.
1/2
Materials
2016, 9, xwith
FOR PEER
REVIEWroot of the scan rates (v
10 of 15
(ip ) compared
the square
) is presented in Figure 9b.
0.90
0.90
0.85
0.85
0.80
ip (mA)
ip (mA)
Current
(A)(A )
Current
0.0012
0.0010 (a)
0.0012
0.0008 (a)
0.0010
0.0006
0.0008
0.0004
0.0006
0.0002
0.0004
0.0000
0.0002
-0.0002
0.0000
0.1
0.2
-0.0004
-0.0002
0.10.3
0.2
-0.0006
-0.0004
0.4
0.3
-0.0008
-0.0006
0.4
-0.0010
-0.0008
2.25 2.50 2.75 3.00 3.25 3.50 3.75 4.00 4.25 4.50 4.75
-0.0010
Potential
(V)
2.25 2.50 2.75 3.00 3.25
3.50 3.75
4.00 4.25 4.50 4.75
(b)
(b)
0.80
0.75
0.75
0.70
0.70
0.65
0.65
0.60
0.60
0.55
0.55
0.50
0.50
0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65
1/2
0.30 0.35 0.40v1/2
0.45
(mv0.50
s ) 0.55 0.60 0.65
v1/2(mv s1/2)
Potential (V)
Figure 9. (a) Cycle voltammetry curve of the prepared LiMnPO4/C; and (b) the plots of peak current
)(a)
asCycle
aCycle
function
of the square
root
of
the
rate
(v1/2LiMnPO
). /C; and
density
(ip(a)
Figure 9.9.
voltammetry
curve
of the
LiMnPO
(b)and
the (b)
plots
peakof
current
voltammetry
curve
ofprepared
thescan
prepared
4 /C;
theofplots
peak
Figure
4
density density
(ip ) as a(ifunction
of the square
of the
ratescan
(v1/2
). (v1/2).
p ) as a function
of the root
square
rootscan
of the
rate
current
The correlation coefficient (R2 = 0.999) indicates the relationship between ip and v1/2 is consistent
withThe
the correlation
Randles-Serick
equation
The Randles-Sevcik
equation
cani p be
as the
2 =[78,79].
2=
1/2expressed
coefficient
0.999)
indicates
relationship
between
v1/2
is consistent
The correlation
coefficient
(R(R
0.999)
indicates
thethe
relationship
between
ip and
vand
is
consistent
with
following
[78,79]:
with
the
Randles-Serick
equation
[78,79].
The
Randles-Sevcik
equation
can
be
expressed
as
the
the Randles-Serick equation [78,79]. The Randles-Sevcik equation can be expressed as the following [78,79]:
/
/
/
following [78,79]:
= 2.69 × 10
(5)
5 53/23/2
1/2 1/2
𝑖𝑖p =
× 10
𝑛𝑛 n 𝐶𝐶LiC𝐴𝐴𝐷𝐷
𝑣𝑣 1/2v1/2
(5)(5)
ip 2.69
= 2.69
×
10
AD
Li
where ip is the peak current density (A·g−1); n is the number of electrons involved in the redox process
where i p is the peak current density (A·g−1); n is the number of electrons involved in the redox process
2+/Mn3+ redox pair); CLi is the−initial
(n
= 1 for
Mn
of electrons
lithium ions
in LiMnPO
4 (0.022
1 ); n isconcentration
2+/Mn
3+ redox
where
ip is
the
peak
current
density
·g initial
the number of
involved
in the
redox
(n
= 1 for
Mn
pair);
C Li is(Athe
concentration
lithium ions
in LiMnPO
4 (0.022
−3); A is the total
2); D is the lithium ion
mol·cm
surface
area
of
the
electrode
active
material
(cm
2+
3+
−3
2
process
(n
=
1
for
Mn
/Mn
redox
pair);
C
is
the
initial
concentration
of
lithium
ions
in
LiMnPO
mol·cm ); A is the total surface area of theLi electrode active material (cm ); D is the lithium ion4
2·s−1); and v is the scan rate. The value of the lithium ion2 diffusion coefficients
diffusion
coefficient
(cm
2·s
−1
(0.022 molcoefficient
·cm−3 ); A (cm
is
the
total
surface
area
of rate.
the electrode
active
material
); D is the coefficients
lithium ion
diffusion
); and
v is the
scan
The value
of the
lithium(cm
ion diffusion
can
be calculated
from
the
ofvEquation
(5)rate.
andThe
the value
value of
of the
thelithium
D for the
prepared
LiMnPO
4/C
2 ·s−slope
1 ); and
diffusion
coefficient
(cm
is
the
scan
ion
diffusion
coefficients
can be calculated from the slope of Equation (5) and the
D for the prepared LiMnPO
4 /C
−15
2
−1
nanorods was 8.61 × 10 cm ·s .
nanorods was 8.61 × 10−15 cm2·s−1.
The Nyquist plot of the prepared LiMnPO4/C and the corresponding equivalent circuit are
The Nyquist plot of the prepared LiMnPO 4 /C and the corresponding equivalent circuit are
shown in Figure 10. The Nyquist curve consists of a compressed semicircle in the high-frequency
shown in Figure 10. The Nyquist curve consists of a compressed semicircle in the high-frequency
region related to charge transfer impedance and a declined line in the low-frequency region ascribed
to the Li+ diffusion resistance (Warburg impedance) in the LiMnPO 4 /C material [30]. The equivalent
circuit of EIS is shown in Figure 10, where R e stands for the resistance of the electrolyte; R ct and Q 1
Materials 2017, 10, 134
11 of 16
can be calculated from the slope of Equation (5) and the value of the D for the prepared LiMnPO4 /C
nanorods was 8.61 × 10−15 cm2 ·s−1 .
The Nyquist plot of the prepared LiMnPO4 /C and the corresponding equivalent circuit are shown
Materials 2017, 10, 134
11 of 15
in Figure 10. The Nyquist curve consists of a compressed semicircle in the high-frequency region
relatedrelated
to charge
transfer
impedance
and aand
declined
line line
in the
low-frequency
region
ascribed
to
region
to charge
transfer
impedance
a declined
in the
low-frequency
region
ascribed
+
thethe
Li Lidiffusion
resistance
material [30]. The equivalent
+ diffusion
4 /C
to
resistance(Warburg
(Warburgimpedance)
impedance)ininthe
theLiMnPO
LiMnPO
4/C material [30]. The equivalent
circuit
of
EIS
is
shown
in
Figure
10,
where
R
stands
for
the
resistance
and Q
circuit of EIS is shown in Figure 10, where Ree stands for the resistance of
of the
the electrolyte;
electrolyte; R
Rct
ct and Q11
(Constant phase
phase angle
angle element)
element) are
are the
the charge
charge transfer
transfer impedance
impedance and
and its
its electric
electric capacitance
capacitance of
of the
the
(Constant
electrode
interface,
respectively;
Z
is
known
as
the
Warburg
impedance;
and
Q
(constant
phase
angle
w
2
electrode interface, respectively; Zw is known as the Warburg impedance; and Q2 (constant phase
element)
represents
the “dispersion
effect” resulting
from thefrom
Li+ diffusion
[69,80–82].
The values
of
angle
element)
represents
the “dispersion
effect” resulting
the Li+ diffusion
[69,80–82].
The
the parameters
of the equivalent
circuit arecircuit
presented
in Table 2. in
AsTable
seen in
Table
2, R
(10.932,Ω)
Rct
values
of the parameters
of the equivalent
are presented
2. As
seen
ine Table
Reand
(10.93
(128
Ω)
of
the
prepared
LiMnPO
/C
nanorods
are
close
to
that
reported
by
Qin
et
al.
[32].
The
smaller
4 LiMnPO4/C nanorods are close to that reported by Qin et al. [32].
Ω) and Rct (128 Ω) of the prepared
values
of Re and
Rct indicate
that
nanorods
with a thin
carbonwith
layera can
The
smaller
values
of Re and
Rctthe
indicate
thatcoated
the nanorods
coated
thinfacilitate
carbon electrolyte
layer can
transport
and
the
charge
transfer
reaction.
facilitate electrolyte transport and the charge transfer reaction.
250
-Z'' (Ohm)
200
150
100
50
0
0
50
100
150
200
250
Z' (Ohm)
Figure 10. Electrochemical impedance spectra of the prepared LiMnPO4/C.
Figure 10. Electrochemical impedance spectra of the prepared LiMnPO4 /C.
Table 2. Parameters of EIS spectroscopy of the prepared LiMnPO4/C.
Table 2. Parameters of EIS spectroscopy of the prepared LiMnPO4 /C.
Rct (Ω)
Element
Re (Ω)
Element
R
(Ω)
Rct (Ω)
e
Values
10.93
128.00
128.00
Error (%) Values0.93 10.93 1.13
Error (%)
0.93
1.13
Q1 (F)
Q1× (F)
1.55
10−6
1.554.15
× 10−6
4.15
n1
n0.8
1
Q2 (F)
Q
2 (F)× 10-3 n2
2.18
0.8
0.59 2.18 ×4.76
10-3
0.59
4.76
0.8
2.67
n2
0.8
2.67
The diffusion coefficient of the lithium ions (DLi) can be calculated according to the following
equation
The [66,75]:
diffusion coefficient of the lithium ions (DLi ) can be calculated according to the following
equation [66,75]:
(6)
2
=
R2 T
2
σ
DLi =
(6)
2A2 n4 F4 C4 σ2
where R is the gas constant; T is the thermodynamic temperature used in the test; A is the reaction
where R is the gas constant; T is the thermodynamic temperature used in the test; A is the reaction area
area of the electrode; n is the number of transferred electrons per LiMnPO4 molecule during
of the electrode; n is the number of transferred electrons per LiMnPO4 molecule during oxidization;
oxidization; F is the Faraday constant; C is the concentration of lithium
ions; and σ represents the
F is the Faraday constant; C is the concentration of lithium ions; and−1/2σ represents the Warburg factor,
Warburg factor, which is the slope of the line
between
Z’
and
ω
.
The
value of the DLi for the
which is the slope of the line between Z’ and ω−1/2 . The−15
value2of−1the DLi for the prepared LiMnPO4 /C
prepared LiMnPO4/C was calculated
to be 8.91 × 10 cm ·s , which is larger than that of the
was calculated to be 8.91 × 10−15 cm2 ·s−1 , which is larger than that of the LiMnPO /C nanorods
LiMnPO4/C nanorods synthesized via a facile EG-assisted solvothermal approach [35]4and is closed
synthesized via a facile EG-assisted solvothermal
approach [35] and is closed to the D value with CV
to the D value with
CV
test
(8.61 × 10−15 cm2·s−1). The low charge transfer resistance and high diffusion
−
15
2
−
1
test (8.61 × 10
cm ·s ). The low charge transfer resistance and high diffusion coefficient further
coefficient further prove that as-prepared LiMnPO4/C nanorods can display good electrochemical
prove that as-prepared LiMnPO4 /C nanorods can display good electrochemical performance.
performance.
4. Conclusions
In this work, we have successfully prepared olivine-type LiMnPO4/C nanorods with an
orthorhombic structure in chloride/ethylene glycol-based DES at 130 °C for 4 h. The prepared
LiMnPO4/C nanorods delivered a discharge capacity of 128 mAh·g−1 with a capacity retention ratio
of approximately 93% after 100 cycles at 1 C. Even at 5 C, the sample still provided a discharge
Materials 2017, 10, 134
12 of 16
4. Conclusions
In this work, we have successfully prepared olivine-type LiMnPO4 /C nanorods with an
orthorhombic structure in chloride/ethylene glycol-based DES at 130 ◦ C for 4 h. The prepared
LiMnPO4 /C nanorods delivered a discharge capacity of 128 mAh·g−1 with a capacity retention ratio
of approximately 93% after 100 cycles at 1 C. Even at 5 C, the sample still provided a discharge capacity
of 106 mAh·g−1 , exhibiting a good capability rate and cycling stability. The improved electrochemical
performance can be attributed to the rod-like nanostructure and a thin carbon layer coated on
the surface of LiMnPO4 . The results provided in this work demonstrate that chloride/ethylene
glycol-based DES can act as a novel structure-directing agent to influence crystal growth orientation
and control the micromorphology of LiMnPO4 . Furthermore, DESs have potential application in the
preparation of olivine-type LiMPO4 and other electrode materials with a special micromorphology
for LIBs.
Acknowledgments: The authors appreciate the financial support from the National Natural Science Foundation
of China (grant No. 21366006, 21606055).
Author Contributions: Y.-X.W. conceived the concept. Z.W., R.-R.Y., H.Y., and Y.-C.X. carried out the experiments.
Z.W., X.-Y.L., Y.-F.Y. and J.S. analyzed the results and co-wrote this paper. All the authors made comments to
the paper.
Conflicts of Interest: The authors declare no conflict of interest. The founding sponsors had no role in the design
of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the
decision to publish the results.
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