Electrochemical performances of Li3V2-(4/3)xTix(PO4)3/C as cathode material for Li-ion batteries synthesized by an ultrasound-assisted sol-gel method

Article abstract of DOI:10.1016/j.jallcom.2015.07.244

Cathode materials Li₃V_2-(4/3)xTi_x (PO₄)₃ (x = 0,0.03, 0.06, 0.09, 0.12) using Polyvinylidene Fluoride (PVDF) as carbon source are synthesized via an ultrasound-assisted sol-gel method. Ultrasound helps to a uniform dispersion of the water insoluble PVDF. X-ray diffraction (XRD) and scanning electron microscopy (SEM) show that samples exhibit pure monoclinic structure and have similar morphology. Electrochemical galvanostatic charging/discharging results show that the capacities at low current density (less than 2C) of all samples are not that different. However, the discharge capacities and cyclic performance are improved at higher current density by a proper amount of Ti⁴⁺ doping (x = 0.03-0.06). The electrochemical performances become a bit worse when x is higher than 0.06. This may be attributed to the uniform lattice distortion caused by the overmuch dopant.

Full text of DOI:10.1016/j.jallcom.2015.07.244

Accepted Manuscript
Electrochemical performance of Li V
ion batteries synthesized by an ultrasound-assisted sol-gel method
Ti (PO ) /C as cathode material for Li-
4 3
3 2-(4/3)x x
Lingfang Li, Changling Fan, Taotao Zeng, Xiang Zhang, Weihua Zhang, Shaochang
Han
PII:
S0925-8388(15)30642-3
10.1016/j.jallcom.2015.07.244
JALCOM 34940
DOI:
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 15 May 2015
Revised Date: 23 July 2015
Accepted Date: 26 July 2015
Please cite this article as: L. Li, C. Fan, T. Zeng, X. Zhang, W. Zhang, S. Han, Electrochemical
performance of Li V
Ti (PO ) /C as cathode material for Li-ion batteries synthesized by
4 3
3 2-(4/3)x x
an ultrasound-assisted sol-gel method, Journal of Alloys and Compounds (2015), doi: 10.1016/
j.jallcom.2015.07.244.
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to
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ACCEPTED MANUSCRIPT
130
125
120
115
110
105
0.2C
1.0C
0.5C
2.0C
100
95
90
x=0
5.0C
x=0.03
x=0.06
x=0.09
x=0.12
85
80
75
70
65
60
55
10C
15C
50
0
5
10
15
20
25
30
35
Cyc lic times
The capacity fading of Li3V2-(4/3)xTix(PO4)3/C becomes larger with increasing x, especially when the
rate is more than 2 C. Sample B and C (x=0.03 & 0.06) have outstanding rate performance that their
specific capacity excesses 100 mAh·g-1 at 15 C.

ACCEPTED MANUSCRIPT
Electrochemical performance of Li3V2-(4/3)xTix(PO4)3/C as cathode material for Li-ion batteries
synthesized by an ultrasound-assisted sol-gel method
Lingfang Lia,b, Changling Fana, Taotao Zenga, Xiang Zhanga, Weihua Zhanga, Shaochang Hana,*
a College of Materials Science and Engineering, Hunan University, Changsha 410082, China
b College of Mechanical Engineering, Hunan University of Arts and Science, Changde 415000, China
* Corresponding author. Tel.:+8613807311980. E-mail: hansc@hnu.edu.cn (Shaochang Han)
Abstract: Cathode materials Li3V2-(4/3)xTix(PO4)3(x=0,0.03, 0.06, 0.09, 0.12) using Polyvinylidene Fluoride
(PVDF) as carbon source are synthesized via an ultrasound-assisted sol-gel method. Ultrasound helps to a
uniform dispersion of the water insoluble PVDF. X-ray diffraction (XRD) and scanning electron microscopy
(SEM) show that samples exhibit pure monoclinic structure and have similar morphology. Electrochemical
galvanostatic charging/discharging results show that the capacities at low current density (less than 2 C) of
all samples are not that different. However, the discharge capacities and cyclic performance are improved at
higher current density by a proper amount of Ti4+ doping (x=0.03~0.06). The electrochemical performances
become a bit worse when x is higher than 0.06. This may be attributed to the uniform lattice distortion
caused by the overmuch dopant.
Keywords: Lithium vanadium phosphates, Polyvinylidene Fluoride, Ultrasound-assisted, Ti-doped, Li-ion
batteries
1. Introduction
Lithium-ion batteries are regarded as the most advanced energy storage systems in recent years[1-3]. The
lithium transition metal phosphates, Li3M2(PO4)3 and systems based on these compounds, have received
considerable attention due to their great thermal stability and competitive energy density[4-8]. In general,
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NASICON-like phosphates have enhanced electronic conductivity and are also have better thermal
stabilities than the oxidative unstable lithium transition metal oxides, such as LiCoO2[9-13]. Li3V2(PO4)3,
which has a especially large theoretical capacity (197 mAhꢀg-1 in 3.2-4.8 V vs.Li or 132 mAhꢀg-1 in 3.2-4.3 V
vs.Li), and other V-based cathodes have been researched intensively these years[14-16]. Nevertheless, the low
intrinsic electronic conductivity (~10-7 Sꢀcm-1) and low Li+ diffusion coefficient (10-10~10-9 cm2 s-1) of
ꢀ
Li3V2(PO4)3 limit its practical application largely[17][18]. Surface coating, especially carbon coating, can
prevent the direct contact of the cathode material and electrolytes and was proves to be an effective way to
decrease the decomposition of the electrolytes electro-catalyzed by transition metals[19-22]. In our previous
study we discussed the influence of pyrolytic carbon of phenolic resin and PVDF on the electrochemical
performance of Li3V2(PO4)3[23][24]. The pyrolitic carbon of the polymer PVDF seems like a folded film and
forms a conductive net to enhance the electrochemical performance of Li3V2(PO4)3 efficiently[24]
.
On the same line this paper describes the preparation and electrochemical performances of Ti dopted
Li3V2(PO4)3/C and using PVDF as carbon source. Ti-Mg, Ti-Fe, Ti-Mn codoped Li3V2(PO4)3 were reported
before but the influences of single Ti to the cyrstal lattice, electroconductivity and electrochemical
performances are not researched systematically[25-26]. An novel ultrasound-assisted sol-gel method was used
and this helps to a uniform dispersion of the water insoluble PVDF. The details will be described in the
experimental part. As a result, a series of compound Li3V2-(4/3)xTix(PO4)3/C (x=0, 0.03, 0.06, 0.09, 0.12) were
synthesized and the influence of cationic substitution in vanadium sublattices and electrochemical
performances of these compounds were investigated.
2. Experimental
2.1. Synthesis of Li3V2-(4/3)xTix(PO4)3/C
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0.01 mol monoclinic Li3V2-(4/3)xTix(PO4)3/C were obtained using a ultrasound-assisted sol-gel method to
mix stoichiometric amounts of V2O5, H2C2O4, Li2CO3, NH4H2PO4, TiO2 and CTAB. TiO2 is dopant and
CTAB is cationic surfactant. Moreover, 1.0 g Polyvinylidene Fluoride (PVDF) was also added in as carbon
source. First, V2O5 and H2C2O4 in a molar ratio of 1:3 were dissolved in deionized water under magnetic
stirring at 60 ꢀ. After about half an hour a clear blue VOC2O4 solution formed
(V2O5+3H2C2O4→2VOC2O4+2CO2+3H2O). Second, Li2 CO3 and NH4H2PO4 were added in this solution in
turn. Both processes were with gas generating and a hydrophilic homogeneous colloid was formed. Third,
the aqueous suspension of Cetyltrimethyl Ammonium Bromide (CTAB, 1 mmol), TiO2 and PVDF (1.0 g),
which was obtained with the help of ultrasound, was added in the colloid. The mixture was magnetic stirred
at 60 ꢀ until the water evaporates mostly. One thing to note here is that the TiO2 and PVDF is water
insoluble, so we put them and CTAB together into the deionized water and
then stirred and ultrasonic dispersed to obtain this aqueous suspension, at 60 ꢀ. Driven by Coulomb force,
CTAB captured the anionic LVP colloid, which rearranged the charge density and formed composite
micelles in the solution[27]. Finally, the green thick sol was dried at 80 ꢀ in vacuum oven and then heated at
400 ꢀ for 5 h and 700 ꢀ for 10 h under Argon flow to synthesize the final composites.
2.2. Preparation of cathode films
The positive electrodes comprises 80-wt.% active materials, 10-wt.% carbon black and
10-wt.% polyvinylidene fluoride (PVDF). These components were mixed inN-methyl-2-pyrrolidone (NMP)
and the resulting slurry was coated onto aluminum foil. After the NMP evaporated, electrodes were pressed
and punched into a disc with an active area of 1.54 cm2 and an active loading of ~5 mg·cm-2. The precise
mass of active material on each positive plate was weighted by an electronic balance with accuracy of
0.0001 g. The electrolyte consisted of 1 M solution of LiPF6 in a mixture of 1:1 by weight of ethylene
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carbonate (EC) and dimethyl carbonate (DMC), and lithium metal was chosen as the negative material. The
coin-cells (CR2016 size) were fabricated in an argon-filled glove box with a porous polypropylene
membrane as the separator.
2.3. Sample characterization
The crystallography of the material was performed using a X-ray diffractometer (D/max 2500VB, Japan)
in the 2θ range from 10°~70°. The lattice parameters was worked out by JADE software. The morphological
characteristics were observed using SEM (QUANTA 200, USA) and TEM (JEOL-2100F, Japan). The
content of residual carbon was tested by carbon and sulfur analyzer (EMIA- 8100, Japan). The
charge-discharge performance was tested on a multi-channel battery test system (CT2001A, China) in the
voltage range of 3.0-4.3 V. Cyclic voltammetry(CV) was performed on the electrochemical workstation
(CHI660C, China) at scanning rate of 0.05 mVꢀs-1, 0.1 mVꢀs-1, 0.3 mVꢀs-1 and 0.5 mVꢀs-1. The electronic
conductivity was performed by electrical resistivity meter (GM-Π, China) and Digit multimeter (Agilent
34401A, America). The Raman spectrum was recorded using Raman spectroscopy (Raman station 400F,
America).
3. Results and discussion
3.1. Structural characterization
Fig.1a shows the XRD patterns of Li3V2-(4/3)xTix(PO4)3/C (x=0, 0.03, 0.06, 0.09, 0.12), which were marked
as A, B, C, D and E in some places below. All the diffraction peaks correspond to a monoclinic Li3V2(PO4)3
structure with the space group of P21/n (PDF#01-074-3236). Samples are found to be single phase with no
detection of impurities or disordered structures. It implies the substituted cations enter into the lattices to
form a solid solution instead of a mixture. The values of 2θ of the first eight peaks are listed in Tab.1. The
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left shift of peaks is unclear in the XRD pattern but these data shows the slight change of crystal face,
especially Li3V1.84Ti0.12(PO4)3/C. It’s worth noting that the peaks’ width is almost the same, although the
diffraction angle changes. It can be deduced that the crystal distortion is uniform[28-30]. To confirum this
conclusion, a full Rietveld refinement was carried out on all these five samples and the results are listed in
Tab.2. Their variation trends in Fig.1b show that doping makes a tiny change of unit cell for the radius of
Ti4+ (0.61Å) is very close to that of V3+(0.64Å)[31]. Furthermore, along with the growth x, the variation of a,
b, c and volume becomes larger. It also illustrates that the content increase of Ti4+ may leads to distortion of
LVP lattice.
895
894
893
892
891
90.55
90.50
90.45
x=0.12
90.40
90.35
8.620
8.615
8.610
8.605
x=0.09
x=0.06
x=0.03
8.600
12.06
12.05
12.04
x=0
8.612
8.610
8.608
8.606
8.604
8.602
8.600
PDF#01-074-3236
10
20
30
40
50
60
70
80
0.00
0.03
0.06
x
0.09
0.12
(b)
2Theta (Degree)
(a)
Fig.1 XRD pattern for the samples (a) and the variation of lattice parameters (b)
Tab.1 Diffraction angle (2θ) of the front eight crystal faces
Sample
PDF
A
B
C
D
E
(h k l)
#01-074-3236
16.272˚
(-1 1 1)
(-1 2 1)
( 0 1 2)
( 1 0 2)
16.281˚
20.7˚
16.22˚
20.58˚
16.22˚
20.598˚
21.842˚
23.018˚
16.18˚
20.599˚
21.802˚
23.001˚
16.202˚
20.562˚
21.792˚
23.002˚
20.673˚
21.952˚
23.102˚
21.894˚
23.098˚
21.841˚
23.018˚
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24.259˚
26.504˚
27.455˚
29.358˚
(-1 3 0)
(-1 3 1)
(-1 2 2)
( 2 0 2)
24.260˚
26.581˚
27.497˚
29.358˚
24.239˚
26.402˚
27.38˚
24.24˚
26.518˚
27.362˚
29.242˚
24.22˚
26.517˚
27.38˚
24.218˚
26.481˚
27.354˚
29.259˚
29.241˚
29.241˚
Tab.2 Lattice parameters and the cell volume for the samples
Samples
a(Å)
b(Å)
c(Å)
γ(˚)
Cell volume(Å3)
Li3V2(PO4)3/C
8.60861
8.60976
8.61205
8.61199
8.60144
12.0586
12.04405
12.04827
12.04714
12.03417
8.61371
8.60104
8.60613
8.61802
8.61291
90.5081
90.5388
90.5266
90.4344
90.3728
894.6
891.9
892.9
894.1
891.5
Li3V1.96Ti0.03(PO4)3/C
Li3V1.92Ti0.06(PO4)3/C
Li3V1.88Ti0.09(PO4)3/C
Li3V1.84Ti0.12(PO4)3/C
3.2. Morphological characterization
From the SEM micrographs in Fig.2(a~e) , sample Li3V2(PO4)3/C consists of particles ranging from 0.2
to 2 ꢁm (Fig.2a) and the other four consist of particles mostly smaller than 1 ꢁm (Fig.2b~e). In contrast,
the particles are smaller than that of Li3V2(PO4)3/C synthesized by solid-state method[32-36].We believe
that the sol-gel method realized the uniform mixing of raw materials in liquid phase and the
rejuvenation effect of surfactant CTAB realized the further refinement of particles. The data of tap
density are listed in Tab.2. Higher tap density means smaller voidage and more uniform particle size of
sample but this series make no great difference. HRTEM micrograph of Li3V2(PO4)3/C ( Fig.2f) shows
that the thickness of carbon layer is about 5-10 nm (part of the layer stripped from the particle in the
process of ultrasonic dispersion). The amount of carbon left in the composite was tested by
carbon-sulfur analyzer and lists in Tab.3. The electronic conductivities are listed in Tab.3 also, which
are about 6 orders of magnitude higher than that of unmodified Li3V2(PO4)3 (2×10-8 Sˑcm-1)[37], and 1~2
orders of magnitude higher than that of Li3V2(PO4)3 samples adopting other carbon sources [38][39]
.
These detail data of electronic conductivity of samples were tested according to the relationship
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, where
=1/ ρ = h / (20× R)
σ represents electronic conductivity (Sꢀ ρ represents resistivity
cm-1),
σ
(
ꢂꢀcm), h is the height of powder under a given pressure and R is powder’s resistance. It is obvious that
doping Ti4+ into the Li3V2(PO4)3/C cell enhances electronic conductivity efficiently and we believe that
the rate ability of cathode material is directly related to that. But the usage of dopant should be proper
because the cell distortion caused by overmuch dopant impedes the transmission of electrons.
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Fig.2 SEM images of Li3V2-(4/3)xTix(PO4)3/C: (a)x=0, (b)x=0.03, (c)x=0.06, (d)x=0.09, (e)x=0.12, and HRTEM image of
Li3V2(PO4)3/C (f)
Tab.3 Electronic conductivity, tap density and carbon content of samples
Sample
A
6.33×10-3
0.893
B
2.83×10-2
0.871
C
6.85×10-2
0.854
D
1.56×10-2
0.836
E
Electronic conductivity(Sꢀcm-1)
Tap density(gˑcm-3)
8.72×10-3
0.852
Carbon content (wt-%)
6.77
6.74
6.65
6.28
7.25
3-
Fig.3 represents Raman spectrums of Li3V2-(4/3)xTix(PO4)3/C samples. The PO4 valency bond vibration
occurs at 870 cm-1. The peak at 1350 cm-1 ( named as D peak) represents the amorphous carbon or sp3
carbon and at 1580 cm-1 (named as G peak) represents the high-conductive graphitized carbon or sp2 carbon.
From the Raman results we can confirm that the graphic carbon is not existed, otherwise the intensity of G
peak will be very strong. The values of ID/IG can be described as the indicator of conductivity[40]. The
3-
smaller this value is, the better the conductivity will be. Moreover, we noticed that the peaks of PO4 , sp2
and sp3 carbon move left a little bit when x=0.03~0.12.
8

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x=0
x=0.03
x=0.06
x=0.09
x=0.12
250
200
150
100
50
Disordered carbon
(D peak, 1350 cm-1)
Graphic carbon
(G peak,1560 cm-1)
(PO43-)
600
800
1000
1200
1400
1600
1800
Wavenumber (cm-1)
Fig.3 Raman spectrums of Li3V2-(4/3)xTix(PO4)3/C (x=0, 0.03, 0.06, 0.09, 0.12)
3.3. Electrochemical properties
The charge/discharge behaviors of these samples are tested in half-cell (Li/LVP-C) under different
current rate and each one was applied for 5 cycles. The potential range was restricted in 3.2-4.3 V vs. Li to
prevent the removal of third Li-ion from crystal structure which leads to crystalloggraphic deformations
leading to loss in cycleability[41]. Fig.4a shows the rate ability of samples and the capacity fading becomes
larger with increasing x, especially when the rate is more than 2 C. Sample B and C (x=0.03 & 0.06) have
outstanding rate performance that their specific capacity excesses 100 mAhꢀ
g-1 at 15 C. It means that Li-ion
batteries using them as cathode materials can be charged in about 75% of its theoretical capacity in 4
minutes (15 C) or 90% in 30 minutes (2 C). Dot graphs in Fig.4b shows the cycle performance of samples
by cycled 50 times at 5 C rate uninterruptedly. Samples A, B and C(x=0, 0.03, 0.06) have capacity retention
close to 100% indicating a highly stable intercalation host. Capacity fluctuations of sample E(x=0.12)
further proves the lattice structural instability caused by overmuch dopant.
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120
115
110
105
100
95
130
125
120
115
110
105
100
95
0.2C
1.0C
0.5C
2.0C
90
90
x=0
85
5.0C
Current density=5C
85
x=0.03
x=0.06
x=0.09
x=0.12
80
75
70
65
60
55
50
80
x=0
75
x=0.03
x=0.06
x=0.09
x=0.12
70
65
60
55
10C
15C
50
0
5
10
15
20
25
30
35
0
5
10
15
20
25
30
35
40
45
50
Cyclictimes
(a)
Cycletime
(b)
Fig.4 Discharge capacities of the Li3V2-(4/3)xTix(PO4)3/C (x=0, 0.03, 0.06, 0.09, 0.12) at different rates (a) and cyclic
performance at 5C (b)
Fig.5a~e shows the charge/discharge profiles of all samples at different current density in 3.2-4.3 V vs. Li.
Each sample, doped or not, shows three plateaus at about 3.6 V, 3.7 V and 4.1 V under small rate. Each
plateau corresponds to a phase transition between two adjacent single phases of LiyV2-(4/3)xTix(PO4)3/C (y=3.0,
2.5, 2.0, 1.0). The plateaus become inconspicuous with the increasing rate and the shape of curves is
changed by Ti4+ doping. For sample D and E (x=0.09 & 0.12), their three plateaus gradually shrink and the
transition zone between two plateaus becomes longer. This phenomenon can be attributed to the decrease of
the amount of active V3+. According to the theoretical oxidation reaction, the amount of extracted Li+ at the
same potential plateau decreases with decreasing amount of active V3+, which leads to the shrinking of the
plateaus. At the same time, the ordering of the Li+ within the structure decreases with increasing x, which
leads to the inclination of the plateaus[23]. In Fig.5e a short transition region at 3.4 V at the 0.2 C charge
curve can be observed. In addition, this curve has a small but observable fluctuation and it illustrates that the
potential can’t keep very steady when charging. These further prove its structure change and irregularity of
crystal cell of Li3V1.84Ti0.12(PO4)3/C
.
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4.4
4.2
4.0
3.8
3.6
3.4
3.2
3.0
4.4
4.2
4.0
3.8
0.2C
0.5C
1.0C
2.0C
0.2C
0.5C
1.0C
2.0C
5.0C
10C
15C
3.6
5.0C
3.4
10C
15C
3.2
3.0
0
20
40
60
80
100
120
140
0
20
40
60
80
100
120
140
-1
-1
Specificcapacity(mAh.g )
SpecificCapacity(mAh.g )
(b)
(a)
4.4
4.2
4.0
3.8
3.6
3.4
3.2
3.0
4.4
4.2
4.0
3.8
3.6
3.4
3.2
3.0
0.2C
0.5C
1.0C
2.0C
5.0C
10C
15C
0.2C
0.5C
1.0C
2.0C
5.0C
10C
15C
0
20
40
60
80
100
120
140
0
20
40
60
80
100
120
140
-1
-1
(c)
SpecificCapacity(mAh.g )
SpecificCapacity(mAh.g )
(d)
4.4
4.2
4.0
3.8
3.6
3.4
3.2
3.0
0.2C
0.5C
1.0C
2.0C
5.0C
10C
15C
0
20
40
60
80
100
120
140
-1
SpecificCapacity(mAh.g )
(e)
Fig.5 Charge/discharge curves for electrochemical cell with cathode Li3V2-(4/3)xTix(PO4)3/C: (a)x=0, (b)x=0.03,
(c)x=0.06, (d)x=0.09, (e)x=0.12
The cyclic voltammetry (CV) is widely used to study the oxidation/reduction or phase transformation
processes in electrode reactions and to estimate quantitatively the Li+ diffusion coefficient[42]. Furthermore,
11

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the electrode reaction rate is limited by the Li+ diffusion inside the electrode system[43]. Fig.6a shows the CV
curves of sample Li3V1.92Ti0.06(PO4)3/C, which owns relatively better electrochemical performances in this
series, at scanning rate 0.05 mVˑ ˑ ˑ ˑ
S-1, 0.1 mV S-1, 0.3 mV S-1, 0.5 mV S-1 between 3.0-4.3 V vs. Li. Three
pairs of anodic/catholic peaks (A1/C1, A2/C2, A3/C3) are in good agreement with the galvanostatic
charge/discharge curves. With the increasing of scanning rate, the height and area of redox peaks increase
also because the area is in direct proportion to the scanning rate. The irreversible behaviors become more
and more obvious with the increasing scanning rate and thus the potential difference of redox peaks becomes
ip = 2.69×105 n3/2 AD1/2V1/2
C
larger. From the peak current (ip) calculation formula
(at room temperature), ip has a
linear relationship with the square root of scanning rate (V1/2), as shown in Fig.6b. In addition, A is the
contact area of electrode with electrolyte (here the geometric area of electrode is used for simplicity, 1.54
cm2); n is the amount of transferred charges; C is bulk concentration of Li+(3.7×10-3 mol/cm3, calculated
from the volume of LVP , 899.8Ǻ3 [37]
)
. Based on the formula and the slope of ip vs. V1/2, the apparent
app
Li+
true
Li+
diffusion coefficient
D
(A3) and the diffusion coefficient
D
(C1) in this sample can be calculated
to be 6.38×10-6 and 4.39×10-5 cm2ꢀs-1 respectively. It is believed commonly that the Li+ diffusion coefficient
directly decides the charge/discharge capacity at large rate and this should be the reason for the nice rate
ability of sample B and C (x=0.03 & 0.06).
3.5
Scanning rate=
A3
A1
3
2
A2
0.05 mV/s
0.1mV/s
0.3mV/s
0.5mV/s
3.0
2.5
2.0
1.5
1.0
0.5
0.0
A3
C1
1
0
-1
-2
-3
i
C2
C1
C3
0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75
3.2
3.4
3.6
3.8
4.0
4.2
4.4
(V, mV/s)1/2
(b)
(a)
Potential (Li+vsLi) /V
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Fig.6 Cyclic Voltammograms at scanning rate of 0.05, 0.1, 0.3, 0.5 mV/s for Li3V1.92Ti0.06(PO4)3/C (a), and the linear
relation of the peak current (ip) and the square root of scanning rate (V1/2) (b)
4. Conclusions
In summary, the monoclinic Li3V2-(4/3)xTix(PO4)3/C with the space group of P21/n can be synthesized by an
ultrasound-assisted sol-gel method and PVDF was used as carbon source to form a electro-conductive net.
The influences of dopant TiO2 on Li3V2(PO4)3/C were analyzed and results proved that Ti4+ doping does not
much affect the size and morphology of composite particles. But proper amount of Ti4+ doping (x=0.03~0.06)
enhances the electronic conductivity and Li+ diffusion coefficient of cathode material and improves the
charge/discharge capacity under high current density. The electrochemical performances become a bit worse
when x is higher than 0.06. This may be attributed to the crystal cell distortion caused by the overmuch
dopant and proved by XRD data.
Acknowledgments
This work was funded by the National Science Foundation of China (Grant No. 51172068 and
No.51472082), the Youth Project of Education Department of Hunan province (Grant No. 14B126), the
Construct Program of the Key Discipline in Hunan Province (Mechanical Design and Theory)
(XJF2011[76]).
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Highlights
1. An ultrasound-assisted sol-gel method was used to synthesize Li3V2-(4/3)xTix(PO4)3/C.
2. PVDF is the carbon source to enhance the electro-conductivity.
3. Li3V2-(4/3)xTix(PO4)3/C has better rate and cyclic performance when x=0.03~0.06.
4. Li3V1.92Ti0.06(PO4)3/C has Li+ diffusion rate of 4.39×10-5 cm2·s-1
.