Poster presented at the Women in Nano Winter School, Kranjska Gora, Slovenia, February 7-9, 200

Cvjetićanin, Nikola

Jančar, Boštjan

Jugović, Dragana

Mentus, Slavko

Mitrić, Miodrag

Uskoković, Dragan

Serbian Academy of Science and Arts Digital Archive (DAIS)

Ultrasound assisted synthesis of nanostructured LiFePO4 /C composite  Dragana Jugović, Miodrag Mitrić *, Nikola Cvjetićanin **, Boštjan Jančar***, Slavko Mentus**, and Dragan UskokovićInstitute of Technical Sciences of the Serbian Academy of Sciences and Arts, Belgrade, Serbia*The Vinča Institute of Nuclear Sciences, Belgrade, Serbia**Faculty of Physical Chemistry,University of Belgrade, Serbia***Jožef Stefan Institute, SloveniaABSTRACT Lithium iron phosphate has become of great interest as storage cathode for rechargeable lithium batteries because of its high energy density, low raw materials cost,  environmental friendliness and safety. At this time, the main obstacle for reaching the theoretical performances of LiFePO4 at ambient temperature is its very low electronic conductivity. Several possible means to overcome this major problem were recently explored: the synthesis of a LiFePO4 /electronic conductor composite compound, selective doping with supervalent cations to increase the intrinsic electronic conductivity, and the achievement of a small and homogeneous particle size distribution. Here we present synthesis conditions and properties of nanostructured LiFePO4 /C composite. Olivine-type lithium iron phosphate partially carbon coated particles were obtained by ultrasound assisted synthesis. A polyvinyl alcohol solution was used as the source of an in situ formed carbon. The as-prepared powders were shortly heat-treated at various temperatures under slightly reductive atmosphere. X-ray powder diffraction confirmed the phase purity. X-ray powder diffraction data were used for the crystal structure refinements, based on Rietveld full profile method. All relevant structural and microstructural crystal parameters that could be significant for electrochemical intercalation/deintercalation processes were determined. Electrochemical characteristics were evaluated by using galvanostatic charge/discharge tests. Particle morphologies of the obtained powders were determined by both scanning and transmission electron microscopies.SYNTHESISThe sonochemical reaction was carried out in a reaction vessel which was provided with two side openings for passing gas through the reaction mixture. The reaction mixture, consisting of 30 ml of 1wt% polyvinyl alcohol (PVA) aqueous solution, 25 ml of 0.2 M aqueous solution of FeSO4 *7H2 O, and appropriate amount of solid Li3 PO4 , was exposed to the ultrasound irradiation. The above mixture was sonicated for one hour by a Vibracell sonicator VCX 750 (Sonics & Materials Inc.) operating at 20 kHz, while reductive gas (75% Ar and 25% H2 ) was bubbled through it. The reaction product was centrifuged, washed with isopropanol, dried at 120 ºC under vacuum, and calcined at 600 ºC in a  slightly reductive atmosphere (95% Ar and 5 % H2 ) for 1.5 hours. s.g. Pnma LiFePO4 /CLattice parameters [Å]a = 10.3149(6)b = 5.9984(4)c = 4.7010(3)Primitive cell volume [Å3] V = 290.86Mean crystallite size [nm] 70(5)Microstrain [%] 0.23(2)Li site occ. by Fe [%] 3R factors [%] Rwp  3.2RB 2.3XRD MEASUREMENTSX-ray diffraction data were collected on a Philips PW 1050 diffractometer with Cu-Kα1,2 radiation (Ni filter) at the room temperature. Measurements were done in 2θ range of 10-120° with scanning step width of 0.02° and 12 s time per step. Crystal structure refinement was based on the Rietveld full profile method using the Koalariet computing program. The  refinement results show that sonochemical reaction gave sample that is well crystallized as single-phase phospho-olivine, without any impurities at detectable level. There is no evidence for the formation of crystalline carbon, so internal carbon could be treated as a contribution to the background. The amount of an in situ formed carbon was determined by heating the powder in air, and the estimated value was 5wt%. The structure of the LiFePO4 powder has been refined in the space group Pnma (D2h16) in olivine type. The observed and calculated X- ray diffraction profiles are given in Figure 1, while the main results of the final Rietveld refinement are presented in Table 1. The cell parameters obtained from Rietveld refinement are a = 10.3149(6) Å, b = 5.9984(4) Å, c = 4.7010(3) Å, and volume V = 290.86(3) Å3, consistent with literature data obtained for the powder synthesized by solid state reaction. The Rietveld refinement also showed additional electron density on the lithium sites, indicating 3 at.% iron on the lithium site. Microstructural parameters data showed that sonochemical method is convenient for obtaining small particles of LiFePO4 , which are suitable for ensuring a good connection with a conductive carbon.SCANNING AND TRANSMISSION ELECTRON MICROSCOPIESFigure 2a shows particle morphology of the sample revealed by scanning electron microscopy. The particles are irregular in shape and agglomerated, with mean size 200-300 nm. By comparing these results with the mean crystallite size of 70 nm obtained from XRPD analysis (Table 1) it can be concluded that polycrystalline particles are composed of large number of crystallites. More detailed analysis of particle microstructure was accomplished using TEM images. Due to pronounced particle agglomeration, grain boundaries are not quite clear, and within a relatively broad grain size distribution,  average grain size was estimated to be about 30 – 40 nm. On the thin particle edges (Figure 2b) carbon film with a typical thickness of several nanometers can be observed. We believe that this carbon layer is responsible for improved electrical conductivity.CHARGE / DISCHARGE TESTSElectrochemical performance of the sample used as a cathode of a Li-ion battery, was examined by charge-discharge tests. The cyclings were done between 2.7 and 4.1 V and the current density was 59.7 mA/g, which correspond nearly to C/3. It is interesting to notice that discharge capacity increases during cycling, starting from the value of 94.1 mAh/g for the first cycle and reaching 99.5 mAh/g at the end of the fiftieth cycle (inset of Fig.3). During structural refinement, it was shown that part of lithium sites (3%) are occupied by iron ions. Iron ion positioned on lithium site could block the whole channel and prevents lithium motion, leaving isles of LiFePO4 domains in the FePO4 surround, and causing smaller specific capacity as a result. Capacity increase observed during cycling suggests structural relaxation most likely through ordering of the iron ions. This may be the explanation for the decrease in microstrain observed in cycled material.CONDUCTIVITY MEASUREMENTSThe measured a.c. conductivity data were arranged in a standard Arrhenius plot σ = σ0exp(-Ea/kBT) (Figure 4), where Ea is the activation energy. These results show an usual increase of the conductivity with increasing temperature. The observed room temperature value was within the order of magnitude of 10-6 Scm-1, which is notably higher than 10−9 S cm−1 characteristic of the carbon-free LiFePO4 sample. Furthermore, plot of log(σ) vs. 1/T was found to be linear in the temperature range considered.Fig. 3 Cycle performance for LiFePO4 /C at C/3 rate. The inset shows capacity vs. cycle number.Table 1. The final results of the structural refinement for LiFePO4 sample.Fig. 4 Arrhenius plot of electrical conductivity for synthesized sample.Fig. 1. The observed (red), calculated (black), and difference (bottom) X-ray diffraction data of LiFePO4 taken at room temperature. Vertical markers bellow the diffraction patterns indicate positions of possible Bragg reflections.Fig. 2a SEM micrograph of composite LiFePO4 /C powder.Fig. 2b TEM image of LiFePO4 /C powder. Arrows point out 2-3 nm thick layer of amorphous carbon.CONCLUSIONWe have shown that olivine nano-structured LiFePO4 /C composite can be successfully synthesized  combining sonochemical precipitation and calcination. The main advantage of this method is the formation of an in situ C-coated LiFePO4 nanostructured powder of high purity, which provides homogeneous material with improved electrical conductivity. This method may also be suitable for selective doping with supervalent cations to increase the intrinsic electronic conductivity of LiFePO4 powder. REFERENCES-J. Moskon, R. Dominko, R. Cerc-Korosec, M. Gaberscek and J. Jamnik, J. Power Sour., (2007),  doi:10.1016/j.jpowsour.2007.06.239. -S. Andersson, B. Kalska, L. Häggström, J. O. Thomas, Solid  State Ionics, 130 (2000) 41-52.-H.M. Rietveld: J. Appl. Cryst. 2 (1969), p. 65.-S.-Y. Chung, J.T. Bloking, and Y.-M. Chiang, Nature Mater., 1, (2002) 123-128.-Z. Chen and J. R. Dahn, J. Electrochem. Soc., 149, (2002) A1184-A1189.-Marca M. Doeff, Yaoqin Hu, Frank McLarnon, and Robert Kostecki, Electrochem. Solid-State Lett., 6, (2003) A207- A209.

English

Ultrasound-assisted synthesis of nanostructured LiFePO4/C composite

Digital Repository of the Institute of Technical Sciences of SASA

Ultrasound assisted synthesis of nanostructured 
LiFePO4 /C composite  
Dragana
 
Jugović, Miodrag Mitrić *, Nikola Cvjetićanin **, Boštjan Jančar***, Slavko Mentus**, and Dragan Uskoković
Institute of Technical Sciences of the Serbian Academy of Sciences and Arts, Belgrade, Serbia
*The Vinča
 
Institute of Nuclear Sciences, Belgrade, Serbia
**Faculty of Physical Chemistry,University
 
of Belgrade, Serbia
***Jožef
 
Stefan Institute, Slovenia
ABSTRACT
 
Lithium iron phosphate has become of great interest as storage cathode for rechargeable lithium batteries because of its high energy density, low raw materials cost,
 
 
environmental friendliness and safety.
 
At this time, the main obstacle for reaching the theoretical performances of LiFePO4
 
at ambient temperature is its very low electronic conductivity.
 
Several possible means to overcome this major problem were recently explored: the synthesis of a LiFePO4
 
/electronic conductor composite compound, selective doping with supervalent
 
cations
 
to increase the intrinsic electronic conductivity, and the achievement of a small and homogeneous particle size distribution.
 
Here we present synthesis conditions and properties of 
nanostructured
 
LiFePO4
 
/C composite. Olivine-type lithium iron phosphate
 
partially
 
carbon coated particles were obtained by ultrasound assisted synthesis. A polyvinyl alcohol solution was 
used as the source of an in situ formed carbon. The as-prepared powders were shortly heat-treated at various temperatures under slightly reductive atmosphere. X-ray powder diffraction 
confirmed the phase purity. X-ray powder diffraction data were used for the crystal structure refinements, based on Rietveld
 
full profile method. All relevant structural and microstructural
 
crystal parameters that could be significant for electrochemical
 
intercalation/deintercalation
 
processes were determined. Electrochemical characteristics were
 
evaluated by using galvanostatic
 
charge/discharge tests. Particle morphologies of the obtained powders were determined by
 
both scanning and transmission electron microscopies.
SYNTHESIS
The sonochemical
 
reaction was carried out in a reaction vessel 
which was provided with two side openings for passing gas 
through the reaction mixture. The reaction mixture, consisting of 
30 ml of 1wt% polyvinyl alcohol (PVA) aqueous solution, 25 ml of
 
0.2 M aqueous solution of FeSO4
 
*7H2
 
O, and appropriate amount 
of solid Li3
 
PO4
 
, was exposed to the ultrasound irradiation. The 
above mixture was sonicated
 
for one hour by a Vibracell
 
sonicator
 
VCX 750 (Sonics & Materials Inc.) operating at 20 kHz, 
while reductive gas (75% Ar
 
and 25% H2
 
) was bubbled through it. 
The reaction product was centrifuged, washed with isopropanol, 
dried at 120 ºC under vacuum, and calcined
 
at 600 ºC in a
 
 
slightly reductive atmosphere (95% Ar
 
and 5 % H2
 
) for 1.5 hours. 
s.g. Pnma LiFePO4
 
/C
Lattice parameters 
[Å]
a = 10.3149(6)
b = 5.9984(4)
c = 4.7010(3)
Primitive cell volume 
[Å3] V = 290.86
Mean crystallite size 
[nm] 70(5)
Microstrain [%] 0.23(2)
Li site occ. by Fe [%] 3
R factors [%] Rwp  3.2RB
 
2.3
XRD MEASUREMENTS
X-ray diffraction data were collected on a Philips PW 1050 diffractometer
 
with Cu-Kα1,2
 
radiation (Ni filter) at the room temperature. Measurements were
 
done in 2θ
 
range of 10-120°
 
with scanning step width of 0.02°
 
and 12
 
s time per step. Crystal structure refinement was 
based on the Rietveld
 
full profile method using the Koalariet
 
computing program. The
 
 
refinement results show that sonochemical
 
reaction gave sample that is well crystallized as 
single-phase phospho-olivine, without any impurities at detectable level. There is no
 
evidence 
for the formation of crystalline carbon, so internal carbon could be treated as a contribution to 
the background. The amount of an in situ formed carbon was determined by heating the 
powder in air, and the estimated value was 5wt%. The structure of the LiFePO4
 
powder has 
been refined in the space group Pnma
 
(D2h16) in olivine type.
 
The observed and calculated X-
 
ray diffraction profiles are given in Figure 1, while the main results of the final Rietveld
 
refinement are presented in Table 1. The cell parameters obtained from Rietveld
 
refinement 
are a = 10.3149(6) Å, b = 5.9984(4) Å, c = 4.7010(3) Å, and volume V = 290.86(3) Å3, 
consistent with literature data obtained for the powder synthesized by solid state reaction. The 
Rietveld
 
refinement also showed additional electron density on the lithium sites, indicating 3 
at.% iron on the lithium site. Microstructural
 
parameters data showed that sonochemical
 
method is convenient for obtaining small particles of LiFePO4
 
, which are suitable for ensuring a 
good connection with a conductive carbon.
SCANNING AND TRANSMISSION ELECTRON MICROSCOPIES
Figure 2a shows particle morphology of the sample revealed by scanning electron microscopy. The 
particles are irregular in shape and agglomerated, with mean size 200-300 nm.
 
By comparing these results 
with the mean crystallite size of 70 nm obtained from XRPD analysis (Table 1) it can be concluded that 
polycrystalline particles are composed of large number of crystallites.
 
More detailed analysis of particle 
microstructure was accomplished using TEM images. Due to pronounced particle agglomeration, grain 
boundaries are not quite clear, and within a relatively broad grain size distribution,  average grain size was 
estimated to be about 30 –
 
40 nm. On the thin particle edges (Figure 2b) carbon film with a typical 
thickness of several nanometers
 
can be observed. We believe that this carbon layer is responsible for 
improved electrical conductivity.
CHARGE / DISCHARGE TESTS
Electrochemical performance of the sample used as a cathode of a
 
Li-ion battery,
 
was examined by 
charge-discharge tests. The cyclings
 
were done between 2.7 and 4.1 V and the current density
 
was
 
59.7 
mA/g, which correspond nearly to C/3. It is interesting to notice that discharge capacity increases during 
cycling, starting from the value of 94.1 mAh/g
 
for the first cycle and reaching 99.5 mAh/g
 
at the end of the 
fiftieth cycle (inset of Fig.3).
 
During structural refinement, it was shown that part of lithium sites (3%) are 
occupied by iron ions. Iron ion positioned on lithium site could
 
block the whole channel and prevents lithium 
motion, leaving isles of LiFePO4 domains in the FePO4
 
surround, and causing smaller specific capacity as 
a result. Capacity increase observed during cycling suggests structural relaxation most likely through 
ordering of the iron ions. This may be the explanation for the decrease in microstrain
 
observed in cycled 
material.
CONDUCTIVITY MEASUREMENTS
The measured a.c. conductivity data were arranged in a standard Arrhenius plot σ
 
= σ0exp(-Ea/kBT) 
(Figure 4), where Ea is the activation energy. These results show an usual increase of the conductivity with 
increasing temperature. The observed room temperature value was within the order of magnitude of 10-6 
Scm-1, which is notably higher than 10−9 S
 
cm−1 characteristic of the carbon-free LiFePO4 sample. 
Furthermore, plot of log(σ) vs. 1/T was found to be linear in the temperature range considered.
Fig. 3 Cycle performance for LiFePO4
 
/C at C/3 rate. The inset shows 
capacity vs. cycle number.
Table 1. The final results of the structural 
refinement for LiFePO4
 
sample.
Fig. 4 Arrhenius plot of electrical conductivity for 
synthesized sample.
Fig. 1. The observed (red), calculated (black), and difference (bottom) 
X-ray diffraction data of LiFePO4
 
taken at room temperature. Vertical 
markers bellow the diffraction patterns indicate positions of possible 
Bragg reflections.
Fig. 2a SEM micrograph of composite LiFePO4
 
/C 
powder.
Fig. 2b TEM image of LiFePO4
 
/C powder. Arrows point out 2-3 nm thick layer 
of amorphous carbon.
CONCLUSION
We have shown that olivine nano-structured LiFePO4
 
/C composite can be successfully synthesized
 
 
combining sonochemical
 
precipitation and calcination. The main advantage of this method is the formation 
of an in situ C-coated LiFePO4
 
nanostructured
 
powder of high purity, which provides homogeneous 
material with improved electrical conductivity. This method may also be suitable for selective doping with 
supervalent
 
cations
 
to increase the intrinsic electronic conductivity of LiFePO4
 
powder. 
REFERENCES
-J. Moskon, R. Dominko, R. Cerc-Korosec, M. Gaberscek
 
and J. Jamnik, J. Power Sour., (2007),
 
 
doi:10.1016/j.jpowsour.2007.06.239. 
-S. Andersson, B. Kalska, L. Häggström, J. O. Thomas, Solid  State Ionics, 130 (2000) 41-52.
-H.M. Rietveld: J. Appl. Cryst. 2 (1969), p. 65.
-S.-Y. Chung, J.T. Bloking, and Y.-M. Chiang, Nature Mater., 1, (2002) 123-128.
-Z.
 
Chen and J. R. Dahn, J. Electrochem. Soc., 149, (2002) A1184-A1189.
-Marca
 
M. Doeff, Yaoqin
 
Hu, Frank McLarnon, and Robert Kostecki, Electrochem. Solid-State Lett., 6, (2003) A207-
 
A209.


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Ultrasound-assisted synthesis of nanostructured LiFePO4/C composite

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