Comparative Study of Potassium Vanadate and Lithiated by fby10358

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									   Proceedings of the International Workshop “Portable and Emergency Energy Sources – from Materials to Systems”
                                        16 – 22 Sept. 2005, Primorsko, Bulgaria


     COMPARATIVE STUDY OF POTASSIUM VANADATE AND LITHIATED
      MANGANESE DIOXIDE FOR EFFECTIVE LITHIUM-ION BATTERIES

       Albena Aleksandrova, Hristina Vasilchina, Anton Momchilov, Branimir Banov*
                Institute of Electrochemistry and Energy Systems (CLEPS)
                               Bulgarian Academy of Sciences
                       *
                         Corresponding author: bbanov@cleps.bas.bg


Abstract

Lithiated manganese dioxide, Li0.3MnO2, as well as vanadium oxides, KV5O13 and K2V8O21,
were investigated as attractive 3V cathode materials for rechargeable Li-ion batteries. The
lithiated MnO2 samples were prepared by the classical solid-state reaction and the conditions
of synthesis were optimized. Two potassium vanadium oxides were obtained by Low External
Temperature Method, using one and the same starting compounds, but in different
stoichiometric ratio. The main idea of the method consists of convective dying in liquid
media, thus allowing fast evaporation of the solvent in "soft" conditions and leading to fin
powder with a spherical shape. The concentration, the temperature, the dropping speed of the
starting solution and the drop's volume, together with the density of the liquid immiscible
media and the temperature are the key factors allowing the preparation of powders with
different grain size. The physicochemical and electrochemical properties of these compounds
were investigated. They were tested electrochemically against spherical graphite as a negative
electrode. The cycling tests have displayed very good electrochemical behaviour and
reversible capacity of these active electrode materials, especially after one-month storage at
elevated temperatures.

Key words: potassium vanadates, cathode material, spherical graphite, lithium-ion batteries

1. Introduction

It is well known that the active electrode material particle size play very important role for the
electrochemical behavior and design of the cell. Lithium electrochemical systems do not defer
from this basic rule. The latest new trends in battery development are battery size diminishing
and using high current drain and fast charge/discharge rates. All these requirements can be
met using new materials or already existing but differently prepared ones. On this base we
have developed a new method for synthesis [1] of electrode materials for lithium batteries
with controlled particle size.
Cathode materials with average working voltage in the range 2.6 – 2.8 V exhibit higher
specific capacity compared to the so-called 4V materials, while the delivered energy is
relatively the same for both groups of materials [2]. Lithiated manganese dioxide, Li0.3MnO2
[3], as well as potassium vanadates, KV5O13 and K2V8O21 [4,5], are promising 3V cathode
materials. Because 3V materials initially do not include for the first lithiation they cannot be
used in Li-ion cells as 4V materials. Nevertheless, the materials have been tested
electrochemically against spherical graphite as a negative electrode.
The aim of this work is the synthesis of vanadium bronzes and Li0.3MnO2 using new methods
and studying their electrochemical behaviour vs. spherolite graphite in Li-ion laboratory cell.




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2. Experimental

Vanadium oxides, KV5O13 and K2V8O21, were obtained by Low External Temperature
Method. The method consists of a two-step process:
         - preparation of precursor by drying droplets of the solutions of KNO3 and NH4VO3
            in stoichiometric ratio K:V=1:5 for KV5O13 and K:V=1:4 for K2V8O21,
            respectively, in an immiscible with the solution liquid drying agent heated at
            2000C;
         - heating of the initial precursor in a gas phase-fluidized bed. The heating process is
            carried out in a furnace at 3000C.
After the synthesis the samples were treated with water for 24h in a hermetically sealed
autoclave at 2000C, with corresponding water vapors pressure of 40 at. As last step of the
procedure the samples were dried at 2000C.
The lithiated manganese dioxide Li0.3MnO2 sample was prepared using MnCO3 and LiNO3
as starting compounds by the following routes [6]:
         - preliminary impregnation and reaction within the entire volume of the particles;
         - grinding and homogenization of the starting precursor;
         - thermal pretreatment at 260-350oC for 24 hours
         - homogenization by grinding and final firing at 400-450oC for a period of 24 hours.
X-ray diffraction (XRD) analysis was carried out on Philips APD 15 diffractometer with
CuKα radiation.
The test electrodes were prepared from the synthesized materials as previously described [4,
7-9]. The materials were tested against lithiated spherical graphite as a negative electrode. The
spherical graphite was preliminary lithiated against Li metal electrode in a laboratory test cell,
an equivalent to CR2032 coin type. After the second lithiation the cell was disassembled in an
argon glove box. The Li metal electrode was replaced with a positive one based on vanadates
or lithiated manganese dioxide. The used electrolyte was 1M LiClO4 in EC/DMC in ratio 1:1
by volume.

3. Results and discussion

The physicochemical characteristics of Li0.3MnO2 were previously studied [3, 6] but it should
be remembered that XRD of this material at low temperatures is reported for the first time in
literature.
The XRD patterns of potassium vanadates are shown in Fig.1. The phase analysis (Fig. 1, A)
determines that besides the main synthesized phase there are impurities from other phase or
phases. Thus we have obtained potassium active material, which is a mixture of KV5O13 with
a small amount of K2V8O21 (10-15%). The pattern on Fig. 1, B suggests the opposite ratio of
the same phases (85 –90% K2V8O21 and 15-10% KV5O13). Nevertheless, we will denote
below samples A and B as KV5O13 and K2V8O21 respectively.




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                                                                                                            B



                                                           (100)




                                                                                                                                                      (100)
                                                                                                                                                                                                  A

                                           Intensity




                                                                                                                                                                2Θ 28.15
                                                                       2Θ 26.24
                                                       2Θ 11




                                                                                                                                                                             2Θ 37.9
                                                                                  2Θ 37.9




                                                               20                 40             60          80                                           20                40          60        80
                                                                                                           Two theta [degrees]


                             Fig.1. Powder X-Ray diffraction pattern of LETM synthesized K2V8O21 and KV5O13. Due to the complex
                                         structure, only the main peaks are shown 100 and the 2Θ positions of others

After LETM synthesis vanadium bronzes have been subjected to an electrochemical cycling
test. The results are summarized on Fig.2. It can be seen that KV5O13 exhibits very good
electrochemical behaviour with stable and reproducible capacity of about 150-160 mAh/g. On
the other side, K2V8O21 delivers initial capacity of 90mAh/g, which is not so stable and
decreases up to 70mAh/g at the 16th cycle.
                                                                                                                                              220
                                                                                                                  Specific capacity [mAh/g]
 Specific capacity [mAh/g]




                             180                                                                 K2V8O21
                                                                                                 KV5O13
                                                                                                                                              200              Ich= I = C/3
                             160                                                                                                                                           dis

                                                                                                                                                                                                   K2V8O21
                                                                                                                                                                                                   KV5O13



                              80                                                                                                              180


                              60
                                   0   2         4     6           8      10          12    14        16   18                                         0                       5              10        15
                                  cycle number                                                                                                                                   Cycle number
                 Fig.2. Galvanostatic cycling tests of potassium
                              vanadates at C/3 rate                                                                                            Fig.3. Specific capacity vs. cycle number of
                                                                                                                                                        treated K2V8O21 and KV5O13

In our previous works [5, 10] it has been shown that some special treatments can sometimes
improve the electrochemical behaviour. That is why we performed water treatment in
autoclave in this case. The results of the thus treated materials are presented on Fig.3. Quite
big improvement in the electrochemical capacity of both materials, especially of K2V8O21 is
observed. The delivered capacity reached twice the amount of the starting delivered capacity
before treatment. The other material, KV5O13, also increased its capacity at about 37%. The
second material was selected for further investigations.


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                                                                                         240
                                                                                                                           I c h a rg e = I d is c h

                                                                                         220
                                                                                                                                                                 I= C /6




                                                             Specific capacity [mAh/g]
                                                                                         200
                                                                                                                       K V 5O 13
                                                                                                                                                                 I= C /3
                                                                                         180

                                                                                                                                                                 I= C /6
                                                                                         160
                                                                                                                       L ix M n O          2

                                                                                                                                                                 I= C /3
                                                                                         140




                                                                                         120
                                                                                               0       10             20                 30                 40             50

                                                                                                            Cycle number [n]



                                                                                          Fig.4. Cyclability of 3 volts cathode materials

Fig.4 presents the comparative study of the electrochemical behaviour of KV5O13 and
Li0.3MnO2 against spherical graphite. Both materials are subjected to the same
charge/discharge rates. Potassium vanadate exhibits very stable electrochemical
characteristics. The delivered capacity is close to 220mAh/g. When the discharge rate is
increased up to C/3 the delivered capacity stays over 180mAh/g. The Li0.3MnO2 presented for
comparison has a little lower specific capacity than vanadate but the value is high enough for
this material. In view of the higher working voltage of lithiated manganese dioxide (2.8 V)
compared to the one of vanadate (2.6 V), both materials deliver specific energy of about 440
mWh g-1 and 550 mWh g-1 respectively (Fig. 5).
                                                                  650
                       Specific gravimetric energy [mWh/g]




                                                                  600
                                                                                                            Ic h a rg e = Id is = C th e o /3
                                                                  550


                                                                                                                                      K V5O 13
                                                                  500



                                                                  450



                                                                  400


                                                                                                                                 L ix M n O 2
                                                                  350

                                                                                                                  sto ra g e
                                                                  300



                                                                  250
                                                                                                   0   10        20               30                   40          50

                                                                                                       Cycle number [n]
            Fig.5. Cyclability of 3 volts cathode materials after 1 month of storage at 55°C

The cycling test of both materials after one-month storage at elevated temperature is presented
on Fig. 5. It is interesting to point out that both active electrode materials are not deteriorated
after storage and preserved their electrochemical behaviour.




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4. Conclusion

A new method of preparation was successfully used for the synthesis of potassium vanadates
with controlled grain size starting from hundreds to thousands of nanometers.
Due to the high homogeneity of the precursor and the small particles reacting volume, the
crystal structure of the small particle size of the synthesized material can be successfully
formed even at low temperatures in contrast to the generally used solid state reaction.
The electrochemical tests have confirmed that 3V electrode materials possess improved
energy efficiency in comparison to materials obtained by solid state reaction.

5. Acknowledgment
The authors acknowledge the financial support by the European Commission (specific
programme “Energy, Environment and Sustainable Development” – Part B: Energy program)
under contract NNE5/2002/18 “Portable and Emergency Energy Sources” for the
participation in the Workshop, by the Bulgarian Science Foundation contracts X-1323/2003,
X-1405/2004 and the WFS Planetary Emergency ”Energy” and by Planet Emergency No4
(Energy) program of World Federation of Scientists – ICSC.

6. References

1. S. Uzunova, B. Banov, B. Puresheva, A. Momchilov, Proceeding of the 2nd Workshop on
    Nanoscience and Nanotechnology, 22-23 November, Sofia, p.133.
2. Momchilov, B. Banov, A. Trifonova, B. Puresheva, “Materials for Lithium-Ion Batteries,
    NATO Science Series, Vol.85, p. 555.
3. Banov, A. Momchilov, A. Trifonova, B. Puresheva, Journal of Power Sources 81-
    82(1999)562.
4. A.Aleksandrova, S. Uzunova, B. Banov, A. Momchilov Proceeding of the International
    Workshop, 4-9 September 2004, Sofia, P3-1.
5. A.Aleksandrova, S. Uzunova, T. Stankulov, A. Momchilov, NATO-ASI “Functional
    Properties of Nanostructured Materials”, Proceeding, 3-14 June 2005, Sozopol, in press.
6. N.Velinova, B. Banov, 6 th Workshop Nanoscience and Nanotechnology, 24 - 27 Nov.
    2004, Sofia, Bulgaria, in press.
7. N. Ilchev, B. Banov, R. Kvachkov, "Physicochemical characteristics and electrochemical
    behavior of MnO2 in primary lithium cells", 6IMLB Ext. Abstr. p.416
8. N. Ilchev, B. Banov, Y. Bourilkov, "Reversible cathode material obtained by
    electrochemical reduction of MnO2 in nonaqueous electrolyte" 6IMLB Ext. Abstr. p.329.
9. N. Ilchev, B. Banov, "Research Studies of the Li/MnO2 System", Progress in Batt. &
    Batt. Materials, JEC Press, Vol.10(1991)232.
10. V. Manev, A. Momchilov, A. Nassalevska, G. Pistoia, M. Pasquali, Journal of Power
    Sources 54 (1995) 501-507.




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