Mixed-metals codeposition as a novel method for the preparation of LiMn2O4 electrodes with reduced capacity fades

Article abstract of DOI:10.1149/1.1580817

Mixed-metals codeposition was introduced as an efficient method for the deposition of cathode materials onto substrate surfaces. The approach proposed was typically used for the deposition of LiMn₂O₄ with codeposition of mixed gold-cobalt. The results obtained for the system under investigation showed that the method employed is applicable for lithium secondary battery performance. In addition to the efficient deposition of the cathode material which is of interest for battery applications, the approach was used to incorporate cobalt oxide into the cathode material. It was demonstrated that the metal oxide deposited with the cathode material significantly reduces the capacity fade of LiMn₂O₄ in both room and elevated temperatures.

Full text of DOI:10.1149/1.1580817

A966
Journal of The Electrochemical Society, 150 ͑7͒ A966-A969 ͑2003͒
0013-4651/2003/150͑7͒/A966/4/$7.00 © The Electrochemical Society, Inc.
Mixed-Metals Codeposition as a Novel Method
for the Preparation of LiMn2O4 Electrodes
with Reduced Capacity Fades
,z
*
Ali Eftekhari
Electrochemical Research Center, Tehran, Iran
Mixed-metals codeposition was introduced as an efficient method for the deposition of cathode materials onto substrate surfaces.
The approach proposed was typically used for the deposition of LiMn2O4 with codeposition of mixed gold-cobalt. The results
obtained for the system under investigation showed that the method employed is applicable for lithium secondary battery perfor-
mance. In addition to the efficient deposition of the cathode material which is of interest for battery applications, the approach was
used to incorporate cobalt oxide into the cathode material. It was demonstrated that the metal oxide deposited with the cathode
material significantly reduces the capacity fade of LiMn2O4 in both room and elevated temperatures.
© 2003 The Electrochemical Society. ͓DOI: 10.1149/1.1580817͔ All rights reserved.
Manuscript submitted October 28, 2002; revised manuscript received February 3, 2003. Available electronically May 30, 2003.
as 1 ␮m can be prepared by this synthesis method,25 small particles
LiMn2O4 is one of the most promising candidates for cathode
materials for lithium secondary batteries due to its advantages such
as low cost, nontoxicity, etc. However, the main disadvantage of
LiMn2O4 , which made it unsuitable for commercial performances,
is its capacity fade during cycling, particularly at elevated tempera-
tures, T ϭ 55°C. Several factors have been reported to be respon-
sible for this behavior: manganese dissolution,1,2 formation of oxy-
gen deficiency,3 electrolyte decomposition,4 Jahn-Teller distortion,5
cation mixing between lithium and manganese,6 and loss of crystal-
linity during cycling.7 Capacity fade of LiMn2O4 at room tempera-
ture can be successfully reduced by cationic substituting the
manganese.8-10 However, the problem of capacity fade at elevated
temperatures is still under investigation. Both chemical and physical
modification of LiMn2O4 with LiCoO2 has been reported to improve
the capacity fading of LiMn2O4 at elevated temperatures.11-13 Other
materials have also been recently reported as candidates of modifiers
to improve capacity fading of LiMn2O4 .14-16
For the practical performance of lithium-secondary batteries, it is
needed to prepare thin films of cathode materials. To this end, sev-
eral methods have been proposed and used for the deposition of
cathode materials. These usual methods for the deposition of cath-
ode materials are based on highly technological techniques such as
electron-beam evaporation,17 rf magnetron sputtering,18 pulsed laser
deposition,19 and chemical vapor deposition ͑CVD͒,20 etc. The main
advantage of these methods is the possibility of adding cation dop-
ants to reduce the capacity fade of LiMn2O4 . However, there is
great interest in finding novel approaches with some special capa-
bilities or simpler procedures.
͑smaller than 100 nm͒ should be used for the present study, as large
particles are not suitable for the codeposition method.26 A platinum
electrode was used as the substrate surface. The deposition of
LiMn2O4 was carried out during codeposition of gold-cobalt on the
substrate electrode. In this case, the electroactive material is at-
tached to the substrate surface as the result of occlusion between the
gold particles deposited. This is an efficient deposition method, as
gold does not provide any electrochemical reaction in the potential
range of our experiments. This process was performed using a so-
lution containing 15 g/L potassium gold cyanide, 80 g/L monopo-
tassium phosphate, 70 g/L potassium citrate, and 1.5 g/L cobalt chlo-
ride with near-neutral pH which was prepared as the deposition
electrolyte. At this pH, cobalt is hydrolyzed to cobalt hydroxide
during the deposition process. The cobalt hydroxide generated dur-
ing the deposition process is transformed to cobalt oxide as the
result of heating in the final step. Although, it is mainly transformed
to the metal oxide, it is hard to claim the absence of any metal
hydroxide. Of course, the existence of metal hydroxide does not
make any problem, as metal hydroxide can improve capacity fading
of LiMn2O4 as well as metal oxides.16 Hydrolization and oxidation
of cobalt during electrodeposition have been extensively described
in the literature.27 On the other hand, metal hydroxides usually exist
in oxide materials; however, we refer the generated cobalt com-
pound as cobalt oxide. The amount of cobalt incorporated was about
4% ͑w/w͒.
A small amount of LiMn2O4 ͑ca. 40 ␮g͒ was also added to the
deposition solution ͑with volume of ca. 5 mL͒. The electrolyte so-
lution was stirred for a few minutes before the deposition process to
reach a well-conditioned suspension. The deposition process was
carried out by applying a cathodic current of 4.0 mA in a warm bath
͑50°C͒. The electrode was washed thoroughly to remove any potas-
sium ion from its surface. Finally, the electrode was heated at 200°C
for 1 h to remove water molecules included to the electroactive film.
The addition of a small amount of cobalt oxide does not make any
noticeable change in the electrochemical behavior and spectroscopic
characteristics of LiMn2O4 , and the results were approximately
similar to those reported for the LiMn2O4 prepared by gold
codeposition.22,23 However, the XRD patterns in the discharged state
after cycling at 60°C for over 100 cycles displays significant peak
broading, indicating structural degradation and a decrease in crys-
tallinity for the LiMn2O4 electrode prepared by the gold codeposi-
tion method, whereas such behavior disappeared for the LiMn2O4
electrode prepared by the mixed-metals ͑gold-cobalt͒ codeposition
method.
In previous works,21,22 we have successfully used a gold codepo-
sition method23 for the preparation of LiMn2O4 cathodes. Here, we
extend this method as an efficient and useful approach for the depo-
sition of LiMn2O4 for lithium battery applications with the possibil-
ity of adding modifiers. In addition to the advantages of available
methods ͑with the possibility of adding dopants͒, the approach pro-
posed here is very simple. Indeed, it is based on a simple electro-
chemical codeposition process, which is indicative of the powerful-
ness of electrochemical methods. As a typical investigation, the
approach was successfully used for the preparation of a LiMn2O4
electrode using a typical metal oxide dopant with excellent electro-
chemical properties for battery purposes.
Experimental
LiMn2O4 spinel was synthesized in a similar manner to previous
work24 with a nanostructure. Although, LiMn2O4 particles as large
The amount of the deposits can be simply determined by weigh-
ing the electrode before and after the deposition process. However,
this amount is related to the mass of the total deposits including the
Au ͑and also cobalt oxide͒ codeposited. The amount of Au codepos-
* Electrochemical Society Active Member.
z E-mail: eftekhari@elchem.org
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Journal of The Electrochemical Society, 150 ͑7͒ A966-A969 ͑2003͒
A967
Figure 2. Capacity as a function of cycle number for the cells with cathodes
of the LiMn2O4 electrode ͑᭺͒ and the cobalt oxide-included LiMn2O4 elec-
trode ͑ᮀ͒ at room temperature.
to the existence of gold within the LiMn2O4 film deposited. As the
spinel particles are incorporated within the Au film deposited, some
active sites of LiMn2O4 are covered by the gold codeposit. This
means that all of the active sites of LiMn2O4 are not able to partici-
pate in the intercalation/deintercalation processes and the electrode
capacity is less than the theoretical capacity of LiMn2O4 . As gold
codeposition is a new method for the deposition of LiMn2O4 , fur-
ther investigations are required to achieve optimum conditions to
gain better battery performances. For example, it has been described
that the capacity of the LiMn2O4 electrode prepared based on gold
codeposition can be improved by 3D deposition of the electroactive
film where more active sites of LiMn2O4 are available to participate
in the intercalation/deintercalation processes.28
Figure 2 illustrates the variation in capacity of the LiMn2O4
electrodes prepared by the gold codeposition and gold-cobalt
codeposition methods during the charge/discharge cycling in the po-
tential range of 3.0-4.3 V with the rate of C/5 at room temperature.
As expected, the LiMn2O4 electrode exhibits a significant loss in the
electrode capacity over 100 cycles indicating a significant capacity
fade, whereas the cobalt oxide-included LiMn2O4 electrode has an
excellent cycleability over 100 cycles. It is obvious here that the
initial capacity of the cobalt oxide-included LiMn2O4 electrode is
higher than that of the LiMn2O4 electrode.
To investigate the capacity fading phenomenon of the LiMn2O4
electrode prepared using the mixed-metals ͑gold-cobalt͒ codeposi-
tion method, the experiments were carried out at an elevated tem-
perature of 60°C ͑Fig. 3͒. The results obviously show that capacity
fade of the LiMn2O4 electrode prepared by the gold-cobalt codepo-
sition method is significantly lesser than that of the LiMn2O4 elec-
trode simply prepared by gold codeposition. Contrary to room tem-
perature, the initial capacity of the cobalt oxide-included LiMn2O4
electrode at the elevated temperature is lower than that of the
LiMn2O4 electrode. As expected, the initial capacities of both elec-
trodes increases by increasing the temperature. However, this in-
crease is stronger for the conventional ͑unmodified͒ LiMn2O4 elec-
trode in comparison with the cobalt oxide-included LiMn2O4
electrode. This can be recognized from a comparative investigation
of the discharge profiles recorded at room and elevated tempera-
tures, because the initial capacity of the LiMn2O4 electrode was
significantly increases, whereas the initial capacity of the cobalt
oxide-included LiMn2O4 was a little different for the experiments
performed at room and elevated temperatures.
Figure 1. Charge/discharge profiles for the cells with cathodes of ͑a͒ the
LiMn2O4 electrode and ͑b͒ the cobalt oxide-included LiMn2O4 electrode
recorded with the rate of C/5 at room temperature presented for different
cycles ͑1, 50, and 100͒.
ited is controlled by the deposition conditions. In our experience,
this amount is approximately equal to the amount of LiMn2O4 at-
tached to the substrate surface. As in the case of the gold-
codeposition method in the absence of cobalt, the amount of
LiMn2O4 attached to the substrate surface was 13 ␮g Ϯ 0.2 and the
film thickness estimated by scanning electron microscopy ͑SEM͒
was about 2.2 ␮m.
Results and Discussion
Figure 1 shows typical charge/discharge characteristics of the
LiMn2O4 electrodes prepared based on the gold codeposition and
mixed-metals ͑gold-cobalt͒ codeposition methods with the rate of
C/5. It can be simply concluded from a comparative investigation of
the curves that the mixed-metals ͑i.e., gold-cobalt͒ codeposition
method improves the electrochemical behavior of LiMn2O4 to gain
better battery performance. Interestingly, the first capacity of the
cobalt oxide-included LiMn2O4 electrode is higher than that of the
LiMn2O4 electrode. This is very interesting, as modification of
LiMn2O4 with LiCoO2 decreases the initial capacity of LiMn2O4 .16
One may think the capacities reported here for LiMn2O4 elec-
trodes are lower than usual for LiMn2O4 electrodes and the theoret-
ical capacity of LiMn2O4 . It is thought that this distinction is related
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A968
Journal of The Electrochemical Society, 150 ͑7͒ A966-A969 ͑2003͒
Figure 3. Capacity as a function of cycle number for the cell with cathodes
of the LiMn2O4 electrode ͑᭺͒ and the cobalt oxide-included LiMn2O4 elec-
trode ͑ᮀ͒ at 60°C.
Figure 5. Discharge capacity vs. number of cycles for the cobalt oxide-
included LiMn2O4 electrode cycled at room temperature, which was stored
in the charged ͑᭺͒ and discharged ͑᭹͒ states for 5 days at 70°C.
Due to the low rate capability of LiMn2O4 in comparison with
LiCoO2 , in order to use LiMn2O4 as a cathode material ͑an alter-
native to LiCoO2), it is also necessary to improve its rate capability.
To this aim, the rate capabilities of the LiMn2O4 electrodes prepared
by the gold-cobalt codeposition and gold codeposition methods were
investigated and compared ͑Fig. 4͒. The results are indicative that
gold-cobalt codeposition significantly improves the rate capability
of LiMn2O4 , which is desirable for its practical performance.
In addition to low cycleability of LiMn2O4 at elevated tempera-
ture, capacity fading during storage at elevated temperature is also
an important obstacle in the commercialization of LiMn2O4 as a
cathode material for lithium secondary batteries. Figure 5 presents
the capacity loss of the cobalt oxide-included LiMn2O4 electrode
after 5 days storage at the elevated temperature of 70°C. As ex-
pected, the gold-cobalt codeposition method also reduces the capac-
ity fading of LiMn2O4 . The electrode stored at the charged and
discharged states shows less than 2 and 3% irreversible capacity
losses over ten charge/discharge cycles, respectively.
Among different possible reasons for capacity fade of LiMn2O4 ,
the main reason is Mn dissolution.29 Indeed, the instability of
LiMn2O4 in the electrolyte solution at elevated temperatures is re-
lated to the disproportionation reaction, 2 Mn3ϩ (insoluble)
→ Mn4ϩ (insoluble) ϩ MnO.30 To overcome this problem, Guyo-
mard and Tarascon31 have described that reducing the surface area
of LiMn2O4 cathodes is the easiest way to decrease Mn dissolution.
However, the best approach is to cover the spinel surface with an
insoluble material, which is usually referred to as surface modifica-
tion. As the cobalt oxide is formed on the depositing LiMn2O4 par-
ticles, surface modification of the spinel with the insoluble oxide is
responsible for the reduced capacity fade of the LiMn2O4 cathode.
The improvement in cycling performance of LiMn2O4 provided by
surface modification can be explained within the context of the dis-
solution mechanism.
We investigated the amount of Mn dissolved during 24 h storage
at 70°C in 10 mL of the electrolyte solution for both LiMn2O4
electrodes. The results showed that 1.87 ␮g Mn is dissolved from
the unmodified LiMn2O4 electrode, prepared by gold codeposition,
where this value is 0.36 ␮g for the modified LiMn2O4 electrode,
prepared by gold-cobalt codeposition. This provides strong evidence
for the fact that surface modification of LiMn2O4 with cobalt oxide
codeposited reduces the Mn dissolution and consequently the capac-
ity fading of the spinel.
Conclusions
A novel method was proposed for the deposition of LiMn2O4 on
substrate surfaces to prepare thin-film cathodes. The method is very
simple in comparison with nonelectrochemical methods for the
preparation of LiMn2O4 thin-film cathodes. It is of interest that the
approach proposed can also be used for other cathode materials to
be deposited on substrate surfaces to fabricate thin-film electrodes.
Another important advantage of the method of mixed metals
codeposition proposed here is its possibility to incorporate modifiers
into the cathode material. The cobalt oxide-included LiMn2O4 elec-
trode prepared based on this method was suitable for the practical
performances, as the electrochemical behavior and cycleability of
LiMn2O4 deposited were significantly enhanced. It was accompa-
nied by high cyclability at elevated temperatures, reduction of ca-
pacity fading during storage at elevated temperatures, and enhance-
ment of the rate capability.
Figure 4. Variations of normalized discharge capacity for the cells with
cathodes of the LiMn2O4 electrode ͑᭹͒ and the cobalt-included LiMn2O4
electrode ͑᭿͒ as a function of charge-discharge rate recorded at room tem-
perature.
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Journal of The Electrochemical Society, 150 ͑7͒ A966-A969 ͑2003͒
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15. D. Zhang, B. N. Popov, and R. E. White, J. Power Sources, 76, 81 ͑1998͒.
References
16. A. M. Kannan and A. Manthiram, Electrochem. Solid-State Lett., 5, A167 ͑2002͒.
17. F. K. Shokoohi, J. M. Tarascon, B. J. Wikens, D. Guyomard, and C. C. Chang, J.
Electrochem. Soc., 139, 1845 ͑1992͒.
18. K.-H. Hwang, S.-H. Lee, and S.-K. Joo, J. Electrochem. Soc., 141, 3296 ͑1994͒.
19. A. Rougier, K. A. Striebel, S. J. Wen, and E. J. Cairns, J. Electrochem. Soc., 145,
2975 ͑1998͒.
20. P. Fragnaud, R. Nagarajan, D. M. Schleich, and D. Vujic, J. Power Sources, 54,
362 ͑1995͒.
21. A. Eftekhari, Electrochim. Acta, 47, 4347 ͑2002͒; Erratum: A. Efekhari, Electro-
chim. Acta, 48, 290 ͑2002͒.
22. A. Eftekhari, J. Electrochem. Soc., Accepted.
23. M.-R. Lim, W.-I. Cho, and K.-B. Kim, J. Power Sources, 92, 168 ͑2001͒.
24. A. Eftekhari, Electrochim. Acta, 47, 495 ͑2001͒.
25. D. S. Ahn and M. Y. Song, J. Electrochem. Soc., 147, 874 ͑2000͒.
26. P. M. Vereecken, I. Shao, and P. C. Searson, J. Electrochem. Soc., 147, 2572
͑2000͒.
27. I. G. Casella and M. R. Guasito, J. Electroanal. Chem., 476, 54 ͑1999͒.
28. A. Eftekhari, Solid State Ionics, Submitted.
1. A. Blyr, C. Sigala, G. G. Amatucci, D. Guyomard, Y. Chabre, and J. M. Tarascon,
J. Electrochem. Soc., 145, 194 ͑1998͒.
2. X. Sun, H. S. Lee, X. Q. Yang, and J. McBreen, Electrochem. Solid-State Lett., 4,
A184 ͑2001͒.
3. Y. Xia, T. Sakai, T. Fujieda, X. Q. Yang, X. Sun, Z. F. Ma, and J. McBreen, J.
Electrochem. Soc., 148, A723 ͑2001͒.
4. G. G. Amatucci, C. N. Schmutz, A. Blyr, C. Sigala, A. S. Gozdz, D. Larcher, and
J. M. Tarascon, J. Power Sources, 69, 11 ͑1997͒.
5. M. M. Thackeray, Y. Shao-Horn, A. J. Kahaian, K. D. Kepler, E. Skinner, J. T.
Vaughey, and S. J. Hackney, Electrochem. Solid-State Lett., 1, 7 ͑1998͒.
6. J. M. Tarascon, F. Coowar, G. Amatucci, F. K. Shokoohi, and D. Guyomard, J.
Electrochem. Soc., 141, 1421 ͑1994͒.
7. H. Huang, C. A. Vincent, and P. G. Bruce, J. Electrochem. Soc., 146, 3649 ͑1999͒.
8. I. J. Davidson, R. S. McMillan, and J. J. Murrar, J. Power Sources, 54, 205 ͑1995͒.
9. M. Yoshio, Y. Xia, N. Kumada, and S. Ma, J. Power Sources, 101, 79 ͑2001͒.
10. S. Myung, S. Komaba, and N. Kumagai, J. Electrochem. Soc., 148, A482 ͑2001͒.
11. J. Cho, G. B. Kim, H. S. Lim, C. Kim, and S. Yoo, Electrochem. Solid-State Lett.,
2, 607 ͑1999͒.
29. P. Arara, R. E. White, and M. Doyle, J. Electrochem. Soc., 145, 3647 ͑1998͒ and
references cited therein.
12. S. Park, Y. Han, Y. Kang, P. S. Lee, S. Ahn, H. Lee, and J. Lee, J. Electrochem.
Soc., 148, A680 ͑2001͒.
30. M. M. Thackeray, P. J. Johnson, L. A. De Picciotto, P. G. Bruce, and J. B. Good-
enough, Mater. Res. Bull., 19, 179 ͑1984͒.
13. Z. Li, H. Wang, L. Fang, J. Y. Lee, and L. M. Gan, J. Power Sources, 104, 101
͑2002͒.
14. P. Arora, B. N. Popov, and R. E. White, J. Electrochem. Soc., 145, 807 ͑1998͒.
31. D. Guyomard and J.-M. Tarascon, J. Power Sources, 54, 92 ͑1995͒.
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