Electrochemical properties of LiMn2O4 cathode material ... - Ali Eftekhari

Journal of Alloys and Compounds 424 (2006) 225–230

Electrochemical properties of LiMn2O4 cathode material doped with an actinide Ali Eftekhari ∗ , Abdolmajid Bayandori Moghaddam, Mehran Solati-Hashjin Laboratory of Electrochemistry, Materials and Energy Research Center, P.O. Box 14155-4777, Tehran, Iran Received 26 May 2005; accepted 17 October 2005 Available online 26 January 2006

Abstract Metal substation as an efficient approach for improvement of battery performance of LiMn2 O4 was performed by an actinide dopant. Uranium as the last natural element and most common actinide was employed for this purpose. Cyclic voltammetric studies revealed that incorporation of uranium into LiMn2 O4 spinel significantly improves electrochemical performance. It also strengthens the spinel stability to exhibit better cycleability. Surprisingly, the capacity increases upon cycling of LiU0.01 Mn1.99 O4 cathode. This inverse behavior is attributed to uniform distribution of dopant during insertion/extraction process. In other words, this is an electrochemical refinement of the nanostructure which is not detectable in microscale morphology, as rearrangement of dopant in nanoscale occurs and this is an unexceptional nanostructural ordering. In addition, uranium doping strengthens the Li diffusion, particularly at redox potentials. © 2005 Elsevier B.V. All rights reserved. Keywords: Actinide alloys and compounds; Electrode materials; Solid-state reactions; Electrochemical reactions

1. Introduction Partial substitution of Mn in LiMn2 O4 spinel by other transition metals is an efficient approach to improve battery performance of this potential cathode material for lithium battery applications [1–9]. In this direction, much attention has been paid to similar transition metals such as Fe, Cr, Co, Ni, etc. In this case, a large amount of dopant is needed, e.g. 0.5 ≥ × ≥ 0.1 in LiMx Mn2−x O4 . This is accompanied by change of conventional structure of the spinel [8,9]. On the other hand, this action may lead to the formation of new redox systems displaying 5 V performance [10–18]. Although, fabrication of 5 V lithium batteries is of particular interest [18], improvement of battery performance of LiMn2 O4 cathode for 4 V operation is necessary. In other words, fabrication of 5 V batteries (or even using 5 V cathodes with 1 V anodes for 4 V batteries) cannot replace current 4 V batteries, as using 5 V cathodes needs additional advancement (e.g. regarding electrolyte solution).

∗

Corresponding author. Tel.: +98 261 621 0009; fax: +98 261 620 1888. E-mail address: [email protected] (A. Eftekhari).

0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2005.10.088

Another type of metal substitution is based on incorporation of small amounts of dopants, which does not change the original redox system of the cathode material, but just enhances the structural stability. Heavier transition metals can also be employed for metal substitution of LiMn2 O4 . However, they do not seem to be suitable alternative (because of size and cost) for Mn in LiMn2 O4 spinel. In this case, only a small amount of dopant is required, since this can strengthen the spinel stability due to stronger M–O bonds for heavy transition metals. However, less attention has been paid to this issue, and only a few reports in the literature are devoted to incorporation of transition metals of the second and third rows of the periodic table into LiMn2 O4 spinel [19,20]. Here, we aim to go further and use an actinide for metal substitution of LiMn2 O4 spinel. For this purpose, uranium was chosen as the most common elements of actinides. On the other hand, uranium is a symbolic element as it is the last natural element. 2. Experimental Uranium-substituted LiMn2 O4 spinels were synthesized by a conventional solid-state procedure. Stoichiometric amounts of the reactants (MnO2 and LiOH) were carefully mixed and appropriate amounts of uranyl acetate (Merck) were added. The mixtures were ball milled to assure uniform distribution of the small amounts of dopants. Then, the mixtures were placed in alumina crucible and

226

A. Eftekhari et al. / Journal of Alloys and Compounds 424 (2006) 225–230

the furnace temperature was slowly increased with a rate of 1 ◦ C/min to reach the reaction temperature of 800 ◦ C. This slow heating provides an opportunity for complete decomposition of raw materials before the final solid-state reaction, which was occurred at 800 ◦ C during 40 h heating. Finally, the furnace was slowly cooled to room temperature. All processes were performed in air atmosphere. The composite electrodes for electrochemical studies were prepared from mixture of active material and acetylene black in the ratio of 1:1. For casting the composite, a drop of melted paraffin was added. Typically, 3 mg of electroactive material was attached to Pt substrate electrode. Cyclic voltammetric studies were performed using a Metrohm 746VA potentiostat. The electrolyte was an aqueous solution of saturated lithium nitrate. Potentiostatic experiments were performed using a Princeton Applied Research potentiostat/galvanostat model 173 (PAR 173) equipped with a model 175 universal programmer in conjunction with CorrView software. In the experimental measurements, all potentials were referenced to a saturated calomel electrode (SCE), but the potentials were calculated in reference to conventional Li/Li+ . Scanning electron microscopic (SEM) investigations were carried out using a Cambridge scanning electron microscope model Steroscan 360. Powder X-ray diffractions (XRD) were recorded using a Phillips PW 1371 diffractometer based on Cu K␣ radiation.

3. Results and discussion Fig. 1 shows XRD patterns of different LiUx Mn2−x O4 samples as x varies from 0 to 0.1. It is obvious that incorporation of such small amounts of dopant has no significant influence on the spinel structure. The XRD patterns of LiUx Mn2−x O4 spinels can be indexed to conventional cubic spinel with space group Fd3m as well as original LiMn2 O4 . However, only a slight decrease in the lattice constant is observed. The lattice constants were 8.234 ± 0.001, 8.235 ± 0.001, 8232 ± 0.001, and 8.231 ± 0.001 for the LiUx Mn2−x O4 with x = 0, 0.01, 0.05, and 0.1, respectively. For common transition metals with ionic radii larger than Mn, the lattice constant increases by adding dopant. This inverse behavior for the case of uranium is due to the strength of bond of actinides with oxygen. This strong oxygen bond stabilizes the spinel stability, as high-temperature solid-state reaction results in uniform distribution of dopant and consequently strong U–O bond.

Fig. 1. XRD patterns of LiUx Mn2−x O4 spinels, as (a) x = 0, (b) x = 0.01, (c) x = 0.05, and (d) x = 0.1. (a)–(d) patterns are from bottom to top, respectively.

Another consequence of U substitution in LiMn2 O4 spinel is significant changes appearing in the powder morphology. According to Fig. 2, even incorporation of such a low amount of U into LiMn2 O4 lattice results in severe changes. This morphological change occurs in accordance with the amount of uranium incorporated. Virgin LiMn2 O4 has a non-uniform morphology with particles with different sizes (Fig. 2a). Whereas, U substitution is accompanied by the formation of rod-like particles, which are attached together. This phenomenon is more obvious for highest concentration of uranium (Fig. 2d). It has been reported [21] that such structure of bunch of tinier rods (of course with nanostructure) is of interest for systems involving diffusion such as Li diffusion in lithium batteries. In fact, such morphological changes indicate the importance of U substitution, even for such a low amount of dopant. Electrochemical studies of LiUx Mn2−x O4 samples can suggest potential application of these cathode materials. Thus, it is useful to investigate their electrochemical behaviors by means of cyclic voltammetry. It has been reported that electrochemical performance in conventional aqueous media can be considered as original electrochemical activity of cathode materials [22], as common non-aqueous media of lithium batteries are subject of side-processes (e.g. electrolyte instability, oxidation at electrode surface, formation of organic passive layer, etc. [23,24]). Fig. 3 presents cyclic voltammograms of three LiUx Mn2−x O4 spinels in an aqueous medium. It is observable that U incorporation does not change the redox couples of LiMn2 O4 . Another consequence of the results reported in Fig. 3 is that concentration of dopant has no influence on the operating voltage of the LiUx Mn2−x O4 cathodes. Nevertheless, increasing the amount of dopant is accompanied by weakening the electrochemical activity as the peak heights decreases. This suggests that incorporation of higher amounts of uranium dopant is not in favor of battery performance of the cathode material. Thus, we come to conclusion that the best amount of uranium dopant for improving battery performance of LiUx Mn2−x O4 cathode material is x = 0.01. Using smaller amounts is accompanied by new difficulties, which will be discussed below. Therefore, we continue the electrochemical studies for the case of LiU0.01 Mn1.99 O4 cathode material. Emerging two redox couples (as occurs for high amounts of uranium dopant) is indicative of deviation from original electrochemical activity of LiMn2 O4 . However, the main problem is the total charge passed through the system, whether during the first step or the second one. It is known that LiMn2 O4 cathodes usually achieve only about 70–80% of their theoretical capacity, and it is an important task to improve this failure [25]. By a simple comparison of the scales of CVs in Fig. 3, it can be concluded that the total charge under the redox peaks is significantly higher for the case of LiU0.01 Mn1.99 O4 cathode material. In other words, uranium doping (of course with this certain concentration) is also in favor of the practical capacity of LiMn2 O4 cathode. Of course, numerical comparison can be made by extensive battery tests, but these preliminary results indicate usefulness of uranium doping to achieve higher capacity for LiMn2 O4 -based cathode.


227

Fig. 2. SEM images of the (a) LiMn2 O4 , (b) LiU0.01 Mn1.99 O4 , (c) LiU0.05 Mn1.95 O4 , and (d) LiU0.1 Mn1.9 O4 samples.

Fig. 4a illustrates three successive cyclic voltammograms of the LiU0.01 Mn1.99 O4 cathode. A peculiar behavior is detected, as the peak currents and the total charges are subject of increase upon cycling. This is indeed an inverse behavior, as the capacity should be gradually lost upon cycling. This is a common behavior for cathode materials including LiMn2 O4 , and such ordinary behavior was also observed for other LiUx Mn2−x O4 cathodes materials synthesized. In fact, this inverse behavior just appears for the case of LiU0.01 Mn1.99 O4 . The results reported in Fig. 4b can explain the occurrence of this inverse behavior. It is obvious that the cathode material capacity increases just in the first cycles (for the system under investigation, five to six cycles). Then, the capacity decreases upon cycling in usual manner. This means that the cathode material is achieving its possible capacity during the first cycles. It has been described that LiMn2 O4 cathodes are subject of severe surface structural changes in the course of cycling particularly during first cycles [26]. This surface structural change may decrease or increase the surface roughness. In other words, successive insertion/extraction process may strengthen order or disorder depending on the current entropy of the system. Of course, this is different from the present case; as such structural changes must be occurred in the bulk, not only on the electrode surface. Since this behavior was only observed for the smallest amount of uranium dopant, it can be concluded that substitution of such

small amount of dopant is not sufficiently uniform. Thus, insertion/extraction in the first cycles provides an opportunity for rearrangement of uranium dopant throughout the cathode material. It is thought that this rearrangement occurs in nanostructure rather than inside each unit cell, as the amount of dopant is significantly lesser than the number of unit cells, and the dopant should be distributed among the material. On the other hand, electrochemical insertion/extraction of diffusing ions provides nano-channels within the electroactive film, which may results in the nanostructural changes quoted above. Of course, it is difficult to detect such rearrangement by means of spectroscopic techniques (and corresponding chemical analysis such as EDAX), as this is just in nanoscale and cannot be detected in microscale morphology. On the other hand, such large particles cannot be handled in TEM study. Of course, it is not claimed that distribution of uranium is not uniform, as a uniform distribution is expected in hightemperature treatment. In fact, this non-uniformity is related to inaccessibility of appropriate intercalating sites for Li insertion/extraction. However, successive Li insertion/extraction in the course of cycling retains such missed sites. In other words, this non-uniformity hinders formation of appropriate intercalating sites during spinel formation in the course of solid-state synthesis. We also synthesized a similar sample, but the dopant was incorporated into previously synthesized LiMn2 O4 . A vir-

228


Fig. 4. Cycleability of the LiU0.01 Mn1.99 O4 cathode. (a) Repetitive cyclic voltammetric behavior of the LiU0.01 Mn1.99 O4 cathode in LiNO3 aqueous solution with scan rate 0.1 mV/s. (b) Cycleability data for LiMn2 O4 (䊉) and LiU0.01 Mn1.99 O4 () cathodes as estimated from the total charge of each cyclic voltammogram.

Fig. 3. Cyclic voltammetric behaviors of the (a) LiMn2 O4 , (b) LiU0.01 Mn1.99 O4 , and (c) LiU0.05 Mn1.95 O4 cathodes in an aqueous solution of saturated LiNO3 . Scan rate 0.1 mV/s.

gin LiMn2 O4 spinel was synthesized, and then the uranium dopant was incorporated during an additional solid-state procedure. In this case, the peculiar behavior reported above was not observed. This suggests that smallness of the dopant is accompanied by a non-uniformity leading to inaccessibility of essential

intercalating sites, which are originally available in LiMn2 O4 spinel. To prove this hypothesis related to non-uniformity of small amounts of the uranium dopant, we also investigated smaller amounts of uranium dopant (i.e. x < 0.01) as such inverse behavior should be observed for them. As expected similar behaviors were also observed for LiUx Mn2−x O4 (where x < 0.01) spinels. Of course, improvement of the cycleability was weaker due to the smallness of the uranium dopant, and consequently, detection of the increase of capacity was difficult. In general, for small amounts of uranium dopant will be uniformly distributed across the cathode in the course of first cycles. As stated above, uranium doping decreases the lattice constant, but a peculiar behavior was observed for the case of LiU0.01 Mn1.99 O4 , as its lattice constant was higher than virgin LiMn2 O4 . Now, it can be understood in the light of the aforementioned hypothesis. In fact, non-uniformity of dopant distribution was the reason for slight increase of the lattice constant. To assure, we also measured lattice constants of all cases after five successive cycles. The lattice constant of


229

It may seem that the discussion made here are simple and speculation. It should be emphasized that this is a preliminary paper aiming to introduce new opportunities. On the other hand, the experimental results reported are strong evident for the practical interests of this novel cathode material. The reported properties of the uranium-doped LiMn2 O4 cathode are warrant of further attention. 4. Conclusion

Fig. 5. Potential dependency of the diffusion coefficient for virgin LiMn2 O4 cathode (䊉) and uranium-doped LiMn2 O4 (i.e. LiU0.01 Mn1.99 O4 ) cathode ().

LiU0.01 Mn1.99 O4 was decreased after cycling, but those of other samples were increased very slightly (it was practically difficult to be detected). In spite of the increase of capacity during first charge/ discharge cycles, uranium-doped LiMn2 O4 cathode exhibits an excellent cycleability in comparison with virgin LiMn2 O4 cathode (Fig. 4b). By considering the capacity increase, the LiU0.01 Mn1.99 O4 cathode retains approximately 100% of its initial capacity after 100 cycles. Even, by eliminating the capacity increase during the first cycles, the rate of capacity fading is very low (in comparison with other metal-substituted LiMn2 O4 cathodes reported in the literature). This is of particular interest from applied point of view. Since the amount of dopant is very low, there is no cost problem, and this is suitable for commercialization. Of course, this is just preliminary report, and further advancements are needed for this purpose. Following the strategy of this research to inspect electrochemical behavior of this novel cathode material before investigations of its battery performance, we also estimate the influence of uranium doping on Li diffusion, which is an important factor for lithium batteries. Diffusion coefficients at various potentials were estimated from chronoamperometric experiments in accordance with a method described elsewhere [15,22]. Fig. 5 shows potential dependency of diffusion coefficients for LiU0.01 Mn1.99 O4 cathode. In general, diffusion coefficient for LiU0.01 Mn1.99 O4 is higher than that for virgin LiMn2 O4 . This indicates that uranium doping is in favor of enhancement of Li diffusion, which is an important process during battery performance. The curve of potential dependency of the diffusion coefficient for LiMn2 O4 is generally associated with two minima at potentials of redox couples [27]. This phenomenon is essential for such electrochemical systems and can be explained from standpoint of statistical mechanic [28]. Since the main reaction of a cathode material occurs at redox potentials, diffusion coefficient at redox potential controls the overall rate of the reaction in the course of battery performance. Interestingly, it can be seen that decrease of the diffusion coefficients at redox potentials is weaker for the case of LiU0.01 Mn1.99 O4 .

It was demonstrated that extremely heavy metals (such as lanthanides and actinides) are also useful for metal substitution of LiMn2 O4 spinel to improve its battery performance. Although, it seems that these metals reduces the theoretical capacity and increases the lattice constant, which are not in favor of practical applications, such heavy metals can stabilize the spinel stability. On the other hand, as only a small amount of dopant is needed they do not decrease the theoretical capacity. Not only they do not increase the lattice constant, but also they decrease it. For the case of LiU0.01 Mn1.99 O4 , which was the core of the present study, higher theoretical capacity and smaller lattice constant were achieved. Since the amount of dopant is needed, this approach is also economic. References [1] Y. Shin, A. Manthiram, J. Power Sources 126 (2004) 169. [2] K. Du, J. Xie, J. Wang, H. Zhang, J. Power Sources 119–121 (2003) 130. [3] M.C. Tucker, L. Kroeck, J.A. Reimer, E.J. Cairns, J. Electrochem. Soc. 149 (2002) A1409. [4] A. Hasegawa, K. Yoshizawa, T. Yamabe, J. Electrochem. Soc. 147 (2000) 4052. [5] Y.-P. Fu, C.-S. Hsu, J. Alloys Compd. 391 (2005) 185. [6] Y. Ito, Y. Idemoto, Y. Tsunoda, N. Koura, J. Power Sources 119–121 (2003) 733. [7] R. Thirunakaran, K.-T. Kim, Y.-M. Kang, J. Young-Lee, Mater. Res. Bull. 40 (2005) 177. [8] Z. Li, J.R. Dahn, J. Electrochem. Soc. 148 (2001) A237. [9] K. Ariyoshi, Y. Iwakoshi, N. Nakayama, T. Ohzuku, J. Electrochem. Soc. 151 (2004) A296. [10] T. Ohzuku, S. Takeda, M. Iwanaga, J. Power Sources 81/82 (1999) 90. [11] H. Kawai, M. Nagata, H. Kageyama, H. Tukamoto, A.R. West, Electrochim. Acta 45 (1999) 315. [12] R. Alcantara, M. Jaraba, P. Lavela, J.L. Tirado, J. Electrochem. Soc. 151 (2004) A53. [13] K.A. Striebel, A. Rougier, C.R. Horne, R.P. Reade, E.J. Cairns, J. Electrochem. Soc. 146 (1999) 4339. [14] Y. Ein-Eli, W.F. Howard Jr., S.H. Lu, S. Mukerjee, J. McBreen, J.T. Vaughey, M.M. Thackeray, J. Electrochem. Soc. 145 (1998) 1238. [15] A. Eftekhari, J. Power Sources 124 (2004) 182. [16] B. Markovsky, Y. Talyossef, G. Salitra, D. Aurbach, H.-J. Kim, S. Choi, Electrochem. Commun. 6 (2004) 821. [17] A. Eftekhari, Chem. Lett. 33 (2004) 616. [18] A. Eftekhari, J. Power Sources 132 (2004) 240. [19] H. Tang, C.Q. Feng, Q. Fan, T.M. Lei, J.T. Sun, L.J. Yuan, K.L. Zhang, Chem. Lett. (2002) 822. [20] G.V. Subba Rao, B.V.R. Chowdari, H.J. Linder, J. Power Sources 97/98 (2001) 313. [21] A. Eftekhari, F. Moztarzadeh, M. Kazemzad, J. Phys. D: Appl. Phys. 38 (2005) 628.

230 [22] [23] [24] [25]

A. Eftekhari et al. / Journal of Alloys and Compounds 424 (2006) 225–230 A. Eftekhari, Electrochim. Acta 47 (2001) 495. D.D. MacNeil, J.R. Dahn, J. Electrochem. Soc. 150 (2003) A21. A. Eftekhari, J. Electrochem. Soc. 151 (2004) A1456. A. Eftekhari, Solid State Ionics 167 (2003) 237.

[26] A. Eftekhari, Electrochim. Acta 48 (2003) 2831. [27] J. Barker, K. West, Y. Saidi, R. Pynenbang, B. Zachau-Christiansen, R. Koksbang, J. Power Sources 54 (1995) 475. [28] A. Eftekhari, Electrochim. Acta 50 (2005) 2541.