Electrochemical properties of LiMn2O4 cathode material doped with an actinide

Electrochemical properties of LiMn2O4 cathode material doped with an actinide
Electrochemical properties of LiMn2O4 cathode material doped with an actinide

Journal of Alloys and Compounds424(2006)

225–230

Electrochemical properties of LiMn2O4cathode

material doped with an actinide

Ali Eftekhari?,Abdolmajid Bayandori Moghaddam,Mehran Solati-Hashjin

Laboratory of Electrochemistry,Materials and Energy Research Center,P.O.Box14155-4777,Tehran,Iran

Received26May2005;accepted17October2005

Available online26January2006

Abstract

Metal substation as an ef?cient approach for improvement of battery performance of LiMn2O4was 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 LiMn2O4spinel signi?cantly improves electrochemical performance.It also strengthens the spinel stability to exhibit better cycleability. Surprisingly,the capacity increases upon cycling of LiU0.01Mn1.99O4cathode.This inverse behavior is attributed to uniform distribution of dopant during insertion/extraction process.In other words,this is an electrochemical re?nement 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.

?2005Elsevier B.V.All rights reserved.

Keywords:Actinide alloys and compounds;Electrode materials;Solid-state reactions;Electrochemical reactions

1.Introduction

Partial substitution of Mn in LiMn2O4spinel by other tran-sition metals is an ef?cient approach to improve battery per-formance 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 LiM x Mn2?x O4.This is accompanied by change of conven-tional structure of the spinel[8,9].On the other hand,this action may lead to the formation of new redox systems displaying 5V performance[10–18].Although,fabrication of5V lithium batteries is of particular interest[18],improvement of battery performance of LiMn2O4cathode for4V operation is neces-sary.In other words,fabrication of5V batteries(or even using 5V cathodes with1V anodes for4V batteries)cannot replace current4V batteries,as using5V cathodes needs additional advancement(e.g.regarding electrolyte solution).

?Corresponding author.Tel.:+982616210009;fax:+982616201888.

E-mail address:eftekhari@merc.ac.ir(A.Eftekhari).

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 struc-tural stability.Heavier transition metals can also be employed for metal substitution of LiMn2O4.However,they do not seem to be suitable alternative(because of size and cost)for Mn in LiMn2O4 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 LiMn2O4spinel[19,20]. Here,we aim to go further and use an actinide for metal substi-tution of LiMn2O4spinel.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 LiMn2O4spinels were synthesized by a conventional solid-state procedure.Stoichiometric amounts of the reactants(MnO2and 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

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

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 ?nal solid-state reac-tion,which was occurred at 800?C during 40h 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 paraf?n was added.Typically,3mg of electroac-tive material was attached to Pt substrate electrode.Cyclic voltammetric studies were performed using a Metrohm 746V A 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 175universal programmer in conjunction with CorrView software.In the experimental measurements,all potentials were referenced to a saturated calomel electrode (SCE),but the potentials were calcu-lated 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 1371diffractometer based on Cu K ?radiation.

3.Results and discussion

Fig.1shows XRD patterns of different LiU x Mn 2?x O 4sam-ples as x varies from 0to 0.1.It is obvious that incorporation of such small amounts of dopant has no signi?cant in?uence on the spinel structure.The XRD patterns of LiU x Mn 2?x O 4spinels can be indexed to conventional cubic spinel with space group Fd 3m as well as original LiMn 2O 4.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±0.001for the LiU x Mn 2?x O 4with x =0,0.01,0.05,and 0.1,respec-tively.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 LiU x Mn 2?x O 4spinels,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 LiMn 2O 4spinel is signi?cant changes appearing in the powder morphology.According to Fig.2,even incorporation of such a low amount of U into LiMn 2O 4lattice results in severe changes.This morpho-logical change occurs in accordance with the amount of uranium incorporated.Virgin LiMn 2O 4has a non-uniform morphology with particles with different sizes (Fig.2a).Whereas,U sub-stitution is accompanied by the formation of rod-like particles,which are attached together.This phenomenon is more obvi-ous 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 morpho-logical changes indicate the importance of U substitution,even for such a low amount of dopant.

Electrochemical studies of LiU x Mn 2?x O 4samples can sug-gest 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 elec-trochemical performance in conventional aqueous media can be considered as original electrochemical activity of cathode mate-rials [22],as common non-aqueous media of lithium batteries are subject of side-processes (e.g.electrolyte instability,oxi-dation at electrode surface,formation of organic passive layer,etc.[23,24]).Fig.3presents cyclic voltammograms of three LiU x Mn 2?x O 4spinels in an aqueous medium.It is observ-able that U incorporation does not change the redox couples of LiMn 2O 4.

Another consequence of the results reported in Fig.3is that concentration of dopant has no in?uence on the operating volt-age of the LiU x Mn 2?x O 4cathodes.Nevertheless,increasing the amount of dopant is accompanied by weakening the electro-chemical 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 LiU x Mn 2?x O 4cathode material is x =https://www.360docs.net/doc/8112133042.html,ing smaller amounts is accompanied by new dif?culties,which will be discussed below.There-fore,we continue the electrochemical studies for the case of LiU 0.01Mn 1.99O 4cathode material.

Emerging two redox couples (as occurs for high amounts of uranium dopant)is indicative of deviation from original electro-chemical activity of LiMn 2O 4.However,the main problem is the total charge passed through the system,whether during the ?rst step or the second one.It is known that LiMn 2O 4cathodes usu-ally 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 signi?cantly higher for the case of LiU 0.01Mn 1.99O 4cathode material.In other words,uranium doping (of course with this certain concentration)is also in favor of the practical capacity of LiMn 2O 4cathode.Of course,numerical comparison can be made by extensive bat-tery tests,but these preliminary results indicate usefulness of uranium doping to achieve higher capacity for LiMn 2O 4-based cathode.

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

227

Fig.2.SEM images of the (a)LiMn 2O 4,(b)LiU 0.01Mn 1.99O 4,(c)LiU 0.05Mn 1.95O 4,and (d)LiU 0.1Mn 1.9O 4samples.

Fig.4a illustrates three successive cyclic voltammograms of the LiU 0.01Mn 1.99O 4cathode.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 behav-ior for cathode materials including LiMn 2O 4,and such ordinary behavior was also observed for other LiU x Mn 2?x O 4cathodes materials synthesized.In fact,this inverse behavior just appears for the case of LiU 0.01Mn 1.99O 4.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 ?rst cycles (for the system under investigation,?ve to six cycles).Then,the capacity decreases upon cycling in usual manner.This means that the cathode material is achiev-ing its possible capacity during the ?rst cycles.It has been described that LiMn 2O 4cathodes are subject of severe surface structural changes in the course of cycling particularly during ?rst 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 suf?ciently uniform.Thus,inser-tion/extraction in the ?rst cycles provides an opportunity for rearrangement of uranium dopant throughout the cathode mate-rial.It is thought that this rearrangement occurs in nanostructure rather than inside each unit cell,as the amount of dopant is sig-ni?cantly 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 ?lm,which may results in the nanostructural changes quoted above.Of course,it is dif?cult to detect such rearrangement by means of spectroscopic tech-niques (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 high-temperature treatment.In fact,this non-uniformity is related to inaccessibility of appropriate intercalating sites for Li inser-tion/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 interca-lating 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 LiMn 2O 4.A vir-

228 A.Eftekhari et al./Journal of Alloys and Compounds424(2006)

225–230

Fig.3.Cyclic voltammetric behaviors of the(a)LiMn2O4,(b)LiU0.01Mn1.99O4, and(c)LiU0.05Mn1.95O4cathodes in an aqueous solution of saturated LiNO3. Scan rate0.1mV/s.

gin LiMn2O4spinel was synthesized,and then the uranium dopant was incorporated during an additional solid-state proce-dure.In this case,the peculiar behavior reported above was not observed.This suggests that smallness of the dopant is accompa-nied by a non-uniformity leading to inaccessibility of

essential Fig.4.Cycleability of the LiU0.01Mn1.99O4cathode.(a)Repetitive cyclic voltammetric behavior of the LiU0.01Mn1.99O4cathode in LiNO3aqueous solution with scan rate0.1mV/s.(b)Cycleability data for LiMn2O4(?)and LiU0.01Mn1.99O4( )cathodes as estimated from the total charge of each cyclic voltammogram.

intercalating sites,which are originally available in LiMn2O4 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 behav-ior should be observed for them.As expected similar behaviors were also observed for LiU x 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,detec-tion of the increase of capacity was dif?cult.In general,for small amounts of uranium dopant will be uniformly distributed across the cathode in the course of?rst cycles.

As stated above,uranium doping decreases the lattice con-stant,but a peculiar behavior was observed for the case of LiU0.01Mn1.99O4,as its lattice constant was higher than vir-gin LiMn2O4.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?ve successive cycles.The lattice constant of

A.Eftekhari et al./Journal of Alloys and Compounds424(2006)225–230

229

Fig.5.Potential dependency of the diffusion coef?cient for virgin LiMn2O4 cathode(?)and uranium-doped LiMn2O4(i.e.LiU0.01Mn1.99O4)cathode( ). LiU0.01Mn1.99O4was decreased after cycling,but those of other samples were increased very slightly(it was practically dif?cult to be detected).

In spite of the increase of capacity during?rst charge/ discharge cycles,uranium-doped LiMn2O4cathode exhibits an excellent cycleability in comparison with virgin LiMn2O4 cathode(Fig.4b).By considering the capacity increase,the LiU0.01Mn1.99O4cathode retains approximately100%of its ini-tial capacity after100cycles.Even,by eliminating the capacity increase during the?rst cycles,the rate of capacity fading is very low(in comparison with other metal-substituted LiMn2O4 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 commer-cialization.Of course,this is just preliminary report,and further advancements are needed for this purpose.

Following the strategy of this research to inspect electro-chemical behavior of this novel cathode material before inves-tigations of its battery performance,we also estimate the in?u-ence of uranium doping on Li diffusion,which is an important factor for lithium batteries.Diffusion coef?cients at various potentials were estimated from chronoamperometric experi-ments in accordance with a method described elsewhere[15,22]. Fig.5shows potential dependency of diffusion coef?cients for LiU0.01Mn1.99O4cathode.In general,diffusion coef?cient for LiU0.01Mn1.99O4is higher than that for virgin LiMn2O4.This indicates that uranium doping is in favor of enhancement of Li diffusion,which is an important process during battery perfor-mance.

The curve of potential dependency of the diffusion coef?-cient for LiMn2O4is 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 coef-?cient 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 coef?cients at redox potentials is weaker for the case of LiU0.01Mn1.99O4.

It may seem that the discussion made here are simple and speculation.It should be emphasized that this is a prelimi-nary 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 LiMn2O4cathode are warrant of further attention.

4.Conclusion

It was demonstrated that extremely heavy metals(such as lanthanides and actinides)are also useful for metal substitu-tion of LiMn2O4spinel 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 stabi-lize the spinel stability.On the other hand,as only a small amount of dopant is needed they do not decrease the theoret-ical capacity.Not only they do not increase the lattice constant, but also they decrease it.For the case of LiU0.01Mn1.99O4, which was the core of the present study,higher theoret-ical capacity and smaller lattice constant were achieved. Since the amount of dopant is needed,this approach is also economic.

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Electrochemical supercapacitors Energy storage

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