Electron. Mater. Lett., Vol. 10, No. 4 (2014), pp. 787-790

The Effect of Na0.44MnO2 Formation in Na+-Modified Spinel LiMn2O4

Lilong Xiong,1 Youlong Xu,

1,* Weiguo Wu,2 Pei Lei,

1 Tao Tao,

1 and Xin Dong

1

1Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education & International Center for Dielectric Research, Xi'an Jiaotong University, Xi'an 710049, P. R. China

2Department of Computer Science & Technology, Xi'an JiaotongUniversity, Xi'an 710049, P. R. China

(received date: 7 June 2013 / accepted date: 6 February 2014 / published date: 10 July 2014)

Na0.44MnO2 impure phase is formed during the synthetic process of Na+-modified spinel LiMn2O4 by solid state

reaction which is confirmed by x-ray diffraction analysis. Scanning electron microscopy, transmission electronmicroscopy and galvanostatic charge-discharge measurements are carried out to investigate the effect of the for-mation of Na0.44MnO2 impurity on the morphology and electrochemical properties of the spinel material. Theresults show that the spinel material with impure phase exhibits improved cyclability compared to that of thepristine LiMn2O4. The improved electrochemical performance is mainly ascribed to the improved crystallinity ofthe spinel particles, enhanced stability of the spinel structure and good electronic conductivity of the composite.

Keywords: inorganic compounds, chemical synthesis, x-ray diffraction, electron microscopy, electrochemicalproperties

1. INTRODUCTION

Lithium ion batteries are considered to be the mostefficient energy storage systems which could be applied inthe electric vehicles (EVs), hybrid electric vehicles (HEVs)and plug-in electric vehicles (PHEVs).[1,2] Spinel LiMn2O4

exhibiting large specific energy, high safety and low costproperties, is regarded as one of the most promising cathodeactive materials for lithium ion batteries used for large-scalepower output or large energy storage.[3] However, LiMn2O4

suffers from severe capacity loss during charge-dischargecycling, which is mainly due to the instability of the spinelstructure and the dissolution of manganese into electrolyte.[4-7]

In efforts to improve the cycling performance of the spinelLiMn2O4, several approaches have been developed tostabilize the spinel structure. Substituting partial Mn ions inoctahedral sites with other cations, such as Al3+,[8] Co3+,[9]

Cr3+,[10] Ni2+,[11] is an effective method to improve thestability of the spinel. Besides the substituting in octahedralsites, the substituting of partial Li+

-ions by K+-ions in

tetrahedral sites in spinel LiMn2O4 has also been reported.[12]

In this study, Na+-modified spinel LiMn2O4 through

substituting partial Li+-ions by Na+

-ions in the tetrahedralsites has been synthesized, and Na0.44MnO2 impurity isformed in the as-prepared material. The effect of the formationof Na0.44MnO2 impurity on the morphology, crystal structureand electrochemical properties of the spinel LiMn2O4 isreported for the first time.

2. EXPERIMENTAL PROCEDURE

2.1 Materials preparation

The Na+-ion modified spinel LiMn2O4 samples wereprepared by simple solid-state reaction. All chemical reagentsused in the experiments were analytical grade. The startingmaterials of Li2CO3, Mn3O4, and Na2CO3 were ball-milledaccording to the stoichiometric formula Li1-xNaxMn2O4

(x = 0, 0.1). The mixture was calcined at 450°C for 6 h. Afterground for 2 h, the precalcined powder was calcined at750°C for 12 h and furnace-cooled to room temperature.

2.2 Materials characterization

The powder x-ray diffraction (XRD) patterns of the as-prepared samples were obtained from a PANalytical Xdiffractometer (X’Pert PRO) equipped with Cu-Kα radiation(λ = 0.15406 nm). The morphology and structure of thepowders were characterized by scanning electron microscopy(SEM, quanta 250, FEI) and Transmission electron microscopy(TEM, JEM-2100, JEOL).

Half cells were assembled to determine the electrochemicalperformance of the Na+-ion modified spinel samples. Thecathode electrodes were fabricated by as-prepared activepowder, carbon black and polyvinylidene fluoride with theweight ratio of 70 : 20 : 10. The mixture was dispersed in N-methylpyrrolidinon to prepare homogeneous slurry, then theslurry was coated onto a pretreated aluminum foil followedby heat-treatmet at 120°C. The cells used metal lithium sheetas anode were assembled in a glove box under a dry argonatmosphere and the electrolyte was 1 M LiPF6 dissolved inethylene carbonate : diethyl carbonate : ethyl methyl carbonate

DOI: 10.1007/s13391-014-3165-z

*Corresponding author: ylxu@mail.xjtu.edu.cn©KIM and Springer

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Electron. Mater. Lett. Vol. 10, No. 4 (2014)

with 1 : 1 : 1 volume ratio. The charge-discharge performanceof the cells was tested with a cut-off voltage limit of 4.3 -3.5 V (versus Li/Li+) using an Arbin Battery Test System toevaluate the effect of Na+-ion on the electrochemicalperformance of the spinel LiMn2O4.

3. RESULTS AND DISCUSSION

Figure 1 shows the powder XRD patterns of the as-prepared samples. The pattern for the Li1-xNaxMn2O4 (x = 0)sample (pristine spinel LiMn2O4) is indexed to be a singlephase of cubic spinel structure with the space group ofFd3m. As seen the pattern for Li1-xNaxMn2O4 (x = 0.1) sample(Na+-modified sample), an impure phase of Na0.44MnO2

(Na4Mn9O18, JCPDS: 00-027-0750) is detected except forthe characteristic peaks for the cubic spinel phase. Calculatedfrom the XRD diffraction data, the lattice size of the spinelphase in the Na+

-modified sample is 8.2312 Å, while that inthe pristine spinel LiMn2O4 is 8.2442 Å. The decrease of thelattice size after Na+-ion doping is mainly due to theformation of the second phase Na0.44MnO2, just as seen inFig. 1. The formation of Na0.44MnO2 in the stoichiometricformula Li0.9Na0.1Mn2O4 sample causes the precipitation ofMn from the spinel phase, resulting in the lacking of Mn inthe spinel structure. The Li:Mn ratio in the spinel wouldincrease due to the formation of the impure phase, whichindicates that the Li+-ions would diffuse into the octahedralsites (16d) to occupy the Mn sites. The Li+-ions doping in theoctahedral sites would reduce the Mn3+-ions amount whileincrease the Mn4+-ions amount in the spinel structure inorder to reach the charge balance. Considering that the radiusof the Mn4+-ion is less than that of the Mn3+-ion, the valencechange of the Mn from trivalence to tetravalence wouldreduce the lattice size of the spinel.

The morphology of the as-prepared powder has been

characterized by SEM and the micrographs are shown inFig. 2. The pristine spinel LiMn2O4 exhibits octahedralparticles in slightly agglomerated state, and the averageparticle size is 250 nm, just as seen in Fig. 2(a). For the Na+-modified spinel sample in Fig. 2(b), both spinel nano-particles and Na0.44MnO2 nano-rods could be observed,which is consistent with the results obtained by XRD analysis.The Na0.44MnO2 rods are distributed among the spinelparticles with the width of 120 - 300 nm and the length of1 μm - 5 μm. It is found that the spinel particles in modifiedsample exhibit more pronounced grown facets and moreuniform distribution than that in the pristine sample. Theaverage particle size of the spinel particles in the modifiedsample is about 200 nm which is smaller than that of thepristine particles. The reduced particle size in the Na+-modified sample is mainly due to the formation of Na0.44MnO2

impurity, which would hinder the spinel-particle growthduring the calcining process.

The nanostructure of the Na+-modified sample is further

Fig. 1. The powder XRD patterns of the prepared materials.

Fig. 2. The SEM micrographs of the as-prepared materials. (a)pristine spinel sample, (b) Na

+-modified sample.

Fig. 3. The TEM analysis. (a) TEM image for the modified sample(b) HRTEM image for Na0.44MnO2, (c) SAD for Na0.44MnO2, (d)EDX spectrum measured at the marked position in Fig. 3(a).

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examined by TEM analysis and the results are shown inFig. 3. Figure 3(a) exhibits the modified sample has twomorphologies: nano-particle and nano-rod, corresponding tospinel LiMn2O4 and Na0.44MnO2 respectively. The spinelparticles show good crystallinity with defined grain boundary.The high resolution TEM (HRTEM) image and selected areadiffraction pattern (SAD) for Na0.44MnO2 are shown inFig. 3(b) and (c) respectively. The diffraction fringes inHRTEM image can be indexed to (140) plane of Na0.44MnO2,corresponding to the diffraction peak of 2θ ≈ 16.6° in XRDpattern of the Na+-modified sample. The SAD resultsindicate the rod-like Na0.44MnO2 is a well ordered singlephase. Elemental point analysis using energy dispersive X-ray spectrometry (EDX) in TEM is carried out to furtherconfirm the structure of the impure phase. Figure 3(d) showsthe EDX spectrum measured at the position of the rod whichis marked as a circle in Fig. 3(a). The EDX analysis revealsthat only Na, Mn and O elements are detected in the nano-rod. The atomic ratio of Na : Mn : O calculated from the

EDX spectrum is 12.19 : 28.33 : 59.48, confirming the surfacecoating layer is composed of Na0.44MnO2.

Figure 4 shows the discharge curves of the as-preparedsamples cycled at rate of C/2 at room temperature. Both thepristine LiMn2O4 (Fig. 4(a)) and Na+-modified spinel (Fig.4(b)) display two voltage plateaus approximately at 4.0 V,suggesting the typical electrochemical characteristics ofspinel materials. The formation of the voltage step is mainlydue to the Li+-ion ordering in tetrahedral sites in spinelstructure.[13,14] However, the voltage step in the modifiedsample is less pronounced than that in the pristine sample,indicating the Li+-ion ordering in the modified sample hasbeen suppressed effectively. As seen in the inset of the Fig.4(a), the pristine sample delivers initial discharge capacityabout 138.5 mAh g−1, and its capacity loss is 21.8% after 100cycles, exhibiting rapid capacity fade. The Na+-modifiedsample delivers initial discharge capacity of 117.1 mAh g−1

and exhibiting 8.6% capacity fade after 100 cycles, just asseen in the inset of Fig. 4(b). The results of the electrochemicalperformance indicate the Na+-modified spinel exhibits greatlyimproved cyclability compared to the pristine LiMn2O4.

The improved cycling performance of the Na+-modifiedspinel sample is attributed to the following reasons: 1) Themodified sample exhibits better crystallinity of the spinelparticles than that of the pristine sample. 2) The formation ofNa0.44MnO2 leads to Li+-ion doping in the octahedral sites,which could enhance the stability of the spinel structure bysuppressing Li+-ion ordering. 3) The Na0.44MnO2 rodsexhibiting high electronic conductivity are distributed amongspinel particles,[15] benefical to electron transport betweenspinel LiMn2O4 particles.

4. CONCLUSIONS

Na+-modified spinel LiMn2O4 has been synthesized bysimple solid state method. The XRD analysis shows that theas-prepared sample consist of two components: spinelLiMn2O4 phase and Na0.44MnO2 phase. The SEM and TEMmeasurements indicate the formation of Na0.44MnO2 impurityduring the synthetic process could suppress the growth of thespinel particles and improve the crystallinity. The formationof the impurity causes the precipitation of Mn from thespinel phase, leading to the Li+-ion diffusion into theoctahedral sites to occupy the Mn sites, which could enhancethe stability of the structure by suppressing Li+-ion orderingin the spinel structure. The improved crystallinity and enhancedcrystal stability is favorable to improving the cyclability ofthe spinel LiMn2O4 material.

ACKNOWLEDGEMENTS

The authors thank the National Natural Science Foundationof China (Grant no. 21203145, 21274115, 51201128 and

Fig. 4. Discharge curves of the (a) pristine spinel sample and (b) mod-ified sample. The inset of Fig. 4(a) is the cycle performance of thepristine spinel sample; the inset of Fig. 4(b) is the cycle performanceof the modified sample.

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Electron. Mater. Lett. Vol. 10, No. 4 (2014)

50902109), the National Science Foundation for Post-doctoral Scientists of China (Grant no. 2013M542340) andthe Natural Science Foundation of Shaanxi Province (Grantno. 2014JQ2079 and 2010JQ6002) for financial support; theauthors also thank Ms. Lu Lu and Ms. Yanzhu Dai for theirhelp in using SEM, TEM at International Center forDielectric Research (ICDR), Xi'an Jiaotong University.

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