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Ceramics International 40 (2014) 6699–6704www.elsevier.com/locate/ceramint

Effect of the stirring rate on physical and electrochemical propertiesof LiMnPO4 nanoplates prepared in a polyol process

Hua-Jun Zhu, Xiao-Min Liu, Hui Yangn, Xiao-Dong Shen

College of Materials Science and Engineering, Nanjing University of Technology, 5 Xinmofan Road, Nanjing, Jiangsu 210009, PR China

Received 23 September 2013; received in revised form 25 November 2013; accepted 25 November 2013Available online 4 December 2013

Abstract

In a polyol approach to make olivine-typed LiMnPO4 nanoplates, the effect of the stirring rate on physical and electrochemical propertiesof the obtained sample has been symmetrically investigated for the first time. The as-prepared powders exhibit well crystalline olivine structure aspresented by X-ray diffraction analysis. The secondary particles with a size of 2–3 μm are composed of aggregated nanoplates as verified byparticle size analysis and field emission scanning electron microscopy measurement. Transmission electron microscopy presents that thenanoplate is about 18 nm thick along a-axis. The discharge capacity of the sample prepared under a stirring rate of 700 rpm reaches 150 mAh g�1

when cycled at 0.05C after a few cycles.& 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: Lithium ion batteries; Lithium manganese phosphate; Polyol process; Nanoplate; Stirring rate

1. Introduction

Co-based cathodes have been widely employed in the lithiumion batteries powering small portable electronic devices (laptops,cell phones and digital cameras, etc. [1]), but they are notappropriate for large scale applications such as hybrid electricvehicles (HEV) and plug-in electric vehicles (PHEV) due to theintrinsic safety issues and cost limitation [2,3]. Olivine metalphosphates (LiMPO4, M=Fe, Mn, Co, Ni) possess high safety,long cycle life and low cost, therefore they have been consideredas the most promising candidates for those applications [4].Among them, LiFePO4 has been extensively studied in the pasttwo decades and now has been commercialized for some high-power applications [5–8]. Encouraged by its success, moreinterests have been focused on LiMnPO4 cathode recently sinceit possesses a higher potential plateau (4.1 V vs. Liþ /Li) and ahigher theoretical energy density (701 Wh kg�1) compared tothose of LiFePO4 (3.45 V vs. Liþ /Li and 586 Wh kg�1) [9–12].Moreover, several reports pointed out that 701 Wh kg�1 isconsidered as the maximum practically achievable energy

e front matter & 2013 Elsevier Ltd and Techna Group S.r.l. All ri10.1016/j.ceramint.2013.11.131

g author. Tel./fax: þ86 258 358 7275.ss: yanghui@njut.edu.cn (H. Yang).

density for the current lithium ion cathodes using carbonateester-based electrolytes [9–13]. However, the inevitable John–Teller effect occurred at the mismatched interface of LiMnPO4/MnPO4 during charge/discharge creates a large kinetic barrier forion and electron hopping [1–15]. This structural drawback limitsthe cycling performance of LiMnPO4 like other Mn-basedmaterials. In addition to that, LiMnPO4 possesses extremelylow electronic conductivity (o10�10 S cm�1) and a very smallLiþ diffusion coefficient (�10–15 cm2 S�1), which are also themajor obstacles hindering its applications [16]. Consequently, thereported capacities are quite low in early studies [1,2]. Upto nowadays, the electrochemical performance of LiMnPO4 isimproved by controlling the particle size and tailoring themorphology of the obtained particles via various methods[17,18]. For example, LiMnPO4 nanoplates prepared via solid-sate reaction in molten hydrocarbon can deliver a capacity up to168 mAh g�1 when cycled at a current density of C/25 [9].LiMnPO4 nanoparticles obtained by a novel precipitation methodexhibit good rate capability, 145 mAh g�1 at C/10 and 62mAh g�1 at 5C [12]. Nanoplate LiFePO4 synthesized via apolyol route possesses a good electrochemical performance,reaching 160 mAh g�1 at 0.15C [19]. It seems that the polyolprocess is a promising method in synthesizing nano-sized

ghts reserved.

Fig. 1. XRD patterns of LiMnPO4 samples obtained under different stirring rates.

H.-J. Zhu et al. / Ceramics International 40 (2014) 6699–67046700

electrode materials of Li-ion cells since it needs no furthercalcination process, therefore preventing grain/particle growthwhich may deteriorate the electrochemical performance.

The polyol method can precipitate nano-sized LiMnPO4

from a polyol (mainly diethylene glycol) based solution in asingle step when the solution is heated at a temperature closeto its boiling point. It is possible to obtain well-crystallineparticles with defined morphology and narrow size distributionunder controlled conditions via this method [20]. Since thepolyol itself acts as a stabilizer, limiting particle growth andprohibiting agglomeration [21], the polyol method is widelyemployed to synthesize many nanometer samples, such asmetallic powders, alloys [22], oxides [23] and phosphates[24,25]. It should be noted that the properties of the finalproduct is sensitive to several parameters, such as the stirringrate, the species of the polyol adopted, the reaction temperatureand the reaction time. However, there is no report system-atically studied the effect of the stirring rate on the propertiesof the synthesized product yet. In this paper, the effect ofthe stirring rate as an individual factor on the particle size, themorphology and the electrochemical performance of theLiMnPO4 has been investigated and discussed in detail.

2. Experimental

2.1. LiMnPO4 preparation and characterization

Two solutions (2 mol L�1) were prepared by dissolvingmanganese acetate tetrahydrate (Mn(CH3COO)2 � 4H2O, AR)and lithium dihydrogen phosphate (LiH2PO4, AR) into deio-nized water. Mn(CH3COO)2 � 4H2O solution of 20 ml waspoured into 200 ml diethylene glycol (DEG, AR) in a three-neck round-bottom flask under various stirring rates, 300, 400,500, 600, and 700 rpm. The resulting solution was heated fromroom temperature to 160 1C. Then 20 ml LiH2PO4 aqueoussolution was dropped into the flask with a speed of 1.5 mlmin�1. After that, the DEG suspension was kept at 160 1Cfor further 3 h before cooling down to room temperature. TheLiMnPO4 particles, collected by centrifugation, were washedseveral times with ethanol to remove the residual organicremnants and dried at 120 1C overnight.

The crystal structure of precursor and synthesized powderswas confirmed by X-ray powder diffraction (XRD, ModelX'TRA) collected by steps of 0.021 in the 2θ range of 5–701.The particle size distribution was measured with a laserparticle size analyzer (Malvern, Mastersizer 2000). Themorphology was characterized by a field emission scanningelectron microscope (FESEM, JSM-6700F, JEOL, Japan)and a transmission electron microscope (TEM, JEM-1010,JEOL, Japan).

2.2. Electrochemical measurement

Electrochemical performance of the obtained LiMnPO4

powders was characterized with CR2032 coin cells assembledin an argon-filled glove box. LiMnPO4/C composites wereprepared using planetary ball-milling of the LiMnPO4 powders

with 20 wt% carbon black. The cathode was prepared bycoating the mixture of LiMnPO4/C, carbon black and PVDF(90.5:2:7.5 wt%) on aluminum foil, followed by vacuumdrying at 100 1C for 10 h. An electrochemical 2032 coin cellconsists of the cathode, lithium foil as anode, 1 M LiPF6 indiethyl carbonate (DEC) and ethylene carbonate (EC) (1:1 byvol) as electrolyte, and the celgard 2400 as separator. The cellswere tested at different current densities in the voltage rangebetween 2.5 V and 4.6 V under room temperature using aNeware BTS-mA computer-controlled battery test system. Theelectrochemical impedance spectroscopy was measured withan electrochemical workstation (CHI650D, Shanghai ChenhuaInstrument Co., Ltd., China) in the frequency ranging from0.01 Hz to 0.1 MHz.

3. Results and discussion

In this study, several nano-sized LiMnPO4 samples areprecipitated via the polyol method under various stirring rates,from 300 rpm to 700 rpm. Each sample is named according tothe stirring rate employed in the preparation. For example, thesample prepared at a stirring rate of 500 rpm is marked asPN500. Fig. 1 presents the X-ray diffraction patterns of allprepared samples. Each sample shows a well-crystalline phasewith all reflections indexing to the orthorhombic structurebased on the space group Pnma (JCPDS# 74-0375). Mean-while, no detectable impurity phase is observed in all XRDpatterns. From the calculated cell parameters as listed inTable 1, it can be concluded that the stirring rate has littleimpact on crystal lattice. In addition, I(020) is higher than bothI(311) and I(200) for all samples, indicating that particle growthof the obtained samples is along b-axis.The SEM images of samples obtained at different stirring

rates are shown in Fig. 2. Apparently, all samples present aplatelet like structure with a thickness of about 20 nm. Multi-ple nanoplates aggregate into large secondary particles with thesize in the range of 2–3 μm. In addition, it seems that theshapes of both nanoplates and secondary particles merelychange with the altering stirring rate. However, it is obviousthat the stirring rate would have a great impact on particle size

Table 2Particle size distribution and dispersal coefficient of as-prepared LiMnPO4

samples.

Samples D10 (μm) D50 (μm) D90 (μm) δ

PN300 1.352 3.053 9.916 2.805PN400 1.159 2.626 7.457 2.398PN500 1.080 2.376 6.078 2.104PN600 1.110 2.369 5.015 1.648PN700 0.991 2.227 4.969 1.786

H.-J. Zhu et al. / Ceramics International 40 (2014) 6699–6704 6701

of the prepared sample. Therefore, in order to describe themonodispersity of the particles quantitatively, the particle sizedispersal coefficient δ is employed here which is defined as

δ¼ D90�D10

D50ð1Þ

where the Dn (n¼10, 50, 90) denotes the cumulative numberpercentage particles with a diameter up to Dn equal to n% [26].Generally speaking, the smaller δ of a crystal sample exhibitsthe narrower particle size distribution it has. The measured Dn

and the calculated δ of the prepared samples are summarizedin Table 2. In addition, the D50 and the δ vs. the stirring rateare plotted in Fig. 3. The particle median size decreasesdramatically, from 3.053 μm to 2.376 μm, when the stirringrate increases from 300 rpm to 500 rpm. However, when thestirring rate further increases to 700 rpm, the particle mediansize only decreases slightly to 2.227 μm. As for δ, it decreaseswith increasing the stirring rate, reaches the minimum value of1.648 at 600 rpm, then, increases with further increasing thestirring rate. The stir has impact on all the steps related to thecrystallization process, including mixing, nucleating, growthand agglomeration. Compared to the low-rate stir, moderate-rate stir improves the mixing, creates more uniform environ-ment for nucleating and particle growth, therefore the resulting

Table 1Calculated cell parameters and relative intensities for as-prepared LiMnPO4

samples.

a (Å) b (Å) c (Å) I(311) I(020) I(200)

#74-0375 10.4600 6.1000 4.7440 100 75.4 28.5PN300 10.4691 6.1153 4.7676 88.4 100 24.2PN400 10.5104 6.1025 4.763 71.9 100 28.1PN500 10.4553 6.1358 4.7593 89.5 100 34.8PN600 10.4386 6.1337 4.7671 93.6 100 39.3PN700 10.4667 6.1228 4.7700 91.9 100 44.4

Fig. 2. SEM images of LiMnPO4 samples obtained under different stirring rates

particles exhibit better monodispersity. However, large parti-cles could be shattered into small ones during the high-ratestirring process, which explains the abnormal increase of δwhen the stir rate changes from 600 rpm to 700 rpm.In order to further confirm the preferred direction for crystal

growth, the crystallographic orientation of LiMnPO4 nano-plates (sample PN700) is analyzed with TEM (Fig. 4a) and the

(a) 300 rpm, (b) 400 rpm, (c) 500 rpm, (d) 600 rpm, and (e) 700 rpm.

Fig. 3. Effect of stirring rate on the median size and dispersal coefficient ofLiMnPO4 samples.

F

Fig. 5. First cycle charge–discharge curves of samples under different stirringrates at 0.05C.

H.-J. Zhu et al. / Ceramics International 40 (2014) 6699–67046702

selected area electron diffraction (SAED) (Fig. 4b). Thenanoplate is about �18 nm thick as observed in Fig. 4(a).The calculated results from the corresponding SAED pattern ofthe dashed area (Fig. 4(b)) can identify the (020), (011), and(01�1) planes of the flat face of nanoplate. Thus, the zoneaxis of the nanoplate was determined to be [100] direction.That is, the shortest crystal orientation of the nanoplate isalong [100] direction, namely, the a-axis.

Carbon is used as an effective conductive additive toimprove the electrochemical performance of LiFePO4 andLiMnPO4 [27,28]. In this study, LiMnPO4/C composites areobtained by ball milling of the resulting LiMnPO4 powderswith 20 wt% acetylene carbon black. Fig. 4(c) shows the TEMimage of one composite sample (PN700 ball milled withacetylene carbon black). The observed particle size is in therange of about 20–60 nm, which is smaller than that of theoriginal PN700. The smaller size resulted from ball millingleads shorter transport path for both lithium ions and electronswhich may facilitate the electrochemical performance of theresulting LiMnPO4. However, the non-uniform layer of carboncoating around the small particles may hinder the kinetics ofcharge transfer.

Fig. 5 presents the first charge–discharge curves of variousLiMnPO4/C cathodes cycled at 0.05C (1C¼171 mAh g�1)between 2.5 and 4.5 V. Each sample shows a typical plateauaround 4.1 V vs. Liþ /Li indicating that the charge–dischargereaction proceeds via a first-order phase transition betweenLiMnPO4 and MnPO4. Apparently, the sample obtained at ahigher stirring rate exhibits higher discharge capacity. Espe-cially, PN700 exhibits the smallest polarization and the largestdischarge capacity up to 129 mAh g�1. The low values ofcoulombic efficiency for all five samples during the first cycleare attributed to the decomposition of the electrolyte. Thisdecomposition could be caused by either the LiMnPO4 nano-plates, or the large amounts of acetylene carbon black [29].

ig. 4. (a) TEM images and (b) SADE pattern of as-prepared PN700 samples, and

Several reports [14] pointed out that the Liþ transport in theolivine LiFePO4 follows a one-dimensional zigzag pathwayalong the b-axis, suggesting that the particles possess a smallersize in the b-axis direction will exhibit better electrochemicalperformance. As observed and discussed in the TEM section,although the shortest crystal orientation is along the a-axis, thesamples still deliver good specific capacities. With the smallestparticle size, PN700 exhibits an initial discharge capacity of129 mAh g�1, which increases to 150 mAh g�1 after a fewcycles due to the electrolyte penetration.The electrical impedance spectroscopy (EIS) is also tested

for all samples and the results are shown in Fig. 6. The Nyqusitplots show a semicircle in the high to medium frequency rangeand a slopping line in the low frequency region. As it iswell known, the semicircle is mainly related to the chargetransfer resistance (Rct, include electron and lithium ion) at theelectrolyte/electrode interface [30]. The slopping line in thelow frequency is referred to the Warburg resistance which is

(c) TEM images of PN700 after ball milling with 20 wt% acetylene back.

Fig. 6. Impedance spectra of samples obtained under different stirring ratesand fitting curves for the inset equivalent circuit. Fig. 7. Cycle performance of samples prepared at different stirring rates.

H.-J. Zhu et al. / Ceramics International 40 (2014) 6699–6704 6703

corresponding to the lithium ion diffusion within the bulkelectrode [31]. An equivalent circuit (the inset of Fig. 6) isused to explain the spectra in which R1 and CPE1 arecorresponding to the surface layer films resistance andcapacitance, R2 and CPE2 are responsible for the lithium ionintercalation/deintercalation resistance and interfacial capaci-tance, respectively, W1 is the Warburg resistance [32]. The fitcurves derived from the equivalent circuit are also presented inFig. 6. The calculated R2 listed in the inset shows that with theincreasing stirring rate, the charge transfer resistance increasesfirst, then, decreases sharply. The sample PN700 demonstratesthe smallest R2, only 62.6 Ω, suggesting that this samplepossesses the fastest kinetics of lithium ions intercalation/deintercalation in all samples, which is in a good agreementwith the largest discharge capacity it exhibits.

The cycling performances of all samples are tested and theresults are presented in Fig. 7. With the increasing cyclenumber, the discharge capacities of PN300, PN400, PN500and PN600 increase first, then decrease. The first increase ofdischarge capacity may be claimed to the gradually permeatingof the electrolyte within the composite electrode during cyclingprocess. And the subsequent fade of capacity could be owingto multiple aspects as follows. During the charge–dischargeprocess, the cell volume changes from 302.82 Å3 for LiMnPO4

to 272.96 Å3 for MnPO4 [14]. This 10% volume change and acooperative John–Teller distortion of Mn3þ induce a largestrain at the interface between two phases. And it increases theactivation energy for Liþ insertion/extraction, lowers the rateof Liþ diffusion [14], and destroys the crystal structure. Thisfatal destroy of structure from volume changes leads todecrease the capacity during the cycles. The capacity ofPN700 increases with the cycle number, reaching 150 mAhg�1 at the 20th cycle, which is close to the theoretical capacity(171 mAh g�1). The stirring rate has significantly impact onthe particle size of the resulting sample: the higher the stirringrate, the smaller the particle size. PN700 obtained under thehighest stirring rate possesses the smallest particle size andperforms the best electrochemical property as discussed inFigs. 5 and 6. Moreover, the capacity fade which caused

partially by volume change during charge–discharge processcould be weakened when the particle size is small.

4. Conclusions

In this study, LiMnPO4 nanoplates are synthesized by apolyol method under various stirring rates, from 300 rpm to700 rpm. The results clearly show that stirring rate has asignificant impact on particle size and electrochemical perfor-mance of the resulting sample. SEM images show that theprimary particles of all samples present a platelet like structure,which further aggregate into large secondary particles. Theparticle size monotonously decreases from 3.053 μm to2.227 μm when the stir rate increases from 300 rpm to700 rpm. However, the particle size dispersal coefficientdecreases with the increasing stirring rate, reaching theminimum value of 1.648 at 600 rpm. TEM results reveal thatthe nanoplates are about 18 nm thick and SADE pattern showsthe well crystalline nanoplates are orientated vertically with a-axis. Although the thinnest direction of the nanoplates is notalong b-axis direction, all samples still deliver good dischargecapacity. The best electrochemical performance is obtained forthe sample prepared at 700 rpm which exhibits 129 mAh g�1

in the first discharge process under a current level of 0.05C.Moreover, the discharge capacity gradually increases to150 mAh g�1 after 20 cycles.

Acknowledgments

This work was supported by the Key Project of NaturalScience Foundation of Jiangsu Province of China (Grant no.BK2011030), Key Project of Educational Commission of JiangsuProvince of China (Grant no. 11KJA430006), Graduate Innova-tion Foundation of Jiangsu Province (CXLX13_406) and thePriority Academic Program Development of Jiangsu HigherEducation Institutions.

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