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Electrochimica Acta 114 (2013) 14– 20

Contents lists available at ScienceDirect

Electrochimica Acta

jou rn al hom ep age: www.elsev ier .com/ locate /e lec tac ta

rinding aid-assisted preparation of high-performancearbon-LiMnPO4

ingbing Ran, Xiaoyan Liu, Qiwei Tang, Kunlei Zhu, Jianhua Tian,iangyong Du, Zhongqing Shan ∗

chool of Chemical Engineering and Technology, Tianjin University, Weijin Road 92#, Tianjin 300072, PR China

r t i c l e i n f o

rticle history:eceived 3 September 2013eceived in revised form4 September 2013ccepted 25 September 2013vailable online 16 October 2013

a b s t r a c t

Carbon-coated LiMnPO4 is prepared by PVP (polyvinyl pyrrolidone)-assisted solid-state reaction. Thestructure and morphology of the materials are characterized by X-ray diffraction (XRD), scanning elec-tron microscopy (SEM), and transmission electron microscopy (TEM). The electrochemical properties ofthe materials are investigated by galvanostatic charge–discharge test. It is found that the introduction ofgrinding aid (PVP) in the precursor caused uniform and smaller particle size in the final products, resultingin a regular carbon coating layer around the LiMnPO4 particles. As a result, an improved electrochemical

eywords:ithium manganese phosphaterinding aidarbonall milling

performance is obtained. The carbon-coated LiMnPO4 exhibits a high capacity, a good cyclability andan excellent rate capability. These excellent results are elucidated by electrochemical impedance spec-troscopy test, which show that there is decrease of charge transfer resistance and faster Li-ion diffusionin carbon-LiMnPO4 cathode materials after adding PVP.

© 2013 Elsevier Ltd. All rights reserved.

olyvinyl pyrrolidone

. Introduction

Recently, the growing large needs for a clean environment andontinuable energy have attracted enormous attention to studyn lithium-ion batteries (LIBs), which are considered to be onef the most promising energy storage systems. Thereby, cheaper,afer, more stable, higher energy, and higher power LIBs have beenidely studied in order to replace the traditional cathode mate-

ials like LiCoO2, LiNiO2 and LiMn2O4 [1]. Since the pioneeringork of Padhi et al., olivine structured LiMPO4 (M = Fe, Mn, Co, Ni)ave been researched intensively as candidate cathode materials

or LIBs because of their low cost, low environmental impact, excel-ent cycle life, and high thermal/chemical stability [2–5]. AlthoughiFePO4 has poor conductivity, its electrochemical properties haveeen greatly improved by cation substitution/doping, carbon coat-

ng and tailored particle-size [6–8].Encouraged by the success of LiFePO4, LiMnPO4 is attracting

ncreasing attention. It has a higher theoretical energy density701 W h kg−1 = 171 mA h g−1 × 4.1 V) due to higher potentialhan that of LiFePO4 (586 W h kg−1 =170 mA h g−1 × 3.45 V). The

otential of this material is also well within the stability windowf well-known carbonate ester-based electrolytes [9]. However,he LiMnPO4 is seriously hindered by its extremely low electronic

∗ Corresponding author. Tel.: +8602227406641.E-mail address: dldc11333@sohu.com (Z. Shan).

013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.electacta.2013.09.142

conductivity. In recent years, tremendous efforts have beenmade to overcome these defects by particle-size minimization,substitutional doping, and carbon coating [10–20]. Therefore,various methods including solid state method and wet chemicalmethod have been adopted to synthesize LiMnPO4 materials.LiMnPO4/C with a reversible capacity of 120 mA h g−1 at 0.1 C wasobtained by a high energy ball mill assisted soft template method[21]. Well-dispersed LiMnPO4 plates with a reversible capacity of119 mA h g−1 at 0.1 C were synthesized by a hydrothermal method[22]. Wu et al. synthetized LiMnPO4/C with a reversible capacity of126 mA h g−1 at 0.1 C by a sol–gel combined ball milling and liquidnitrogen quenching method [23]. A novel acetate-assisted antisol-vent precipitation method combined with ball milling and heattreatment was developed for LiMnPO4 with a reversible capacity of134 mA h g−1 at 0.2 C [24]. Yoshida et al. synthetized LiMnPO4/C byone-step mechanical method that showed a discharged capacityof 100 mA h g−1 at 0.2 C [25]. Wet chemical method is availableto synthesize uniform nanoparticle. However, the disadvantageof this method is the complex process. Besides, the LiMnPO4materials synthesized by the wet chemical method need to becalcined in order to improve the crystallinity and form a morethermodynamically stable structure with stronger bonding.

Complex chemical routes are short of attraction in real-world

applications because simplicity of the synthesis process is impor-tant for commercializing LIBs. From this perspective, solid statemethod is propitious to reach its industrialization. For example,LiMnPO4/C with a reversible capacity of 111.3 mA h g−1 at 0.1 C

L. Ran et al. / Electrochimica Acta 114 (2013) 14– 20 15

Table 1The reversible capacity of LiMnPO4/C synthetized by various methods.

Refs. Method Capacity (mA h g−1) Discharge rate (C)

[21] Soft template method 120 0.1[22] Hydrothermal method 119 0.1[23] Sol–gel method 126 0.1[25] Mechanical method 100 0.2

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[26] Solid-state method 111 0.1

ur result: 134 mA h g−1 at 0.1 C.

as obtained by solid-state method [26]. Fang et al. synthetizediMnPO4/C with 121 mA h g−1 at 0.1 C via solid-state method [27].he disadvantage of this method is the inhomogeneous distribu-ion of the particles [28]. Besides, it is not conducive to synthesis

aterials with small particle. However, the solid-state method istill attractive, owing to simplicity and easy operation. Therefore, its valuable to synthesis materials with small particle and efficientarbon coating via solid-state reaction. The reversible capacity ofiMnPO4/C synthetized by various methods is shown in Table 1.

Here, we report a new simple method to resolve this prob-em. A grinding aid (PVP) is admixed in the process of ball milling,

hich contributes to form uniform carbon coating and smallerarticle. Uniform carbon coating provides good electrical con-uct between particles and current collector. The smaller particlenhances Li ion diffusion as well as structural flexibility towards lat-ice deformation [29], thus leading to the enhanced electrochemicalerformance.

. Experimental

The carbon-containing LiMnPO4 composite (LMP–PVP/C) wasrepared by a solid-state reaction by adding sucrose and PVP to theynthetic precursors. Stoichiometric amount of C4H6MnO4·4H2O,iH2PO4 were mixed with 14 wt% of sucrose and 2 wt% of PVP byall milling for 6 h. The LMP–PVP/C was synthesized by sinter-

ng the precursor in a purified N2 gas stream at temperature of55 ◦C for 3 h, followed by heated up to 650 ◦C for 8 h. The sam-le (LMP/C) without adding PVP was also prepared for comparison

n the same way. The carbon content was about 8 wt% while thearbon pyrolyzed from the PVP was neglected [30,31].

The thermal-gravimetric analysis (TGA, TA, USA) was used totudy the decomposition and reaction of the precursor and deter-ine the synthesis temperature of Carbon-LiMnPO4. The precursoras heated from room temperature to 800 ◦C in N2 at a heating rate

f 10 ◦C min−1.The final Carbon-LiMnPO4 composite cathode material was

tudied by X-ray diffraction (XRD, Bruker AXS, Germany) with Cu� radiation. The morphology and particle size parameters werebserved by field-emission scanning electron microscopy (FE-SEM,EGA TS-5130SB). Its microstructure was examined by transmis-ion electron microscopy (TEM) on a HILIPS TECNAI G2 F20 with anccelerating voltage of 200 kV.

Electrochemical characterization was accomplished by assem-ly of 2032 coin-type cell with a lithium metal anode. Theathode was made by mixing active material, acetylene black andolyvinylidene fluoride (PVDF) in a weight ratio of 8:1:1. Activeaterial, acetylene black, and PVDF were mixed in an N-methyl-2-

yrrolidone (NMP) solvent to form slurry, and then this slurry waspread uniformly onto an aluminum foil using the doctor-bladeechnique and then dried in vacuum for 2 h at 60 ◦C. This compos-te sheet was cut into a circular shape and compressed at 100 kgf

−2

m in a hydraulic press. After pressing, the composite sheet wasried in a vacuum oven at 120 ◦C for 8 h. The cell was assembled

nside a glove box filed with high-purity argon gas (99.999%). Thelectrolyte was a solution of 1 M LiPF6 in a 1:1 mixture of EC/DMC.

Fig. 1. Thermo-gravimetric analysis (TGA) of precursor with PVP after ball milling.

The cells were tested on a Land battery program-controlled testsystem in the voltage range of 2.0–4.5 V at 25 ◦C. Electrochemicalimpedance spectroscopy (EIS) was carried out in a frequency rangefrom 0.1 Hz to 1 MHz with an Ac signal of 5 mV.

3. Results and discussion

Fig. 1 shows the thermal-gravimetric analysis (TGA) curveobtained from the precursor for synthesizing LMP–PVP/C after ballmilling. The total weight loss is about 40%, and weight loss doesnot occur at temperatures over 650 ◦C. The TG curve reveals fourmain stages of weight loss. From 30 to 165 ◦C, the precursor losesabout 14% weight, which is mainly attributed to the loss of crystal-lization water in Mn(CH3COO)2·4H2O [32]. From 165 to 240 ◦C, theprecursor loses about 15% weight, which is mainly ascribed to thethermal decomposition of LiH2PO4 and Mn(CH3COO)2. The weightloss in the temperature range of 240–355 ◦C is attributed to thedecomposition of sucrose and the continuous thermal decompo-sition of Mn(CH3COO)2·4H2O and LiH2PO4. The weight loss in thetemperature range of 355–470 ◦C is mainly ascribed to the thermaldecomposition of PVP. A continuous weight loss (∼1.7%) between470 and 650 ◦C is related to the removal of remaining organics. Fur-ther heating at higher temperatures only improves the crystallinityof the samples.

According to the TGA results, the decomposition of precur-sor and the reaction mainly occur before 355 ◦C. Therefore, atwo-stage high temperature sintering is adopted to synthesisCarbon-LiMnPO4 [32]. The precursor is heated at 355 ◦C for 3 h tomake it decompose completely, then heated up to 650 ◦C for 8 h.

The XRD patterns of the samples are shown in Fig. 2. All thepatterns show well-resolved diffraction peaks indexed to olivineLiMnPO4 (ICDD 33-0804), and no impurities are detected. Thereare no carbon peaks in the XRD patterns of the two samples, whichshows that the carbon pyrolyzed from the sucrose and PVP is amor-phous.

SEM was applied to investigate the morphology and particle sizeof the samples. The SEM images of (a) Mn(CH3COO)2·4H2O and (b)LiH2PO4 is shown in Fig. 3. The pristine materials are composed ofmany large micron-size grains. Fig. 4(a) represents the SEM imageof the precursor with PVP after ball milling, (c) represents the SEMimage for the precursor without PVP after ball milling. The images

show that the precursor is composed of agglomerated nanoparti-cles. As shown in Fig. 4(a), the SEM image of powders shows smallerand uniform particle dimensions, the average particle size of whichis about 150 nm. In Fig. 4(c), the image shows quite large aggregates

16 L. Ran et al. / Electrochimica

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Fig. 2. XRD patterns of the samples.

nd the particle sizes are larger, whose average particle size is about40 nm.

The reason why there are quite large aggregates is the recom-ination of the resultant small granules driven by the large surfacenergy. The particle size is decreased and the specific surface area isncreased with increasing grinding time in the superfine pulveriza-ion. As a result, the surface energy is increased, the interactionetween particles is large and the agglomeration phenomenonecomes more obvious. After a certain time, the grinding andgglomerate reach equilibrium. That is the main reason why theres a lowest particle size limit.

Grinding � Agglomerate. In order to break the balance, promot-ng the balance move to the right, the most important methods adding a grinding aid to the pristine material. Grinding aid

olecules adsorbed on the particle surfaces reduce surface energynd weaken the interaction between particles, which helps toeduce agglomeration.

Fig. 4(b) and (d) presents the SEM images of LMP–PVP/C andMP/C, respectively. In Fig. 4(d), the LMP/C consists of irregular

ggregated nanoparticles with an average particle size of 600 nm.s a contrast, Fig. 4(b) shows a relatively smaller particle size,ith an average particle size of 175 nm. As discussed before, PVP

an effectively reduce the particle size in the process of milling,

Fig. 3. SEM images of pristine material: (a

Acta 114 (2013) 14– 20

leading to fine-sized particles and uniform distribution. Duringheat treatment, the sucrose and PVP decomposed and finallyformed a carbon layer via solid-state reactions, which is helpful toincrease the electrical conductivity of the synthesized LiMnPO4.

Fig. 5 shows the SEM images of (a) LMP/C, (b) LMP–PVP/C, andEDS mappings of carbon for each. It is obvious that the carbon dis-tribution of the LMP–PVP/C is more uniform than LMP/C, whichindicates that the nanoparticles of LMP prepared by PVP-assistedsolid-state reaction are uniformly dispersed in the carbon matrix.It’s proved by the TEM images of LMP–PVP/C. The carbon matrixconstitutes the network structure, providing good electronic con-ductivity between the particles. As the mapping of carbon for LMP/Cshows, the carbon is dense in some places but sparse in some otherplaces. In the dense places, the electronic conductivity is higher,while in the sparse places, the electronic conductivity is low. As awhole, it is not conducive to improve the conductivity of LMP/C.

Fig. 6 shows the TEM images of the samples. Fig. 6(a) shows aTEM image of the LMP/C. It reveals that most particles are about600 nm. The TEM images of LMP–PVP/C are presented in Fig. 6(b)and (c). Fig. 6(b) reveals that most particles are about 170 nm,and they are interconnected by carbon. A high-magnification TEMimage (Fig. 6(c)) reveals the thickness of the uniform carbon layeris around 5 nm. As discussed before, uniform carbon coating isanother critical factor in determining the electrochemical perfor-mance of the LiMnPO4 material. Besides, Fig. 6(c) exhibits clearlattice fringes, indicating the single-crystalline nature of the par-ticles.

The electrochemical properties of LMP–PVP/C and LMP/C areshown in Fig. 7. The initial charge–discharge curves of thesamples are compared in Fig. 7(a). The cells were charged at0.05 C (1 C = 150 mA h g−1) to 4.5 V, held at 4.5 V until the currentdecreased to 0.02 C, and then discharged at 0.05 C to 2.0 V. All theelectrodes exhibited electrochemical activity with charge and dis-charge plateaus around 4.1 V vs. Li/Li+. These plateaus correspondto the Mn3+/Mn2+ redox couple accompanied by Li-ions extractionand insertion from/into LiMnPO4. The initial discharge capacity at0.05 C is 154 mA h g−1 for LMP–PVP/C, and 113 mA h g−1 for LMP/C.It is clear that the electrochemical activity of LMP can be sig-nificantly enhanced by adding PVP. As we discussed before, theLMP–PVP/C has a smaller particle size than LMP/C. This reduces the

diffusion path length for Li-ions in the cathode material. Besides,with the same amount of carbon for coating, smaller particles leadto higher degree of percolation of conductive network and thishelps electron transfer.

) Mn(CH3COO)2·4H2O, (b) LiH2PO4.

L. Ran et al. / Electrochimica Acta 114 (2013) 14– 20 17

Fig. 4. SEM images of prepared samples: (a) the precursor with PVP, (b) LMP–PVP/C, (c) the precursor without PVP, (d) LMP/C.

Fig. 5. SEM images of (a) LMP/C and (b) LMP–PVP/C; EDS mappings of carbon for (c) LMP/C and (d) LMP–PVP/C.

18 L. Ran et al. / Electrochimica

Fig. 6. TEM images of prepared samples: (a) LMP/C, (b) LMP–PVP/C, (c) high-magnification image of the LMP–PVP/C.

Fig. 7. (a) Initial charge–discharge curves at 0.05 C (1 C = 150 mA h g−1), (b) Rate performa(c) Cycling performance of the samples at 0.1 C by cc–cv mode, (d) Initial charge–discharg

Acta 114 (2013) 14– 20

Rate capabilities for the samples are shown in Fig. 7(b). It is evi-dent that the LMP–PVP/C shows a higher capacity than LMP/C atvarious rates. The reversible capacity of LMP–PVP/C could reach134 mA h g−1 at 0.1 C, 120 mA h g−1 at 0.5 C, 108 mA h g−1 at 1 C. Thereversible capacities of LMP/C are 105, 92, and 88 mA h g−1 at 0.1 C,0.5 C and 1 C, respectively. It is clear that increasing particle sizehas a great impact on the capacity of LMP. With the increase of theparticle size, lithium diffusion becomes increasingly difficult owingto both the diffusion limitation of Li-ions within a single large par-ticle and the difficulty of electron transport through the bulk of thismaterial.

The cycling performance of the samples at 0.1 C rate and 25 ◦Cis shown in Fig. 7(c). All samples exhibit an excellent cycling per-formance, and almost no capacity loss is observed after 35 cycles.Fig. 7(d) shows the initial charge–discharge curves of LMP–PVP/C atvarious rates. The LMP–PVP/C showed good discharge performancefrom 134 mA h g−1 at 0.1 C to 66 mA h g−1 at 5 C.

To explain further the different electrochemical behaviorsbetween LMP/C and LMP–PVP/C, electrochemical impedance spec-troscopy plots are recorded after the cells were cycled for 35 times.The electrochemical impedance spectroscopy of both samples wasmeasured in the frequency range from 0.1 Hz to 1 MHz in a three-electrode cell with lithium foil as counter and reference electrodes.Fig. 8 shows typical Nyquist plots of LMP/C and LMP–PVP/C. Both

profiles exhibit a semicircle in the high frequency region and aninclined line in the low frequency region. The semicircle is approxi-mately related to the charge transfer resistance (Rct) [33], while theinclined line is referred to the Warburg impedance, corresponding

nce of the samples charged at 0.1 C by cc–cv mode and discharged at various rates.e curves of LMP–PVP/C at various rates.

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ig. 8. The measured Nyquist plots and the calculated values of LMP–PVP/C andMP/C with the equivalent circuit shown in the figure.

o Li-ion diffusion in the bulk of the electrode materials [34].nterpretation of the impedance spectra is based on the equivalentircuit in the inset of Fig. 8. The equivalent circuit is obtained byubstituting the double layer-capacitance of the Randles equiva-ent circuit to the constant-phase-angle element (CPE) [35]. Theymbols Re, Rct, CPE, and Zw denote the solution resistance, chargeransfer resistance, capacitance of the double layer, and Warburgmpedance, respectively. The numerical value of the diameter ofhe semicircle on the Zre axis is approximately equal to the chargeransfer resistance (Rct) [33]. We fitted these impedance data withhe equivalent circuit shown in the inset of Fig. 8 to estimate theharge transfer resistance, Rct. The calculated values of LMP/C andMP–PVP/C are 402.9 � and 129.5 �, respectively, which indicateshat there is a remarkable decrease in Rct after adding PVP. A lowct means a large exchange current density, indicating the fasteriffusion of Li-ions across the LMP–PVP/C-electrolyte interfacehan across the LMP/C-electrolyte interface. So, adding PVP is moreavorable for the insertion and extraction of Li-ions during theharge and discharge process. The faster interfacial diffusion forMP–PVP/C is due to its smaller particle size (with a large specificurface area) that maximizes the contact with the electrolyte.

As can be seen from Fig. 8, the slope of the impedance ofMP–PVP/C is bigger than that of LMP/C, indicating that addingVP is able to enhance the electrochemical activity of Carbon-iMnPO4. According to the above analysis, it can be concluded thathe decrease of charge transfer resistance and faster Li-ion diffu-ion results in an improved electrochemical performance, which isnduced by adding PVP.

As the carbon content in the synthesized sample and the con-ition applied for test are different, a direct comparison of theerformance of our samples with the previous reported results isifficult. Anyway, the electrochemical properties of LMP–PVP/C arexcellent as compared to the results of the very recent literature27,28,36]. All electrochemical results indicate that the LMP–PVP/Cas a much improved electrochemical performance.

. Conclusions

In this work, we proposed the method to restrict the growthf the particle size of the precursors. The precursor with small

article size and homogeneous morphology was formed by addingVP in the process of ball milling, and this helps to synthesizeiMnPO4 with smaller and uniform particle size and regular carbonoating layer. The carbon-coated LiMnPO4 prepared by adding

[

Acta 114 (2013) 14– 20 19

PVP exhibited an improved electrochemical performance. Thereversible capacity could reach 134 mA h g−1 at 0.1 C. Besides, thesynthesized LiMnPO4 exhibited stable cycling behavior and goodrate capability. It may be not as effective as some wet chemicalmethods that can elaborately tailor the particle size and morphol-ogy of the final products, but it is a routine route that makes thisimproved method attractive, particularly in consideration of thesimplicity, easy operation and inexpensive synthesis procedure.This method is also applicable to other electrode materials withpoor conductivity. In this manuscript, we do not study the effectof PVP content on the electrochemical performance of LiMnPO4,and, in our subsequent study, we will continue to explore thisquestion.

Acknowledgments

This work was supported by the National Basic Research Pro-gram of China (2009CB220105), the International CooperationProgram with Germany (2012DFG61480), the International Coop-eration Program with France (2011DFA70570-4), and The NationalHigh Technology Research and Development Program of China(2013AA050901).

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