Cite this: CrystEngComm, 2013, 15,7808

Hydrothermal synthesis and electrochemical propertiesof dispersed LiMnPO4 wedges3

Received 21st May 2013,Accepted 1st August 2013

DOI: 10.1039/c3ce40890f

www.rsc.org/crystengcomm

Zhi Gao,* Xiaoliang Pan,* Heping Li, Shikun Xie, Rongxi Yi and Wei Jin

A simple one-pot hydrothermal approach was employed to synthesise novel dispersed LiMnPO4 wedges at

200 uC for 10 h. The phase and the morphology of the sample were characterized by X-ray diffraction

(XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It was shown that

the morphology of the dispersed wedges could be directly tuned by varying the reagents amounts which

altered the oversaturation of the solution and consequently the splitting rate as well as degree of LiMnPO4

crystals. The dispersed wedges, the dendritic microspheres and the coarse dendritic microspheres were

evaluated electrochemically by charge–discharge measurements. The results showed that the dispersed

wedges displayed better electrochemical properties than those of the two microspheres, which could be

reasonably ascribed to its great dispersibility and small crystal size. This study can open a new route to

fabricating LiMnPO4 crystals with designed morphology for lithium ion batteries.

1. Introduction

Lithium transition metal phosphates LiMPO4 (M = Mn, Fe, Coand Ni) with ordered olivine structures have attracted muchattention as promising alternative cathode materials to replacethe high cost and toxic LiCoO2 for lithium ion batteries due totheir low price, environmental friendliness, excellent cycle lifeand superior safety properties.1–4 Among them, the LiFePO4

cathode has been successfully developed5,6 and is beingcommercially produced for cells toward large-scale applica-tions, such as automotive and stationary grid-storage.7–10

Nevertheless, LiFePO4 is known to have a low energy densityowing to its low voltage window (3.5 V vs. Li/Li+). LiMnPO4

exhibits a voltage plateau at 4.1 V vs. Li/Li+, which makes thetheoretical energy density about 1.2 times larger than that ofLiFePO4.11 Moreover, the mild voltage of LiMnPO4 is withinthe stable window of commercial electrolytes used in lithiumion batteries, while LiCoPO4 (4.8 V vs. Li/Li+) and LiNiPO4 (5.1V vs. Li/Li+) are not easily compatible with these presentelectrolytes.12–15 However, LiMnPO4 suffers from very poorelectronic and ionic conductivities caused by the Jahn–Telleranisotropic lattice distortion in Mn3+ sites and the largevolume change between the LiMnPO4 phase and MnPO4

phase, leading to low electrochemical activities in lithium ionbatteries.16–18 Therefore, extensive research has been carriedout to overcome such intrinsic obstacles. The most effectivestrategies mainly include particle-size minimization,19,20 elec-

tronically conductive coatings21,22 as well as cationic dop-ing.23,24 In addition, a review reported by Aravindanhighlighted the overview of current research activities onLiMnPO4 cathodes in both native and substituted forms alongwith carbon coating synthesized via various synthetic techni-ques.25

Recent research studies demonstrated that optimization ofmorphology was a critical factor in determining the electro-chemical properties of LiMnPO4 cathodes.26–29 Accordingly,various synthetic routes were applied to control the morphol-ogies of LiMnPO4 crystals for improving the electrochemicalproperties. For example, the authors had already reported thatLiMnPO4 microspheres assembled by plates, wedges andprisms were synthesized via a hydrothermal method, and thesynthesized LiMnPO4 microspheres assembled with platesexhibited higher discharge capacity, more stable cyclingstability as well as better rate capability.30 The large clustersof flower-like particles and the agglomerations of 20–30 nmparticles were synthesized by the polyol method and solid-state reaction, respectively, and the large clusters of flower-likeparticles delivered better cathode performance than thesample prepared by the solid-state method.31 The cluster-likeand the rod-like morphologies of LiMnPO4 samples wereobtained via solvothermal processes in water–organic solventmixtures, and the cluster-like nanoplates delivered muchhigher discharge capacity and rate capability than thenanorods.32 LiMnPO4/C nanocomposites were prepared by acombination of spray pyrolysis and wet ball-milling followedby heat treatment in a range of spray pyrolysis temperaturesfrom 200 to 500 uC, and the sample obtained at 300 uC showedthe best electrochemical performance due to the largestspecific surface area, the smallest primary particle size and a

School of Mechatronics, Jinggangshan University, Jian 343009, China.

E-mail: zhigaomail@gmail.com; xiaoliangpanmail@gmail.com; Fax: +86 796

8100455; Tel: +86 13097226610

3 Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ce40890f

CrystEngComm

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good distribution of carbon.33 Although the above-mentionedLiMnPO4 samples exhibited remarkable achievements forimproving performance, most of their morphologies werecomposed of self-assembled architectures or agglomerations,decreasing the contact area between the cathodes and theelectrolytes, and thus resulting in high electrode polarizationin lithium ion batteries. Thereby, enhancing the dispersibilityof the morphology for LiMnPO4 samples can further increasethe electrochemical reaction zone, leading to low currentdensities and high electrochemical activities. On the otherhand, the hydrothermal process has many advantages overother processes such as its cost, convenience, short reactiontime, mild reaction temperature, and fine crystallinity withhigh purity.

Herein, we report a facile hydrothermal method for thesynthesis of dispersed LiMnPO4 wedges at 200 uC for 10 h. Theeffects of the reagents concentrations in the hydrothermalreaction on the morphologies of the samples were discussed.The possible formation mechanism of the samples wasproposed. The resulting LiMnPO4 samples exhibited consider-able morphology-dependent electrochemical properties.

2. Experimental

2.1. Synthesis of LiMnPO4 samples

All the reagents were commercially available and used asreceived unless otherwise stated. LiMnPO4 samples weresynthesized by a hydrothermal method at 200 uC for 10 h.Na2S?9H2O was employed as an alkaline reagent, since alkalineconditions were necessary for precipitating LiMnPO4 samplesin the hydrothermal synthesis.34,35 In a typical reaction, 17mmol Na2S?9H2O, 40 mmol Li2SO4?H2O, 20 mmol MnSO4?H2Oand 20 mmol NH4H2PO4 were added in sequence in a 40 mLTeflon liner with 30 mL distilled water under constant stirringconditions for 30 min, and the resulting suspension wastransferred into a Teflon-lined stainless steel autoclave, sealedand heated at 200 uC for 10 h. The resulting precipitate wasfiltered, and washed with de-ionized water and ethanol severaltimes, respectively. Then the powder was dried in air at 60 uCovernight.

2.2. Characterizations

The phase of the sample was characterized by X-ray diffraction(XRD, Rigaku D/max-rA diffractometer, Cu Ka radiation, l =1.5406 Å). The morphology of the sample was examined usinga field-emission scanning electron microscope (FE-SEM, FEIQuanta 200F) and transmission electron microscope (TEM) ona JEOL JEM-2100 microscope. The chemical composition ofthe sample was analyzed by inductively coupled plasma-opticalemission spectrometry (ICP-OES, Leeman Labs).

2.3. Electrochemical measurements

The electrochemical measurements were performed with aLand-CT2001A battery test system (Jinnuo Wuhan Corp.,China) by assembly of 2032 coin-type cells. The as-preparedLiMnPO4 sample was mixed with 20 wt% of Super P by ballmilling. Then, the mixture was annealed at 700 uC for 10 h in

an Ar atmosphere to obtain the final LiMnPO4/C composites.The cathode was prepared by mixing 87.5 wt% LiMnPO4/Ccomposites, 2.5 wt% Super P and 10 wt% polyvinylidenefluoride (PVDF) to make the final active material, carbon andbinder in a weight ratio of 70 : 20 : 10, respectively. Thecathode was dried at 120 uC for 10 h under vacuum beforeassembly into a coin-type cell in an argon-filled glovebox(Mbraun, Unilab, Germany). The electrolyte was composed of 1M LiPF6 solution in ethylene carbonate (EC)–dimethyl carbo-nate (DMC) (1 : 1 by volume). The lithium metal was used asthe anode and Celgard 2400 (Celgard polypropylene) wasemployed as a separator. Electrochemical impedance spectro-scopy (EIS) was undertaken on a CHI 660E electrochemicalworkstation (Chenhua Instruments Shanghai Inc., China). Anac voltage signal of 5 mV was used in the frequency range of1022 to 105 Hz. All the electrochemical measurements werecarried out at room temperature.

3. Results and discussion

Fig. 1 shows a typical XRD pattern of the dispersed wedgesprepared via a hydrothermal route at 200 uC for 10 h. Alldiffraction peaks of the sample can be indexed as olivine-typeLiMnPO4 in accordance with the Pmnb of the orthorhombicstructure (at the bottom of Fig. 1) without any visible peaksdue to impurities. The lattice parameters of the dispersedwedges are a = 10.4859 Å, b = 6.12357 Å, c = 4.76822 Å, whichare close to the values in ref. 21, 36 and 37. Similar latticeparameters can be also found in other LiMnPO4 samples (seeTable S1 in ESI3 for details).

SEM images and TEM image of the dispersed LiMnPO4

wedges are shown in Fig. 2. Fig. 2a and 2b are the overall SEMimages, which reveal that the wedges have homogeneousparticle size distribution, uniform shape and well-dispersedmorphology. Upon closer observation of Fig. 2c, the shape ofthe wedges can be seen owing to the gradual change ofthickness along the length direction. The dispersed wedges

Fig. 1 XRD pattern of the dispersed wedges synthesized with 17 mmolNa2S?9H2O, 40 mmol Li2SO4?H2O, 20 mmol MnSO4?H2O and 20 mmolNH4H2PO4 by a hydrothermal method at 200 uC for 10 h.

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with a mean width of 200 nm and length of 1.5 mm graduallychange in the thickness direction, which can be furtherconfirmed by TEM image in Fig. 2d.

According to our previous related work,38 Na2S?9H2O wasemployed as an alkaline reagent to adjust the pH value of thesolution in the synthesis, whereas undesired phases andcoarse crystals were obtained in the absence of Na2S?9H2O asseen in Fig. 3. Moreover, the splitting process was proposed toelucidate the growth mechanism of the LiMnPO4 crystalsprepared with Na2S?9H2O. Crystal splitting depends stronglyon the reagent concentration associated with the solutionoversaturation. Hence, the effect of the reagent concentrationon the morphology of the sample was investigated as follows.

3.1. Effect of lithium precursor concentrations

The effects of Li2SO4?H2O concentrations in the startingsolution on the morphology of the wedges were considered.By keeping other conditions unchanged except strategicallyvarying Li2SO4?H2O concentrations, the morphologies fromthe dispersed wedges to the flower-like microsphere could becontinuously changed, as shown in Fig. 4. In detail, while thereaction was carried out with 30 mmol Li2SO4?H2O, SEM

images of the obtained sample (Fig. 4a and 4b) clearly revealedthat its morphology consisted of dispersed wedges andmicrospheres. By further decreasing the Li2SO4?H2O amountto 20 mmol, dendritic microspheres were obtained as shownin Fig. 4c and 4d. The microspheres with diameters of about10–20 mm were assembled by the wedge-like crystals witharound 500 nm width and 4 mm length. The XRD pattern of thedendritic microspheres could also be indexed to olivineLiMnPO4 without any observable impurity phase (show inFig. S1a, ESI3). These results clearly exhibited that the amountof Li2SO4?H2O could drastically change the morphologies ofthe LiMnPO4 samples under hydrothermal conditions.

3.2. Effect of the starting reagents amounts

To further modify the morphology, the influence of thestarting reagents amounts on the nature of the sample wasinvestigated. In a synthesis using half the concentrations ofthe dispersed LiMnPO4 wedges, coarse dendritic microsphereswith diameters of about 10–20 mm were obtained, as shown inFig. 5. Upon closer observation of Fig. 5b, it can be clearly seenthat the morphological features of the constituent units werenot changed. Moreover, the constituent units of the micro-

Fig. 2 SEM images (a, b and c) and TEM image (d) of the dispersed LiMnPO4

wedges.

Fig. 3 XRD pattern (a) and SEM image (b) of the as-synthesized sample withoutusing Na2S?9H2O in the hydrothermal synthesis.

Fig. 4 SEM images of the LiMnPO4 samples with different amounts of lithiumprecursor: (a and b) 30 mmol; (c and d) 20 mmol.

Fig. 5 SEM images of the coarse dendritic microspheres obtained in a synthesisusing half the concentrations of the dispersed LiMnPO4 wedges.

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spheres were changed into shorter and wider shapes withabout 1 mm width and 2 mm length. The XRD pattern of thecoarse dendritic microspheres perfectly matched the standardorthorhombic LiMnPO4 (show in Fig. S1b, ESI3). These resultsclearly demonstrated that the sizes of the wedges in micro-spheres could be greatly affected by the variations in theconcentrations of the starting precursors.

The chemical compositions of the dispersed wedges, thedendritic microspheres and the coarse dendritic microsphereswere confirmed by ICP, as shown in Table 1. The observedchemical compositions show fair agreement with the stoichio-metric one within 3% deviation.

3.3. Formation mechanism

It is interesting that the changes in the precursors amountscan result in differences in the LiMnPO4 morphologies. Tounderstand the phenomenon of changes in morphologies, welooked into the relationships of concentration, oversaturationand splitting rate as well as degree. The higher the Li2SO4?H2Oconcentration in the mixtures, the larger the number oflithium ions in the reaction medium, which resulted in agreater oversaturation of solution for LiMnPO4. Generallyspeaking, a higher oversaturation of solution is associatedwith a faster crystal growth, which can cause that splitting rateand the degree of crystals are increased.39,40 When the speedof LiMnPO4 crystal growth could not afford a connectionbetween neighboring splitting crystals under the effect of thesplitting mechanism, the wedge-like crystals were split fromsplitting blocks, leading to the formation of the dispersedmorphology. Conversely, the dendritic microspheres could beobtained while the splitting rate and degree were decreased.Moreover, the wedge-like crystal sizes of dispersed morphology

were smaller than that of the dendritic microspheres due to ashorter growth period on splitting blocks associated withfaster splitting rate. Obviously, the splitting rate was remark-ably slowed down when the hydrothermal reaction was

Table 1 Chemical compositions of as-prepared LiMnPO4 samples

Samples Li : Mn : P

Dispersed wedges 1.00 : 0.97 : 1.00Dendritic microspheres 1.00 : 0.99 : 1.00Coarse dendritic microspheres 0.98 : 1.00 : 1.00

Fig. 6 Schematic illustration of the variation of the morphologies for LiMnPO4

samples with the effect of the reagents concentrations.

Fig. 7 The rate charge–discharge capability plots of LiMnPO4 electrodes withdifferent morphologies. (a) The dispersed wedges; (b) the dendritic micro-spheres; (c) the coarse dendritic microspheres; (d) magnification of the charge–discharge plateaus in (a, b and c).

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conducted with half of the starting reagents concentrations,resulting in the formation of the coarse dendritic micro-spheres. On the basis of the above discussions, the strategy tocontrol the morphologies of LiMnPO4 crystals is summarizedin Fig. 6.

3.4. Electrochemical properties

The as-prepared LiMnPO4 samples with different morpholo-gies were mixed with Super P by the ball milling, and thencalcined at 700 uC for 10 h in an Ar atmosphere to obtain theLiMnPO4/C composites. Fig. S2 in ESI3 clearly displays that themorphological features of the samples remained unchangedafter the ball milling and the calcination. Electrochemicalmeasurements were performed to test the electrochemicalproperties of LiMnPO4 electrodes with different morphologiesat room temperature. The cells were charged in galvanostaticmode to 4.5 V, held at 4.5 V until 0.01 C, and then dischargedin galvanostatic mode to 2.4 V.

Fig. 7 is the rate charge–discharge capability plots ofLiMnPO4 electrodes with different morphologies. All of theelectrodes clearly displayed a flat plateau around 4.1 V vs. Li/Li+. This is a typical character of two-phase lithium insertionand extraction processes, suggesting the good development ofthe crystalline phase of LiMnPO4. Furthermore, it was foundthat the discharge capacities were strongly influenced by themorphologies of LiMnPO4 crystals, and the capacity of thedispersed wedges was obviously higher than that of the twomicrospheres. In detail, the dispersed wedges (Fig. 7a)exhibited the discharge capacities of 124 mA h g21 at 0.05 C,107 mA h g21 at 0.1 C, 97 mA h g21 at 0.2 C, 87 mA h g21 at 0.5C, and 82 mA h g21 at 1 C. As to the dendritic microspheres(Fig. 7b), the discharge capacities were 103, 93, 80, 69 and 61mA h g21 at 0.05, 0.1, 0.2, 0.5, and 1 C, respectively.Additionally, the discharge capacities of 74, 69, 61, 52, and45 mA h g21 were reached for the coarse dendritic micro-spheres (Fig. 7c) at a discharge rate of 0.05, 0.1, 0.2, 0.5, and 1C, respectively. The plateaus voltage difference (DV) betweencharging and discharging plateaus reflects the polarizationeffect during the electrochemical reaction, which is associatedwith the diffusion of lithium ion and the electronic con-ductivity of cathodes.41,42 The values of DV, i.e., with capacityof 50 mA h g21 at the charge–discharge rate of 0.05 C (Fig. 7d),for the dispersed wedges, the dendritic microspheres and thecoarse dendritic microspheres were 101, 197 and 389 mV,respectively, which indicated that the kinetics of the cells weregreatly improved for the dispersed wedges. Furthermore, the

comparisons between the discharge capacities of LiMnPO4

cathodes reported by other groups and our work are listed inTable 2. It can be seen that the dispersed LiMnPO4 wedges inthis work exhibited comparable discharge capacity with thatreported in the references in Table 2. It demonstrates that thehydrothermal method is a promising process to obtainLiMnPO4 cathodes with high discharge capacities.

The cycling performance (a) and coulombic efficiency (b) ofLiMnPO4 cathodes with different morphologies was alsocompared, as shown in Fig. 8. After a total of 50 cycles at arate of 0.1 C in the cell potential range of 2.4–4.5 V, thedischarge capacity of 98 mA h g21 was still achieved for thedispersed wedges, and 91.2% of the initial discharge capacitywas retained. By contrast, the dendritic microspheres and the

Fig. 8 The cycle performance (a) and coulombic efficiency (b) curves of LiMnPO4

electrodes with different morphologies at a rate of 0.1 C in the cell potentialrange of 2.4–4.5 V.

Table 2 Comparisons of discharge capacity of the dispersed LiMnPO4 wedges and previous reported data at room temperature

Reference Synthesis route Rate/C Discharge capacity/mA h g21

31 Polyol method 1/13 12032 Solvothermal process 0.05 12033 Spray pyrolysis 0.05 12343 Ultrasonic spray pyrolysis 0.05 11844 Supercritical ethanol process 0.05 y13045 Ionothermal synthesis 0.05 8246 Solvothermal method 0.05 108.2This work Hydrothermal method 0.05 124

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coarse dendritic microspheres displayed greater capacityfading over cycles, exhibiting discharge capacities of 84 and61 mA h g21 at the 50th cycle with capacity retentions of 90.8and 88.7%, respectively. The contrasting results clearlyrevealed that the dispersed wedges showed a higher cyclingstability than those of the two microspheres. Additionally, theenhancement of the coulombic efficiency for the dispersedwedges and the dendritic microspheres (Fig. 8b) could beobserved during the first few cycles. Gradually, the cycling databecame stable in the subsequent cycles, reaching over 98%.The low coulombic efficiency was limited to the first fewcycles, suggesting that any surface reactions occurred onlyduring the initial cycling. More studies will be done todetermine the exact processes in the future.

Nyquist plots obtained from LiMnPO4 electrodes withdifferent morphologies in cells after 50 cycles are shown inFig. 9. It was found that the size of the semicircle was stronglydependent on the LiMnPO4 morphology. A smaller semicirclewas obtained from the LiMnPO4 electrode with dispersedwedges. This could indicate the decreasing of lithium ionmigration and/or charge transfer resistance in the LiMnPO4

electrode with dispersed wedges.From the above results, it can be safely concluded that the

resulting LiMnPO4 samples exhibit considerable morphology-dependent electrochemical properties. Furthermore, the dis-persed wedges clearly exhibit a superior electrochemicalperformance over the two microspheres in terms of dischargecapacity, rate capability and cycling stability. The excellentelectrochemical performance of the dispersed wedges can bereasonably attributed to its morphology advantages. Greatdispersibility and small crystal size for the dispersed wedgeswhich are easily verified by the SEM images are beneficial tofast Li+ ion diffusion rate, short diffusion length as well aslarge contact area between the electrode and electrolyte. As aresults, both the iR potential drop and Li+ diffusion flux at thesurface of the wedges can be decreased, resulting in fastinterfacial charge transfer and low electrode polarization.18

Therefore, morphology advantages of the dispersed wedgesafford the overall achievement of its excellent electrochemicalperformance. Additionally, the above results clearly demon-

strate the electrochemical performance of the LiMnPO4

cathode can be indeed improved by optimization of morphol-ogy.

4. Conclusions

In summary, dispersed LiMnPO4 wedges have been success-fully synthesized by a facile hydrothermal method at 200 uC for10 h. It has been demonstrated that a small amount ofLi2SO4?H2O is in favor of forming the dendritic microspheresassembled with the wedges crystals. It has been found that theconcentrations of starting reagents were very important for theformation of the dispersed morphology. The morphologies ofLiMnPO4 samples are proved to be important for electro-chemical performance. The dispersed wedges clearly exhibitrelatively high electrochemical performance in terms ofdischarge capacity and rate capability, having a dischargecapacity of 124, 107, 97, 87, and 82 mA h g21 at 0.05, 0.1, 0.2,0.5, and 1 C, respectively. The good electrochemical perfor-mance of the dispersed wedges is attributed to its greatdispersibility and small crystal size.

Acknowledgements

We gratefully thank Prof. L. Zhen and Associate Prof. C. Y. Xufrom Harbin Institute of Technology for their valuablecomments and financial assistance. We are also thankful toProf. H. T. Fang from Harbin Institute of Technology for helpin coin cell assembly.

Notes and references

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7814 | CrystEngComm, 2013, 15, 7808–7814 This journal is � The Royal Society of Chemistry 2013

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