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Materials Chemistry and Physics 148 (2014) 933e939

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Materials Chemistry and Physics

journal homepage: www.elsevier .com/locate/matchemphys

Controllable growth of LiFePO4 microplates of (010) and (001) latticeplanes for Li ion batteries: A case of the growth manner on the Li iondiffusion coefficient and electrochemical performance

Yongqiang Wang a, Dongyun Zhang a, *, Chengkang Chang a, *, Lin Deng a, Kejun Huang b

a School of Materials Science and Engineering, Shanghai Institute of Technology, 100 Haiquan Road, Shanghai 200235, Chinab Brighsun New Energy Pty Ltd, 50 Hidden Grove Bvd, Keysborough, Vic, Australia

h i g h l i g h t s

� LiFePO4 microplates grown in (010) and (001) lattice planes were prepared by a controllable method.� Two growth mechanisms based on nucleation and recrystallization process were proposed.� (010) microplates have a higher diffusion coefficient than the (001) ones.� (010) microplates exhibit high specific capacity, showing great potential for application in LIBs.

a r t i c l e i n f o

Article history:Received 23 May 2014Received in revised form16 July 2014Accepted 31 August 2014Available online 16 September 2014

Keywords:NanostructuresChemical synthesisElectron microscopyDiffusionElectrochemical properties

* Corresponding authors.E-mail addresses: dyzhang@sit.edu.cn (D.

(C. Chang).

http://dx.doi.org/10.1016/j.matchemphys.2014.08.0710254-0584/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

Two different LiFePO4 microplates grown in parallel with (010) and (001) lattice planes were obtainedusing a controllable hydrothermal process. The as-synthesized materials were characterized physicallyand electrochemically. XRD, SEM and HRTEM investigations confirmed the preferred growth and twocorresponding growth mechanisms based on nucleation and recrystallization process were proposed.The calculation of Li ion diffusion coefficient along [001] and [010] directions using CV and EIS resultsrevealed that (010) microplates have a higher diffusion coefficient than the (001) ones, implying a goodelectrochemical performance for (010) microplates over the (001) ones. Capacity tests confirmed theabove assumption. Both the two cathode materials showed high specific capacities, and the (010)microplates exhibit a value around 158 mAh g�1 at a rate of 0.5C, showing a great advantage of (010)microplates for future application in LIBs for EV application.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Since the first report of phosphate-based cathodic materials byPadhi et al. [1], the olivine type phosphate LiFePO4 has beenconsidered as an important cathodic material in the lithium-ionbatteries. Such materials have been expecting to have an impor-tant application inhybrid electric vehicles andpure electric vehicles,because of its environmentally friendliness, high theoretical specificcapacity (170 mAh g�1), stable operating voltage (3.5 V vs Li�/Liþ)and good resistance to overcharge and thermal degradation.

However, one of the key drawbacks blocking the widespreadapplication of LiFePO4 is its low intrinsic electronic conductivity

Zhang), ckchang@sit.edu.cn

(10�9e10�10 S cm�1) [2e4], which will cause considerable capacityfading after long term cycling. In order to overcome this problem, alot of methods including doping with supervalent cations [5],coatingwith electronically conducting agents such as graphene andnano carbon tubes [6e9], and nanostructuring the materials havebeen developed recently [10e12].

Among the strategies, nanostructuring LiFePO4 has beenreceiving particular attention because it can be an effective way tobring the batteries a better rate behavior and longer lifetime, for itprovides a short diffusion path for both electron transport and Liþ-ion diffusion for the electrode reactions. Therefore, a lot of syn-thesis methods of the nanostructured LiFePO4 have been devel-oped, such as polyol process, vapor deposition, hydrothermalprocess, solegel, co-precipitation, and microemulsion. Forexample, Wang et al. [13] prepared well-crystallized plateletnanoparticles with a ~30 nm thickness and b orientation. Thismaterial exhibits a specific capacity of 145 mAh g�1 at C/10 and a

Y. Wang et al. / Materials Chemistry and Physics 148 (2014) 933e939934

very long cyclic life. Recently, Saravanan et al. [14] reported amethod to derive hierarchical plate-like LiFePO4/C with a thicknessof 30 nmwhich can achieve a specific capacity of 167mAh g�1 and astable cycle performance. Uchiyama et al. [15] also preparedLiFePO4 mesocrystals consisting of nanorods elongated in the bdirection whose size decreased with an increase of ascorbic acidaddition. The data reported in the documents imply that the elec-trochemical performance could be improved by controlling thegrowth manner (grain size and orientation) of the cathodic mate-rial. However, the mechanism has not been clarified yet.

Herein, we propose a simple hydrothermal process to synthesizeLiFePO4 powder in which the growth manner is controlled byadjusting the molar ratio of the initial materials. Two differentLiFePO4 microplates grown in parallel with (010) plane and (001)plane were obtained. Capacity tests and kinetics calculation revealedthat the difference in electrochemical performance was caused bythe different diffusion coefficients along the b and c axes. LiFePO4microplates grew in (010) plane showed better specific capacity andcyclic behavior than that grew in (001) plane, suggesting great po-tential to serve as cathode materials in LIBs for EV applications.

2. Experimental procedures

LiFePO4 microplates were synthesized by hydrothermalmethod. All reagents were of analytical grade. Two differentmicroplates were obtained by using different ratio of initial mate-rials. For preparation of ac plane microplates, hereafter (010)microplates, a typical process was conducted as follows. 27.8 gferrous sulfate (FeSO4$7H2O) was dissolved in 50mL distilled water,then 11.53 g H3PO4 (85%) was added into the solution. Then 50 mLLiOH solution (6 mol/L) was added into the solution dropwisely,and finally a slight gray suspensionwas achieved. Themolar ratio ofLiþ, Fe2þ, and PO4

3� was controlled at 3:1:1. To increase the elec-tronic conductivity of the cathode material, 5 g sugar was added to

Fig. 1. XRD patterns and SEM micrographs of the prepared LiFePO4 powders. (a) XRD pattemicroplates composed with rectangle plates, and (d) micrograph of (010) microplates.

the suspension and calcinated to form conductive carbon. Thesuspensionwas aged for 0.5 h before it was transferred into a Teflonlinked autoclave and heated in an electric furnace at 160 �C for 5 h.The product was filtrated and washed with distilled water forseveral times, and then dried at 60 �C for overnight.

The preparation of (001) microplates was similar to the syn-thesis route but with minor modification. In a typical process,4.06 g ammonium phosphate tribasic ((NH4)3PO4$3H2O) weredissolved in 20 mL distilled water. Then, 20 mL LiOH solution(2 mol L�1) and 2 g sugar were added into this solution slowly withcontinuous stirring. Finally, 10 mL FeSO4$7H2O (2 mol L�1) solutionwas added with stirring. The molar ratio of Liþ, Fe2þ, to PO4

3� wascontrolled at 2:1:1 in this case. The resulted suspension was heattreated at 200 �C for 18 h.

X-ray diffraction was employed to study the crystal structureand phase purity of the prepared LiFePO4 microplates using Cu Karadiation with a step of 0.02� over the 2q range of 10e70� at roomtemperature. Morphology and particle size of powders wereinvestigated using a scanning electron microscopy (FEI SIRION200).The fine structures and the growth manner of the as-preparedLiFePO4 microplates were analyzed by a transmission electronmicroscopy (JEOL JEM-2100F).

The electrochemical behaviors of the hydrothermally preparedLiFePO4 microplates were evaluated with coin type cells (2016type). Cathodes were prepared by mixing the LiFePO4 powders,acetylene black and polyvinylidene fluoride (PVDF) with a massratio of 80:10:10. N-methyl pyrrolidone (NMP) was used as thesolvent. Cathode films were made by casting the slurry on analuminum current collector using a doctor's blade. The thickness ofthe films was measured to be 20 mm after a cool rolling. Electrodeswith the diameter of 12 mmwere cut and then assembled into thecoin type cells in an argon filled glove box, and lithium metal wasused as anode. The loading of active LiFePO4 material for each coincell is about 4.52 mg for (010) microplates and 4.68 mg for (001)

rn of (001) microplates, (b) XRD pattern for (010) microplates, (c) micrograph of (001)

Y. Wang et al. / Materials Chemistry and Physics 148 (2014) 933e939 935

microplates, that is 0.01265 mol/cm3 for (010) microplates and0.0131 mol/cm3 for (001) microplates. 1 M LiPF6 in ethylene car-bonate/dimethyl carbonate (EC/DMC 1:1 in volume) was employedas the electrolyte. A Celgard 2502 membrane was used as theseparator. The cell was cycled galvanostatically between 2.5 and4.0 V (versus Li/Liþ) at a current rate of 0.5C, or current density70e80 mA g�1. The cyclic voltammogram (CV) tests were carriedout using an electrochemical workstation (CHI604D) at differentscanning rates with the voltage ranging from 4.5 to 2.5 V. Theelectrical impedance spectroscopy (EIS) was evaluated from0.01 Hz to 100 kHz at fully discharging condition using the sameCHI604D electrochemical workstation.

3. Results and discussion

3.1. Phase determination and morphology confirmation

XRD investigations were conducted in order to explore thecrystalline properties of the prepared LiFePO4 cathodematerials. Asshown in the X-ray diffraction patterns of the LiFePO4 powders inFig. 1a and b, the strong and narrow peaks in the obtained XRDpatterns indicate the as synthesized materials are well-crystallized.Referred to the reference data (PDF83-2092), all the samples showapure phase of LiFePO4 with olivine structure that can be indexedinto Pnma space group of orthorhombic system as denoted on thefigures. When compared with the standard XRD profile, reflectionfrom (002) plane is identified in the XRD pattern for (001) micro-plates, as shown in the inset of Fig.1a.While this peak can hardly be

Fig. 2. TEM observations of the (001) microplates. (a) An individual (001) microplate. (b) Enfor one nanorod, showing fine fringes with b axis parallel to the growth direction, and (d)

observed in Fig. 1b, because the characteristic is very weak due tothe atomic absorption of the X-rays. In the standard PDF profile, 83-2092, the intensity of the reflection from (002) plane, withd ¼ 2.3465, weights only 1.4% of the strongest reflection from (311)plane. However, on (001) microplates, the peak intensity of (002)plane increases up to 19.3% of that of (311) plane. Also, an increaseof intensity of (020) plane is observed for (010) microplates asshown in Fig. 1b, and the intensity of (020) reflection peak becomesthe strongest. Both the results imply the preferred growth of thecrystals was obtained through the hydrothermal process.

To confirm the preferred growth of the cathode powders, SEMwas performed and the results are shown in Fig. 1c and d. For (001)microplates, the micrograph shown in Fig. 1c reveals a rectangleplaty shape, with average particle size of 11.8 mm. For (010)microplates, as shown in Fig. 1d, platy crystals are observed with anaverage particle size of 2.3 mm. The results from morphologicalobservation confirmed the preferred growth of the microplates andfurther indicated different growth manner for the two microplates.

3.2. Growth manner determination by HRTEM

High-resolution TEM were conducted to determine the growthmanner of the microplates. Fig. 2a is an overall image of a single(001) microplate where the platy shape was further confirmed.From an enlarged micrograph shown in Fig. 2b, LiFePO4 platelet isassembled by oriented rods of 200 nm inwidth and 3 mm in length.Fig. 2c is the HRTEM image of one single nano rod, onwhich a set oflattice is clear. Electron diffraction pattern of the nanorod is shown

larged micrograph, showing platelets oriented in the aeb plane. (c) HRTEM micrographelectron diffraction pattern, showing the [001] growth manner of the platy crystals.

Y. Wang et al. / Materials Chemistry and Physics 148 (2014) 933e939936

in Fig. 2d, which can be indexed into the orthorhombic structure ofLiFePO4 phase from the [001] axis direction. Therefore, the growthdirection of the nanorods is confirmed as the b axis direction. TEMimages reveal that the microplate is a kind of mesocrystal with thelarge plate surface lies in the aeb plane, viewed along the c axisdirection, with the thickness of 100e200 nm for the platelets. Sucha growth manner provides a short migration path (100e200 nm)for the Liþ ions along the C axis direction.

High-resolution TEM was also conducted to analysis (010)microplates, which is shown in Fig. 3. Fig. 3a is an overall image ofthe powder, while Fig. 3b reveals a single (010) plate. From the highresolution micrograph in Fig. 3c, a clear set of lattice fringes is quitevisible. The spacing between the two fringes is measured as0.47 nm and 1.04 nm, corresponding to the d values of (001) and(100) lattice planes. Therefore, the microplate shows a preferredgrowth within the ac plane. Electron diffraction pattern furtherconfirmed the above results (Fig. 3d). The clear pattern from thesingle crystalline microplate could be indexed into the ortho-rhombic structure of LiFePO4 phase from the [010] axis direction.Therefore, the growth manner of the microplate is confirmedwithin the ac lattice plane. Such a growth manner is somewhatdifferent form the (001) microplates, by providing a migration pathfor the Liþ ions along the b axis direction.

The above two different growth behaviors therefore could beexplained by the effect of the difference in starting materials. Ac-cording to the earlier report by Dokko et al. [16], the growth ofLiFePO4 crystals is greatly influenced by the chemical environment inwhich the crystals were prepared. Simply adjusting the Liþ, Fe2þ, and

Fig. 3. TEM images for (010) microplates. (a) An overview of the microplates, (b) TEM ofrepresent the (100) and (001) lattice plane, and (d) electron diffraction pattern for the LiFe

PO43� molar ratio will lead to different shape (needles to plates) for

the crystals. Wang et al. [17] also found that nano plate LiFePO4 insize around 200 nm could be obtained at molar ratio of 3:1:1 whenuse of PEG as surfactant. For the formation of dumbbell-like micro-structures in larger size, Su [18] and Yang [19] proposed a five stepsgrowth mechanism, which is from the initial nucleation of nanocrystals to the final recrystallization of the micro-sized plates. Basedon these researchworks, the growth behavior of LiFePO4microplatesin our case could be explained schematically in Fig. 4. For the for-mation of (001) microplates, when the Liþ, Fe2þ, and PO4

3� molarratio was kept as 2:1:1, the initial nucleation occurs along the b axis,and the growth along the other two directions was suppressed.Therefore LiFePO4 nanorods were obtained. With time goes on, self-assembly or aggregation begins with the aid of sugar as the surfac-tant. The nanorods were driven by the surfactant and formed aregular arrangement, and finally plate-like microplates grown in abplane were obtained, as illustrated in Fig. 4a. For the (010) micro-plates, as shown in Fig. 4b, since the original molar ratio was fixed at3:1:1, it is reasonable that, microplate crystals, rather than thenanorods will be obtained after the initial nucleation [17]. Such platycrystals grow in ac plane, while the growth along b direction wassuppressed. Therefore, after the self-assembly and recrystallization,LiFePO4 microplates grown in ac plane was obtained.

3.3. Texture electrodes formed after the doctor's blade method

The cathodic electrodes were prepared in the experiment byusing the two different LiFePO4 microplates with the doctor's blade

an individual LiFePO4 microplate, (c) HRTEM of the LiFePO4 microplate, the fringesPO4 microplate.

Fig. 4. Schematic illustrations for LiFePO4 microplates grown in different manner. (a) The formation of LiFePO4 platelets in (001) lattice plane and (b) the formation of LiFePO4 platesin (010) lattice plane.

Fig. 5. XRD patterns for the electrodes of (001) and (010) microplates.

Y. Wang et al. / Materials Chemistry and Physics 148 (2014) 933e939 937

method. After the thoroughly drying of the electrodes, a rollingtreatment was performed to improve the electronic conductivity.Then the electrodes were subjected to XRD measurements beforethe electrochemical tests, and the results are shown in Fig. 5. To oursurprise, the reflection intensities changed a lot, although the peakposition remained the same. On Fig. 5a for the electrode made from(001) microplate, increased (002) reflection was observed. Also, forthe electrode made from (010) microplate, an obvious increasing in(020) reflection was confirmed. Such results could be explained by“shape effect”. Since the powders are consisted of platy particles,during the preparation of the electrodes, the platy particles tends tomove under the shear stress with their surface parallel to the stressdirection, so that electrodes with strong anisotropy were formed.Such textured structures along c and b directions are beneficial forthe electrochemical performance because the migration of Li ionstakes place mainly along b and c axis directions.

Fig. 6. CV curves for the prepared LiFePO4 cathodes at different sca

3.4. Calculation of Li ion diffusion coefficient from CV and EIS curves

The textured electrodes provided a chance to measure thediffusion coefficient along [001] and [010] directions for the twodifferent microplates. Fig. 6 shows the CV curves of the (001)microplates and (010) microplates, respectively at scan ratesfrom 0.2 mV s�1 to 0.02 mV s�1 after the cells were activated at ascan rate of 0.01 mV s�1 for two cycles. The good symmetry ofthe oxidation and reduction peaks in the CV plots reveals a goodreversibility of lithium extraction/insertion reactions in thecathode materials and therefore employed for calculating thediffusion coefficient of Li ions in different directions, the [001]and [010] channels. According to Randles Sevcik equation, thepeak currents Ip (Amperes), during anodic scans with differentscan rates, can be used to extract the Li-ion diffusion coefficient D(cm2 s�1) [20]:

n rates. (a) For (001) microplates and (b) for (010) microplates.

Table 1Li ion diffusion coefficients calculated from CV results under different scan rates.

(001) microplates (010) microplates

Ip (A) v (V s�1) D (cm2 s�1) Ip (A) v (V s�1) D (cm2 s�1)

Dischargingprocess

0.00196 2E-4 1.21E-9 0.00268 2E-4 2.42E-90.00134 1E-4 1.13E-9 0.00184 1E-4 2.28E-98.94E-4 5E-5 1.02E-9 0.0013 5E-5 2.29E-95.8E-4 2E-5 1.06E-9 8.4E-4 2E-5 2.39E-9

Average 1.105E-9 2.35E-9Charging

process0.00191 2E-4 1.15E-9 0.00239 2E-4 1.93E-90.00124 1E-4 9.69E-10 0.00169 1E-4 1.92E-98.7E-4 5E-5 9.54E-10 0.00118 5E-5 1.89E-95.7E-4 2E-5 1.02E-9 7.4625E-4 2E-5 1.88E-9

Average 1.03E-9 1.91E-9

Table 2Diffusion coefficient calculated from EIS results.

(001) microplates (010) microplates

A (U s�1/2) 14.33 7.56dE=dx (V mol�1) 1.32 1.19DLi (cm2 s�1) 6.95 � 10�10 2.04 � 10�9

Y. Wang et al. / Materials Chemistry and Physics 148 (2014) 933e939938

Ip ¼ 2:69� 105 � SCD1=2n3=2v1=2 (1)

where S is the electrode area (1.13 cm2 for electrodes with diameterof 12 mm), C is the shuttle concentration, 0.01265 mol cm�3 for(010) microplates and 0.0131 mol cm�3 for (001) microplates,calculated from the weight of the cathode film, n is the number ofelectrons involved in the redox process (in the case of LiFePO4, n¼ 1for Fe2þ/Fe3þ pair), and v is the potential scan rate (V s�1).

Table 1 shows the calculated diffusion coefficients for twodifferent microplates. The values vary from 9.54 � 10�10 cm2 s�1 to2.42 � 10�9 cm2 s�1, and remain almost the same order of thevalues as the reported elsewhere [17], where diffusion coefficient of4.2�10�9 cm2 s�1 was obtained. By comparing the data obtained inour experiment, Li ion diffusion coefficient along [010] direction, ismuch high than that along [001] direction, both in the dischargingprocess and in the charging process, indicating a fast Li ion diffusionalong [010] direction than the [001] direction. Such result is inconsistence with the theoretical prediction by Islam [3] that thediffusion energy along [010] direction is the lowest among thepossible three diffusion paths (0.55 V for [010] direction, 2.89 V for[001] direction and 3.36 V for [101] direction).

We carried out electrochemical impedance spectra (EIS) tocalculate the diffusion coefficients. Fig. 7a shows the Nyquist plotsunder open-circuit (3.5 V) condition for the twomicroplaty cathodematerials at fully discharged condition. The semicircles have a high-frequency intercept that identifies the ionic conductivity of theelectrolyte. At lower frequencies, the resistance is related to thecharge transfer between the electrolyte and the active material. Atvery low frequencies (0.01 Hze1 Hz in our case), a typical Warburgbehavior could be attributed to the diffusion of lithium ions in theLiFePO4 cathode material. By using the model proposed by Ho et al.[21], the diffusion coefficient of Li ions can be calculated by usingEq. (2):

Fig. 7. Electrochemical impedance results. (a), Nyquist plot of the impedance spectroscopytionship between the Warburg impedance and the inverse square root of angular frequenc

DLi ¼ 1 2½ðVM=SFAÞðdE=dxÞ�2 (2)

.

where VM is the phosphate molar volume (44.11 cm3 mol�1 forLiFePO4), S is the contact area between electrolyte and sample(1.13 cm2), F the Faraday constant (96486C mol�1), and dE=dx theslope of the coulometric titration curve can be obtained from thecyclic curves. The constant A from the Warburg impedance can beobtained according to the following equation:

W ¼ Au�1=2 (3)

u is the angular frequency, and can be express as a function of thefrequency employed in the test by the following equation:

u ¼ 2pf (4)

Fig. 7b shows the plot of the imaginary resistance determined byIS as a function of the inverse square root of the angular frequencyfor the two microplaty cathode materials. A linear behavior wasobserved for frequency values ranging from 0.01 Hz to 1 Hz, with aslope of 14.33 U s�1/2 for (001) microplates and 7.56 U s�1/2 for(010) microplates. Therefore, the diffusion coefficients can becalculated, and listed below in Table 2. It's obvious that EIS resultsalso suggest high diffusion coefficient for (010) microplates.

3.5. Electrochemical performances of the microplates

The difference in growth manner greatly influences electro-chemical performance. Charging and discharging curves of the twohydrothermally prepared LiFePO4 microplates are shown in Fig. 8.The tests were carried out at 0.5C (80 mA/g). In both cases, veryflat plateaus were observed at the potential of 3.4e3.5 V versus Li/Liþ. These flat plateaus can be explained by the solid-state redox ofFe2þ/3þ in the LiFePO4 matrix, accompanying with Liþ ion extrac-tion and insertion [1]. The initial discharge capacity of the (001)microplate was 150 mAh g�1, which was a little bit smaller than thetheoretical capacity (170 mAh g�1). After 10 cycles, the degradationin capacity was observed as shown in Fig. 8a. Such electrochemicalperformance for LiFePO4 materials in micro size is acceptable, dueto the very low electronic conductivity and slow diffusion of Liþ ion

(IS) of coin cell assembled with (001) and (010) microplates, and (b), the linear rela-y, the slopes of the simulated lines are the Warburg constant for the microplates.

Fig. 8. Electrochemical performances for (001) and (010) microplates. (a), Chargingedischarging plots at the first ten cycles for (001) microplates, (b), Chargingedischarging plotsfor (010) microplates and (c), the cyclic performance of the two different microplates.

Y. Wang et al. / Materials Chemistry and Physics 148 (2014) 933e939 939

in the olivine structure [2�4]. On the contrary, the electrochemicalperformance for the (010) microplate shown in Fig. 8b revealedvery high initial capacity of 160 mAh �1, and the fading in specificcapacity was hardly observed after 10 cycles. A detailed comparisonof the chargingedischarging datawas illustrated in Fig. 8c, inwhichthe difference in electrochemical behavior of the two materials isquite visible. Such results demonstrate better electrochemicalperformance of (010) microplates than the (001) microplates,indicated the advantage of (010) microplates over the (001)microplates and further suggested great potential of (010) micro-plates for future application in LIBs for EV application.

4. Conclusions

In summary, we presented in this paper a controllable hydro-thermal route to tune the growth manner of plate like LiFePO4cathode materials with a good electrochemical performance. XRDand SEM results indicated preferred growth of themicroplates withdifferent growth manner, which are in (001) lattice plane and theother in (010) lattice plane. Such results were further confirmed byhigh resolution TEM observations. The difference could be ascribedto the slight difference in the molar ratio of Li: Fe: P in the startingmaterials during the hydrothermal process. The two directionswere calculated from CV and EIS curves. The calculated valuesindicated that (010) microplates have a high diffusion coefficientthan the (001) ones, and therefore a good electrochemical perfor-mance for (010) microplates over the (001) ones are expected.Capacity tests indicated that both the two cathode materials showgood specific capacities and the (010) microplates have a value

around 158 mAh g�1 at a rate of 0.5C, showing a great potential forfuture application in LIBs for EV application.

Acknowledgments

The authors are grateful to the financial supports from thegrants with contract No. NSFC212031201, 11ZR1435900,10ZR1415400, 10JC1406900, 12YZ163.

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