Qian Xie ,Jihua Zhu ,Chengyun Wang ,Kaibin Fang ,Wei Yang, *,Quanbing Liu *,Yali Wang,Shengzhou Chen

1 School of Chemistry and Chemical Engineering,Guangzhou University,Guangzhou 510006,China

2 Sunwoda Electronics Co.,Ltd.,Shenzhen 518107,China

3 GAC Automotive Research &Development Center,Guangzhou 511434,China

4 School of Chemical Engineering and Light Industry,Guangdong University of Technology,Guangzhou 510006,China

Keywords:LMnPO4 Polyols Lithium-ion battery Phosphate material Crystal

ABSTRACT Phosphate material LiMnPO4 is popular for its high energy density (697 W.h.kg-1) and safety.When LiMnPO4 crystal grows,the potential barrier along b and c axis is strong,which makes the crystal grow along b axis to form a one-dimensional chain structure.However,the main migration channel of lithium ions in olivine structure is plane(0 1 0).By shortening the growth in the direction of b axis and enhancing the diffusion along the directions of a and c,two-dimensional nanosheets that are more conducive to the migration of lithium ions are formed.The dosage of polyols is the key factor guiding the dispersion of the crystals to the (0 1 0) plane.X-ray diffraction (XRD),Scanning electron microscopy (SEM),transmission electron microscopy(TEM)and other means are used to characterize the samples.After experiments,we found that when the ratio of polyol/water was 2:1,the morphology of the synthesized sample was 20–30 nm thick nanosheets,which had the best electrochemical performance.At 0.1C,the discharge specific capacity reaches 148.9 mA.h.g-1,still reaches 144.3 mA.h.g-1 at the 50th cycle.and there is still 112.5 mA.h.g-1 under high rate (5C).This is thanks to the good dispersion of the material in the direction of the crystal plane(0 1 0).This can solve the problem of low conductivity and ionic mobility of phosphate materials.

1.Introduction

In 1997,Goodenough‘s team discovered LiMPO4(M is Fe,Mn and other transition metal elements),in which lithium iron phosphate has been applied successfully,and has reached large-scale production in recent years [4].It became the main positive electrode material of 3.4 V platform.However,in areas such as electric vehicles that require more powerful batteries,the 3.4 V LiFePO4platform cannot meet this requirement.Therefore,people began to develop positive electrode materials for high voltage charging and discharging platforms(4 V)such as LiMnO4,LiCoO2or ternary materials.All of these materials were produced on a large scale,and because of platform differences,lithium iron phosphate is not common to them.The discharge platform of lithium manganese phosphate can reach 4.1 V,which solves the problem of incompatibility between lithium iron phosphate and layered LMO (M=Ni,Co,Mn) [1–5].

Moreover,the olivine-structured LiMnPO4cathode material has the advantages of low material cost,high energy density,stable chemical structure and environmental friendliness.However,when LiMnPO4crystals grow,the potential barrier diffusing along Axis a and c is relatively strong,which makes the crystals grow in the direction of Axis b in a preferred direction and form a onedimensional chain structure.This means that the transport of lithium ions can only depend on (0 1 0) plane.Octahedron LiO6is arranged as a chain in the direction of axis b with common sides,and Li+performs one-dimensional diffusion in the direction of plane (0 1 0) [6,7].This diffusion pattern tends to result in low ion mobility.Therefore,research and modification of LiMnPO4has become a hot topic for lithium battery workers [8–17].

The synthesis method of LiMnPO4includes a solid phase method,a sol–gel method,a hydrothermal method,a template method,and the like.Among them,in the hydrothermal method or solvent-thermal method,the crystal in the hydrothermal reactor through nucleation →growth →aggregation →recrystallization development process[18].It has been reported in many literatures that hydrothermal method or solvent-thermal method can control the crystal morphology more simply and can usually prepare smaller size particles.Pan et al.[19] used a simple hydrothermal method to synthesize highly dispersible LiMnPO4plate-like structure.At 0.05 C,25 °C,the specific discharge capacity reached 139.2 mA.h.g-1.Liu et al.[20]successfully prepared nanometer lithium manganese phosphate(LiMnPO4)sheet-like particles by solvothermal method in ethylene glycol solvent.The discharge specific capacity reached 130 mA.h.g-1at 0.1 C.Su et al.[21].used diethylene glycol as a solvent and reacted at 190 °C for only 5 h via solvent-thermal method to obtain an irregular sheet structure which was aggregated into a particle group of about 3 μm in diameter similar to an embroidered sphere.The material has a specific discharge capacity of 131.4 mA.h.g-1at 0.1 C.Moreover,the retention rate reached 99% after 50 cycles at 1 C.Therefore,the kind of the solvent has a great influence on the structure of the synthesized LiMnPO4.The participation of the organic solvent in the solvothermal reaction can not only control the morphology of the crystal but also provides a high boiling point and an improved crystal growth dual role and can inhibit the formation of Mn2+impurities as a stabilizer for crystal growth [22–25].

In the process of crystal growth,nucleation is the first process.Once the crystal nucleus is formed,a crystal-liquid two-phase interface is formed,and the atoms and ions composing the crystal are piled up to form the crystal according to the arrangement of crystal structure [26–32].At the two-phase interface,the orientation of crystal growth can be changed by some factors,media or modifiers.Adjusting the phase and morphology of the product,the organic solvent acts as a soft template to play a decisive role in the growth of the material particles.In addition,its high viscosity reduces the diffusion rate of the atoms and ions that make up the crystal.It can reduce the growth rate of the nanocrystals and adjust the orientation of the crystal growth,so that the nanoparticles are further closely arranged to form different morphologies of precursor [24,33].

After investigation,we know that the preferential orientation of LiMnPO4crystal of orthogonal crystal system along the plane direction of(0 1 0)is more conducive to the release and embedding of electrochemical active lithium ions [2,15,34–37].The diffusion of ions on some of the adsorbed plane that is adsorbed by a certain modifier is restricted to enhance the diffusion on the orthogonal plane.Xu et al.[38],when tetragonal perovskitelead titanate(PZT) was prepared by hydrothermal method,PVA was adsorbed on the(1 0 0) and(0 1 0) surfaces to help the crystal surface grow along the (0 0 1) direction.They also reported the crystal growth trend along the direction of (0 1 0) in the preparation of LiMnPO4by ethylene glycol system (1:1).However,they attributed most of the reason to the binding of MnO6octahedral parallel to the(0 1 0) crystal plane with CH3COO-ions,thus inhibiting the decomposition along the (0 1 0) direction [33].In order to explore the influence of the selective adsorption of polyol organic solvent on the morphology and electrochemical properties of LiMnPO4crystal,we set up several organic solvent systems with different concentrations to conduct experiments on the basis of the predecessors.

It has been reported in many literatures that the adsorb-ability of polyols plays a decisive role in the preferred orientation of crystal growth.Li et al.[39,40]investigated the preparation of vaterite by solvent-thermal method for polyols such as ethylene glycol.The results show that the influence of polyols on vaterite formation lies in the adsorption of vaterite crystal nucleus by electric field generated by hydroxyl group in polyols.The results show that the influence of polyols on vaterite formation lies in the adsorption of vaterite crystal nucleus by electric field generated by hydroxyl group in polyols.Liange Shi et al.[12]also modified strontium carbonate by solvent thermal method with ethylene glycol.In their experiment,when the polyol dosage was 30 ml,the adsorption capacity was weak and the expected product was not achieved.Only when the amount of polyol reaches 60 ml can the polyol have strong adsorption effect,and a six-sided ellipsoid with tapering ends at the top of the dense surface is synthesized.All of these indicated that polyols had specific adsorption and guided the preferred orientation of a certain crystal surface during crystal growth.

Herein,polyethylene glycol 600(PEG-600),polyethylene glycol 400 (PEG-400) and glycerol were used as solvents to investigate the effect of polyol adsorption on crystal formation of LiMnPO4(LMPO) by solvothermal method.We used different polyol dosage(the proportion of polyol/water) to investigate its adsorption capacity.

2.Experimental

2.1.Sample preparation

The sample was prepared with LiOH.H2O dissolved in a certain proportion of water-organic solvent binary solvent was prepared as 1 mol.L-1solution A.H3PO4(85% (mass)) was dissolved in the same binary solvent to obtain 0.5 mol.L-1solution B.Add 20 ml solution B to solution A at A rate of 1 ml?min-1,and stir continuously for 10 min to obtain white suspension C.Different organic solvents(PEG-600,PEG-400,glycerol)were mixed with water into 5 binary solvents in different proportions in polyol/water ratios of 3:1,2:1,1:1,1:2,and 1:3.0.01 mol MnSO4was dissolved in 20 ml binary solvent to obtain solution D,and a small amount of(0.005 mol.L-1) L-ascorbic acid was added as the metal antioxidant.Solution D was then drip-added to the suspension C at a rate of less than 2 ml?min-1,and strongly stirred for 15 min before being transferred to the inner lining of the reactor.After the heat preservation reaction at 180 °C for 10 h,the resulting liquid was taken out for high-speed centrifugation,washed several times with deionized water and ethanol respectively,and dried for 12 h to obtain all kinds of LMPO samples.Samples obtained in different solvent systems such as PEG-600,PEG-400 and glycerol were marked as A-LMPO,B-LMPO,C-LMPO.The samples obtained by different proportions (1:3,1:2,1:1,2:1,3:1) of polyol/water are marked as X-LMPO-1,X-LMPO-2,X-LMPO-3,X-LMPO-4 and XLMPO-5 (X=A,B,C),respectively.The blank group of samples was named as LMPO-0.

2.2.Material carbon cladding

Using glucose as the carbon source,LMPO was mixed with grape sugar ball mill and dried,then transferred to a tube furnace,which was raised from room temperature to 600 °C at 3 °C.min-1,roasted in nitrogen atmosphere for 8 h,and cooled to room temperature to obtain LMPO/C.

2.3.Material characterization

Fourier transform infrared spectrometer(U.S.Brooke FT-IR Tensor Ⅱtype) was used to characterize molecular structure of LMPO.The scanning wavelength range is 4000–400 cm-1.The crystal structure of LMPO/C samples was characterized by X-ray diffraction (PANalytical Xpert Pro,Netherlands).The Cu target excited the Kα ray source,the incident wavelength=0.15406 nm,the scanning Angle 2θ=5°–80°,and the scanning speed was 10 (°).min-1.JADE was used to refine the XRD pattern to estimate the lattice parameters.Scanning electron microscopy(SEM,JSM-7001F,JEOL,Japan) was used to characterize the morphology of the samples.The material was uniformly dispersed with ethanol solution and observed at an acceleration voltage of 10–20 kV.The microstructure and crystal structure of the samples were identified by transmission electron microscopy (TEM).

2.4.Electrochemical measurement

First,N-methyl pyrrolidone (NMP) was used as dispersant to mix 80% (mass) active material,10% (mass) acetylene black and 10%(mass)polymer binder(PVDF)evenly.Then the paste is coated on the aluminum foil and cut into a diameter of 12 mm circle to obtain a positive plate.Then the paste is coated on the aluminum foil and cut into a diameter of 12 mm circle to obtain a positive plate.A circular monolayer polypropylene with a diameter of 16 mm is used as the diaphragm.Constant current charge–discharge is performed in the voltage range of 2.5–4.5 V (vs.Li/Li+)to test the cycling and multiplier properties of the material (Neware BTS-5V-10 mA).The electrochemical workstation (CHI600E,China) tests the cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS).The voltage window of CV is set to 2.5–4.5 V(vs.Li/Li+)at the scanning rate of 0.1 mV?s-1.The amplitude of EIS is 5 mV,and the frequency range of it is 10-2-105Hz.All of these tests were preceded at 25 °C.

3.Results and Discussion

3.1.Structural characterization of materials

In the FTIR spectra of LMPO/C samples (shown as Fig.1a),it is composed of four parts.The band at 980 cm-1corresponds to the symmetrical extension mode of O—P—O,and the band at 1060,1100 cm-1belongs to the asymmetric stretching vibration of P—O.The wavebands around 640 cm-1are in asymmetric bending mode,while 580,550,500 and 455 cm-1are in PO43-bending mode.All the materials prepared by different solvent thermal methods have complete IR spectra,and the absorption spectra of the materials show that all the absorption bands conform to the characteristics of LMPO/C.

The LMPO/C samples synthesized under different solvent systems were characterized by XRD.The A-LMPO/C samples synthesized under different solvent systems were characterized by XRD(Fig.1b).The XRD patterns of B-LMPO/C and C-LMPO/C are shown in the supporting document (Fig.S1a and Fig.S1b in Supplementary Matrial).All the characteristic peaks of Samples were matched with the standard card (JCPDS NO.33-3080),which indicated that we successfully prepared the ordered olivine structure LMPO/C with Pnmb space group.According to the position of the diffraction peak,the sharpness of the peak and the intensity of the peak,it is judged that they have high purity and good crystallinity.From another perspective,the XRD peak strength of different samples varies,which indicates that LMPO/C in different systems grows along different preferential orientations.However,what is most noteworthy is that with the increase of polyol dosage,the peak corresponding to the (2 0 0) plane is relatively higher.

From the XRD patterns,we can see that the crystal crystallinity of the sample with a high proportion of polyols is relatively good in the direction of plane (2 0 0).This implies good diffusion in the(0 1 0) oriented plane orthogonal to (2 0 0) during crystal growth,which must result in an increased exposure rate of the primary particle surface(0 1 0).That’s what we want.The ratio of key peaks is also indicative of crystal growth.The ratio of I200to I020(Table 1)indicates the orientation growth of the crystal.The higher the numeric value,the higher the exposure rate of the (0 1 0) plane parallel to the(0 2 0) plane.In addition,the larger numeric values of I200/I111and I200/I131are,indicating that crystal growth along the orthogonal (0 1 0) plane orientation scilicet inhibits the growth along the (0 1 0) plane direction.It is highly possible to obtain(0 1 0) plane dominant flat particles.With the increase of the dosage of polyols,these numeric values also increase,which indicating that the adsorption of polyols affected the growth of preferred orientation of crystals.And it can be seen that the amount of polyol is the key to the strength of adsorption.This is similar to the results reported in the literature [12].

Table1 Ratios of key peaks for different samples

3.2.Morphology characterization

The SEM images of LMPO-0/C and each sample of A-LMPO/C are shown in Fig.2 (The SEM images with different proportions of BLMPO/C and C-LMPO/C are shown in Fig.S2 and Fig.S3,respectively).First of all,it is clear that the LiMnPO4obtained via solvent thermal reaction has smaller particles.The most important information in the SEM images is that when the water ratio is large,the system is dominated by water,and the morphology is like a bar (length 200–300 nm,diameter 80–100 nm,Fig.2a,b and c).When the ratio of water to polyols was 1:1,the length of the nanorods was significantly reduced (length of 120–150 nm,diameter of 50–60 nm,Fig.2d),because the contribution of polyols was manifested,and the increased viscosity and adhesion changed theorientation of crystal growth.When the ratio of polyols exceeds that of water,polyols occupy a dominant position in the solvent thermal reaction,and the morphology of LMPO-1/C is similar to that of 2D nanosheets(size 120 nm×150 nm and thickness 20–30 nm,Fig.2e and f).The morphology changed with the increase of polyol dosage,which perfectly verified the XRD pattern.

Fig.1.(a) FTIR of samples in different solvent systems;(b) the XRD pattern of A-LMPO/C samples.

Fig.2.(a) The SEM image of LMPO-0/C;(b,c,d,e,f) the SEM images of A-LMPO-1/C,A-LMPO-2/C,A-LMPO-3/C,A-LMPO-4/C,A-LMPO-5/C,respectively.

The morphology changes of SEM images are highly consistent with the characterization results of XRD patterns,which is closely related to the adsorption of polyols.Because of the difference in viscosity,boiling point and other physical properties,the shape and size of samples prepared by different types of polyols are slightly different.Most importantly,with the increase of polyol dosage,the morphology of LMPO primary particles in the three systems changed from columnar to 2D nanosheet structure,which has proved that the function of PEG600 system is not unique.We hypothesized the growth process of LiMnPO4in a solvent thermal reaction dominated by polyols(see Fig.3).The adsorption of polyols leads the LiMnPO4to grow into a 2D nano-laminae in the direction we expect.The plane with the largest area is dominated by(0 1 0).Due to the adsorption of polyols on the lower(0 1 0)plane,the growth of crystals on the (0 1 0) surface is restricted.So the crystal grows in the direction of its orthogonal planes (1 0 0) and(0 0 1).This method of controlling crystal growth is similar to the literature [33,41],and the different results of different polyol dosage also explain that sulfate isn’t the dominant factor.Growth restriction along the (0 1 0) plane of the crystal inevitably results in the formation of primary structures of 2D laminae with high exposure rates of (0 1 0) plane.

In order to prove that the growth process dominated by polyol was correct,HRTEM tests were performed on the front of the nanosheet (see Fig.4b).SEAD (Fig.4c) images can indicate that HRTEM images are formed by individual primary particles.Lattice fringes spaced at 0.302 nm correspond well to(0 1 0)plane.Moreover,the surface of the nanosheets are covered by a uniform carbon coating with a thickness of about 1.8 nm.The carbon conductive layer is conducive to improving the electronic conductivity of the (0 1 0) plane.These representations are consistent with the process we derive.The 2D nanosheet structure was obtained by selective adsorption of polyols on the (0 1 0) plane to restrict its growth (Fig.4d).

3.3.Electrochemical test and others

Fig.3.The growth process of LiMnPO4 nanosheets material dominated by polyols.

Fig.4.(a)SEM image of nanosheets A-LMPO/C;(b)HRTEM of nanosheet A-LMPO/C;(c)SEAD images of the corresponding sample;(d)Growth process of lamellar LiMnPO4.

Fig.5.(a) The charge–discharge curve of LMPO-0/C sample;(b–f) The charge–discharge curve of A-LMPO-1/C,A-LMPO-2/C,A-LMPO-3/C,A-LMPO-4/C,A-LMPO-5/C,respectively;(g)Rate capability of LMPO-0/C and A-LMPO/C samples at different discharge rates;(h)Cycling capability of relevant samples;(i)Rate capability of the literature[13].

Fig.6.(a and b) SEM and charge and discharge curves of B-LMPO-4/C samples;(d and c) SEM and charge and discharge curves of C-LMPO-4/C samples.

All LMPO samples were loaded into CR2032 battery for charging-discharge test,and the charging-discharge curve and rate performance diagram of were obtained.The discharge capacity of the LMPO-0/C sample was only 84.5 mA.h.g-1at 0.1 C (1 C=170 mA.g-1,see Fig.5a).It’s probably the tiny rod-shaped particles at either end that are not conducive to Li+migration.After the introduction of polyol,the charge and discharge capacity of the material was improved significantly.When the volume ratio of water to PEG-600 was 3:1,the discharge capacity was 104.5 mA.h.g-1(See Fig.5b).With the increase of polyol dosage,the material capacity becomes higher and higher.When the volume ratio of water/PEG-600 was 1:1,the charging capacity was 140.0 mA.h.g-1while the discharge capacity was 130.0 mA.h.g-1,and the coulomb efficiency(CE)was 92.9%(see Fig.5d).This is due to the increased exposure rate of(0 1 0)because of adsorption of polyols.When the volume ratio of water/PEG-600 was 1:2,the dominant position of solvent thermal reaction was taken over by PEG-600,resulting in 2D nanosheets self-assembled particles with high exposure(0 1 0)plane.Compared with the nanorods,the nanosheets exhibit better electrochemical performance,which is almost consistent with the situation in the literature [42].The one-dimensional Li+channel dominated by plane (0 1 0) is opened to the maximum extent.The capacity of the material was well exerted,the discharge capacity reached 148.9 mA.h.g-1(as shown in Fig.5e),and the CE reached 93.6%.This is basically consistent with the results predicted in XRD and morphology characterization.

When the volume ratio of water to polyol is 1:2,the discharge specific capacity of different polyol samples is the best.At 0.1 C,the discharge capacity of B-LMPO-4/C sample and C-LMPO-4/C sample reached 145.4 mA.h.g-1and 145.2 mA.h.g-1,respectively(Fig.6b and c).Other electrochemical data of B-LMPO/C samples and C-LMPO/C samples can be seen on the supporting document(shown as Fig.S4 and Fig.S5,respectively).This well proves that the crystal structure modification is the selective adsorption of hydroxyl groups with strong adsorption by polyols,rather than PEG-600 alone.Thanks to its short-range and spacious Li+migration channel,nanosheets materials have excellent Rate properties.(see Fig.5g) Under 5 C,the discharge capacity also had 112.5 mA.h.g-1.And it also has good cycling performance.The retention rate of 100 cycles under 1 C is more than 90% (see Fig.5h).The performance of our LiMnPO4using a simple solvothermal method is comparable to that of LiMnPO4modified with a 3D honeycombstructure of LiAlO2[13].However,when the proportion of polyols is larger,the capacity of the prepared material decreases.This is most likely attribute to the large area and excessive surface energy of the 2D nanosheet,which causes the structure of the material to collapse during the electrochemical process.

Through the preliminary test of the electrochemical performance of the material,we judge that the dosage of polyol should not be too large.Furthermore,we found that when the ratio of polyol to water was 7:1,the preferred orientation of crystal growth changed significantly,and the morphology changed to a spindle structure (see Fig.S6).The Li+migration channel will become worse.

In the test of cyclic voltammetric (CV) characteristics shown as Fig.7a,the REDOX reaction voltages of Mn3+/Mn2+for the positive electrode LiMnPO4were 4.36 V and 4.12 V respectively,and the pressure difference was 0.24 V.When the volume ratio of water to polyol is 1:2,the peak current of A-LMPO-4/C material is the largest according to Eq.(1),indicating that the Li+diffusion rate of the material is the fastest.At the same time,the voltage difference of the REDOX reaction is small,indicating that the material has low polarization and good cycling stability.This also confirms once again that 2D nanosheets with a high surface exposure rate of(0 1 0) have a higher Li+mobility.

At 25°C,the positive pole peak current value(Ip)is proportional to the square root of the scanning rate(υ),Randles-Sevcik equation can be expressed as:

where Ipis current maximum in amps,n is number of electrons transferred in the redox event (usually 1),A is electrode area in cm2,D is diffusion coefficient in cm2.s-1,C is concentration in mol.cm-3,v is scan rate in V.s-1.

Fig.7.(a)CV curves of synthesized LMPO-1/C samples;(b)EIS of the LMPO-1/C samples;(c)CV curves and EIS of B-LMPO-1/C,B-LMPO-4/C and B-LMPO-6[When the volume ratio of polyols to water is 5:1,the sample is labeled as X-LMPO-6(X=B,C).]/C,respectively;(d)CV curves and EIS of C-LMPO-1/C,C-LMPO-4/C and C-LMPO-6/C,respectively.

The Li+diffusion impedance in LMPO/C was assessed using a button cell test EIS in a fully discharged state.The simulated equivalent circuit diagram of the internal impedance of the battery was shown in Fig.7b.In the EIS diagram,the diameter of the semicircle corresponds to the Li+diffusion impedance.When the volume ratio of water to polyol is 1:2,the high frequency to medium frequency semicircle diameter is the smallest among all the samples(see Fig.7b).It shows that the LMPO/C sample with 2D nanoplate morphology is more conducive to improving the Li+transfer between material interfaces and improving the dynamics of Li+de-nesting/insetting.Coincidentally,under the same conditions(polyol/water=2:1),samples of PEG-400 system (B-LMPO-4/C)and glycerol system (C-LMPO-4/C) showed the same results on CV and EIS (shown as Fig.7c and d).

Why the ratio of polyols to water shows the best performance at 2:1 is a question worth pondering.There is no doubt that the nano-sheet structure with favourable monodispersity is indeed conducive to the diffusion and penetration of Li+,which is consistent with the conclusion in this literature [43].When the proportion of polyols is too large (polyols/5:1),the electrochemical properties become worse.Fig.8a and b perfectly illustrate the cause of this problem.In the SEM images of samples B-LMPO-6/C and C-LMPO-6/C,the stacking (PIP of Fig.8a) and agglomeration(PIP of Fig.8b)of nanosheet can be seen.As indicated by the ratios of key peak of the XRD patterns,too large dosage of polyol leads to synthesize the oversize of the LMPO nanosheet.In other words,if the size of plane dominated by (0 1 0) is too large,the surface energy will become higher,which will easily lead to stacking phenomenon,and even cause the nanosheet to break and collapse.Because of so,the monodispersity of the LMPO nanosheet becomes low,leading to the deterioration of the electrochemical performance of the material,which is the analogy as the conclusion in the literature [41].The value of REDOX peaks in the CV curve can also explain this.The value of Ipdecreases when the polyol dosage exceeds 2:1 (Ipvalue is proportional to

4.Conclusions

Fig.8.(a) SEM of the sample B-LMPO-6/C,which partially stacked;(b) The agglomeration of sample C-LMPO-6/C was shown by SEM.

In summary,as reported in the literature,the growth of LiMnPO4crystals was affected to varying degrees by different dosage of polyols in solvothermal reactions.With the increase of polyol dosage,the morphology gradually changed from rod shape to lamina shape.When the volume ratio of water to polyol is 1:2,the LiMnPO4samples prepared is only 20 nm thick and the twodimensional plane size is 150 nm × 120 nm,which has the best electrochemical performance.This indicates that a large number of hydrogen bonds carried by hydroxyl groups in polyols are selectively adsorbed on the low barrier (0 1 0) plane.While inhibiting the growth of crystals in the direction of(0 1 0),the preferred orientation of the (1 0 0) and (0 0 1) planes orthogonal to it is enhanced.This increases the exposure rate of the main Li+onedimensional channel (0 1 0),thus enhancing the material’s Li+migration rate and reducing the solid–liquid surface Li+migration impedance.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors gratefully acknowledge the financial support from Natural Science Foundation of Guangdong Province(2018A030313423),Key Research and Development Program of Guangdong Province (2020B090919005) and Pearl River Science and Technology New Star Project (201806010039).

Supplementary Material

XRD,SEM and related electrochemical test of samples of PEG400 solvent system (B-LMPO) and Glycerin solvent system(C-LMPO) and morphology of the ratio about polyol/water at 7:1 were shown in Supplement Material.Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2020.10.027.