简介概要

Hydrothermal synthesis mechanism and electrochemical performance of LiMn_0.6Fe_0.4PO₄ cathode material

来源期刊：Rare Metals2019年第1期

论文作者：Chang-Chang Xu Ying Wang Li Li Yi-Jing Wang Li-Fang Jiao Hua-Tang Yuan

文章页码：29 - 34

摘要：Monocrystal LiMn_{0. 6}Fe_{0. 4}PO₄ cathode material was obtained via hydrothermal method at 180℃ for 10 h without any surfactant. The effects of hydrothermal time on the phase and morphology of the material were discussed.By controlling the reaction solutions, the rodlike, flowerlike,and strawlike LiMn_0.6Fe_0.4PO₄ cathode materials were synthesized. Electrochemical performances show that the rodlike LiMn_{0. 6}Fe_{0. 4}PO₄ has the best electrochemical properties. The initial discharge capacity of the rodlike structure is 106.4 mAh·g^-1, which is higher than those of flowerlike and strawlike materials.

稀有金属(英文版) 2019,38(01),29-34

Hydrothermal synthesis mechanism and electrochemical performance of LiMn_0.6Fe_0.4PO₄ cathode material

Chang-Chang Xu Ying Wang Li Li Yi-Jing Wang Li-Fang Jiao Hua-Tang Yuan

Institute of New Energy Material Chemistry,Nankai University

Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University

Key Laboratory of Advanced Energy Materials Chemistry,Nankai University

Tianjin Key Laboratory on Metal and Molecule-based Material Chemistry, Nankai University

作者简介：*Yi-Jing Wang e-mail:wangyj@nankai.edu.cn;

收稿日期：23 December 2013

基金：financially supported by the National Natural Science Foundation of China (Nos. 21231005 and 51071087);the Major State Basic Research Development Program of China(Nos. 2011CB935900 and 2010CB631303);the Discipline Innovative Engineering Plan (B12015);the Research Fund for the Doctoral Program of Higher Education of China (No.20120031110001);the Tianjin Science & Technology Project (No. 10SYSYJC27600);

Hydrothermal synthesis mechanism and electrochemical performance of LiMn_0.6Fe_0.4PO₄ cathode material

Chang-Chang Xu Ying Wang Li Li Yi-Jing Wang Li-Fang Jiao Hua-Tang Yuan

Institute of New Energy Material Chemistry,Nankai University

Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University

Key Laboratory of Advanced Energy Materials Chemistry,Nankai University

Tianjin Key Laboratory on Metal and Molecule-based Material Chemistry, Nankai University

Abstract：

Monocrystal LiMn₍0. 6)Fe₍0. 4)PO₄ cathode material was obtained via hydrothermal method at 180℃ for 10 h without any surfactant. The effects of hydrothermal time on the phase and morphology of the material were discussed.By controlling the reaction solutions, the rodlike, flowerlike,and strawlike LiMn_0.6Fe_0.4PO₄ cathode materials were synthesized. Electrochemical performances show that the rodlike LiMn₍0. 6)Fe₍0. 4)PO₄ has the best electrochemical properties. The initial discharge capacity of the rodlike structure is 106.4 mAh·g^-1, which is higher than those of flowerlike and strawlike materials.

Keyword：

LiMn_0.6Fe_0.4PO₄; Hydrothermal method; Cathode material; Lithium-ion batteries; Synthesis mechanism;

Received： 23 December 2013

1 Introduction

Lithium-ion batteries,which have high energy density and efficiency,become more and more popular [ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10] .As a typical cathode,LiMn_χFe_1-χPO₄ materials have advantages of environmental benignity,low cost,high capacity,excellent cycle life,and thermal stability;all of these make them be promising materials.The redox potential of Mn³⁺/Mn²⁺versus Li⁺/Li is 4.1 V,higher than that of Fe³⁺/Fe²⁺.It indicates that Mn-based olivine structural LiMn_χFe_1-χPO₄ offers a higher energy density than LiFePO₄.Meanwhile,LiMn_χFe_1-χ.PO₄ shows much better electrochemical property compared with LiMnPO₄.The main reason is that doping Fe²⁺into olivine structure significantly reduces the volume changes caused by the Jahn-Teller effect around Mn³⁺.Therefore,great attention was paid to Mn-based olivine-structured LiMn_χFe_1-χPO₄materials [ 11, 12, 13] .

Recently,LiMn_χFe_1-χPO₄ materials with various morphologies were successfully synthesized due to the effect of shape on the electrochemical property.Many experiments suggested that LiMPO₄ (M=Mn,Fe) materials assembled by different shapes were prepared with various surfactants [ 14, 15, 16, 17, 18, 19] ,but the discussion about the effects of the morphology of the materials on the electrochemical performances should eliminate the differences of surfactants.So it is essential to overcome challenges in the preparation of monocrystal LiMn_χFe_1-χPO₄ without any surfactant.

In this work,a facile hydrothermal method to prepare monocrystal LiMn_0.6Fe_0.4PO₄ material without any surfactant was reported.The possible synthesis and formation mechanism of strawlike LiMn_0.6Fe_0.4PO₄ were proposed on the basis of time-dependent experiments.Moreover,the influential factor such as solvent was also investigated.Rodlike,flowerlike,and strawlike LiMn_0.6Fe_0.4PO₄ materials were synthesized,and their differences on the electrochemical performances were studied.

2 Experimental

2.1 Synthesis of LiMn0.6Fe0.4PO4 materials

LiMn_0.6Fe_0.4PO₄ cathode materials were synthesized by a facile hydrothermal method.Li₂SO₄·H₂O,NH₄H₂PO₄,MnSO₄·H₂O,and (NH₄)2Fe(SO₄)2.6H₂O with mole ratio of 2.0:1.0:0.6:0.4 were dissolved into 27 ml distilled water with magnetic stirring,resulting in a transparent liquid.And then 1 ml hydrazine hydrate was dropped into the solution.At last,the mixture was transferred into40-ml Teflon-lined autoclave,which was maintained at180℃for 10 h and cooled down to room temperature naturally.The product was filtered,washed,and dried under vacuum at 80℃for 10 h.After annealing in argon-hydrogen atmosphere at 300℃for 5 h,the sample was achieved.

2.2 Characterization

The crystalline forms of the synthesized materials were characterized by X-ray power diffraction (XRD,Rigaku D/Max-2500,Cu Kαradiation) with scanning rate of1.2 (°).min^-1 and 2θrange of 3°-80°.Scanning electron microscopy (SEM,Hitachi X-650) equipped with energy dispersive spectrometer (EDS),transmission electron microscopy (TEM,JEOL JEM-2100),and X-ray photoelectron spectrometer (XPS,PHI 5000 Versaprobe,ULVACPHI) were used to analyze the surface morphology,element compositions,and valence states of the sample.

2.3 Electrochemical property

The as-prepared sample was mixed with 20%acetylene black and heated at 600℃for 10 h in Ar/5%H₂ gas flow.Then,they were combined with polyvinylidene fluoride(PVDF) in weight ratio of 90:10,pasted on an aluminum foil and finally dried in vacuum oven at 100℃for 10 h.Electrolyte cells were assembled with the cathodes asfabricated,lithium foil Celgard 2300 film separator and1 mol·L^-1 LiFP₆ that was dissolved into a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC)(1:1;volume ratio).After that,the cells were charged with C/20 rate to 4.5 V,kept at 4.5 V until C/80 rate,and then discharged to 2.3 V at C/20 rate on a LAND battery-test instrument (CT2001A).All the tests were carried out at room temperature.

3 Results and discussion

3.1 Materials characterization

Figure 1 displays the XRD patterns of the synthesized LiMn_0.6Fe_0.4pO₄.All the diffraction peaks of LiMn_0.6-Fe_0.4PO₄ materials can be identified to LiMn_0.6Fe_0.4PO₄phase with an orthorhombic olivine structure,and its space group is Pnma according to the standard card (JCPDS74-375).The lattice parameters can be obtained by calculating from the XRD pattern of LiMn_0.6Fe_0.4PO₄ composite on general structure analysis system (GSAS).The lattice parameters are a=0.6058 nm,b=1.0423 nm,and c=0.4736 nm,which are close to the previous results [ 20] .The valence states of Mn and Fe are illustrated in Fig.2.As shown in Fig.2a,a single peak with binding energy of641.8 eV demonstrates that the valence state of Mn is+2.Correspondingly,a wide peak of binding energy at 711.4 eV declares that the oxidation state of Fe is+2,in agreement with the report of Zhang et al. [ 18] .Moreover,the EDS spectrum of the material is shown in Fig.2c.It reveals that Mn,Fe,P,and O elements are all observed in LiMn_0.6-Fe_0.4PO₄ composite except Li due to its light weight.Furthermore,the relative ratio of Mn and Fe is 6:4 from EDS analysis.Therefore,it can be confirmed that pure LiMn_0.6-Fe_0.4PO₄ material is successfully synthesized.

SEM and TEM images of LiMn_0.6Fe_0.4PO₄ material are shown in Fig.3a and b.It is found that strawlike LiMn_0.6Fe_0.4PO₄ assembled by rods is about 10-20μm in diameter.Moreover,the SAED pattern is shown in Fig.3c,it is found that there are a series of periodic two-dimensional diffraction points,which demonstrate that the structure of synthesized LiMn_0.6Fe_0.4PO₄ materials is single crystal.

3.2 Hydrothermal synthesis mechanism

A set of experiments with different hydrothermal time (0,2,4,6 h) were carried out in order to discuss the synthesis mechanism of the LiMn_0.6Fe_0.4PO₄ material.The effect of hydrothermal time on the phase and morphology were characterized by XRD and SEM analysis.

Fig.1 Rietveld refinement patterns of XRD data for LiMn_0.6-Fe_0.4PO₄ powder

Fig.2 XPS spectra of a Mn 2p3/2 and b Fe 2p_3/2 of LiMn_0.6Fe_0.4PO₄ sample,and c EDS result of LiMn_0.6Fe_0.4PO₄ sample

Fig.3 SEM image a,TEM image b,and corresponding SAED pattern c of LiMn_0.6Fe_0.4PO₄ sample

Fig.4 XRD patterns of samples prepared for different hydrothermal time

The XRD patterns of the samples prepared for different hydrothermal time are shown in Fig.4.After the addition of hydrazine hydrate,the pH value of the solution was adjusted to 9-10.There is very little amount of HPO₄^2-in the solution.Moreover,according to the solubility product constants (K_sp),it is found that theK_sp_of MnHPO₄ and FeHPO₄ are very close to each other and they are both smaller than those of Mn(OH)2,Fe (OH)2,NH4MnPO₄,and NH₄FePO₄.Therefore,based on the fundamental of chemical precipitation,Mn²⁺and Fe²⁺could react with HPO₄^2-simultaneously.Thus,Mn_0.6Fe_0.4HPO₄ composite is achieved.When hydrothermal reaction time is 2,4,or 6 h,the peaks of Mn_0.6Fe_0.4HPO₄appear and coincide with the standard card (JCPDS 74-199) [ 16] ,indicating that amorphous Mn_0.6Fe_0.4HPO₄ gradually transforms into crystal Mn_0.6Fe_0.4HPO₄.Moreover,the diffraction peaks of LiMn_0.6Fe_0.4PO₄ matched with JCPDS74-375 are also observed.With the increase of the hydrothermal time,the amounts of diffraction peaks of LiMn_0.6-Fe_0.4PO₄ increase gradually,while those of Mn_0.6Fe_0.4HPO₄disappear little by little.

Based on these analyses,the possible hydrothermal synthesis mechanism of LiMn_0.6Fe_0.4PO₄ material is proposed as follows:at the early reaction stage,hydrazine hydrate is hydrolyzed to yield hydroxyl,and then the hydroxyl reacts with H₂PO₄.Thus,HPO₄^2-is obtained as shown in Reactions (1) and (2).Amorphous Mn_0.6Fe_0.4HPO₄ firstly forms through Reaction (3).As the hydrothermal reaction goes on,LiMn_0.6Fe_0.4PO₄ finally generates after the Mn_0.6Fe_0.4HPO₄reacts with Li⁺through Reaction (4).

Meanwhile,Fig.5 shows the possible formation mechanism of strawlike LiMn_0.6Fe_0.4PO₄ materials.Amorphous Mn_0.6Fe_0,4HPO₄ is composed of nanoparticles before the hydrothermal reaction.As the hydrothermal reaction goes on,the Mn_0.6Fe_0.4HPO₄nano-particles disappear,while self-assembly structures appear gradually.Strawlike LiMn_0.6Fe_0.4PO₄ assembled with rods was prepared at last.The possible formation mechanism is also illustrated in Fig.6.

Fig.5 SEM images of samples prepared for different hydrothermal time:a 0 h,b 2 h,c 6 h,and d 10 h.Inserts in c and d being corresponding enlarged images

Fig.6 Schematic illustration of hydrothermal synthesis mechanism of LiMn_0.6Fe_0.4PO₄ material

3.3 Effect of solvent

In order to investigate the effect of the reaction solvent on the morphology of the samples,a series of experiments were carried out.The distilled water was replaced by water-ethanediol (2:1;volume ratio) or water-isopropanol(2:1;volume ratio),while the other factors were kept the same.Using water-ethanediol (2:1;volume ratio) as the reactive solvent,flowerlike LiMn_0.6Fe_0.4PO₄ materials can be achieved (Fig.7b).The particle size of the flowerlike LiMn_0.6Fe_0.4PO₄ material is about 5μm.Figure 7c illustrates that rodlike LiMn_0.6Fe_0.4PO₄ can form by employing the mixture solution of water-isopropanol (2:1;volume ratio).It is found that the width of the rods is about 300 nm and the length is around 1μm.Obviously,LiMn_0.6-Fe_0.4PO₄ materials with various shapes were prepared with different solvents such as water,water-ethylene glycol,or isopropanol were utilized.It may be related to the polarities of solvents [ 21] .The polarities of solvents may have an effect on the growth of LiMn_0.6Fe_0.4PO₄ material.The sequence of the polarities of solvents is water,ethylene glycol,and isopropanol from large to small order.Combining the polarities of solvents with the SEM results of LiMn_0.6Fe_0.4PO₄ materials prepared under different solvents,it declares that LiMn_0.6Fe_0.4PO₄ materials may prefer to self-assemble in the bigger polarity of solvents such as water and ethylene glycol.

Moreover,the electrochemical performances of rodlike,flowerlike,and strawlike LiMn_0.6Fe_0.4PO₄ materials were tested at a 0.05C rate.The first charge/discharge capacity and cycling properties of the above LiMn_0.6Fe_0.4PO₄materials are illustrated in Fig.8a and b.It is found that there are two characteristic potential plateaus related to Fe and Mn in Fig.8a.As to the first plateau near 3.5 V,it is associated to the redox process of Fe³⁺/Fe²⁺.Similarly,the redox process of Mn³⁺/Mn²⁺causes the second plateau at around 4.0 V.Obviously,the capacity related to Mn³⁺/Mn²⁺is low,it is preferable to account it to the polarization effect caused by the diffusion rate of lithium ions and the poor electro-conductivity of LMFP material [ 22] .The initial discharge capacities of rodlike,flowerlike,and strawlike LiMn_0.6Fe_0.4PO₄ materials are only 106.4,101.9,and 82.0 mAh·g-l,respectively.It can be confirmed that rodlike LiMn_0.6Fe_0.4PO₄ material has the best electrochemical property.The reason may be that the size of rodlike material is much smaller than those of others.Furthermore,the ideal crystal of the rods is beneficial to the diffusion of lithium ions [ 17] ,which also plays a significant role in enhancing the electrochemical performance.

Fig.7 SEM images of LiMn_0.6Fe_0.4PO₄ samples obtained from different reaction solvents:a water,b water-ethanediol,and c water-isopropanol

Fig.8 Charge-discharge curves a and cycling performance plots b of LiMn_0.6Fe_0.4PO₄ with strawlike,flowerlike,and rodlike at 0.05C rate

4 Conclusion

In this study,a facile hydrothermal method for the preparation of monocrystal LiMn_0.6Fe_0.4PO₄ materials was demonstrated.Based on the time-dependent experiments and the fundamental of chemical precipitation,the possible synthesis mechanism was proposed.Rodlike,flowerlike,and strawlike LiMn_0.6Fe_0.4PO₄ materials were synthesized by controlling the composition of the solution.In comparison with flowerlike and strawlike LiMn_0.6Fe_0.4PO₄materials,rodlike LiMn_0.6Fe_0.4PO₄ materials have the best electrochemical property due to its small particle size and the special shape.