1 Introduction

Lithium-ion batteries, which have high energy density and efficiency, become more and more popular [1–10]. As a typical cathode, LiMnxFe1−xPO4 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 LiMnxFe1−xPO4 offers a higher energy density than LiFePO4. Meanwhile, LiMnxFe1−xPO4 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 LiMnxFe1−xPO4 materials [11–13].

Recently, LiMnxFe1−xPO4 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 [14–19], 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 LiMnxFe1−xPO4 without any surfactant.

In this work, a facile hydrothermal method to prepare monocrystal LiMn0.6Fe0.4PO4 material without any surfactant was reported. The possible synthesis and formation mechanism of strawlike LiMn0.6Fe0.4PO4 were proposed on the basis of time-dependent experiments. Moreover, the influential factor such as solvent was also investigated. Rodlike, flowerlike, and strawlike LiMn0.6Fe0.4PO4 materials were synthesized, and their differences on the electrochemical performances were studied.

2 Experimental

2.1 Synthesis of LiMn0.6Fe0.4PO4 materials

LiMn0.6Fe0.4PO4 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 into 40-ml Teflon-lined autoclave, which was maintained at 180 °C for 10 h and cooled down to room temperature naturally. The product was filtered, washed, and dried under vacuum at 80 °C for 10 h. After annealing in argon–hydrogen atmosphere at 300 °C 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 Kα radiation) with scanning rate of 1.2 (°)·min−1 and 2θ range 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 °C 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 °C for 10 h. Electrolyte cells were assembled with the cathodes as-fabricated, lithium foil Celgard 2300 film separator and 1 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 LiMn0.6Fe0.4PO4. All the diffraction peaks of LiMn0.6Fe0.4PO4 materials can be identified to LiMn0.6Fe0.4PO4 phase with an orthorhombic olivine structure, and its space group is Pnma according to the standard card (JCPDS 74-375). The lattice parameters can be obtained by calculating from the XRD pattern of LiMn0.6Fe0.4PO4 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 of 641.8 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 LiMn0.6Fe0.4PO4 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 LiMn0.6Fe0.4PO4 material is successfully synthesized.

Fig. 1
figure 1

Rietveld refinement patterns of XRD data for LiMn0.6Fe0.4PO4 powder

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Fig. 2
figure 2

XPS spectra of a Mn 2p3/2 and b Fe 2p3/2 of LiMn0.6Fe0.4PO4 sample, and c EDS result of LiMn0.6Fe0.4PO4 sample

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SEM and TEM images of LiMn0.6Fe0.4PO4 material are shown in Fig. 3a and b. It is found that strawlike LiMn0.6Fe0.4PO4 assembled by rods is about 10–20 μm 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 LiMn0.6Fe0.4PO4 materials is single crystal.

Fig. 3
figure 3

SEM image a, TEM image b, and corresponding SAED pattern c of LiMn0.6Fe0.4PO4 sample

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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 LiMn0.6Fe0.4PO4 material. The effect of hydrothermal time on the phase and morphology were characterized by XRD and SEM analysis.

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 HPO42− in the solution. Moreover, according to the solubility product constants (Ksp), it is found that the Ksp 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 HPO42− simultaneously. Thus, Mn0.6Fe0.4HPO4 composite is achieved. When hydrothermal reaction time is 2, 4, or 6 h, the peaks of Mn0.6Fe0.4HPO4 appear and coincide with the standard card (JCPDS 74-199) [16], indicating that amorphous Mn0.6Fe0.4HPO4 gradually transforms into crystal Mn0.6Fe0.4HPO4. Moreover, the diffraction peaks of LiMn0.6Fe0.4PO4 matched with JCPDS 74-375 are also observed. With the increase of the hydrothermal time, the amounts of diffraction peaks of LiMn0.6Fe0.4PO4 increase gradually, while those of Mn0.6Fe0.4HPO4 disappear little by little.

Fig. 4
figure 4

XRD patterns of samples prepared for different hydrothermal time

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Based on these analyses, the possible hydrothermal synthesis mechanism of LiMn0.6Fe0.4PO4 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, HPO42− is obtained as shown in Reactions (1) and (2). Amorphous Mn0.6Fe0.4HPO4 firstly forms through Reaction (3). As the hydrothermal reaction goes on, LiMn0.6Fe0.4PO4 finally generates after the Mn0.6Fe0.4HPO4 reacts with Li+ through Reaction (4).

$${\text{N}}_{ 2} {\text{H}}_{ 4}^{ + } + {\text{H}}_{ 2} {\text{O}} \to {\text{N}}_{ 2} {\text{H}}_{ 5}^{ + } + {\text{OH}}^{ - }$$
(1)
$${\text{OH}}^{\text{ - }} + {\text{H}}_{ 2} {\text{PO}}_{ 4}^{\text{ - }} \to {\text{HPO}}_{ 4}^{{ 2\text{ - }}}+{\text{H}}_{ 2} {\text{O}}$$
(2)
$$0. 6 {\text{ Mn}}^{ 2+ } + \, 0. 4 {\text{ Fe}}^{ 2+ } + {\text{ HPO}}_{ 4}^{ 2- } \to {\text{Mn}}_{0. 6} {\text{Fe}}_{0. 4} {\text{HPO}}_{ 4} \downarrow$$
(3)
$${\text{Mn}}_{0. 6} {\text{Fe}}_{0. 4} {\text{HPO}}_{ 4} + {\text{ Li}}^{ + } \to {\text{LiMn}}_{0. 6} {\text{Fe}}_{0. 4} {\text{PO}}_{ 4} \downarrow + {\text{ H}}^{ + }$$
(4)

Meanwhile, Fig. 5 shows the possible formation mechanism of strawlike LiMn0.6Fe0.4PO4 materials. Amorphous Mn0.6Fe0.4HPO4 is composed of nano-particles before the hydrothermal reaction. As the hydrothermal reaction goes on, the Mn0.6Fe0.4HPO4 nano-particles disappear, while self-assembly structures appear gradually. Strawlike LiMn0.6Fe0.4PO4 assembled with rods was prepared at last. The possible formation mechanism is also illustrated in Fig. 6.

Fig. 5
figure 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

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Fig. 6
figure 6

Schematic illustration of hydrothermal synthesis mechanism of LiMn0.6Fe0.4PO4 material

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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 LiMn0.6Fe0.4PO4 materials can be achieved (Fig. 7b). The particle size of the flowerlike LiMn0.6Fe0.4PO4 material is about 5 μm. Figure 7c illustrates that rodlike LiMn0.6Fe0.4PO4 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 1 μm. Obviously, LiMn0.6Fe0.4PO4 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 LiMn0.6Fe0.4PO4 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 LiMn0.6Fe0.4PO4 materials prepared under different solvents, it declares that LiMn0.6Fe0.4PO4 materials may prefer to self-assemble in the bigger polarity of solvents such as water and ethylene glycol.

Fig. 7
figure 7

SEM images of LiMn0.6Fe0.4PO4 samples obtained from different reaction solvents: a water, b water–ethanediol, and c water–isopropanol

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Moreover, the electrochemical performances of rodlike, flowerlike, and strawlike LiMn0.6Fe0.4PO4 materials were tested at a 0.05C rate. The first charge/discharge capacity and cycling properties of the above LiMn0.6Fe0.4PO4 materials 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 LiMn0.6Fe0.4PO4 materials are only 106.4, 101.9, and 82.0 mAh·g−1, respectively. It can be confirmed that rodlike LiMn0.6Fe0.4PO4 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. 8
figure 8

Charge–discharge curves a and cycling performance plots b of LiMn0.6Fe0.4PO4 with strawlike, flowerlike, and rodlike at 0.05C rate

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4 Conclusion

In this study, a facile hydrothermal method for the preparation of monocrystal LiMn0.6Fe0.4PO4 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 LiMn0.6Fe0.4PO4 materials were synthesized by controlling the composition of the solution. In comparison with flowerlike and strawlike LiMn0.6Fe0.4PO4 materials, rodlike LiMn0.6Fe0.4PO4 materials have the best electrochemical property due to its small particle size and the special shape.