A Simple and Facile Preparation of LiFePO₄ by a One-step Microwave Hydrothermal Method

Abstract

A novel method was used to synthesize LiFePO₄, using inorganic salts as raw materials, and PEG-4000 as the surfactant. The results show that LiFePO₄ powders with various morphologies were prepared by microwave hydrothermal method, and it is very important to synthesize the LiFePO₄ powders with well-defined shape and size controlling experimental conditions, such as the solution pH and surfactant. The modified preparation of LiFePO₄ was built. The coating carbon on LiFePO₄ powders as a core–shell structure was carried out by annealing in 3%H₂/97%N₂ at 700 °C for 2 h. As a result, the diffusion coefficient of lithium ions can be increased, and the reversibility of lithium intercalation and deintercalation can be improved markedly. In addition, LiMn_0.08Fe_0.92PO₄ powders were synthesized, which were observed in an ordered olivine structure, but great changes occurred in morphology. Doping Mn²⁺ does not destroy the lattice structure and enlarges the lattice volume. Consequently, the conductivity can be enhanced, and the lithium ion diffusion coefficient can be boosted. Initial discharge capacity is improved obviously, and increases to 99.6 mA h g⁻¹ and 93.8 mA h g⁻¹ respectively. The microwave assisted hydrothermal approach presented here opens a potential avenue to explore the synthesis of LiFePO₄ powders.

1 Introduction

With the wide using of mobile phones, laptops, electric cars and other electronic equipments, the high-performance secondary batteries grow rapidly year by year. Developing a new generation of rechargeable type high-energy batteries becomes an urgent task. LiFePO₄ has attracted great attention due to a high theoretical specific capacity (≈170 mAh g⁻¹), and it is the most attractive recently because of its high stability, low cost and high compatibility with environment [1, 2], but it also has small lithium ion the diffusion coefficients, low electronic conductivity, leads to its poor electrochemical properties at normal temperature [3–5]. To solve these problems, various chemical routes are employed to increase the electronic conductivity of LiFePO₄ [6–9].

It is still a challenge for our researchers to seek for a simple and economically efficient route at relatively low reaction temperature for the synthesis of LiFePO₄ particles with well-defined shape and size. In this paper, LiFePO₄ was synthesized by a novel method of microwave hydrothermal at a low temperature, and it was controllable synthesis of regular LiFePO₄ powders with various morphologies. It still did not demand a subsequent heating treatment with additional reducing agents and inert N₂ atmosphere to obtain the crystalline LiFePO₄ powders. Then, further improvements were carried out, which depend on advanced electrode materials providing better performance in terms of energy and power density, rate capability, and cycle number [10, 11]. The microwave hydrothermal synthesis is a useful method to prepare fine particles and has some advantages such as simple synthesis process and low energy consumption, compared to high annealing temperature and long time using the conventional solid state reaction [12].

2 Experimental

2.1 Materials Synthesis

All of the chemical reagents were of analytical grade and used without further purification. The experimental details are as follows: inorganic salts as raw materials were dissolved in 60 ml distilled water according to the Fe–P–Li molar ratio of 1:1:2, then appropriate polyethylene glycol (PEG-4000) and manganese salt were added to the solution, respectively. After 30 min stirring, the above milky solution was transferred into a double-walled vessel, which has an inner liner and a cover made of Teflon perfluoroalkoxy (PFA) polymer resin and an outer high strength sleeve. The vessel was sealed and maintained in a Microwave Accelerated Reaction System (MARS-5, CEM Corporation, USA) at 180 °C for 1 h. After the reaction was complete, the resultant black precipitates were centrifuged, washed with distilled water and then ethanol to minimize the extra ions in the final products, and dried at 80 °C in vacuum. In addition, for the ex situ Ex Situ coating of carbon on LiFePO₄, the LiFePO₄ nanocrystals obtained by the microwave assisted hydrothermal process were mixed with glucose powders and carbonized at 700 °C for 2 h in a flowing atmosphere with volume ratio of 2% H₂/98% Ar to achieve the carbon coating.

2.2 Materials Characterization

The morphology and structure of the as-prepared samples were characterized by powder X-ray diffraction (XRD, DS-advance, Cu Kα radiation, 40 kV, 25 mA), field emission scanning electron microscopy (FESEM, QUANTA FEG250, 15 kV), energy dispersive spectrometer (EDS, X-MAX50, 30 kV). For LiFePO₄/C samples, they were fixed on conductive copper tape while they were characterized by FE-SEM and EDS. The electrochemical measurements were carried out via BT2000 (Arbin, America) with lithium metal as the counter and reference electrode at room temperature. The working electrode consisted of 85 wt% active material, 10 wt% carbon black, and 5 wt% polyvinylidene difluoride (PVDF). The electrolyte was 1 M/L LiClO₄ in a 1:1 v/v mixture of ethylene carbonate (EC) and diethyl carbonate (DEC). Cells were assembled in an argon-filled glove box (Mbraun, Germany). Charge–discharge cycles were measured between 2.5 and 4.1 V at the current rate of 50 mA g⁻¹.

3 Results and Discussion

3.1 Structural Analysis and Morphology Characterization

Figure 1 shows the XRD patterns of the samples, and the peaks in the XRD spectra are identical and can be perfectly indexed as LiFePO₄ phase [13]. All diffraction peaks at (200), (101), (201), (211), (301), (311), etc., can be attributed to LiFePO₄ with the an ordered olivine structure, which are consistent with the values in the standard card (JCPDS card No. 81-1173), agree very well with the literature data [14]. No peaks of all samples from other phases such as Li₃PO₄ and others have been detected, indicating that the LiFePO₄ powders are of high purity. Additionally, the peak strength of samples (A, B, C, and D) has a tendency to become larger, meaning the increasing trend in crystallinity. Thereby, a nearly pure phase and well-crystallized product is obtained by a one-step microwave hydrothermal synthesis.

Fig. 1

XRD patterns of LiFePO₄ powders prepared at 180 °C for 1 h by microwave irradiation under various conditions: a XRD pattern of LiFePO₄ using raw materials of LiOH·H₂O, NH₄H₂PO₃ and FeSO₄·7H₂O, b XRD pattern of LiFePO₄ using raw materials of H₃PO₃, LiOH·H₂O and FeSO₄·7H₂O, c XRD pattern of LiFePO₄ using raw materials of NH₄H₂PO₃, FeSO₄·7H₂O and Li₂CO₃, d XRD pattern of LiFePO₄ using raw materials of NH₄H₂PO₃, FeSO₄·7H₂O and Li₂CO₃, and polyethylene glycol (PEG-4000) as the surfactant

FE-SEM images of the LiFePO₄ samples are shown in Fig. 2. Before microwave irradiation, the solution pH values of sample (A), (B), and (C) were 8, 6, and 7, respectively. The raw materials have an important impact on the morphologies of LiFePO₄ powders, which also may greatly depend on the solution pH [15, 16]. Sample (A) has certain long flake structure with an edge length beyond 1 μm. The LiFePO₄ particles congregate together and grow up to large second particles by self-assembly. The sample (B) exhibits regular lozenge-like morphology with a size of 1–2 μm. However, it has a single grain structure which is different with the former sample. Sample (C) presents regular spindle-like structure. The size of most particles with an edge length of about 5 μm. High magnification FE-SEM image of sample (C) displayed in Fig. 2d shows an individual LiFePO₄ spindle-like microstructure with the second particles self-assembly. The first particles are single crystal nanorods with about 300 nm in length and 100 nm in diameter. Sample (D) synthesized with PEG-4000 consists of hexagon nanoplates and agglomerated particles. The morphology of sample (D) changes obviously adding PEG-4000 as the surfactant. Figure 2f shows a typical high-magnification FE-SEM image of the LiFePO₄ sample (D). It can be clearly seen that the sample (D) is mainly composed of flat and spindle-like microstructure. The average grain size of sample (D) is around 2 μm in the length and around 200 nm in the thichness, showing that the LiFePO₄ microstructures are hierarchically constructed with hexagon nanoplates attached side by side in an ordered fashion. It is found that the currently used synthesis strategy provides a powerful means to tailor the composition, purity, and assembly of LiFePO₄ microstructures in dependence of reaction parameters, such as the raw materials and addition of PEG-4000. Figure 2e and f present that the presence of PEG-4000 is beneficial to construct the hierarchically self-assembled microstructures and crucial to control the morphology of LiFePO₄ particles [17, 18]. Formation mechanism may well be that, the initially LiFePO₄ nanoplates formed substantially, and PEG-4000 amorphous shell around the LiFePO₄ crystal lattice, then to reduce the surface energy, the initially nanoplates began to assemble in edge-to-edge and layer-by-layer growth style, just because of the PEG-4000 interaction [14, 19]. In addition, the research is not done on the amount change of PEG-4000 now, and it highly possible plays an important role in both the morphology and size of the LiFePO₄ powders.

Fig. 2

Morphological and structural characterizations of LiFePO₄ powders generated using different raw materials at 180 °C for 1 h by microwave irradiation: a FE-SEM image of LiFePO₄ sample A, b FE-SEM image of LiFePO₄ sample B, c FE-SEM image of LiFePO₄ sample C, d High magnification FE-SEM image of sample C, e FE-SEM image of LiFePO₄ sample D, f High magnification FE-SEM image sample D

3.2 Modification Research

To prepare LiFePO₄/C composite materials, LiFePO₄ sample (D) powders were mixed with glucose powders and carbonized at 700 °C for 2 h in a flowing atmosphere with volume ratio of 2% H₂/98% Ar to achieve the carbon coating. XRD patterns of LiFePO₄/C composite materials are shown in Fig. 3. The peaks in the XRD patterns are identical and can be perfectly indexed as LiFePO₄ phase. However, the diffraction peaks attributed to carbon can not be found, which is probably due to the amorphous and low crystalline of carbon, or the formed carbon is too small or too thin on the surface of LiFePO₄ powders [20, 21].

Fig. 3

XRD patterns of LiFePO₄/C samples calcined at 700 °C for 2 h: sample E is LiFePO₄/5wt% C, and sample F is LiFePO₄/10wt% C

To gain a better understanding of LiFePO₄/C composite materials, more other analysis were carried out such as FE-SEM, EDS, and corresponding EDS mappings. Figure 4 shows the FE-SEM images, the corresponding EDS and EDS mappings analysis of the carbon coated LiFePO₄ powders. It is well-known that the carbon distribution affects electrochemical performances of materials. The FE-SEM images of carbon-coated LiFePO₄ powders show that the carbon completely coats the surface of LiFePO₄ particles. The LiFePO₄/C composite materials are homogeneous with a uniform core–shell microstructure. In order to confirm the prepared powders covered by carbon and how the carbons are distributed with the LiFePO₄ powders, the EDS analysis and EDS mappings were carried out, and the results are shown in Fig. 4d–h. It is clear from the images that the surface of the LiFePO₄ powders is modified by carbon. It can be seen that the sample (F) has a more thick carbon layer covering on a crystalline particle than the sample (E). The mapping analysis shows the distribution of C, P, Fe, and O elements is uniform obviously, and the distribution area for carbon is also homogeneous. As mentioned above, the carbon shell is amorphous, and lithium ions can pass through the micropores easily [15, 21, 22]. On the other hand, the carbon coating is also expected to reduce the particle-to-particle contact resistance and facilitates the transport of electrons from the current collector. Furthermore, the carbon shell acts as an electronically conducting network which increases the rate of movement of lithium ions in the depth of the electrode [23, 34]. The core–shell structure can improve the electrochemical performance of LiFePO₄/C composite materials markedly [19, 24, 25].

Fig. 4

FE-SEM images, EDS analysis and corresponding mappings of LiFePO₄/C composite materials: a FE-SEM image of sample E with 5 wt% carbon, b and c FE-SEM images of sample F with 10 wt% carbon, d EDS analysis of sample F corresponding to point 1 in c, e–h EDS mappings of elements in sample F

It is observed that XRD pattern of the as-prepared LiMn_0.08Fe_0.92PO₄ powders is consistent of the typical fingerprint of LiFePO₄ phase, which is observed in an ordered olivine structure. No other peak is observed in the XRD pattern. It is obvious that the Mn²⁺ doping LiFePO₄ as LiMn_xFe_1−xPO₄ structure can be synthesized by microwave hydrothermal method at 180 °C for 1 h. Also, Mn²⁺ embedded in LiFePO₄ particles leads to lattice defect, which increases the migration possibility of lithium ion significantly.

As can be seen from Fig. 5b, due to the Mn²⁺ doping, great changes occurred in morphology from spindle-like shape in Fig. 2e to now rectangular structure. The content analysis of each element was investigated by EDS from the particle to the surface clearly combined with XRD quantitative analysis, the data lead us to the conclusion that LiMn_0.08Fe_0.92PO₄ powders are synthesized. EDS mappings were carried out and the results are shown in Fig. 5d–g. The mapping images obtained for Mn, Fe, P, and O elements match with that of the corresponding FE-SEM image. Since Mn element also follows the same uniform distribution and image contrast as that of the other elements, it is concluded that LiMn_0.08Fe_0.92PO₄ contains Mn element in a uniformly distributed manner.

Fig. 5

XRD pattern, FE-SEM image, EDS analysis and corresponding mappings of LiMn_0.08Fe_0.92PO₄ powders: a XRD pattern of LiMn_0.08Fe_0.92PO₄ powders prepared at 180 °C for 1 h by the microwave hydrothermal synthesis based on Sample D, b FE-SEM image of LiMn_0.08Fe_0.92PO₄ powders, c EDS analysis of LiMn_0.08Fe_0.92PO₄ powders corresponding to point 2 in b, d–g EDS mappings of elements in LiMn_0.08Fe_0.92PO₄ powders

Following this doping process to get LiMn_xFe_1−xPO₄ structure materials, Mn²⁺ can be further doped into them to effectively improve the kinetics of intercalation and deintercalation of lithium ions, since Mn²⁺ does not destroy the lattice structure and enlarges the lattice volume of LiFePO₄ [26]. During deintercalation and intercalation of lithium ions, the dopant manganese atoms protect the LiFePO₄ crystal from shrinking. This kind of “pillar” effect provides larger space for the movement of lithium ions. Consequently, the conductivity is enhanced, and the lithium ion diffusion coefficient is boosted after doping [26–30]. These favorable changes are beneficial to the improvement of the electrochemical performance of LiFePO₄, including discharge capacity and rate capability.

Charge–discharge curves tested at 0.1 C-rate in the initial cycle of the electrodes of lithium cell with LiFePO₄ cathode are shown in Fig. 6. The discharge capacities of sample A and sample B are very poor, but the discharge capacity of sample C is 79.6 mA h g⁻¹ with big promotion, and the sample D is 82.4 mA h g⁻¹, which is closed to sample C. After coating with 10 wt% carbon (sample F in Fig. 4) and doping with 8 mol% manganese (sample LiMn_0.08Fe_0.92PO₄ in Fig. 5), discharge capacities increase to 99.6 and 93.8 mA h g⁻¹, respectively. These show clearly that the carbon coating and manganese doping can markedly improve the reversible capacity of LiFePO₄ powders by increasing the kinetics of lithium intercalation and deintercalation. The performance during the first cycle is also retained well during the subsequent cycles.

Fig. 6

Initial charge–discharge curves of LiFePO₄ samples

4 Conclusion

In this paper, LiFePO₄ powders with uniform size and shape were successfully synthesized by microwave hydrothermal method. The morphology of LiFePO₄ powders is closely related to raw materials and PEG-4000, respectively. Compared to the LiFePO₄ powders without adding PEG-4000, the presence of PEG-4000 plays an important role in the construction of the hierarchically self-assembled nanostructures, and can inhibit grain growth more effectively. After coating a nanolayer of carbon, core–shell structured LiFePO₄/C composite materials are formed, and the carbon shell acts as an electronically conducting network, which increases the rate of movement of lithium ions. For LiMn_0.08Fe_0.92PO₄ structure, great changes occur in morphology, but Mn²⁺ does not destroy the lattice structure, and it enlarges the lattice volume and lattice defects existed. Therefore, they improve the reversible capacity of LiFePO₄ powders obviously by increasing the kinetics of intercalation and deintercalation of lithium ions.