Synthesis and Caracterization of Mesoporous FePO4 as Positive Electrode Materials for Lithium Batteries

⎯ Mesoporous iron phosphates were synthesized using sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB) as surfactants. The material synthesized in the presence of SDS was not applied as a positive electrode active material of a lithium battery. The results show that the obtained FePO4 has a mesoporous structure with a specific surface area of 70 m2 g and a dominant pore diameter of 3 nm. Those mesoporous were characterized by different microstructural and electrochemical analyzes. Among the materials studied under different conditions, those calcined at 450°C preserve mesoporous structures and exhibit the best electrochemical performance when used as active materials of the positive electrodes of lithium batteries. Effectively, a relatively high specific capacity of 135 and 122 mAh g was registered at C/20 collected experimentally by the samples synthesized in the presence of SDS and CTAB, respectively.


INTRODUCTION
The iron-based cathode materials, such as FePO4 and LiFePO4, are attractive for usage in lithium batteries because they are low-cost and environmentally friendly and have a high theoretical capacity [1][2][3][4][5][6][7][8]. For instance, FePO4 has a theoretical capacity of 178 mAh g −1 per 1 mol of the lithium intercalated and a discharge voltage from 3.5 to 2.5 V [1]. However, the behavior of these materials has two disadvantages: a slow diffusion of Li+ in the structure and a very low electronic conductivity. Therefore, the combination of a porous structure and an electronic conductor (carbon) can overcome these drawbacks.
Recently, FePO4 mesoporous materials with a large specific surface have found many applications, including in lithium batteries [9][10][11][12][13][14][15]. However, few synthesis methods of getting mesoporous FePO4 with a large specific surface, using surfactants, are indicated [16][17][18]. A mesoporous FePO4 was synthesized as cathode material using CTAB as templating agentand the compatibility of this compound with other components of lithium cells was demonstrated [15]. A surfactant (EO20-PO70-EO20, Pluronic P123) was used to show the role of the mesoporous structure in improving the kinetics of intercalation of lithium ions in the host material [17]. Later, other authors [19,20] were able to increase the specific surface area of these materials by improving the synthetic process, which engendered in a better electroactivity. In addition, using SDS as surface active agent made it possible to prepare mesoporous FePO4 with a large surface area [16]. However, these materials have not been characterized electrochemically.
In this work, we aimed at synthesizing mesoporous FePO4, using SDS as structuring agent, and at demonstrating its use as an active material for the positive electrode in lithium batteries. A comparison of the electrochemical performance of this material with that of a material synthesized in the presence of CTAB and the effect of the heat treatment was a part of our objective.

Synthesis
The synthesis of a mesoporous structure was based on the procedure described in 1 The article is published in the original. [16]. To this end, 8.08 g of Fe(NO3)3 (99%) and 7.16 g of Na2HPO4 (99%) were dissolved separately in 80 g of distilled water; the two solutions were then stirred and mixed. The resulting precipitate (FePO4) was recovered by centrifugation and washed with distilled water, then dispersed in 20 g of distilled water. Finally, 1.32 g of HF (40%) was added under rigorous agitation. After obtaining a clear solution, 2.88 g of SDS (99%) was dissolved in 10 g of distilled water and then added to the above solution with stirring; the mixture was agitated for 30 min at room temperature and then transferred to an oven at 60 C for 2.5 h. After cooling to room temperature, a light yellow precipitate was collected by centrifugation and washed several times with water and acetone. The collected solid was dried at room temperature. The as synthesized product (denoted SDS-B) was the base of the first series of our samples.
The second series of samples was prepared by the same previous procedure, but using CTAB instead of SDS as the surfactant [15]. After the step of cooling at ambient temperature, no precipitate was observed.Thereby, increasing the pH solution from 2 to 10 byadding 1 M of tetraethylammonium solution yields ayellow precipitate that was collected by centrifugation and washed several times with distilled water and acetone and then allowed to air dry (denoted CTAB-B).
In order to extract surfactants, SDS-B and CTAB-B have been submitted to ion exchange according to the protocol proposed in [21]. Typically, 0.

Cell Fabrication and Testing
The electrochemical measurements (cyclic voltammetry and chronopotentiometry) were performed on lithium cells with two electrodes. These cells were subjects to the following electrochemical chain:

Physicochemical Characterization
The XRD patterns of mesoporous materials are very special because they have few peaks relatively wide and located at the small angle. Figures 1 and 2 show the diffractograms obtained.
A single diffraction peak at 2θ=1.2º is clearly observed on the diffractogram of the SDS-B sample ( Fig. 1, line a), characteristic of a mesoporous iron phosphate [15,16,22,23] with a disordered arrangement of the channels [8,24]. However, the diffractogram of the SDS-E sample shows two reflections at 1.18 and 1.48 ( Fig. 1, line b), and that of SDS-B-450-at 0.9º and 1.5º (Fig. 1, line c).
This confirms the fact that the mesoporous structure is preserved after the elimination of the surfactant or calcination at 450 C. The appearance of a second peak may be explained by a better pore organization due to the surfactant removal by chemically or thermally [8].
The XRD patterns at small angles of the samples synthesized using CTAB (CTAB-B, CTAB-E and CTAB-B-450) are shown in Fig. 2, with three diffractograms exhibiting similar behavior of the samples. In the diffraction patterns, only one peak around 0.8º is present. This is in good agreement with the results in the diffractograms of the mesophases of CTAB-E and CTAB-B-450 indicates that they are less ordered than those of SDE-E and SDS-B-450 [15].
The nitrogen adsorption-desorption isotherms are shown in Figs. 3 and 4, exhibiting a typical shape of a mesoporous material, which is in agreement with literature [25][26][27]. Indeed, a saturation bearing develops at a high relative pressure and this saturation corresponds to the activities of mesoporous materials. Furthermore, a clear hysteresis is observed between the adsorption and desorption curves corresponding to an irreversible adsorption-desorption phenomenon. According to the IUPAC classification, the obtained isotherms are typical type IV isotherms. In the case of SDS-E (Fig. 3) and CTAB-E (Fig. 4), the hysteresis loop appears at the relative pressure P/P0 higher than 0.8. This is due to the swelling of the sample during the adsorption (swelling intergranular); a similar effect has been already observed for different mesoporous materials: among them MCM-41 mesoporous silica [28,29].
The isotherms related to SDS-B-450 and CTABB-450 samples calcined at 450ºC under argon are also of type IV, with a lower closure point of the hysteresis at P/P0 ≈ 0.4. The same phenomenon has been reported in literature [7,8,30,31]. However, for as the synthesized samples of SDS-B and CTAB-B, the mesoporosity does not seem to be accessible duringadsorption. This is presumably due to the occupation of the pores by surfactants.
The pore size distribution curves (insets of Figs. 3and 4) show that the samples eliminated the pores with diameters in a range of 1.5 and 4.0 nm, which is conventional with the pore size of mesoporous FePO4 [20,32]. The pore size distribution curve is heterogeneous, with a broad peak centered at 3.0 nm. The obtained nano-FePO4 has a mesoporous structure with the intra-particle porosity rather than the inter-particle one [20,32]. However, micropores of a large size can be observed beside mesopores whose diameter is about 1.0 nm. Similar results have been reported elsewhere [32]. The insets of Figs.
3 and 4 show that the pores diameters for SDS-B-450 and CTAB-B-450 samples are getting smaller. This is probably due to the contraction of the mesoporous structure [8,17,31]and the formation of the residual carbon in the pores [19]. These results are supported by those evidenced by isotherms and the XRD data.
The physical characteristics deducted from the nitrogen adsorption-desorption isotherms of the samples synthesized (Table 1) show that among the studied samples, SDS-E and CTAB-E have the most important porous volumes and pores with diameters due to the removed of the organic matter.
However, the calcination of as synthesized samples at 450 C generates an enlargement of the specific surface area and a decrease of the total volume and of the average diameters of mesopores.
This can be explained by the fact that the existing surfactant, in the pores and between the particles, can be transformed into porous carbon after the heat treatment [19]. Moreover, the samples prepared in the presence of CTAB show specific surface areas and pore volumes higher than those of the samples obtained using SDS, which may be due to the lengths of the chains of two surfactants. possess nanometric particles, slightly agglomerated; each agglomeration is constituted of several primary particles, thereby the mesostructure is entirely similar to that found in a mesoporous iron phosphate [8,17,19,20].
The images in Figs. 5b, 5d allow confirming that particles of small sizes (some tens of nm) are assembled to form porous aggregates. However, the heat treatment engenders modifications in the mesostructured and a significant reduction in the size of the interparticle pores. This is likely due to the pyrolysis of structuring on the FePO4 particle surface.
In order to confirm the result of elimination of surfactant pores of the mesoporous structure, we applied the FTIR spectroscopy to the as synthesized FePO4 and to that which sustained the eliminating of structuring via chemical or thermal treatment (Fig. 6). The infrared spectra obtained are practically similar to that of iron phosphate [15,17,19,33].

Electrochemical Properties of the Synthesized Materials
The electrochemical behavior of the FePO4 mesoporous materials synthesized with the SDS and CTAB as surfactant agents, used as positive electrodes for Li batteries, is investigated by cyclic voltammetry (CV), realized at room temperature into cells in two electrodes with lithium metal as negative electrode (Fig. 7). The reduction and oxidation peaks are well defined in the voltage range of 2.5 to 3.5 V, attributed to the Fe(III)/Fe(II) redox couple corresponding to the lithium insertion and extraction in the FePO4 crystal structure [14,17,30,35,36]. This process appears to be reversible as shown in the voltammograms. These voltammograms, except that of the CTAB-B-600 sample, have a single pair of reduction and oxidation peaks, indicating that there are no other electrochemically active species in the range of the potential chosen [30,37]. The potential difference between the cathodic and anodic peaks is about 0.3 V comparable to that mentioned in literature [8,[38][39][40][41][42][43]. The voltammogram of CTAB-B-600 reveals that the redox reaction is effected in two successive steps.
This indicates that the heat treatment at 600 C engenders the formation of a closed second phase (FePO4) or other species into impurity (FexOy) [8,17,20,44].  are satisfactory and are similar to the best results reported for iron phosphate [15,17].
In contrast, the capacity at the end of the first discharge does not exceed 25 mAh g −1 (13% of the theoretical capacity) for the as synthesized phases calcined at 600 C. Also, the experimentally recovered capacity is approximately 10 mAh g −1 for the removed phases (5.2-5.8% of the theoretical capacity). Electrochemical cells using these samples as positive electrodes demonstrate very low electrochemical performance compared to that of the cells with the samples calcined at 450ºC.
The electrochemical study shows that the best performance of lithium batteries is produced with as synthesized materials calcined at 450ºC for both surfactants used. So, it appears that the optimum calcination temperature for this type of materials is around 450ºC. Indeed, the heat treatment at temperatures above 450ºC engenders lowering of the performance of the positive electrode generally attributed to the formation of a glassy surface phase [17,20,45,46].
To provide more information about the electrochemical property of SDS-B-450 sample, cycling and rate capability were checked (Fig. 9). Figure 9a shows the discharge/charge curves at the first, 10th, 20th, 30th, 40th and 50th cycles; the material delivered a discharge capacity of 135.0, 134.5, 133.4, 132.3, 131.0 and 129.6 mAh g-1, respectively, at C/20, exhibiting good cycling stability and high reversible capacity. In addition, the voltage difference between the charge and discharge pseudoplateaus did not increase significantly with increasing numbers of cycles, implying that the material possessed good electronic conductivity and high reaction reversibility. Figure 9b presents the cycling performance of the SDS-B-450 sample at C/20; the first discharge capacity is above 135.0 mAh g-1. After 50 cycles, the capacity retention rate is 96%. It exhibits much better cycling stability.