A novel environment-friendly synthesis of high purity micron iron phosphate and its application as a precursor of lithium iron phosphate

This essay studies a novel environment-friendly synthesis method for the high purity micron iron phosphate. Compared with traditional synthesis method, this new method can greatly reduce the amount of industrial sewage, hence it is an environment-friendly production technology which can effectively improve the products’ purity, structure stability and morphological consistency. This new method includes two-step reactions: first, phosphoric acid, iron powder and hydrogen peroxide are used as raw materials to obtain iron phosphate dihydrate intermediate, which is then calcined at high temperature to obtain anhydrous iron phosphate. The results show that the iron phosphate prepared for this new method is a hexagonal system-structured iron orthophosphate with high purity and without any impurities, exhibiting a regular and primary spherical particle morphology, at the average size of 3.096μm in particle and the specific surface area reaching 39.1765 m2 g−1. With these excellent characteristics, it has better application potentials in the field of electrode material preparation. The electrochemical performance of lithium iron phosphate anode materials synthesized with it as the precursor is superior to those synthesized with the iron phosphate prepared by the traditional method as precursor. The first specific capacity of discharge at 0.5C and 10C can be as high as 154.3989 or 102.9326 mAh g−1, increasing by 2.74% and 8.03%, respectively.


Introduction
Iron phosphate, an important raw material for elementary chemical industry, has been widely applied to photocatalysis, sewage treatment, synthesis of lithium battery cathode material and other fields [1][2][3]. There are plenty of researches shown that the purity, structure, morphology, particle size and other indicators of iron phosphate have a great influence on its catalytic performance and electrochemical performance [4][5][6][7][8][9]. Therefore, how to improve the purity, refine the particle size and improve the morphological consistency and structural stability of iron phosphate has always been a hot point concerned by the researchers.
The traditional preparation methods for iron phosphate mainly include solid phase method and liquid phase method [10,11]. Solid phase method prepares iron phosphate through calcination at high temperature with iron oxide (such as iron oxide red) and phosphate (such as ammonium phosphate) as raw materials. However, with great energy consumption and poor production consistency, it is gradually replaced by the liquid phase methods. Liquid phase methods include hydrothermal method, coprecipitation method, and controlled crystallization method. These three methods adopt ferric salt and phosphate as raw materials, so as to obtain basic ferric ammonium phosphate or ferric hydroxyphosphate and other intermediate products through reactive synthesis under liquid phase conditions, which are then calcined into iron phosphate at high temperature [12][13][14][15]. Though liquid phase method can overcome the disadvantages of solid phase method, it still has some problems remaining to be solved. For example, to obtain iron phosphate products with high morphological consistency and good crystallinity, it usually requires longer reaction time so as to not only increase the production cost, but also improve the probability of the products wrapping or absorbing byproducts; thereby the products' purity is reduced. At present, reducing the impurity elements of package and improving products' purity and morphology consistency, chemical reagents or templates are usually used as morphology aids [16,17] to control the crystallization process, or the great amount of pure water is employed to wash products. However, such methods have increased the complexity of the technology and generated plenty of industrial sewage containing abundant nitrogen and sulphur, resulting in great environmental pressure. At the same time, the intermediate product obtained by the traditional method is basic ferric ammonium phosphate or ferric hydroxyphosphate, which will have thermal decomposition reaction during calcination at high temperature [18][19][20]. Compared with the desorption reaction of crystal water at high temperature, ammonia gas generated by severe thermal decomposition will destroy the crystal morphology of iron phosphate. In case of the inadequate intermediate decomposition, the residual impurities could affect stability of products structure and lower reactivity of iron phosphate. These situations will impact the performance and application of iron phosphate.
Therefore, in order to solve the dilemma of traditional preparation method for iron phosphate, there is a new method is put forward in this essay. First, phosphoric acid and iron powder are used as raw materials to synthesize ferrous phosphate, and then hydrogen peroxide is added to prepare iron phosphate dihydrate intermediate, and few CTAB is added to make products particle disperse more evenly. Finally, they are calcined at high temperature to prepare anhydrous iron phosphate. This new method can block the introduction of impurities and reduce the consumption of sewage, resulting in great reduction to industrial sewage. At the same time, the sewage without any other impurities can be recycled, lowering the production cost, so it is an environment-friendly production technology. The intermediate products applied this new method are iron phosphate dihydrates instead of basic iron phosphates or iron hydroxyphosphates generated by the traditional method, so that no severe thermal decomposition reaction would occur when anhydrous iron phosphate is prepared through high-temperature calcination, only a small amount of vapour escaping. In this way, the products morphology and structure are rarely destroyed, which can improve the morphological consistency and structural stability of the products. Hence, it is predicted that such environment-friendly and preparation technology at high-quality of iron phosphate would have a favorable prospect in industrial application.

Preparation of iron phosphate
Traditional method: add 760 g ferrous sulfate (with the mass concentration of 2%) and 40 hydrogen peroxide solution (with the mass concentration of 10%) into a 2L beaker, stir at the speed of 200 rpm for 15 min and the liquid turns to yellow brown, then, add 53.2 g of ammonium dihydrogen phosphate solution (with the mass concentration of 25%) and 105g of ammonia solution (with the mass concentration of 5%) slowly, and the solution turns white gradually. Having it react continuously for 10h at 60°C and the reaction liquid turns pale green, then filtering it after the reaction. Washing several times with deionizzed water, and then the filter cake is dried up at 120°C for 2 h to obtain the intermediate product which is marked as FP-a-t, and then dehydrated at 550°C for 4h to obtain the iron phosphate which is marked as FP-b-t. The technological process is shown in figure 1(a). The main reaction equation is: The new method: add 11.2 g iron power and 1200 g deionized water into 2L beaker, stir and add 78.4 g phosphoric solution slowly (with the mass concentration of 25%). The reaction temperature is controlled at 80°C and the reaction time is 2 h. Then, the solution turns brown-green. Leaching out the residual scrap iron and add 70 g of hydrogen peroxide solution slowly (with the mass concentration of 10%) and 5 ml of CTAB solution (with the mass concentration of 0.1%) into the filter liquor. The temperature of the reaction is controlled at 60°C and the reaction time is 4h. Then, the solution becomes white. Filtering it after reaction, and washing several times with deionizzed water, and then the filter cake is dried up at 120°C for 2 h to obtain the intermediate products which will be marked as FP-a-n, then dehydrated at 550°C for 4h to obtain iron phosphate which will be marked as FP-b-n. The technological process is shown in figure 1(b). The main reaction equation is:

Representation and test
The elementary composition of sample is analyzed by the inductively coupled plasma emission spectrometer (American PE, Optima 7000) and x-ray energy spectrometer (Oxford, England, INCA X-MAX50); C content of the sample is tested by a carbon-sulfur analyzer (Nanjing Jinshi, JS-HW2000A); N content of the sample is tested by a Kjeldahl nitrogen analyzer (Shanghai Xinjia KDN-C); the sample structure is represented by an x-ray diffractometer (Panaco, Netherlands, X'Pert Powder III); the functional group structure of the sample is represented with infrared spectrometer (Thermo Fisher, Nicolet-IS10); the water content and thermal stability of the sample are tested by the synchronous thermal analyzer (Germany Netsch, STA449C); the shape and particle size of the sample is observed by the tungsten filament scanning of electron microscope (German Zeiss, SIGMA-500); the particle size of the sample is tested by a laser particle size analyzer (Malvin, UK, Mastersizer 2000); the dispersion of the sample is tested by a Zeta potentiometer (Malvin, UK, Zetasizer Nano S90); and the specific surface area and pore size distribution of the sample are tested by a specific surface and pore size analyzer (Kanta, NOVA1000E).

Test of electrochemical property
With FP-b-t and FP-b-n prepared above as precursors, it adds certain amount of lithium carbonate and glucose. The relative ratio of quantity between the added lithium carbonate and iron phosphate is 0.52:1 and the addition of glucose is 10% of the mass of iron phosphate. After stirring and mixing evenly, it is pre-calcined in nitrogen atmosphere at 350°C for 4 h, then calcinated at 750°C for 8 h, cooled and ground to prepare LiFePO 4 /C composite material. The composite material synthesized with FP-b-t as the precursor is marked as LFP/C-t, and the synthesis with FP-b-n as the precursor is marked as LFP/C-n. Preparing the slurry with LFP/C-t and LFP/ C-n as active substances, acetylene black as the conductive agent and polyvinylidene fluoride as the adhesive agent at the mass ratio of 75:15:10, and N-methyle-2-pyrrolidone as the solvent, which is then painted on aluminum foil, dried for 12 h at 120°C and made into disc to be used as the positive plate. With lithium plate used as the negative electrode, 1mol/L LiPF 6 ethylene carbonate/dimethyl carbonate (with the volume ratio of 1:1) solution as electrolyte, polypropylene microporous membrane as the battery separator, the battery is assembled in the glove box. The Wuhan Landian battery test system (LAND-BT2013A) is used to conduct the constant current charging-discharging and cycle performance is tested at room temperature.

Results and discussions
Figures 2(a) and (b) are XRD spectrograms of intermediates FP-a-t and FP-a-n, which are prepared according to the experimental methods described in 2.1, as well as the products FP-b-t and FP-b-n, respectively. Table 1 is the corresponding composition analysis of the elements. As can be seen from the figures, the structures of the intermediates prepared by different methods are basically same. No diffraction high temperature the structure of the obtained products is basically same. No diffraction peak is observed in XRD spectrogram of intermediate FP-a-n, which shows typical amorphous substance structure [21], while FP-a-t has sharp characteristic diffraction peak, indicating that intermediate FP-a-t has high crystalline. After comparison it can be found its characteristic diffraction peak in accordance with the substance diffraction peak of JCPDS card 01-082-1164, suggesting its molecular formula is Fe 2 (NH 4 )OH(PO 4 ) 2 (H 2 O) 2 and belonging to monoclinic system, whose space group is P2 1 /C. When both intermediates are calcined at high temperature, the products FP-b-n and characteristic diffraction peaks, and the spectrograms are basically the same, indicating that the phase structure of the products is the same. After comparison it is found the characteristic diffraction peaks of both products are  in accordance with the spectrogram of standard substance JCPDS 50-1635, indicating that the obtained products are ferric orthophosphates with the molecular formula of FePO 4 and the space group of P3 1 21, belong to hexagonal, indicating that both intermediates FP-a-n and FP-a-t can be transmitted into positive FePO 4 after high-temperature calcination and dehydration. Though the dehydrated structures are the same, differences in the intermediate structure will cause varied product purity and impurity contents, which are verified by the data listed in table 1. As can be seen from table 1, FP-a-t has higher impurity contents, among which N covers 3.49%, and S covers 0.0631%. Combining the analysis results of XRD spectrogram, it can be known that N mainly comes from ferric ammonium phosphate composite, and S comes from the package and adsorption of impurity elements during the reaction. After dehydrating there are still slight N and S impurities account for 0.0024% and 0.0028%, respectively. However, no impurity elements like N and S are detected in FP-a-n and F-b-n, indicating that the high-purity iron phosphate prepared by the new method is superior. The reason for the above differences is that when iron phosphate is prepared by the traditional method, the iron phosphate generated in the reaction and the ammonium ion in the solution will still have coordinate reaction and produce a complex of ammonium iron phosphate, which can be seen from the XRD spectrogram ( figure 2(a)). Therefore, the amorphous iron phosphate dihydrate is changed into the crystalline ammonium iron phosphate [14]. During the formation of the crystal, there is a small amount of byproduct (NH 4 ) 2 SO 4 is wrapped, introducing impurity elements of N and S. However, when iron phosphate is prepared by the new method proposed in this essay, since there is no impurity elements such as N and S in the reaction raw materials, mixture of impurity ions is eliminated from the source. In the reaction, the iron powder first reacts with phosphoric acid to generate ferrous phosphate, which can generate amorphous iron phosphate dihydrate after oxidating with hydrogen peroxide. It is just a difference in the preparation mechanism of both methods that causes the variance in the structure and impurity contents of the intermediates. However, after calcinating at a high temperature, the ammonium ferric phosphate intermediate will happen thermal decomposition reaction and generate crystalline ferric orthophosphate. At the same time, the impurity content decreases due to the escape of ammonia gas, but there are still some wrapped N and S impurities retained. On the other hand, the iron phosphate dihydrate intermediate happens crystallization water-removal reaction, and the iron orthophosphate of the same crystal structure is obtained, but the product is high in purity without N and S impurities which can be verified by the test results shown in figure 3. Figure 3 is the EDS atlas of the sample, in which figure 3(a) is the EDS atlas of the intermediates FP-a-t and FP-a-n, figure 3(b) is the EDS atlas of products FP-b-t and FP-b-n. As can be seen from figure 3(a) that except the energy spectrum of Fe, P and O, there is also an energy spectrum of impurities N and S in intermediate FP-a-t, while in intermediate FP-a-n only the energy spectrum of Fe, P and O can be observed (C is introduced during sample preparation). The results in figure 3(b) show that impurity elements in the iron phosphate obtained after high-temperature of intermediate FP-a-t are removed to some degree. Although it is suggested in the results listed in table 2 that there are a small amount of residual N and S impurities, perhaps because the residual amounts are few, and their energy spectrum cannot be observed in the EDS atlas. The EDS atlas of product FP-b-n shows that its elementary compositions include Fe, P and O. Table 2 shows the content of the elements in the industrial sewage obtained by different preparation methods, which is composed of reaction filtrate and washing water. It can be seen from the data in table 2 that there are a large amount of N and S impurity ions in the sewage when iron phosphate is prepared by the traditional methods, covering 0.11% and 0.82% respectively, hence they can neither be directly used, nor meet the Chinese discharge standards of industrial sewage (GB8978-1996). Therefore, it should be treated before it is recycled or discharged. On the other hand, there is no such impurity in the sewage generated in preparation of iron phosphate with the proposed technology, and can be recycled as the mother liquor, which can reduce greatly the production cost and save water resources, hence it is an environment-friendly production technology. At the same time, it is observed that both types of sewage contain a certain amount of Fe and P, this is caused by some tiny iron phosphate particles which are left in the mother liquor. Figures 4(a) and (b) are the TG-DSC curves of FP-a-t, FP-a-n, FP-b-t and FP-b-n, respectively. It can be known from figure 4(a) that TG and DSC curves of FP-a-t and FP-a-n vary greatly, and TG curve of FP-a-t loses weight at three stages with the total weight loss rate of 19.83%, while FP-a-n loses weight at two stages, with the total weight loss rate of 19.54%. This is caused by different intermediate structures. FP-a-n loses weight due to removal of crystal water, while FP-a-t loses weight due to decomposition of ferric ammonium phosphate and crystal water, which can also be proved by the DSC curve. FP-a-t reaches endothermic peaks at 140.43, 288.5 and 537.13°C, corresponding to the removal of crystal water, thermal decomposition of intermediate and transformation of the phase structure of the products [22,23], while FP-a-n has significant endothermic peaks at 142.56 and 557.1°C, corresponding to the removal of crystal water and transformation of the phase structure of the product [24]. It can be seen from figure 4(b) that the TG-DSC curves of FP-b-t and FP-b-n are basically the same, suggesting that after high-temperature calcination, both become anhydrous FePO 4 . The data in the figure shows that their weight loss rates are 0.32% and 0.41% respectively, which may be caused by the sample containing a small amount of free water. At the same time, both samples have an endothermic peak at about 719  °C, corresponding to transformation of the phase structure of the material [25]. Thus, compared with traditional methods the intermediates prepared by the new method are ferric phosphate dihydrates which are better in thermal stability. The reaction at high-temperature calcination is crystal water removal reaction. In contrast, the intermediate prepared by traditional methods is ferric ammonium phosphate. At the hightemperature calcination, severe intermediate decomposition could occur, which will damage the morphological consistency of products and affect the performance of materials to a certain degree. Figures 5(a) and (b) are respectively FT-IR atlases of FP-a-t, FP-a-n and FP-b-t, FP-b-n at the wave number of 400-4000cm −1 . As can be seen from figure 5(a), FP-a-t and FP-a-n have the same vibration peaks at the wave numbers of 526.13, 1047.63 and 1428.66 cm −1 , belonging to the symmetrical stretching vibration peak of P-O bond; while FP-a-t is also asymmetrical in the stretching vibration peak of P-O bond at 984.13 cm −1 [26], which is caused by the influence of the introduction of N on the structure of intermediate FP-a-t. On the other hand, intermediate FP-a-n has high purity, with no N impurity, so, such peak is not observed in it. FP-a-t and FP-a-n have vibration peaks belonging to O-H at 1645.87 and 3225.87cm −1 [14, indicating that the both contains a certain crystal water, which accords with the TG curve results shown in figure 4. The wave number of FP-a-t has vibration peak belonging to N-H at 3188.04 cm −1 [27], while FP-a-n has not. This is resulted from NH 4 + action in FP-a-t, and the result accords with XRD results shown in figure 2. It can be seen from figure 5(b) that FP-b-t and FP-b-n obtained after high-temperature calcination have similar FT-IR spectrogram. The vibration peaks of P-O functional group are retained, but the peaks of O-H and N-H disappear, which also verifies the crystal water of both intermediates which are completely removed after high-temperature calcination. At the same time FP-a-t intermediate has thermal decomposition and ammonia gas escapes, resultingthe disappearance of N-H peak. Figures 6(a)-(d) are respectively the microtopographies of FP-a-t, FP-b-n and FP-a-n, FP-b-n, from which can be seen both FP-a-t and FP-a-n showing spherical particle morphology. The sphericity of FP-a-t is poorer and there are some irregular particles, with the particle size ranging within 4-10μm, and wider particle size distribution. On the other hand, FP-a-n has better sphericity, basically all of spherical morphology with fine particle size of about 3-5μm are distributed more evenly. This is because the surfactant CTAB is added in the proposed method. During the growth process of the crystal, the cationic surfactant CTAB has a micellar shell formed by hydrophilic head and hydrophobic tail. The hydrophobic tail group of the micelle shell faces inward towarding the central particle, while the hydrophobic head group diffuses toward the water phase. On the one hand, it can protect iron phosphate from aggregation; on the other hand, it makes the surface of crystal nucleus of iron phosphate has a positive charge CTA+, forming outer spherical complex through electrostatic interaction, which is helpful to form spherical morphology [28]. However, when iron phosphate is prepared by the traditional method, the iron phosphate and ammonia have further coordinated reactions and can form an ammonium iron phosphate complex. The crystal growth is mainly limited by the reaction conditions. Under strong stirring conditions, the intermediate particles form near-spherical morphology gradually, which has no the function as morphology inducer, hence the morphology consistency is poor. After high-temperature calcination the morphology of FP-b-t is destroyed more seriously, but FP-b-n is preserved well. This is because during the high-temperature calcination, compared with the new method and except the removal of crystal water of iron phosphate prepared by the traditional method, thermal decomposition of the intermediate is more intense, which is more destructive to morphology. This result also shows that the iron phosphate prepared by the new method performs better in controlling morphology consistency.
The particle size, dispersion and specific surface area of products FP-b-t and FP-b-n are tested further, and the results are shown in table 3 and figure 7. From table 3 it can be seen that D 50 value of FP-b-t and FP-b-n is 9.138 and 3.096μm, respectively, zeta potential is 24.6 and 42.7 mv and the specific surface area is 28.1903 and 39.1765 m 2 /g respectively. This indicates that the products prepared with the proposed method are smaller in particle size, and have better dispersity and larger specific surface area, which is helpful for iron phosphate products' application in the field of synthesis of lithium iron phosphate cathode materials, as it improves the electrochemical property of lithium iron phosphate cathode materials. Figure 8 shows the electrochemical performance of LFP/C-t and LFP/C-n synthesized with FP-b-t and FPb-n as the precursors. Figures 8(a) and (b) are the first charge-discharge curves of LFP/C-t and LFP/C-n at low magnification (0.5C) and high magnification (10C). It can be seen from the figures that the charge-discharge voltage of both materials is close. However, their specific charge-discharge capacity varies greatly. It is measured that the first specific discharge capacity of LFP/C-t and LFP/C-n at 0.5C and 10C magnification is 150.2792, 154.3989 mAh g −1 and 149.9487 and 154.1146 mAh g −1 , respectively, indicating that LFP/C-n synthesized with the proposed method has better specific discharge capacity no matter at low magnification (0.5C) and high magnification (10C), increasing respectively 2.74% and 8.03%. This can also be verified by the data shown in figure 8(c), which is the rate performance curve of LFP/C-t and LFP/C-n, from which it can be seen that the specific discharge capacity of LFP/C-t and LFP/C-n starting at 0.5C, experiencing 1C, 5C and 10C and then returning to 0.5C is 150. It can be seen that under all rate conditions, the first specific discharge capacity of LFP/C-n is better than those of LFP/C-t. In addition, the first specific discharge capacity returning to 0.5C change little, almost the same as the initial value, indicating that shock of large current has little influence on the electrochemical performance of LFP/C-n. The specific discharge capacity can basically recover and the irreversible capacity is only 0.2843 mAh g −1 which is better than 0.3305 mAh g −1 of LFP/C-t. Figure 8(d) is the rate performance curve of LFP/C-t and LFP/C-n at 1C circulating 300 times, from which it can be seen that after 3oo cycles of charge and discharge, LFP/C-n exhibits a more excellent cycle performance and its discharge curve is still steady. The specific capacity attenuation rate after 300 cycles is 3.44%, better than 4.15% of LFP/C-t. The iron phosphate synthesized by the proposed method is smaller in particle size and has larger specific surface area and more consistent spherical morphology, so that to provide a larger contact area with the electrolyte during the electrochemical reaction, which can increase the electrochemical reactivity. At the same time, smaller particle size and high-consistent spherical morphology have shortened the transmission distance of Li + , thus improving the electrochemical property of materials [29,30].

Conclusions
In this study, a new synthesis method for the high-purity micron iron phosphate is provided. The mass percentage compositions of Fe and P in the prepared iron phosphate are 37.03% and 20.51%, respectively. The products are high in purity without any other impurities. The products are regular micron spherical particles at the average particle size of 3.096 micron, and the specific surface area reaching 39.1765 m 2 g −1 . Compared with the traditional method, the iron phosphate product prepared with this new method is higher in purity, has finer particle size and better morphological consistency and structural thermostability, which are beneficial for improving its application performance of battery material. The electrochemical performance of the lithium iron phosphate cathode material synthesized with it as the precursor is better than that of the lithium iron phosphate  cathode materials synthesized with iron phosphate as the precursors by the traditional method. At the same time, this new method can reduce the water consumption for washing greatly during the preparation process of iron phosphate, and the sewage can be recycled as it contains no other impurities and can reduce the production cost and is environment-friendly. Therefore, the proposed method would have a bright prospect in the industrial application.