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• In recent years, more and more concerns have been paid to environmental protection and energy conservation, therefore, the development of new energy technology are speeding up. With the advantages of low cost, environmental friendliness, high safety, good cycle stability and high specific capacity (170 mAh·g^-1), lithium iron phosphate (LiFePO₄) has established itself as a potent competitor of cathode material for Lithium ion battery (Li-ion battery) since the olivine-type LiFePO₄ was reported by Padhi et al. in 1997^[1-6]. In recent years, LiFePO₄ has been applied on the electric vehicle, especially on electric bus due to its relatively stable nature. However, relatively high cost of LiFePO₄ still prevents it from super extensive application. Recently, Hu et al. reported LiFePO₄ cathode materials using a precursor Fe₂P₂O₇ in Li-ion batteries^[7]. To furtherly cut down the cost, our group developed a method of synthesizing LiFePO₄ using Fe-P waste slag as Fe and P source. Fe-P waste slag used in this method comes from the by-product of yellow phosphorus industry, which is composed of iron and phosphor. Fe-P waste slag is plentiful, cheap and commonly used as building materials. The compositional formula of the Fe-P slag used throughout this work was determined as Fe_1.5P^[8]. This method proposed in this work can be able to dramatically cut back the cost of LiFePO₄ and promote its extensive application^[9-12].

  Generally, some iron compound such as FeC₂O₄, Fe₂O₃ and even expensive FePO₄ have been commonly used as raw materials to synthesize LiFePO₄^[11-18]. Recently, Fe₂P₂O₇ was demonstrated as a novel precursor for LiFePO₄. Compared with FePO₄ and other precursors, Fe₂P₂O₇ has the following advantages: the same ratio of n_Fe: n_P, the same Fe and P chemical valence and similar crystal structure with LiFePO₄^[19-20]. In the synthesis of LiFePO₄ using Fe₂P₂O₇ as precursor, only lithium source is required, which give this method a simplified property.

  Carbon dioxide (CO₂), which is the main greenhouse gas, is drawing more and more attention due to its effect on global warming. The CO₂ capture and utilization are one of the most promising strategies to reduce the CO₂ concentration in the atmosphere. In this work, CO₂ is utilized as oxidizing agent for preparing Fe₂P₂O₇. Also, Fe-P slag, which is a waste sourced from yellow phosphorus industry, is used as Fe and P source in this work. Both of them give the method proposed in this work an environmental-friendly properties.

  In the work, a one-step solid-phase method was developed for synthesis Fe₂P₂O₇ using Fe-P waste slag and CO₂, which can be furtherly used as precursor for LiFePO₄ preparation. Also, the as-synthesized LiFePO₄ was characterized as cathode of Li-ion battery. The synthesis method for Fe₂P₂O₇ is optimized and its influence on the properties of LiFePO₄ is investigated. In addition, the compositions and microstructures of the as-synthesized Fe₂P₂O₇ and LiFePO₄ samples are characterized by TG/DSC, XRD and SEM.

  1   Experimental

  1.1   Preparation

  In this work, LiFePO₄/C composites were prepared from Fe₂P₂O₇, which was synthesized by solid-state reaction from Fe_1.5P slag and phosphoric acid, as shown in Fig. 1. Fe_1.5P slag, which is a kind of Fe-P alloy, is a byproduct from the electrothermal reduction process for manufacturing yellow phosphorus. Fe_1.5P slag used as Fe and P source in this method, which was pulverized and ground finely simultaneously before use.

  图1 Flow chart for synthesis of LiFePO₄/C using Fe₂P₂O₇ as precursor Figure1. Flow chart for synthesis of LiFePO₄/C using Fe₂P₂O₇ as precursor

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  In the first step, phosphoric acid (~85%) was used as the extra P source and mixed with Fe_1.5P to keep n_Fe: n_P=(2~1): 1. The mixture was thoroughly ground with ethanol in an agate mortar, and then dried in an oven at 70 ℃. After that, the mixture was heat treated at 700~800 ℃ in a quartz tube furnace flushed by a CO₂ flow (100 mL·min^-1) to start the reaction, and finally Fe₂P₂O₇ was collected. In the second step, Fe₂P₂O₇, stoichiometric Li₂CO₃ and glucose were mixed thoroughly with ethanol in an agate mortar. After being dried, the mixture was calcined at 700 ℃ in a quartz tube furnace in argon flow (100 mL·min^-1), and finally LiFePO₄ sample was generated.

  1.2   Material characterization

  Thermogravimetric and differential scanning calorimetric analyses (TG/DSC) were carried out on a NETZSCH STA 499F3 instrument, where the sample was examined by heating from ambient to 850 ℃ with a heating rate of 10 ℃min^-1 in a CO₂ atmosphere. The phase structures were analyzed by X-ray diffraction (XRD, Philips X′Pert Pro, Holland, Cu Kα radiation, λ=0.154 06 nm) with a step of 0.04°·s^-1 from 10° to 70° at the power of 35 kV and 25 mA. The samples were examined and photographed with scanning electron microscope (SEM, Hitachi S-4800, Japan) operating 15 kV.

  1.3   Electrochemical measurements

  2025 type coin cells using the as-prepared LiFePO₄/C as cathode were assembled in a glovebox filled with argon (≥99.99%). The cathode was prepared by mixing 83%(w/w) LiFePO₄ powder with 10%(w/w) of conductive acetylene black and 7%(w/w) of commercial available LA-132 binder (Chengdu Indigo Power Sources Co. Ltd., China) to form rheological phase slurry, which was coated onto aluminum foil current collector with a loading density of (1.04±0.32) mg·cm^-2. After being dried at 100 ℃ under vacuum for 10 h, it was cut into round wafers (about 1.2 cm²) as working electrodes. Lithium metal was applied as both the counter electrode and the reference electrode and Celgard 2300 film was used as the separator. 1.0 mol·L^-1 solution of LiPF₆ in ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) (1: 1: 1, V/V, Shenzhen Capchem Chemicals Co. Ltd., China) was used as electrolyte. Galvanostatic chargedischarge measurements were conducted on a Neware battery-testing instrument (Shenzhen Neware Technology Ltd., China) in the voltage range of 2.4~4.2 V vs Li⁺/Li at room temperature. Electrochemical impedance spectroscopic (EIS) characterization was carried out on an electrochemical workstation cont-rolled by the Powersuit software (Princeton Applied Research, United States).

  2   Results and discussion

  XRD and TG/DSC measurements were performed to understand the reaction between Fe_1.5P and H₃PO₄ under CO₂ atmosphere, and its results were presented in Fig. 2. In this research, Fe_1.5P and H₃PO₄ mixture was calcined at 700 and 800 ℃ for 6 h in a CO₂ atmosphere. The as-synthesized sample powder was collected for XRD analysis, and its XRD patterns were presented in Fig. 2a. There are obvious peaks of Fe₂P₂O₇ (PDF No.76-1762) in all the samples. Therefore, it′s reasonable to conclude that Fe₂P₂O₇ was generated in the calcination of Fe_1.5P and H₃PO₄ mixture at 800 ℃ in a CO₂ atmosphere. However, there are still some Fe-P alloy remain in the sample due to its incomplete reaction. Comparing the XRD patterns, it is found that the sample fabricated at 800 ℃ shows a more intense peak, indicating a relatively higher temperature can enhance the generation reaction of Fe₂P₂O₇. Based on this analysis, the possible reaction in the this process was described in reaction (1).

  图2 (a) XRD patterns of Fe_1.5P and H₃PO₄ mixture calcined in CO₂ at different temperatures; (b) Thermal analysis of mixture Fe_1.5P and H₃PO₄ heated in CO₂ Figure2. (a) XRD patterns of Fe_1.5P and H₃PO₄ mixture calcined in CO₂ at different temperatures; (b) Thermal analysis of mixture Fe_1.5P and H₃PO₄ heated in CO₂

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  Fig. 2b shows the TG and DSC curves of Fe_1.5P and H₃PO₄ mixture in a CO₂ atmosphere. The weight loss of the mixture below 600 ℃ is nearly 9.4%(w/w) as labeled in the TG curve. As for the DSC pattern, there is one remarkable exothermic peak at ~140 ℃ and two remarkable endothermic peaks located at ~100 and ~176 ℃ respectively. The endothermic peaks located at ~100 and ~176 ℃ can be assigned to the evaporation of residual water and solvents in the mixture. It is noticed that there is another exothermic peak located at 627 ℃, while the weight stop decreasing and starts increasing after the temperature goes up to 700 ℃. In the reaction showed in equation (1), the mixture of Fe_1.5P and H₃PO₄ react with CO₂ along with a mass increasing due to the participation of CO₂ and only CO generated. From this analysis, it is confirmed that the reaction of Fe_1.5P, H₃PO₄ and CO₂ can be started above 700 ℃.

  To optimize the method of synthesing Fe₂P₂O₇, different raw material ratio (n_Fe1.5P: n_H3PO4=1: 1, 1.5: 1, 2: 1) was applied in the preparation of Fe₂P₂O₇. The XRD patterns of the as-prepared samples are showed in Fig. 3a. It can be found that the main phase of all the samples can be identified as Fe₂P₂O₇ (PDF No.76-1762)^[9]. Comparison with these samples, the sample with n_Fe1.5P: n_H3PO4=1: 1 has the most intense Fe₂P₂O₇ peaks, indicating it′s the most suitable starting material ratio for Fe₂P₂O₇ preparation in this research.

  图3 XRD patterns: (a) As-prepared Fe₂P₂O₇ with different molar ratios of Fe_1.5P and H₃PO₄; (b) LiFePO₄/C composites from Fe₂P₂O₇ synthesized with different molar ratios of Fe_1.5P and H₃PO₄; (c) LiFePO₄/C samples for different calcinating times Figure3. XRD patterns: (a) As-prepared Fe₂P₂O₇ with different molar ratios of Fe_1.5P and H₃PO₄; (b) LiFePO₄/C composites from Fe₂P₂O₇ synthesized with different molar ratios of Fe_1.5P and H₃PO₄; (c) LiFePO₄/C samples for different calcinating times

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  Furtherly, the as-prepared Fe₂P₂O₇ was used for preparing LiFePO₄/C composite. Also, Li₂CO₃ and glucose were used as lithium source and carbon source respectively. In this research, LiFePO₄/C was synthesized by annealing Fe₂P₂O₇ and Li₂CO₃ mixture at 700 ℃ in an argon atmosphere for 6, 8, 10 h, respectively. The XRD patterns of the as-synthesized LiFePO₄/C composites with different Fe₂P₂O₇ and calcinating times are shown in Fig. 3b and Fig. 3c respectively. It is noted that all the samples exhibit pounced crystallographic control, and all the indexed peaks of 10 h sample matches well with the standard peaks of LiFePO₄, indicating that LiFePO₄ was generated in the calcination process^[14]. However, there was some Fe-P alloy still remained in the products while the reacting time was 6 or 8 h, indicating that calcination for 10 h is necessary for synthesizing LiFePO₄ in this research.

  Theoretically, Fe₂P₂O₇ has the same valence of Fe and P element with LiFePO₄, also share the same ratio of n_Fe: n_P with LiFePO₄, which makes the second step to be a complex reaction. Its the main reason for using Fe₂P₂O₇ as an intermediate product for LiFePO₄ preparation in this work. Furtherly, Li₂CO₃ was introduced as lithium source to synthesize LiFePO₄. Also, glucose was used as carbon source for improving the composites conductivity. Based on the XRD analysis, it was proved that LiFePO₄ was generated from the reaction between Fe₂P₂O₇ and Li₂CO₃ successfully. The possible reaction in this process was described in reaction (2).

  The morphology of the as-prepared LiFePO₄/C composites was performed by SEM in the same magnification. Fig. 4(a, b), (c, d), (e, f) show the SEM images of LiFePO₄/C composites with calcination time of 6, 8 and 10 h, respectively. The SEM images show that all the samples have a similar particle size of 2~5 μm, and there are some agglomeration especially for 6 and 10 h sample. However, it is obvious that the sample with 10 h calcination time show a smoother surface than that of 6 and 8 h calcination time. It indicating a better carbon coating surface gives an improved electrochemical properties to LiFePO₄/C composites.

  图4 SEM images of the as-synthesized LiFePO₄/C materials with different calcinating times: (a, b) 6 h; (c, d) 8 h; (e, f) 10 h Figure4. SEM images of the as-synthesized LiFePO₄/C materials with different calcinating times: (a, b) 6 h; (c, d) 8 h; (e, f) 10 h

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  To evaluate the electrochemical energy storage properties of the as-prepared LiFePO₄/C samples, coin-cell type Li-ion batteries was assembled with lithium metal counter electrode. Fig. 5a shows the voltage profiles of LiFePO₄/C samples cycled at 0.1C. Obviously, all the voltage patterns exhibit flat plateaus, which correspond to the lithium ion′s extraction and insertion in LiFePO₄/C cathode. For all the voltage patterns, their charge plateaus located at about 3.45 V and discharge plateaus located about 3.42 V. However, specific capacity of LiFePO₄/C samples with different calcining times is quite different. The discharge capacities of the LiFePO₄/C samples of 6, 8, and 10 h calcining time at 0.1C are 99, 98 and 130 mAh·g^-1, while their coulombic efficiencies are 94.46%, 94.35%, and 95.13%, respectively. Consequently, the sample with 10 h calcining time exhibits the most excellent discharge capacity and columbic efficiency.

  图5 (a) Galvanostatic charge-discharge curves of the LiFePO₄/C composites synthesized with different calcinating time; (b) Cycling performance of LiFePO₄/C composites at 0.1C, 0.2C, 0.5C, 1C; (c) Galvanostatic charge-discharge curves of LiFePO₄/C composites with different current rate; (d) EIS plots of LiFePO₄/C composites with 10 h calcining time and the inset is the corresponding equivalent circuit model Figure5. (a) Galvanostatic charge-discharge curves of the LiFePO₄/C composites synthesized with different calcinating time; (b) Cycling performance of LiFePO₄/C composites at 0.1C, 0.2C, 0.5C, 1C; (c) Galvanostatic charge-discharge curves of LiFePO₄/C composites with different current rate; (d) EIS plots of LiFePO₄/C composites with 10 h calcining time and the inset is the corresponding equivalent circuit model

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  The cycle performances of the LiFePO₄/C composites with different C-rates are present in Fig. 5b. The sample with 10 h calcining times has the a higher discharge capacity than other samples, which shows 130, 126, 117, and 108 mAh·g^-1 at 0.1C, 0.2C, 0.5C and 1C, respectively. The discharge specific capacities of the LiFePO₄/C samples at 0.1C keep increasing in the first few cycles due to cathode activation process. The galvanostatic charge/discharge curves of the LiFePO₄/C sample with 10 h calcining time at 0.1C, 0.2C, 0.5C, and 1C are illustrated in Fig. 5c. The coulombic efficiencies at the rate of 0.1C, 0.2C, 0.5C, and 1C are 95.13%, 97.64%, 99.13%, and 99.15%, respectively. Based on this analysis, it is proved that the as-prepared LiFePO₄/C sample with 10 h calcining time has great energy storage capacity as cathode for Li-ion battery.

  To investigate the electrode reaction process and diffusion behavior, EIS measurement was conducted under the open-circuit potential of coin cells. The Nyquist plots of 10 h sample with the equivalent circuit is presented in Fig. 5d. The EIS plots is well fitted by the R_s(Q_d(R_ctW))(Q_fR_f) equivalent circuit model using ZSimpWin software. In this equivalent circuit, R_s, R_ct, Q_d, and Q_f denote the solution resistance, charge -transfer resistance, constant phase element of the electrolyte film/electrode interface, and the constant phase element of the film, respectively^[14]. Herein, constant phase element (CPE) is used instead of capacitance because the electrode film is not continuous and the sizes of particles vary around an average. The simulated results show that the values of R_s and R_ct are 4.28 and 87.05 Ω respectively. The low and stable interface resistance of the LiFePO₄/C cathode indicates that the as-prepared LiFePO₄/C composites has fast reaction kinetics.

  3   Conclusions

  A novel solid-state method was developed to synthesize Fe₂P₂O₇ using Fe-P waste slag and CO₂ as raw materials. Furtherly, the as-synthesized Fe₂P₂O₇ was used as precursor for synthesizing LiFePO₄ by adding Li₂CO₃ as lithium source. As for synthesis of Fe₂P₂O₇, the optimized synthesis procedure is Fe_1.5P and H₃PO₄ mixture (n_Fe1.5P: n_H3PO4=1: 1) be heat treated at 800 ℃ for 6 h. The as-prepared LiFePO₄ can get capacities of 130, 126, 117, and 108 mAh·g^-1 at 0.1C, 0.2C, 0.5C and 1C, while the corresponding coulombic efficiencies are 95.13%, 97.64%, 99.13%, and 99.15%, respectively. Consequently, in this work, a novel simplified and environmentally friendly route is successfully developed to synthesize Fe₂P₂O₇ precursor for LiFePO₄.

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  Figure 1  Flow chart for synthesis of LiFePO₄/C using Fe₂P₂O₇ as precursor

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  Figure 2  (a) XRD patterns of Fe_1.5P and H₃PO₄ mixture calcined in CO₂ at different temperatures; (b) Thermal analysis of mixture Fe_1.5P and H₃PO₄ heated in CO₂

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  Figure 3  XRD patterns: (a) As-prepared Fe₂P₂O₇ with different molar ratios of Fe_1.5P and H₃PO₄; (b) LiFePO₄/C composites from Fe₂P₂O₇ synthesized with different molar ratios of Fe_1.5P and H₃PO₄; (c) LiFePO₄/C samples for different calcinating times

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  Figure 4  SEM images of the as-synthesized LiFePO₄/C materials with different calcinating times: (a, b) 6 h; (c, d) 8 h; (e, f) 10 h

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  Figure 5  (a) Galvanostatic charge-discharge curves of the LiFePO₄/C composites synthesized with different calcinating time; (b) Cycling performance of LiFePO₄/C composites at 0.1C, 0.2C, 0.5C, 1C; (c) Galvanostatic charge-discharge curves of LiFePO₄/C composites with different current rate; (d) EIS plots of LiFePO₄/C composites with 10 h calcining time and the inset is the corresponding equivalent circuit model

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