A Novel Strategy for the Synthesis of Fe3(PO4)2 Using Fe-P Waste Slag and CO2 Followed by Its Use as the Precursor for LiFePO4 Preparation.

A novel method whose starting materials was Fe-P waste slag and CO2 using a closed-loop carbon and energy cycle to synthesize LiFePO4/C materials was proposed recently. In the first step, Fe-P slag was calcinated in a CO2 atmosphere to manufacture Fe3(PO4)2, in which the solid products were tested by XRD (X-ray diffraction) analysis and the gaseous products were analyzed by the gas detection method. In the second step, as-synthesized Fe3(PO4)2 was further used as the Fe and P source to manufacture LiFePO4/C materials. Also, the influence of the preparation conditions of Fe3(PO4)2, including calcination time and calcination temperature, on the energy storage properties of as-obtained LiFePO4/C was investigated. It was found that the LiFePO4/C materials, which was synthesized from Fe3(PO4)2 obtained by calcining Fe-P waste slag at 800 °C for 10 h in CO2, exhibited a higher capacity, better reversibility, and lower polarization than other samples. The discharge capacity of as-obtained LiFePO4/C can reach 145 mAh/g at 0.1 C current rate. This work puts forward an environment-friendly method of manufacturing LiFePO4/C cathode materials, which has a closed-loop carbon and energy cycle.

Introduction

Since lithium iron phosphate (LiFePO4, LFP) was reported as a novel cathode material for lithium ion battery (Li-ion battery) by Padhi et al., it has been intensively studied during the past decade.[1] Due to its high theoretical specific capacity (170 mAh/g) and environmental friendliness,[2−4] LiFePO4 has been one of the most widely used cathode materials for Li-ion battery powering electric vehicles, hybrid electric vehicles, aerospace devices, and military devices. Until now, LiFePO4 was mainly synthesized by the method of solution phase or solid phase. Solution-phase methods, including sol–gel method,[5] solvothermal method,[6] emulsion drying method, co-precipitation method,[7,8] etc., was not perfect for large-scale industrial production because of its dependence and complicated preparation route. In contrast, the solid-phase synthesis methods were feasible and straightforward when compared to the solution method. Generally, ferrous compounds, such as FeC2O4·2H2O, Fe2O3, or FePO4,[9] are mainly used as raw materials to synthesize LiFePO4. However, Fe(III) in FePO4 has to be reduced to Fe(II) for the manufacturing of LiFePO4, which will complicate the roasting process. Also, as the commercialization of LiFePO4, the prices of FePO4 has been rising steadily.[10] Other Fe(II) compounds such as FeC2O4·2H2O and Fe(CH3COO)2 are not only expensive but also toxic, which make it an inconvenience for producing LiFePO4. Therefore, Fe3(PO4)2 is a promising alternative Fe and P source for LiFePO4 production.[11]

Fe–P waste slag was a solid waste sourced from the yellow phosphorus industry, which is an important chemical industry, especially in western China.[12,13] In the traditional industry, Fe–P waste slag cannot be efficiently utilized to manufacture other products, while it was commonly used as a low-quality resource for the steel-making industry. It is urgent to develop a novel method to make both Fe and P in Fe–P slag comprehensively utilized, which can benefit both the environment and the related industry. As for the yellow phosphorus industry, about 150 kg of Fe–P slags will be generated for manufacturing 1 ton of yellow phosphorus. Therefore, Fe–P slag is of low cost (no more than $500/ton,) and a huge production (more than 140,000 tons per year in western China) resource in western China.[12,13] In our previous research, Fe–P slag was determined as Fe1.5P, which can be treated as a mixture of FeP and Fe2P.[10] Also, CO2 is a conventional waste gas, which is gradually accumulating in air and considered as the primary greenhouse gas.[14,15] In recent years, both chemical and electrochemical methods were reported to transfer CO2 to other chemicals, which show great potential to reduce the CO2 concentration in air. However, almost all the method faces huge cost problems for practical applications.[16,17]

In our previous research, we have demonstrated that Fe–P waste slag can react with Li2CO3 and H3PO4 under a CO2 atmosphere to generate LiFePO4.[13] However, the chemical mechanism insight of this process is unclear. In this work, we develop a two-step method, inspired by our previous work, of synthesizing LiFePO4. At first, Fe–P slag was calcined under a CO2 atmosphere for producing Fe3(PO4)2. In the second step, as-synthesized Fe3(PO4)2 is further used as the Fe and P source for synthesizing LiFePO4. Also, the CO gas, generated in this step, is determined by a home-made online CO detection system. Finally, we found that this two-step method has a closed-loop carbon and energy cycle, while the CO gas generated in the first step is enough to be used as a fuel in the second step. Also, CO2, as the gaseous product in the second step, can feedback to the first step as the gaseous reactant.

Results and Discussion

Initial experiments were carried out by calcining Fe–P slag under a CO2 atmosphere to study the reaction between Fe1.5P and CO2. In this test, Fe1.5P powder was respectively calcined at 750, 800, 850, and 900 °C for 10 h under a CO2 atmosphere, and the as-obtained solid products were characterized by X-ray diffraction (XRD) analysis. The XRD patterns of the as-obtained products are shown in Figure . As shown in Figure a, Fe–P waste slag was composed of FeP and Fe2P, which was also confirmed in our previous research.[10] There were three diffraction peaks appeared when the roasting temperature increased to 750 °C, which agreed well with standard diffraction peaks of Fe3(PO4)2 (JPCDS no, 49-1087).[13] This indicates that Fe1.5P can be oxidized to Fe3(PO4)2 by CO2 with a temperature above 750 °C. The typical diffraction peaks of Fe3(PO4)2 centered at 2θ ≈ 29.5°, 30.7°, and 42.0°, and their peak intensities increase, whereas those of Fe2P and FeP decrease while increasing the calcining temperature. When the calcining temperature is above 850 °C, the solid products did not show any other diffraction peaks except that of Fe3(PO4)2, indicating that Fe3(PO4)2 is the only solid product for this reaction in the test condition.

Figure 1

XRD patterns of the solid products from roasting Fe–P slag under a CO2 atmosphere with (a) different calcination temperatures and (b) different calcination times.

XRD patterns of the solid products from roasting pan class="Chemical">Fe–P slag under a CO2 atmosphere with (a) different calcination temperatures and (b) different calcination times.

Furthermore, Fe1.5P powder was calcined at 800 °C for 5, 10, and 15 h in a CO2 atmosphere, and the XRD patterns of the as-obtained solid products are shown in Figure b. Herein, the as-obtained solid products were respectively labeled as FP800°C5h, FP800°C10h, and FP800°C15h. It was shown that FP800°C5h exhibited the typical diffraction peaks of Fe3(PO4)2 (JPCDS #49-1087). Also, there were typical peaks attributed to Fe2P, while peaks for FeP were not identified, indicating that FeP was entirely consumed by CO2 in the first 5 h, but Fe2P was not. As the calcination time extends to 10 and 15 h, the peak intensity of Fe3(PO4)2 increase obviously, while that of Fe2P decrease dramatically, implying that Fe2P reacted with CO2 and generated Fe3(PO4)2.

A home-made online gaseous detection system determined the gaseous product for the reaction between Fe–P slag and CO2. The schematic diagram of this online CO testing system is illustrated in Figure a. In this test, a coil heat exchanger was employed to cool down the outlet gas from the tube furnace followed by flowing it into a CO sensor with a resolution of 0.5 ppm (Membrapor, Switzerland). In this test, Fe1.5P powder was roasted in a CO2 flow (200 mL/min), and the temperature was raised from room temperature to 900 °C with a slope of 10 °C/min. Also, the temperature compensation of this system was carried out with an empty sample crucible, which is shown in Figure S1. The CO concentration in off-gas was tested, and its concentration is shown in Figure b. It was shown that CO liberation started with a temperature of about 600 °C, which indicated that the reaction between Fe–P slag and CO2 was started at about 600 °C. As the rising of roasting temperature, the CO concentration increased dramatically to about 4500 ppm with a temperature of about 700–80 °C, indicating that the reaction between Fe–P slag and CO2 got a fast reaction stage in this temperature range. As the consumption of Fe–P slag, the CO concentration decreased dramatically to about 0 ppm at 900 °C. According to this result, it is safe to conclude that CO was generated from the reaction between Fe1.5P and CO2, while this reaction can be started at about 600 °C and got to a fast reaction stage after 700 °C.

Figure 2

(a) Schematic diagram of the online CO detection system. 1: tube furnace; 2: thermocouple; 3: intelligent temperature controller; 4: sample crucible; 5: SiC heating component; 6: quartz tube; 7: coil heat exchanger; 8: CO sensor. (b) CO concentration in the off-gas for roasting Fe–P slag in a CO2 atmosphere.

According to the above product analysis for this reaction, its solid product and gpan class="Chemical">aseous product were Fe3(PO4)2 and CO, respectively. Basically, the overall reaction equation was proposed based on the analysis of products for this reaction between Fe1.5P and CO2, while the reaction equation was shown as

As shown in this equation, pan class="Chemical">Fe1.5P was oxidized to Fe3(PO4)2 by CO2 under the testing condition, as CO2 was reduced to CO at the same time. This is the general chemistry for preparing Fe3(PO4)2 in this method.

In this work, Fe3(PO4)2 was further used as the iron and phosphorus source for manufacturing LiFePO4. As for the preparation of LiFePO4/C, Li2CO3 was used as the lithium source, while H3PO4 was used as the phosphorus source to make up the content difference of iron and phosphorus in Fe3(PO4)2. In this work, Fe3(PO4)2 with different roasting temperatures and roasting times was used for the preparation of LiFePO4/C. The XRD patterns of the LiFePO4 synthesized are shown in Figure a. Herein, as-synthesized LiFePO4 was labeled as LFP-FP750°C10h, LFP-FP800°C10h, LFP-FP850°C10h, and LFP-FP900°C10h, corresponding to their Fe3(PO4)2 precursors manufactured by calcining at 750, 800, 850, and 900 °C, respectively. The XRD patterns of LiFePO4/C obtained from Fe3(PO4)2 prepared with different calcination times are shown in Figure b. Also, the LiFePO4/C samples obtained from Fe3(PO4)2 precursors prepared with different calcination times were labeled as LFP-FP800°C5h, LFP-FP800°C10h, and LFP-FP800°C15h. As shown in Figure , all of the indexed peaks match well with the standard peaks of LiFePO4 (JPCDS #40-1499),[10] indicating that the as-prepared samples were orthorhombic LiFePO4 crystals.

Figure 3

XRD patterns of LiFePO4/C synthesized from Fe3(PO4)2 prepared at (a) different temperatures and (b) different calcination times.

XRD patterns of LiFePO4/C synthesized from pan class="Chemical">Fe3(PO4)2 prepared at (a) different temperatures and (b) different calcination times.

Therefore, the possible chemical reaction occurred in the synthesizing of LiFePO4 pan class="Chemical">could be reasonably described as

As shown in this reaction equation, this reaction relepan class="Chemical">ased CO2 and H2O from the decomposition of Li2CO3 and H3PO4. Also, glucose was used as the carbon source in this experiment, which is the most commonly used cheap and effective carbon source for LiFePO4.[18] Therefore, the LiFePO4 samples obtained in this work were labeled as LiFePO4/C.

Further, the average grain sizes of the as-obtained pan class="Chemical">LiFePO4/C samples were calculated using Scherrer’s formula,[19] and the results are shown in Tables S1 and S2. The average grain sizes of the as-prepared LiFePO4/C samples arranged from 50 to 70 nm and have a larger crystal size when roasting with a longer calcination time or a higher calcination temperature.

The scanning electron microscopy (SEM) images of both Fe3(PO4)2 and LiFePO4/C are shown in Figure . The Fe3(PO4)2 samples, including FP800°C10h, FP800°C15h, and FP900°C10h, are shown in Figure (a-1), (b-1), and (c-1), respectively, while the corresponding LiFePO4/C samples, including LFP-FP800°C10h, LFP-FP800°C15h and LFP-FP900°C10h, are given in Figure (a-2), (b-2), and (c-2), respectively. As shown in Figure , all the Fe3(PO4)2 and LiFePO4/C samples showed an irregular shape, and their particle dimension arranged from several micrometers to decades of micrometers. Besides, it was found that the smaller particle size Fe3(PO4)2 generated the smaller particle size LiFePO4/C product, while there was no apparent agglomeration that occurred in the synthesis of LiFePO4/C from Fe3(PO4)2. Also, the particle size of FP900°C10h was relatively bigger than that of FP800°C10h, indicating that increasing the calcination temperature can increase the sample size.

Figure 4

SEM images of Fe3(PO4)2 (a-1) FP800°C10h, (b-1) FP800°C15h, and (c-1) FP900°C10h and the corresponding LiFePO4/C products (a-2) LFP-FP800°C10h, (b-2) LFP-FP800°C15h, and (c-2) LFP-FP900°C10h.

SEM images of Fe3(PO4)2 (a-1) FP800°C10h, (pan class="Gene">b-1) FP800°C15h, and (c-1) FP900°C10h and the corresponding LiFePO4/C products (a-2) LFP-FP800°C10h, (b-2) LFP-FP800°C15h, and (c-2) LFP-FP900°C10h.

Further, the influence of the preparing condition for pan class="Chemical">Fe3(PO4)2 on the energy storage properties of the corresponding LiFePO4/C was studied using a galvanostatic charge/discharge test, cyclic voltammograms, and EIS measurements, and the results are shown in Figure a–f.

Figure 5

(a) Galvanostatic charge/discharge curves, (b) CV curves, and (c) EIS plots of LFP-FP750°C10h, LFP-FP800°C10h, LFP-FP850°C10h, and LFP-FP900°C10h. (d) Galvanostatic charge/discharge curves, (e) CV curves, and (f) EIS plots of LFP-FP800°C5h, LFP-FP800°C10h, and LFP-FP800°C10h.

The galvanostatic charge/discharge curves of the LiFePO4/C samples are shown in Figure a. This test was performed in the voltage range of 2.4–4.2 V at 0.1 C rate. All the curves exhibit a similarly steady charge plateau at ∼3.44 V and a discharge plateau at ∼3.41 V, which correspond to the extraction and insertion of Li ion in LiFePO4, respectively. As shown in Figure a, the discharge capacities of LiFePO4/C depended on the calcination temperatures for manufacturing Fe3(PO4)2. The initial discharge capacities of LFP-FP750°C10h, LFP-FP800°C10h, LFP-FP850°C10h, and LFP-FP900°C10h were 93, 145, 108, and 104 mAh/g, while the corresponding Coulombic efficiencies were 88, 95, 88, and 86%, respectively. This noted that the sample from 800 °C calcination delivered both maximum discharge capacity and Coulombic efficiency among all the samples. The galvanostatic charge/discharge curves of as-synthesized LiFePO4/C from Fe3(PO4)2 with different calcination times are shown in Figure d. The initial discharge capacities of LFP-FP800°C5h, LFP-FP800°C10h, and LFP-FP800°C15h at 0.1 C are 92, 145, and 122 mAh/g, and the corresponding Coulombic efficiencies are 91, 95, and 91%, respectively. This noted that the sample with 10 h calcination time delivered higher capacity and Coulombic efficiency than other samples. The cycle performance of the sample LFP-FP800°C10h at 0.1 C current rate was subsequently investigated as given in Figure S2. After 25 cycles at 0.1 C rate, the capacity retention ratio remains at about 97%, indicating that the LFP-FP800°C10h sample has a distinguish reversibility. Also, according to the XRD and SEM experimental results discussed above, the LFP-FP800°C10h sample has a smaller average grain size, which may benefit the transportation of Li ion. Also, an appropriate degree of crystallinity is another critical factor that influences the capacity of LiFePO4. In this method, the LFP-FP800°C10h sample showed the optimized capacity under the influence comprehensively by both factors.

The cyclic voltammograms of as-synthesized LiFePO4/C, from Fe3(PO4)2 precursors prepared with different calcination temperatures and calcination times, performed at a scan rate of 0.1 mV/s are given in Figure b,e, respectively. One pair of symmetric oxidation/reduction peaks in each CV curve indicates that as-synthesized LiFePO4/C has a reversible two-phase reaction between LiFePO4 and FePO4. The oxidation and reduction peaks refer to Li-ion extraction from LiFePO4 and insertion into FePO4, respectively. The oxidation potential (EO), reduction potential (ER), separation of redox peak potentials (ΔE), oxidation peak current (IO), and reduction peak current (IR) are summarized in Tables S3 and S4. It is shown that ΔE of as-synthesized LiFePO4/C increases as the calcination temperature and calcination time increases. Also, the peak current intensity decreases with longer calcination time for synthesizing Fe3(PO4)2. By considering the particle dimension distribution analysis, it was inferred that the larger LiFePO4/C particle size would be detrimental to the transportation of lithium ion in the cathode, while the larger particle size was caused by longer calcination time and calcination temperature in the synthesis of Fe3(PO4)2.

The Nyquist plots of as-synthesized LiFePO4/C, from Fe3(PO4)2 prepared with different calcination temperatures and calcination times, and its equivalent circuit are shown in Figure c,f. All curves exhibit a similar shape with a depressed semicircle in the high-to-medium frequency region and an oblique line in the low-frequency region. The EIS curves were fitted using the R(QR)(Q(RW)) model provided by the ZsimpWin software. The constant phase element (CPE) is defined as Q to substitute for capacitance if taking the nonhomogeneity, such as porosity, roughness, and geometry, in the system into account.[10,20]Rs, Rct, W, Rf, Qd, and Qf refer to the ohmic resistance of the electrolyte, charge-transfer resistance, Warburg impedance, and the resistance of the solid electrolyte film (SEI), constant phase element of the film, and the electrolyte film/electrode interface, respectively. The simulated results of Rct, W, Rf, Qd, and Qf are summarized in Tables S5 and S6. The charge-transfer reaction impendence Rct of LFP-FP800°C10h has the lowest value among the obtained samples; however, the Warburg impedance W of that is the highest one, implying that LFP-FP800°C10h has fast electrode reaction kinetics but a low lithium diffusion rate in the bulk electrode. Rct and W show the smallest value for the LFP-FP800°C10h sample, indicating that it has a better dynamics property than other samples. As a result, the LiFePO4/C sample from Fe3(PO4)2 obtained by calcinating Fe1.5P powder at 800 °C for 10 h in a CO2 atmosphere, exhibits preferable electrochemical performances including high charge/discharge capacity, low polarization degree, and fast electrode reaction kinetics.

Based on this work, an environment-friendly method of manufacturing LiFePO4 is put forward using Fe–P and CO2, and this is described as flowing steps. The schematic description of the preparation process is illustrated in Figure . At first, an airslide disintegrating mill was employed to pulverize Fe–P waste slag to a 2000 mesh size. The powdered Fe–P slag from the mills was carried to the rotary kiln, and Fe–P powder was calcined in a CO2 atmosphere at 800 °C for 10 h to manufacture Fe3(PO4)2. The Fe3(PO4)2 manufactured in the first step was further ball milled with H3PO4/Li2CO3 in a molar ratio of 1:1:1.5, and another 10 wt % glucose was added as the carbon source. The mixture was calcined at 700 °C for 6 h in an Ar atmosphere in a furnace, and the final product LiFePO4/C was subsequently obtained.

Figure 6

Schematic description of the method for preparing LiFePO4/C.

Fe–P slag used in this method wpan class="Chemical">as a widespread waste slag in western China. Through this method, Fe–P slag can be utilized for manufacturing the high value-added LiFePO4/C. Furthermore, the CO2 gas, generated in the second step, can be used as a reaction gas in the first step. Also, the CO gas generated in the first step can be collected and used as a fuel for this process. It gives this method an environmentally friendly and high value-added property.

In this work, CO2 was utilized as a reactant, while it was reduced to CO in the synthesis of Fe3(PO4)2. However, the CO gas in the obtained gaseous products can be used as a fuel for the next step of manufacturing LiFePO4. Also, its combustion product is CO2, which can be feedback to the first step as the reactant gas. In the reaction of the synthesis of Fe3(PO4)2, every kilogram of Fe–P slag will consume about 779 m3 of CO2 and generate about 779 m3 of CO, which can be collected using gas separation technology and feedback to oxidize Fe–P slag in the first step. The detailed calculation process was given as below. The volume of CO generated in the synthesis of Fe3(PO4)2 was first calculated. As for the reaction showed as formulation 3, for 1 kg of Fe–P slag, the volume of CO generated in this reaction was calculated asIn which, VCO is the volume of CO generated in this method for 1 kg of Fe–P slag. MFe1.5P is the relative molecular mass of Fe1.5P (115 g/mol). According to formulation 3, the volume of CO generated in this reaction is equal to that of CO2 as the reactant gas. Therefore, about 779.1 m3 CO2 will be consumed for the synthesis of Fe3(PO4)2, and it will generate 779.1 m3 of CO at the same time. Considering that the combustion heat of CO is 12.64 MJ/m3, the overall combustion heat for 779.1 m3 CO (QCO) was calculated as

This design can make carbon dioxides circulate in a closed-loop carbon cycle of this industry process. This closed-loop carbon cycle can effectively reduce CO2 emissions and give this method an environmental characteristic, avoiding the massive emissions of CO2. By considering that the combustion heat of CO was 12.64 MJ/m3, the CO gas generated in the first step can generate 9846.6 MJ heat for 1 kg of Fe1.5P. The corresponding thermodynamic parameters and calculation process are given in the Supporting Information. This amount of energy can be used in the next step to synthesize the LiFePO4/C composite. It was estimated that this amount of CO generated in the first step was enough to drive the reaction in the second step. Therefore, this built a closed-loop energy cycle for this method, as shown in Figure . This design supplies a green strategy for manufacturing Fe3(PO4)2 and LiFePO4.

In this method, all the Fe and P from Fe–P slag was fully utilized for manufacturing LiFePO4, which provided a new way for the multipurpose use of Fe–P waste slag. Also, the CO gas generated in the first step can be used as a fuel in the second step. Also, CO2, as the gaseous product in the second step, can feedback to the first step as the gaseous reactant. Therefore, it avoided massive emissions of CO2. Due to this closed-loop carbon cycle and energy cycle, this method has environmentally friendly property, which can significantly reduce the energy consumption and CO2 emission.

Conclusions

A novel method with a closed-loop carbon and energy cycle was proposed to synthesize LiFePO4/C materials for Li-ion batteries. At first, Fe3(PO4)2 was synthesized by calcining Fe–P slag in a CO2 atmosphere followed by as-obtained Fe3(PO4)2 reacting with Li2CO3 and H3PO4 for synthesizing LiFePO4/C cathode materials. Further, it was found that LiFePO4/C materials, synthesized from the Fe3(PO4)2 obtained by calcining Fe–P waste slag at 800 °C for 10 h in a CO2 atmosphere, exhibited a high capacity (145 mAh/g at 0.1 C rate), good reversibility, and low polarization degree. In this method, the CO gas generated in synthesizing Fe3(PO4)2 can be used as a fuel in the second step. While its combustion product, CO2, can be feedback to the first step as the reactant gas. It formed a closed-loop carbon and energy cycle, which can dramatically reduce the energy consumption and CO2 emission for this method. This work puts forward an environment-friendly method for manufacturing LiFePO4/C cathode materials, which can remarkably reduce its energy consumption and CO2 emission.

Experimental Section

Methods

Fe–P waste slag used throughout this work was previously determined as a mixture of FeP and Fe2P, which is denoted as Fe1.5P for simplification in this paper.[10] Fe1.5P (∼98.0%) powder was first calcined in a CO2 atmosphere at 750–900 °C for 5–15 h to synthesize Fe3(PO4)2, which was further used as the Fe and P source in the next step for manufacturing LiFePO4. As-synthesized Fe3(PO4)2 was mixed with H3PO4 (∼85%)/Li2CO3 (∼99.6%) in a molar ratio of 1:1:1.5, and 10 wt % glucose was added as the carbon resource. The mixture was ball milled using ethanol as the dispersing agent to form a rheological phase. After being dried, the mixture was calcined at 700 °C for 6 h in a quartz tube furnace flushed by argon to manufacture LiFePO4/C. As for the tail gas treatment for this work, a mixture of CO2 and CO was separated using a method of alkali absorption, and as-obtained CO was collected and centralized treated in a torch.

Characterization

The phase structures of pan class="Chemical">as-synthesized Fe3(PO4)2 and LiFePO4/C were tested by X-ray diffraction analysis (XRD, Philips X’Pert Pro, Holland) with a step of 0.04° s–1 in the range of 10–80° using Cu Kα radiation at the power of 40 kV × 40 mA. The morphologies and particle sizes were observed using a scanning electron microscope (JSM-7500F, Japan).

Electrochemical Measurements

The electrochemical performances of as-synthesized LiFePO4/C were investigated as the cathode material for Li-ion batteries using 2032-type coin cells. The galvanostatic charge/discharge test, cyclic voltammetric (CV) test, and electrochemical impedance spectroscopy (EIS) measurements were used to analyze the electrochemical properties of the as-synthesized LiFePO4/C materials. For preparing the LiFePO4/C electrode, the as-synthesized LiFePO4/C materials were well mixed and ground with 10 wt % conductive acetylene black and 7 wt % commercial LA132 binder (Chengdu Indigo Power Source Co. Ltd., China), and then the mixture was coated on washed aluminum foil. After being dried in vacuum at 100 °C for 10 h, the coated foil was subsequently cut into round wafers as the working electrode. Metal lithium was applied as both the reference electrode and the counter electrode. A Celgard 2300 microporous polyethylene membrane was employed as the separator, and the electrolyte was 1 M LiPF6 dissolved in the mixed solution of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) (1:1:1 by volume, Shenzhen Capchen Chemicals Co. Ltd., China). CR2025-type coin cells were assembled in an argon-filled glove box. Galvanonstaic charge/discharge measurements were performed in the range of 2.4 to 4.2 V versus Li+/Li at room temperature on a Neware battery-testing instrument (Shenzhen Neware Tech-144 nology Ltd., China). EIS and CV measurements were carried out using an electrochemical workstation controlled by the Powersuit software (Princeton Applied Research, USA). The EIS analysis was performed in a frequency range from 10 mHz to 100 kHz with an amplitude of 5 mV, and the CV measurements proceeded between 2.4 and 4.2 V.