Preparation of LiFePO₄/C Cathode Materials via a Green Synthesis Route for Lithium-Ion Battery Applications

Abstract

In this work, LiFePO₄/C composite were synthesized via a green route by using Iron (III) oxide (Fe₂O₃) nanoparticles, Lithium carbonate (Li₂CO₃), glucose powder and phosphoric acid (H₃PO₄) solution as raw materials. The reaction principles for the synthesis of LiFePO₄/C composite were analyzed, suggesting that almost no wastewater and air polluted gases are discharged into the environment. The morphological, structural and compositional properties of the LiFePO₄/C composite were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM), Raman and X-ray photoelectron spectroscopy (XPS) spectra coupled with thermogravimetry/Differential scanning calorimetry (TG/DSC) thermal analysis in detail. Lithium-ion batteries using such LiFePO₄/C composite as cathode materials, where the loading level is 2.2 mg/cm², exhibited excellent electrochemical performances, with a discharge capability of 161 mA h/g at 0.1 C, 119 mA h/g at 10 C and 93 mA h/g at 20 C, and a cycling stability with 98.0% capacity retention at 1 C after 100 cycles and 95.1% at 5 C after 200 cycles. These results provide a valuable approach to reduce the manufacturing costs of LiFePO₄/C cathode materials due to the reduced process for the polluted exhaust purification and wastewater treatment.

Highlights:

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LiFePO₄/C is synthesized by using a green route where almost no wastewater and air polluted gases are discharged into the environment.

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The reaction principles for the synthesis of LiFePO₄/C are analyzed.

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LiFePO₄/C exhibits uniform nano-structure and carbon layer.

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LiFePO₄/C shows excellent rate capability and cycling capability.

1. Introduction

Olivine-type LiFePO₄ is considered as one of the most promising cathode materials for Li ions batteries owing to its high operating voltage (~3.4 V vs. Li/Li⁺), high theoretical capacity (~170 mA h/g), low cost and no environmental pollution [1,2,3,4,5,6,7]. However, bare LiFePO₄ materials suffer from many disadvantages, such as low conductivity and sluggish diffusion rate of Li⁺ ions coupled with low tap density [6,7]. Recently, many efforts have been made to improve its conductivity and accelerate the diffusion rate of Li⁺, including coating the conducting materials on the surface of LiFePO₄ materials [8,9,10,11], reducing particle size [12], doping transition metals ions [13], etc. Ultimately, high-quality LiFePO₄ materials have been successfully developed and commercialized in energy storage and electric vehicles (EVs).

However, there still exist some challenging problems for the commercialization of LiFePO₄ materials in the next generation of lithium-ion batteries. Firstly, complex fabrication procedures such as ingredients, pulping, coating, tableting, winding and assembly welding, further need to be simplified and optimized [7,14]. Secondly, understanding the kinetic behavior of LiFePO₄ material for the lithium-ion batteries is of fundamental importance [2,6], including the conductive pathway with conducting materials coated on its surface, the Li⁺ ions diffusion dynamics with the transition metal atoms doping, the Li⁺ ions diffusion pathway during the insertion/extraction process, etc. Thirdly, reducing the manufacturing costs of LiFePO₄ materials and preventing environmental pollution are quite important. Currently, it is noted that the synthesis LiFePO₄ materials always uses solid state reaction method [15,16], liquid phase method [17], sol-gel method [18], hydrothermal method [19,20] and spray pyrolysis method [21,22]. Almost all these methods can produce wastewater containing excessive anions impurities such as SO₄²⁻, Cl⁻ and NO₃⁻, and contaminated gas (N_xO_y, CO, and NH₃), which need additional apparatus to deal with them and increase the manufacturing cost. Therefore, seeking approaches to further reduce the manufacturing cost of LiFePO₄ materials synthesis and preventing environmental pollution are still highly pursued by materials scientists.

Here, we developed a green route to synthesize the LiFePO₄/C composite by using Iron (III) oxide (Fe₂O₃) nanoparticles, Lithium carbonate (Li₂CO₃), glucose powder and phosphoric acid (H₃PO₄) solution as raw materials. We first synthesized the FePO₄·2H₂O precursor by the reaction of Fe₂O₃ nanoparticles with H₃PO₄ solution. The wastewater was water and excessive H₃PO₄ solution which could be recycled next time. Second, we synthesized the LiFePO₄/C composite by annealing the mixtures composed of FePO₄·2H₂O precursor, Li₂CO₃ and glucose powder at a high-temperature process, where only CO₂ gas and water vapor were discharged. Therefore, all the reaction processes were environmentally friendly. The morphological, structural, compositional properties of the synthesized LiFePO₄/C composite were characterized. Lithium-ion batteries using such composite as cathode active materials were fabricated, and the corresponding electrochemical performance were discussed.

2. Experimental Section

2.1. Preparation of LiFePO₄/C Composite

Iron (III) oxide (Fe₂O₃) powder (~800 nm (ACS, 99.99%)), phosphoric acid (H₃PO₄) solution (85%), Lithium carbonate (Li₂CO₃) and glucose powder were purchased from Sigma Aldrich (Shanghai, China) and used without further purification unless stated otherwise. The FePO₄·2H₂O precursor was prepared by the chemical reaction of Fe₂O₃ powder with H₃PO₄ solution where the molar ratio of the Fe/P was 1:1.05. In a typical procedure, 16 g of Fe₂O₃ powder and 14.4 mL H₃PO₄ solution were added into 20 mL deionized water in a flask followed by ultrasonic dispersion for 30 min. Then, the mixed slurries were transferred into a ball mill tank and ball-milled for additional 9 h. After that, the mixed slurries were filtered and allowed to heat up to 85 °C for 5 h forming a suspension, followed by cooling down to room temperature. The white precipitate (FePO₄·2H₂O precursor) was collected and separated by centrifugation and washed with water for several times, and then dried in a blast drying box for 24 h. The LiFePO₄/C composite were prepared by using stoichiometric amounts of FePO₄·2H₂O precursor, Li₂CO₃ and glucose powder (60.0 g glucose/1 mol FePO₄·2H₂O precursor) as the starting materials, followed by high temperature sintering. First, stoichiometric amounts of FePO₄·2H₂O precursor and Li₂CO₃ were added into water-dissolved glucose solution in a flask followed by ultrasonic dispersion for 30 min. Then, the mixed slurries were dried in a blast drying box for 24 h. After that, the mixture was sintered at 650 °C in a tube furnace for 10 h under argon flow to obtain LiFePO₄/carbon composite.

2.2. Characterization of LiFePO₄/C Composite

The crystallinity was estimated by using X-ray diffraction (XRD, D/Max-IIIC, Rigaku Co., Tokyo, Japan) equipped with a Cu-Kα source of wavelength λ = 1.54060 Å and operated at 40 kV and 20 mA. The top-view SEM images were taken on a Hitachi S-4800 (Hitachi Limited, Tokyo, Japan), and the attached energy dispersive spectrometer (EDS (Hitachi Limited, Tokyo, Japan) analyzer was used to analyze the composition distribution of carbon. The transmission electron microscopy (TEM) images were acquired on a FEI Talos F200X (FEI, Hillsboro, OR, USA) with an acceleration voltage of 200 kV. Thermo-Gravimetric coupled with Differential Scanning Calorimetry (TG-DSC, Netzsch Scientific Instruments Trading Co., Ltd., Shanghai, China) was used to measure the carbon content in the LiFePO₄/carbon composite. Raman spectra were tested at room temperature equipping with 514 nm laser excitations. X-ray Photoelectron Spectroscopy (XPS) measurement was performed on a SPECS HSA-3500 (SPECS, Berlin, Germany) to determine the valence state of each element of the samples.

2.3. Electrochemical Measurements

The electrochemical measurements were performed using a CR2032 coin-type cell assembled in an argon-filled glove-box (MIKROUNA, Guangzhou, China). For fabricating the working electrodes, a mixture of active materials (LiFePO₄/C composite), conductive carbon blacks (Super-P, Shenzhen, China), and polyvinylidene fluoride (PVDF) binder at a weight ratio of 80:10:10 was coated on aluminum foil and dried in vacuum at 120 °C for 12 h. The thickness and loading level were 48 µm and 2.2 mg/cm², respectively. The lithium pellets were used as the counter and reference electrode. The electrolyte consisted of a solution of 1 mol/L LiPF₆ in ethylene carbon (EC)/dimethyl carbonate (DMC) (1:1 w/w). A celguard 2300 microporous film was used as separator. The cells were galvanostatically charged and discharged between 2.5 V and 4.2 V versus Li/Li⁺ on a battery cycler (LAND, CT2001A, Wuhan, China). Cyclic voltammogram (CV) measurements were carried out using a Multi Autolab electrochemical workstation (Metrohm, Guangzhou, China) at a scanning rate of 0.1–0.5 mV s⁻¹. Electrochemical impedance spectra (EIS) were also characterized by Autolab electrochemical workstation adjusting amplitude signal at 5 mV and frequency range of 0.01 Hz–100 kHz.

3. Results and Discussions

Figure 1 shows the schematic diagram of the preparation of LiFePO₄/C composite. Firstly, an excess of phosphoric acid solution reacted with Iron (III) oxide (Fe₂O₃) powder to form FePO₄·2H₂O precursors. Subsequently, these precursors were mixed with Lithium carbonate (Li₂CO₃) and glucose powder followed by high temperature sintering to form LiFePO₄/C composite. The reaction equations are shown below.

$${\mathrm{Fe}}_2{\mathrm{O}}_3+2{\mathrm{H}}_3{\mathrm{PO}}_4+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{FePO}}_4·2{\mathrm{H}}_2\mathrm{O}.$$

(1)

$$2{\mathrm{FePO}}_4·2{\mathrm{H}}_2\mathrm{O}+{\mathrm{Li}}_2{\mathrm{CO}}_3+{\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6\to 2{\mathrm{LiFePO}}_4/\mathrm{C}+\text{volatile matter}$$

(2)

Equation (1) shows the synthesis process of FePO₄·2H₂O precursors. The reaction mechanism is referred to the reported literature [23], where the Fe salts are used and the reaction equations are shown below:

$${\mathrm{Fe}}^{3+}+{\mathrm{H}}_2\mathrm{O}=\mathrm{Fe}{\left(\mathrm{OH}\right)}^{2+}+{\mathrm{H}}^{+}$$

(3)

$$\mathrm{Fe}{\left(\mathrm{OH}\right)}^{2+}+{\mathrm{H}}_2\mathrm{O}={\mathrm{Fe}{\left(\mathrm{OH}\right)}_2}^{+}+{\mathrm{H}}^{+}$$

(4)

$${\mathrm{Fe}}^{3+}+{\mathrm{H}}_3{\mathrm{PO}}_4={\mathrm{FeH}}_2{{\mathrm{PO}}_4}^{2+}+{\mathrm{H}}^{+}$$

(5)

$${\mathrm{FeH}}_2{{\mathrm{PO}}_4}^{2+}+{\mathrm{H}}_3{\mathrm{PO}}_4={\mathrm{Fe}{\left({\mathrm{H}}_2{\mathrm{PO}}_4\right)}_2}^{+}+{\mathrm{H}}^{+}$$

(6)

$$\mathrm{Fe}{\left(\mathrm{OH}\right)}^{+}+{\mathrm{Fe}{\left({\mathrm{H}}_2{\mathrm{PO}}_4\right)}_2}^{+}=2{\mathrm{FePO}}_4+2{\mathrm{H}}_2\mathrm{O}+2{\mathrm{H}}^{+}$$

(7)

In this work, we replaced the Fe salts with the Fe₂O₃ powder as raw materials to synthesize the FePO₄·2H₂O precursors because the wastewater from the above method contains impurity ions such as SO₄²⁻, Cl⁻ and NO₃⁻ which are not environmentally friendly, although Fe salts are very cheap. Due to the complex reaction processes, we add Equations (3)–(7) to get Equation (1) where the Fe³⁺ ions are replaced by Fe₂O₃. In addition, it takes a little time to dissolve the Fe₂O₃ powder in the acid solution, leading to a lower reaction rate of our method in comparison with that of the method described above. In the environmental protection perspective, our method is favorable to the commercialization of future products. The reason is that there are no metal ion and anion impurities left in the wastewater solution. Although the phosphoric acid solution is excessive, it can be recovered and recycled. Equation (2) displays the synthesis process of LiFePO₄/C composite. The reaction equation is balanced according to the stoichiometric values of Li, Fe and P elements, where LiFePO₄/C is the final product while the volatile matter represents the volatile gases, such as CO₂ gas and H₂O vapor, even a small amount of CO gas, other C_xH_yO_z, etc. To determine whether the CO gas or C_xH_yO_z is present in the exhaust during the synthesis of LiFePO₄/C composite, thermodynamic Gibbs free energies for the formation of CO and C_xH_yO_z were calculated. Because the precursors were annealed in inert gas, the thermal decomposition products of the FePO₄·2H₂O are FePO₄ and H₂O vapor, while those of Li₂CO₃ are Li₂O and CO₂, and those of C₆H₁₂O₆ are C and H₂O vapor. Therefore, the formation of CO or C_xH_yO_z can be derived from the reaction of C with H₂O vapor or CO₂ gas. By thermodynamic Gibbs free energies calculation, the temperature for the formation of CO gas must be more than 980.6 K. In our experiment, the sintering temperature for the precursors was 650 °C (923 K), i.e. lower than 980.6 K. Thus, CO gas was not generated. C_xH_yO_z was also not generated due to absence of CO and H₂. In other words, our work is a green route to synthesize the LiFePO₄/C cathode materials for lithium ion battery application.

Figure 2a shows the XRD pattern of LiFePO₄/C composite. As can be seen, all XRD peaks match well with the standard data JCPDS (Joint Committee on Powder Diffraction Standards) card No. 81-1173, demonstrating the formation of LiFePO₄ with orthorhombic structure. The lattice parameters are a = 10.342 Å, b = 6.021 Å, and c = 4.699 Å, respectively. The main XRD peaks are strong and sharp, suggesting good crystallinity of LiFePO₄/C composite. The XRD peaks assigned to the carbon are not detected due to its amorphous state [22]. Moreover, its low content also plays an important role. Figure 2b shows the TG/DSC curves to estimate the carbon content in the LiFePO₄/C composite. As can be seen, the weight gain of 3.62% below 550 °C is assigned to the oxidation of LiFePO₄/carbon to the Li₃Fe₂(PO₄)₃ and Fe₂O₃ [24,25]. Above 550 °C, there is almost no weight change, indicating that the LiFePO₄/C composite are fully oxidized where the carbon is oxidated to the CO₂ gas. According to the total weight gain of 5.07% for pure LiFePO₄ in theory [26], the amount of carbon in the LiFePO₄/C composite is about 1.45%. Figure 2c shows the Raman characterization of LiFePO₄/C composite. The Raman spectrum exhibits two peaks at 1351 cm⁻¹ and 1605 cm⁻¹ corresponding to the D band (disordered carbon, sp3) and G band (graphite, sp2) for amorphous carbon, respectively [27,28,29,30]. The observed D band and G band indicate the existence of carbon in the LiFePO₄/C composite. A lower relative intensity ratio of D/G band corresponds to a higher order carbon arrangement. As can be seen, the relative intensity ratio of D/G is 0.66 and the G band shows a smaller full-width half-maximum compared to that of the D band, indicating high graphitization of C in the LiFePO₄/C composite. Although the Raman spectrum shows a sharp graphitic carbon peak, the carbon remains in the amorphous state in the LiFePO₄/C composite. Therefore, it is not detected by XRD characterization. Figure 2d shows EDS mapping to estimate the composition distribution of carbon element in the LiFePO₄/C composite. As can be seen, the carbon is uniformly distributed across the whole surface, which is beneficial to the conductivity properties of LiFePO₄ and improves electrochemical performance of Lithium-ion battery.

Figure 3a shows the SEM images of LiFePO₄/C composite. As can be seen, the LiFePO₄/C composite exhibit uniform particle size distribution ranging from 100 to 200 nm. The small grain sizes of LiFePO₄/C composite are attributed to the carbon coating on the surface of the LiFePO₄ nanoparticles that prevents their quick growth. This phenomenon can be explained by the space steric effect which increases the diffusion activation energy of the reactants and slows down the growth rate of grains [31]. Therefore, the carbon coating layer is quite important in controlling particle size. The small grain sizes are conducive to shortening the migration paths of lithium ions and electrons during the lithiation/delithiation process and as a result, improve the electrochemical performances of LiFePO₄/C composite efficiently [32]. Further characterization was carried out by TEM and the corresponding images of the LiFePO₄/C composite are shown in Figure 3b–d. The carbon layer on the LiFePO₄ nanoparticles surface is uniform, showing a thickness of about 2–3 nm, which demonstrates that the carbon exists in the LiFePO₄/C composite. This result is consistent with the previous TG-DSC analysis and Raman characterization. The effect of the carbon layer is beneficial to smoothing electron migration for the reverse reaction of Fe³⁺ to Fe²⁺. In addition, the carbon layer can supply a better electronic contact between the LiFePO₄ nanoparticles, which ensures that the electrons are able to migrate quickly enough from all sides [32,33,34]. Meanwhile, the lattice fringes corresponding to the (011) crystal plane demonstrate the formation of olivine-type LiFePO₄.

Figure 4 shows the high-resolution X-ray photoelectron spectroscopy (XPS) spectra of the Li 1s, Fe 3p, Fe 2p, P 2p, O 1s and C 1s core levels to determine the oxidation states of the elements in the LiFePO₄/C composite. The peak at 56.5 eV, corresponding to the lithium of the LiFePO₄/C composite, cannot be seen due to the superposed iron peak of Fe 3p [35,36]. The peak intensity of Fe 3p is higher than Li 1s because the Fe 3p has greater relative atomic sensitivity than that of Li 1 s [37,38]. The Fe 2p shows two peaks at 710.1 (2p3/2) and 724.1 eV (2p1/2) with a splitting energy of 14.0 eV, which is close to the standard splitting energy of 19.9 eV, demonstrating the oxidation state of Fe²⁺ [36,38]. Moreover, two small peaks at high binding energy of 713.9 and 728.5 eV are the characters of transition metal ions with partially filled-d orbits, which are assigned to the multiple splitting of the energy levels of Fe ion [37,38]. The peaks representing the other valence states of Fe ions cannot be seen, revealing that only Fe²⁺ ions exist in the LiFePO₄/C composite. The P 2p shows a peak at 132.9 eV, revealing that the valence state of P is 5+ [38]. The O 1s shows a peak at 531.0 eV, confirming that the valence state of O in the LiFePO₄/C composite is divalent. The two shoulder peaks at 531.9 and 533.0 eV are attributed to the C–O and C=O bands arising from functional groups absorbed on the sample surface [39]. The C 1s shows peaks at 284.0 and 284.4 eV, which correspond to the short-order sp2-coordinated and sp3-coordinated carbon atoms [38]. The additional peak at 288.2 eV is the C=O band arising from functional groups absorbed on the sample surface. These results confirm that the LiFePO₄/C composite was synthesized.

Figure 5a shows the cyclic voltammetry curves of lithium ion batteries using the LiFePO₄/C composite as the cathode active materials. N peak appears at 2.63 V (characteristic of Fe³⁺ in Fe₂O₃), indicating that all the iron atoms in the LiFePO₄/C composite are Fe²⁺ [40]. The two peaks around at 3.34 and 3.53 V (vs. Li⁺/Li) are attributed to the Fe²⁺/Fe³⁺ redox reaction, which corresponds to lithium extraction and insertion in LiFePO₄ crystal structure [41]. Furthermore, the two peaks show a narrow potential separation of 0.19 V and exhibit good symmetric and poignant shape, which imply a good electrochemical performance for lithium ion batteries. Figure 4b further shows the evolution of the cyclic voltammetry curves of LiFePO₄/C composite in the scanning rate ranging from 0.1 to 0.5 mV·s⁻¹. The peak position shifts and the potential separation between two peaks broadens gradually as the scan rate increases. Previous literature has reported that the diffusion coefficient of lithium ions (D_Li) can be determined from a linear relationship between peak currents (i_p) and the square root of the scan rate (v^1/2) based on the Randles–Sevcik equation [41,42,43]:

$${\mathrm{I}}_{\mathrm{p}}=2.69\times {10}^5{\mathrm{n}}^{3/2}{\mathrm{ACD}}^{1/2}{\mathrm{v}}^{1/2}$$

(8)

where I_p (A) is the current maximum, n is the number of electrons transfer per mole (n = 1), F (C/mol) is the Faraday constant, A (cm²) is the electrode area (1.77 cm²), C (mol/cm³) is the lithium concentration in the LiFePO₄/C composite, v (V/s) is the scanning rate, D_Li (cm²/s) is the lithium diffusion coefficient, R (J/K·mol) is the gas constant, and T (K) is the temperature. Figure 4c shows the linear relationship between peak currents (I_p) and the square root of the scan rate (v^1/2). The diffusion coefficient D_Li are calculated to be 4.35 × 10⁻¹³ and 2.57 × 10⁻¹³ cm²/s for the charge and discharge processes, respectively, which are comparable to the previous reported literature [43,44,45]. This confirms that Li ions show excellent transmission performance, suggesting excellent electrochemical performance of our Li-ion batteries. Figure 4d,e shows the charge/discharge curves of lithium ion batteries at current rate from 0.1 C to 20 C. Apparently, at a low current rate of 0.1 C, the batteries deliver a discharge capacity of 161 mAh·g⁻¹, corresponding to 95% of the theoretical capacity (170 mAh·g⁻¹) of LiFePO₄. With the current rate increasing, the discharge capacity continually decreases, which is attributed to the low electronic conductivity and ion diffusion coefficient coupled with low tap density [32,38]. Despite this, the discharge capacity of our lithium ion batteries can reach 119 and 93 mAh·g⁻¹ at high current rate of 10 C and 20 C. In addition, our batteries retain an approximate discharge capacity of 161 mAh·g⁻¹ at the current rate of 0.1 C after the batteries are tested at the current rate of 20 C. This indicates that our batteries are highly structural stability, which can be suitable for the large current discharge. Figure 4f displays the cyclic performances and the coulombic efficiency of the lithium ion batteries. It is found that the batteries show a discharge capacity of 142 mAh·g⁻¹ with a capacity retention of 98% after 100 cycles at 1 C. When the rate reaches at 5 C, the batteries even show discharge capacity of 125 mAh·g⁻¹ with a capacity retention of 95.1% after 200 cycles. The coulombic efficiency with a value of 99% almost remains constant. These results demonstrate the high cycling stability of our batteries.

The electrochemical impedance spectra (EIS) technology is one of the most powerful tools to study electrochemical reactions, such as the processes occurring at the interface between electrodes and electrolyte, and the Li⁺ intercalation/de-intercalation in the interior of cathode/anode materials [46,47]. Figure 6a shows the EIS curve of lithium ion batteries using the LiFePO₄/C composite as the cathode active materials after 10 cycles at rate of 1 C. Clearly, the EIS curve consists of a semicircle in the high-frequency region followed by a straight line in the low-frequency region. The former is related to the charge-transfer process at the electrode/electrolyte interfaces, while the latter represents the Warburg impedance associated with the Li⁺ diffusion in the LiFePO₄ crystal lattice [48,49]. The radius of the semicircle in the EIS curve for the LiFePO₄/C composite is 60.2 Ω. As a comparison, the EIS curve of the commercial LiFePO₄/C materials is also plotted in Figure 6a. All the procedures for the fabrication of lithium ion batteries are completed under identical conditions. In addition, the loading level of commercial LiFePO₄/C composite as active materials is also 2.2 g/cm². The commercial LiFePO₄/C materials with the carbon content of about 1.44% are purchased from the Optimumnano Energy Co., Ltd. (Shenzhen, China). The grain size of the LiFePO₄/C is 200–300 nm, as shown in Figure 6b. As can be seen, the radius of the semicircle in the EIS curve is 124.2 Ω. This indicates that our LiFePO₄/C composite shows better electrical properties than that of the commercial LiFePO₄/C materials. One of the possible reasons is that our LiFePO₄/C composite (100–200 nm) exhibits relatively smaller grain sizes and higher specific surface area (Figure 6b) in comparison with that of the commercial LiFePO₄/C materials. This is because the small grain sizes are conducive to shortennig the migration paths of lithium ions and electrons during the lithiation/delithiation process [38]. In addition, the carbon content is very similar between our LiFePO₄/C composite and the commercial LiFePO₄/C materials. The diffusion coefficient of Li⁺ (D) can also be calculated form the EIS curve by using the following equation [49,50]:

$$\mathrm{D}={\mathrm{R}}^2{\mathrm{T}}^2/2{\mathrm{A}}^2{\mathrm{n}}^4{\mathrm{F}}^4{\mathrm{C}}^2{\mathsf{\sigma }}^2$$

(9)

where R is gas constant (8.314 J·mol⁻¹·k⁻¹), T is the absolute temperature (298.15 K), A is the area of the tested electrode surface (cm²), n is the number of electrons involved in the redox process (n = 1 in this work), C is the molar concentration of Li⁺ in the tested electrode, F is the Faraday constant, and σ is the Warburg impedance coefficient [46,47]. By linear fitting the relation plot between Z_Re and ω^−1/2 (the reciprocal square root of the angular frequency ω) (as shown in Figure 6b) to estimate the Warburg impedance coefficient σ, the diffusion coefficient of Li⁺ (D) could be obtained from the above equation. By calculation, the diffusion coefficient of Li⁺ (D) for our LiFePO₄/C composite is 3.17 × 10⁻¹³ cm²/s. This result is consistent with the previous calculation using the Randles–Sevcik equation. The D value for the commercial LiFePO₄/C materials is also calculated to be 2.34 × 10⁻¹³ cm²/s. For a comparison, our LiFePO₄/C composite shows a relatively higher D value, which is assigned to the smaller grain sizes that are conducive to shortening the migration paths of lithium ions [38].

4. Conclusions

In conclusion, high-quality LiFePO₄/C composite were synthesized via a green route in which no wastewater or air polluting gas is discharged into the environment. The synthesized LiFePO₄/C composite exhibited excellent nanoscale particle size (100–200 nm) showing uniform carbon coating on the surface of LiFePO₄ nanoparticles, which effectively improved the conductivity and diffusion of Li⁺ ions of LiFePO₄. Consequently, lithium ion batteries using the as-synthesized LiFePO₄/C composite as cathode materials exhibit superior electrochemical performance, especially for high rate performance. More importantly, this work provides a valuable method to reduce the manufacturing cost of the LiFePO₄/C cathode materials due to the reduced process for the polluted exhaust purification and wastewater treatment, which is highly desired for applications such as large-scale energy storage and electric vehicles.