Green and High-Yield Recovery of Phosphorus from Municipal Wastewater for LiFePO4 Batteries

Yijiao Chang; Xuan Wang; Bolin Zhao; Anjie Li; Yiru Wu; Bohua Wen; Bing Li; Xiao-Yan Li; Lin Lin

doi:10.1016/j.eng.2024.05.018

Engineering ›› 2025, Vol. 45 ›› Issue (2) :247 -256. DOI: 10.1016/j.eng.2024.05.018

Research Environmental Engineering—Article

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Green and High-Yield Recovery of Phosphorus from Municipal Wastewater for LiFePO₄ Batteries

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Abstract

The rapidly growing demand for lithium iron phosphate (LiFePO₄) as the cathode material of lithium-ion batteries (LIBs) has aggravated the scarcity of phosphorus (P) reserves on Earth. This study introduces an environmentally friendly and economical method of P recovery from municipal wastewater, providing the P source for LiFePO₄ cathodes. The novel approach utilizes the sludge of Fe-coagulant-based chemical P removal (CPR) in wastewater treatment. After a sintering treatment with acid washing, the CPR sludge, enriched with P and Fe, transforms into purified P–Fe oxides (Fe_2.1P_1.0O_5.6). These oxides can substitute up to 35% of the FePO₄ reagent as precursor, producing a carbon-coated LiFePO₄ (LiFePO₄/C) cathode with a specific discharge capacity of 114.9 mA·h·g⁻¹ at current density of 17 mA·g⁻¹), and cycle stability of 99.2% after 100 cycles. The enhanced cycle performance of the as-prepared LiFePO₄/C cathode may be attributed to the incorporations of impurities (such as Ca²⁺ and Na⁺) from sludge, with improved stability of crystal structure. Unlike conventional P-fertilizers, this P recovery technology converts 100% of P in CPR sludge into the production of value-added LiFePO₄/C cathodes. The recovered P from municipal wastewater can meet up to 35% of the P demand in the Chinese LIBs industry, offering a cost-effective solution for addressing the pressing challenges of P scarcity.

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Municipal wastewater / Chemical phosphorus removal sludge / Lithium iron phosphate / Lithium-ion batteries / Phosphorus recovery

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Yijiao Chang, Xuan Wang, Bolin Zhao, Anjie Li, Yiru Wu, Bohua Wen, Bing Li, Xiao-Yan Li, Lin Lin. Green and High-Yield Recovery of Phosphorus from Municipal Wastewater for LiFePO₄ Batteries. Engineering, 2025, 45(2): 247-256 DOI:10.1016/j.eng.2024.05.018

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1. Introduction

Rock phosphorus (P), a critical and finite resource, is necessary for the production of industrial chemicals and agricultural fertilizers [1], [2], [3]. Commercially available P resources are estimated to be depleted within the next 50–100 years at the current mining speed [4], [5]. In fact, the international market price of 30% grade P ore has more than doubled to 160 USD·t⁻¹ in the past decade [6]. In China, P ore has been listed in the Strategic Minerals Catalogue since 2016 to conserve this valuable resource.

Furthermore, the demand for P for power storage batteries has surged with the increasing application of green and renewable energy for achieving carbon neutrality goals and addressing global warming concerns. Among various types of lithium-ion batteries (LIBs), lithium iron phosphate (LiFePO₄ or LFP) batteries are commonly used, with a market share of approximately 60%, owing to their cost-effectiveness and high safety [7], [8], [9]. The annual consumption of LiFePO₄ for LFP battery production in China reached 1 200 000 t in 2022 [10], corresponding to a need for 240 000 t of P, inevitably necessitating an increase in P resource exploration. Thus, P recycling and reuse become increasingly important, and several strategies have been proposed for recycling P from various waste sources, such as LiFePO₄ cathode materials from spent LFP batteries [11], [12], steelmaking slag [13], and dephosphorization waste [14], to ensure resource conservation and environmental sustainability.

According to the Ecological Environment Bulletin Statistics 2022 of China, food consumption and chemical applications have led to over 250 000 t of P entering municipal wastewater as a primary nutrient pollutant. This quantity is more than the entire annual P consumption for LiFePO₄ synthesis in China. To meet stringent discharge standards (P ≤ 0.5 mg·L⁻¹) [15], the chemical P removal (CPR) process using Fe³⁺- or Al³⁺-based coagulates has been commonly adopted after biological treatment [16], [17], [18]. The CPR eliminates P from the effluent by transferring it via precipitation from the liquid phase to the sediment sludge. For example, the FeCl₃-based CPR process implemented at the southern wastewater treatment plant (WWTP) in Shenyang, China, lowers the P concentration in the treated effluent to 0.2–0.5 mg·L⁻¹, generating 1800 m³·d⁻¹ of P-rich CPR sludge [19]. This sludge, a byproduct of Fe-based CPR, contains up to 10 wt% of P in dry solid, comparable to the content in low-grade natural P rock (8–13 wt%), along with approximately 20 wt% of Fe.

Currently, the dominant method of P recovery from wastewater sludge in WWTPs is based on the dissolution–precipitation approach, which requires a low pH condition to dissolve P from the sludge or sludge ash after incineration, followed by the precipitation of PO₄³⁻ or HPO₄²⁻ ions through the addition of specific metal ions in alkaline conditions [20], [21]. The main outputs of this P recovery approach include raw P-fertilizers such as struvite (MgNH₄PO₄) [22], vivianite (Fe₃(PO₄)₂) [23], and calcium phosphate (Ca₃(PO₄)₂) [24]. However, the low market price of P-fertilizers (approximately 400 USD·t⁻¹) cannot offset the cost of P recovery from wastewater sludge, impeding its widespread industrial application [25]. Furthermore, the co-precipitation of heavy metals (e.g., Zn, Cu, Ni, Ni, and Pb) leached from sludge poses challenges to the application of the recovered P-fertilizers in agriculture [26]. Given the scarcity of P resources and the low price of P-fertilizers, more effective and economically sound technologies are urgently needed to improve P recovery from sludge to obtain more value-added products. In comparison, commercial FePO₄ for batteries holds a significantly higher market value of 3500 USD·t⁻¹ [27], rendering it a more feasible and attractive application of the P recovered from wastewater and sludge.

Therefore, this paper introduces a novel route for high-value P recovery from municipal wastewater. The proposed approach converts the Fe-based P-rich CPR sludge to a raw material for producing LiFePO₄ cathode material. The sludge is refined through a simple sintering treatment and mild-acid washing, resulting in P–Fe oxides. Based on material characterizations, electrochemical measurements, and economic analyses, the quality and feasibility of the recovered product for use in LiFePO₄ synthesis are demonstrated. This research provides an innovative technical strategy with significantly superior economic incentives for P recovery from wastewater and sludge, contributing to the reduction of urban waste and alleviation of the global P crisis.

2. Material and methods

2.1. Samples and materials

The secondary effluent after biological treatment at the Nanshan WWTP in Shenzhen, China, was collected as the wastewater sample for enhanced P removal and sludge preparation. Anhydrous FeCl₃ as the coagulant was dosed at 6 milligram Fe per liter water (mg-Fe·L⁻¹; based on Fe:P molar ratio equals 1.7) to 70 L of wastewater in a flocculation tank, followed by rapid stirring at 200 r·min⁻¹ for 1 min and slow stirring at 30 r·min⁻¹ for 15 min. Subsequently, the mixture was allowed to settle for 1 h [28]. Following Fe-based CPR treatment, the P concentration in the effluent decreased from approximately 2 to (0.2 ± 0.05) mg·L⁻¹, meeting the discharge standard of 0.5 mg·L⁻¹ (Class 1A in GB18918-2002). After sedimentation, 69.7 L (i.e., 99.5%) of the supernatant was withdrawn, and the remaining 0.3 L of sediment (i.e., 0.5%) was obtained as raw CPR sludge (RS). Eight coagulation tests were conducted in parallel to produce P-rich sludge, which was then mixed to obtain a sufficient amount of RS for the experimental study. The obtained sludge was first dewatered in an oven at 105 °C for 24 h and then sintered in a muffle furnace at 600 °C in air for 2 h to form the sintered sludge containing P–Fe oxides. For purification, the sintered sludge was washed with an HCl solution (pH 3) for 24 h and then with deionized water to remove the residual acid. After vacuum filtration and drying, the purified sintered sludge, labeled SS, was reserved for subsequent experiments.

2.2. Carbon-coated LiFePO₄ synthesis

To fulfill stoichiometry requirements, chemical reagents, including ferric phosphate (FePO₄), ammonium dihydrogen phosphate (NH₄H₂PO₄), lithium carbonate (Li₂CO₃), and organic carbon, as well as the sludge-derived SS, were used to form the precursor for the synthesis of carbon-coated LiFePO₄ (LiFePO₄/C) according to the reaction specified in Eq. (1). Notably, NH₄H₂PO₄ reagent was added into the P–Fe containing SS to provide the chemical P source and regulate the Fe:P molar ratio from approximately 0.5 in SS to 1 in the precursors for LiFePO₄/C synthesis.

(1)

F e P O_{4} + {L i}_{2} {C O}_{3} + C_{6} H_{12} O_{6} \to L i F e {P O}_{4} / C + C O + {C O}_{2} + H_{2} O

The commercial FePO₄ and commercial NH₄H₂PO₄, collectively referred to as P-containing reagents (RE), underwent subsequent procedures. First, RE was mixed with the SS sample at different P molar ratios, and the mixture was placed in a ball mill jar with Li₂CO₃ at a molar ratio of Li:P = 1:1. Additionally, 20 wt% anhydrous glucose was introduced as the carbon source for carbon coating on the surface of primary LiFePO₄ particles [29]. The ball mill was operated at 400 r·min⁻¹ for 8 h, using ethanol as the dispersant. After ball milling, the slurry was placed in a blast drying oven at 80 °C for 12 h to remove ethanol. The dried precursor was then fully ground and placed in a tube furnace for calcination with Ar/H₂ (95/5 v%) as the carrier gas. The temperature program was set at 400 °C for 4 h and 700 °C for 12 h, with a heating rate of 5 °C·min⁻¹. The oven was allowed to naturally cool to room temperature, and the resulting black LiFePO₄/C powders were labeled as SP-0, SP-5, SP-15, SP-25, SP-35, and SP-50 according to the percentages of P provided by the SS sample (from 0 to 50%) in the precursors. These LiFePO₄/C products were obtained as the active substance for LFP batteries and thoroughly ground and stored for subsequent battery tests.

2.3. Material characterization

The elemental compositions of RS, SS, and LiFePO₄/C samples were determined through inductively coupled plasma optical emission spectrometry (ICP-OES; Avio 200, PerkinElmer, USA). The structure and phase of the materials were identified through powder X-ray diffraction (XRD; D8 Advance, Bruker, Germany) in the range of 10° to 80° with Cu Kα radiation (λ = 1.5418 Å). The XRD results were refined using general structure analysis system (GSAS) and a graphical user interface for GSAS (EXPGUI, USA) software to determine the crystalline parameters and impurity contents of the samples. The micromorphology of powder samples was observed by scanning electron microscopy (SEM; Apreo 2S, Thermo Fisher Scientific, USA) at an accelerating voltage of 10 kV, and the surface element distribution was assessed by energy dispersive spectroscopy (EDS). High-resolution transmission electron microscopy (HRTEM; Tecnai F20, Frequency Electronics Inc., USA) was applied to investigate the microscopic morphology and crystal structure of the prepared LiFePO₄/C samples, along with the thickness of the carbon coating on the primary LiFePO₄ particles. An X-ray photoelectron spectroscopy (XPS; K-Alpha, Thermo Fisher Scientific) analysis was performed to determine the contents and valence states of elements on the solid surface. Fourier-transform infrared spectrometry (FTIR; iS50, Thermo Fisher Scientific) was used to analyze the functional groups on the surfaces of RS and SS samples. The amount of carbon coating on the sample surface was measured using an infrared carbon/sulfur analyzer (CS844, LECO, USA).

2.4. Electrochemical measurements

The active cathode substance, such as as-prepared LiFePO₄/C powder, was mixed with conductive carbon (Super P, TIMICAL, Switzerland) and binder (polyvinylidene fluoride, Arkema, Franch) at a weight ratio of 8:1:1 and thoroughly ground in an agate mortar for 15 min. Subsequently, N-methyl pyrrolidone (Macklin, China) was added, and the mixture was ground to form a slurry [30]. This slurry was applied to Al foil, dried in an oven at 80 °C for 12 h, and then cut into circular pieces with a diameter of 12 mm to obtain a cathode with approximately 2 mg·cm⁻² of dry mass coated, which is comparable to the mass loading reported in the literature [30], [31]. Coin-type 2025 half-cells were assembled in an Ar-filled glovebox, using the prepared cathode, a lithium foil of the same shape as the anode, and a separator (Celgard 2500; 19 mm diameter, USA). The electrolyte consisted of 1 mol·L⁻¹ LiPF₆ in a mixture of ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate at a volume ratio of 1:1:1. Charge–discharge measurements were performed on LANHE battery-test systems (CT 3002A, China) between 2.0 and 4.2 V vs Li⁺/Li, with current density ranging from 17 mA·g⁻¹ (based on the theoretical capacity of LiFePO₄ is 170 mA·g⁻¹) [32]. Electrochemical impedance spectroscopy (EIS) measurements were performed on an electrochemical workstation (Princeton, PARSTAT MC, USA) with testing frequencies from 10 mHz to 100 kHz under a 5 mV alternating current (AC) amplitude signal. The EIS results were fitted using Zview (Solartron, UK) software to analyze the ohmic resistance (R_s, including the electrolyte impedance, liquid phase impedance, and contact impedance), charge transfer resistance (R_ct), and Warburg impedance. The Warburg coefficient σ was calculated from the slope of Z′ (real impedance) with respect to ω^−1/2 (ω is the angular frequency) in the Warburg region of the EIS measurement using Eq. (2), and the diffusion coefficient of lithium ions in the materials was calculated using Eq. (3):

(2)

Z^{'} = R_{s} + R_{c t} + σ ω^{- 1 / 2}

(3)

D_{{L i}^{+}} = {\frac{1}{2} [\frac{R T}{n^{2} F^{2} A C σ}]}^{2}

where R is the gas constant (8.314 J·mol⁻¹·K⁻¹), T is the absolute temperature (298.15 K), n is the number of electron transfers (n = 1 in this study), F is the Faraday constant (96 485 C·mol⁻¹), A is the geometric area of the electrode (1.13 cm²), C is the concentration of lithium ions in the electrode, and D_Li⁺ indicates the Li⁺ diffusion coefficients. For electrochemical performance tests, 10 cells were assembled for each condition, and measurement errors did not exceed 3%.

3. Results and discussion

3.1. Characterizations of raw and sintered sludge

The weight contents and molar ratios of chemical elements in the as-obtained RS and SS samples are presented in Fig. 1 and Table S1 in Appendix A. Among the various inorganic elements in RS (Fig. 1(a)), Fe was dominant (296.4 mg·g⁻¹, on average), followed by P (80.5 mg·g⁻¹), Ca (64.9 mg·g⁻¹), Na (24.8 mg·g⁻¹), and Si (19.0 mg·g⁻¹). Thus, the molar ratio of Fe to P in the CPR sludge was approximately 2:1. After undergoing air sintering and acid washing, the organic components in the RS samples were completely combusted and removed. This process led to a significant increase in the sum of Fe and P from 376.9 mg·g⁻¹ in RS to 590.1 mg·g⁻¹ in SS (Fig. 1(b)), while their molar ratio remained 2:1, suggesting an overall form of Fe_2.1P_1.0O_5.6 for the P–Fe oxides in SS. Additionally, the relative contents of impurities such as Ca, Na, Si, B, Mg, Al, and K compared to the sum of P and Fe decreased from 37.4% to 20.3%, indicating that air sintering and acid washing can effectively remove a large portion of impurities from the sludge through volatilization and dissolution. The XRD patterns of the RS and SS samples (Fig. 1(c)) did not reveal sharp characteristic peaks upon retrieval, suggesting the absence of a crystal structure. The FTIR spectra of both RS and SS samples (Fig. 1(d)) showed that the absorption peaks at 3260 and 1642 cm⁻¹, corresponding to bending vibrations of O–H and C=O bonds, respectively, disappeared after sintering, indicating organic volatilization. The decrease in the peak intensity at 981 cm⁻¹, corresponding to the stretching vibration mode of PO₄³⁻, indicated a change in the binding forms of P in the SS sample. According to the SEM and EDS images (Figs. 1(e) and (f) and Fig. S1 in Appendix A), the SS sample was composed of numerous irregularly shaped nanoscale primary particles agglomerated together. A comparison of Figs. 1(e) and (f) showed that the morphology and grain size (∼100 nm) of the particles did not change significantly after air sintering and acid washing. The EDS results revealed the uniform distribution of elements Fe, P, O, Ca, Na, Si, Mg, Al, and K on the surfaces of the RS and SS powders, suggesting their entrapment in the particle aggregates during Fe-induced chemical precipitation.

3.2. Characterization of sludge-derived LiFePO₄/C

Fig. 2(a) shows the XRD results and 44°–45° magnification of the sludge-derived LiFePO₄/C crystals. In comparison with the standard card (PDF#83-2092), the SP-5 LiFePO₄/C sample (i.e., 5% of P from the sludge) demonstrated matching and sharp characteristic peaks without other impurities, suggesting its high purity and quality. When the SS dosing ratio increased to 25%–50%, the XRD patterns of the obtained LiFePO₄/C still aligned with the standard card. However, a new characteristic peak at 44.7°, corresponding to elemental Fe, appeared (Fig. 2(a); 44°–45° magnification). The XRD spectrum was further refined using GSAS and EXPGUI software, and the refinement results are shown in Figs. 2(b) and (c), Fig. S2, and Table S2 in Appendix A. The cell parameters of sludge-derived LiFePO₄/C, including a, b, c (axis length of the crystal), and cell volume (V), increased with the SS ratio. This increase may be attributable to the incorporation of Ca²⁺ and Na⁺ as major impurities from the sludge into the LiFePO₄ crystal lattice (Table S3 in Appendix A). As Ca²⁺ and Na⁺ have larger ionic radii than Fe²⁺ (1.00 vs 0.78 Å) and Li⁺ (0.97 vs 0.68 Å), respectively, their proper intercalation into the LiFePO₄ lattice can provide more space for ion movement [32], [33]. Other impurities like Mg²⁺, Al³⁺, and K⁺ may also contribute beneficially, to decrease the diffusion resistance of Li⁺ ions and improve the electronic conductivity of the material [34], [35], [36].

The valence states of chemical elements in the sludge-derived LiFePO₄/C were investigated using XPS (Fig. 3(a)). The analysis revealed Li, Fe, and P as the fundamental constituents. The characteristic peaks of Li 1s, Fe 2p, and P 2p at approximately 55 eV, 724 eV, and 133 eV, respectively, confirmed the successful incorporation of Li⁺, Fe²⁺, and P⁵⁺ in the LiFePO₄ lattice (Figs. 3(b)–(d)). In other words, the sludge-derived SS could serve as a favorable source of P and Fe for LiFePO₄ synthesis. Additionally, characteristic XPS peaks of Ca and Na were observed in the wide-scan spectrum when SS was used for LiFePO₄/C synthesis. The core level spectra of Ca 2p and Na 1s are shown in Figs. 3(e) and (f), respectively. Consistent with the ICP-OES results (Table S3), the peak intensities of Ca 2p_3/2 (∼347 eV) and Ca 2p_1/2 (∼351 eV), corresponding to Ca in the +2 state, and the peak intensity of Na 1s (∼1071 eV), corresponding to Na in the +1 state, significantly increased with increasing SS ratio. It was reported that LiFe_0.99Ca_0.01PO₄/C material with 1 mol percentage Ca exhibited excellent cycling performance, as the introduction of Ca²⁺ increased the cell volume, electronic conductivity, and Li⁺ diffusion coefficient of the material [37]. Moreover, it was found that the introduction of Na into LiFePO₄ (Li_0.97Na_0.03FePO₄/C) expanded the lattice volume and improved the electronic conductivity from 10⁻³ to 10⁻² S·cm⁻¹, without altering the crystal structure [38]. Therefore, for the LiFePO₄/C produced in this study, the incorporation of minor amounts of Ca and Na from the sludge can potentially enhance the electrochemical properties of the cathode materials.

Figs. 4(a)–(f) shows SEM images of LiFePO₄/C samples with P sourced from the SS and chemical reagent. The primary particles of LiFePO₄/C appeared nanosized and tended to agglomerate and stack to form secondary clustering particles. As the SS dosing ratio increased, the primary particles decreased in size and became more compact. Figs. 4(g)–(i) show HRTEM images of the SP-0, SP-25, and SP-50 LiFePO₄/C samples. Clear lattice stripes in the LiFePO₄/C bulk and an apparent carbon coating layer (thickness ranging from 2 to 8 nm) on the surface can be observed. An accurate quantification of the carbon coating content of the LiFePO₄/C material shows that the carbon content is about 4.89% (Table S3). The insets in Fig. 4(g) show the fast Fourier transform results, where the clear dot distribution of LiFePO₄/C and ring diffraction image of the amorphous carbon coating layer are easily distinguishable.

3.3. Electrochemical performance of sludge-derived LiFePO₄/C

The electrochemical performance of the sludge-derived LiFePO₄/C materials was evaluated, as shown in Fig. 5. This evaluation included galvanostatic–potentiostatic charge–discharge tests, cycling tests, AC impedance tests, differential capacity analysis, and rate tests. Notably, the sludge, serving as a P and Fe source, could effectively substitute the commercial FePO₄ reagent in the synthesis of LiFePO₄/C cathodes. The SP-25 LiFePO₄/C demonstrated an initial specific discharge capacity of 118.8 mA·h·g⁻¹ (Fig. 5(a)), suitable for large-scale energy storage systems (ESSs). A decrease in the SS dosing ratio, as in the cases of SP-15 and SP-5, led to an enhanced initial specific discharge capacity (134.1 and 149.9 mA·h·g⁻¹, respectively), thus meeting the requirements of power batteries used in electric vehicles. When the SS ratio in the precursor exceeded 25%, as in SP-35, the cathode exhibited an initial specific discharge capacity of 114.9 mA·h·g⁻¹, suitable for energy storage batteries with slightly lower power density requirements. All sludge-derived LiFePO₄/C cathodes demonstrated excellent cycling performance, with the coulombic efficiencies remaining 93.1%–99.8% for each charge/discharge process. For example, SP-5 LiFePO₄/C exhibited an initial specific discharge capacity close to that of a pure chemical-reagent-based cathode (154.8 mA·h·g⁻¹), with enhanced cycling performance (95.2%). The cathode with a 15% P ratio from SS exhibited an initial specific discharge capacity of 134.1 mA·h·g⁻¹, with a value of 132.6 mA·h·g⁻¹ maintained after 100 cycles at current density of 17 mA·g⁻¹. As the P ratio from SS increased to 50%, the discharge capacity retention of the LiFePO₄/C cathode increased to 102.3% after 100 cycles with 17 mA·g⁻¹ (Fig. 5(b) and Table S4 in Appendix A). This improved cycle stability was likely to be attributable to the incorporation of a moderate amount of impurities, including Ca²⁺, Na⁺, Mg²⁺, Al³⁺, and K⁺ from the sludge into the LiFePO₄ lattice, which increased the cell parameter (a, b, c) and expanded the layer space, thereby stabilizing the crystal structure during Li⁺ insertion/extraction [33], [34], [35], [36], [39]. Thus, the LiFePO₄/C cathodes with higher SS dosing ratios demonstrated improved cycle stabilities.

However, the contents of major impurities Ca and Na in the sludge-derived LiFePO₄/C apparently exceeded the reported optimum doping ratio (6%–18% vs 1% compared to Fe and Li, respectively) for LiFePO₄ cathodes (Table S3). The excess impurity content would hinder Li⁺ diffusion and increase the solid–liquid interface resistance, resulting in a diminished initial specific discharge capacity and Columbic efficiency (Fig. 5(a) and Table S4). According to the impedance spectra shown in Fig. 5(c), R_ct value fitted from the semicircle was 830.5 Ω for the SP-50 LiFePO₄/C cathode, in comparison with 465.0 Ω for the control of SP-0. Thus, based on the σ obtained from Fig. 5(d) and calculations using Eq. (2),

D_{{Li}^{+}}

for the SP-0 and SP-50 LiFePO₄/C cathodes were determined to be 1.4 × 10⁻¹⁵ and 2.9 × 10⁻¹⁶ cm²·s⁻¹, respectively, confirming increased resistance to Li⁺ insertion/extraction in the LiFePO₄/C lattice. Furthermore, Fig. 5(e) shows that the voltage difference between the anode and cathode peaks (ΔV) of the LiFePO₄/C cathodes increased when more SS was dosed, suggesting a greater polarization risk due to the excessive impurity contents [30], [37], [40], [41]. Additionally, the capacity retention at current density of 170 and 17 mA·g⁻¹ for the SP-0 and SP-50 LiFePO₄/C cathodes decreased from 79.1% to 50.8% (Fig. 5(f)). Therefore, the dosing ratio of sludge-based SS must be controlled to ensure that the quality and properties of the sludge-derived LiFePO₄/C cathode material would not be significantly compromised. According to the present study, a dosing ratio below 35% in terms of the P percentage can yield a specific discharge capacity of nearly 120 mA·h·g⁻¹ with excellent cycle and rate properties, while maintaining the negative effects of Ca and Na impurities from the sludge within acceptable levels.

The municipal wastewater treatment processes are standardized to ensure a stable effluent quality, with P levels consistently below 2 mg·L⁻¹ after secondary biological treatment. During the production of CPR sludge, the Fe-based coagulant dosage is precisely adjusted to maintain a stable P-to-Fe molar ratio of 1.5 to 2.0. This controlled approach results in consistent P and Fe content in the CPR sludge. Generally, P in sludge can be purified through a dissolution-based regeneration approach, which requires a large amount of strong acid and multiple processing steps, rendering it expensive and complex [42]. In contrast, in this study, the CPR sludge was subjected to a simple sintering process, which required minimal chemical reagents and conserved nearly 100% of the P in the P–Fe product for LFP batteries. Moreover, the air sintering and acid washing further enhance the stability and purity of these components, ensuring uniform quality in the SS used for LiFePO₄/C synthesis. For large-scale ESSs requiring low power densities (e.g., 110 mA·h·g⁻¹) [43], up to 35% of P and 70% of Fe can be sourced from municipal sludge for the synthesis of LiFePO₄/C cathodes, with significant cost savings and resource conservation.

3.4. Environmental significance

To achieve carbon neutrality goals, the energy infrastructure must undergo the transition from high-carbon fossil fuels to low-carbon, clean, and renewable energy sources. To this end, high-performance batteries for energy storage and power supply must be developed. However, the battery industry requires significant amounts of Li and P resources, which are typically mined. Given the crisis of P depletion on Earth, the recovery and recycling of P from various waste sources has become essential. CPR sludge, a byproduct of P removal in municipal wastewater treatment, is enriched with P content comparable to that in low-grade P ore (∼10 wt%). Fig. 6 illustrates the material flow in the process of P recovery from the secondary effluent of municipal WWTP for utilization in the form of LiFePO₄/C. Consider a WWTP with a wastewater treatment capacity of 10⁶ m³·d⁻¹ in a large city as an example. Through chemical precipitation with 6 mg-Fe·L⁻¹ of FeCl₃ as the coagulant, the P concentration in wastewater can be decreased from 2 to 0.5 mg·L⁻¹, meeting the discharge standard (GB18918–2002). Consequently, 75% of P in the wastewater and 88% of Fe from the coagulant are concentrated into the CPR sludge generated daily, with P and Fe contents of 1.5 and 5.3 t, respectively. After dewatering, sintering, and acid washing (assuming no P loss), the SS product can be a feedstock to provide 50% P and 100% Fe to substitute up to 14.3 t of the FePO₄ reagent and produce 14.9 t of LiFePO₄/C as the cathode material of LFP batteries, which are capable of providing 6000 kW·h of electricity (400 kW·h·t⁻¹ LiFePO₄). Additionally, this route can contribute to a reduction of 17.2 t in CO₂ emissions from ore mining and chemical production processes (calculations are shown in the Supplementary Text in Appendix A) [44], [45]. The total amount of municipal wastewater treatment in China was 62 580 million m³ in 2022 [46]. If all these wastewater effluents are treated with CPR for enhanced P removal, a total of 94 000 t P would be retained in the CPR sludge (equivalent to ∼39% of P demand for LFP batteries in China), which is sufficient to provide up to 35% of P (i.e., the acceptable P substitution ratio) for the LiFePO₄/C cathode material of LFP batteries in the same year.

At present, the recovery of P from sludge is not widely practiced, and typical methods for P recovery involve the extraction of P from sludge by chemical means before incorporating it into products. Common approaches for P extraction from sludge mainly include wet chemical P extraction from sludge or sludge ash [20]. However, this route suffers from limited extraction efficiency, a high treatment cost, and low-value products such as P-fertilizers with a risk of soil contamination. The P recovery technology introduced in this work involves thermal treatment at a low temperature of 600 °C to convert all of the P in CRP sludge into the P–Fe precursor. The mild-acid washing adopted for removing impurities from P–Fe oxides is also more economical and environmentally friendly than acid-leaching treatments, which involve large amounts of acids and bases, with additional costs for heavy metal removal [47]. The biological method based on anaerobic digestion for P extraction from sludge has been noted to exhibit low extraction efficiencies (typically 10%–30%) [20]. Additionally, the use of chemical reagents (e.g., Mg salts) to produce P-fertilizers such as struvite and the need for further purification to remove heavy metals for safe agricultural applications contribute to increased P recovery costs [5], [48]. Table S5 in Appendix A outlines the results of a cost-income analysis of the abovementioned P recovery and processing methods. Although the thermal treatment in the proposed method is more expensive than the biological method based on anaerobic digestion (98.9 vs 37.8∼44.3 USD·t⁻¹ dry sludge), the high P recovery efficiency (100% vs 30%) and higher value-added product (battery material vs MgNH₄PO₄ P-fertilizers, 3500 vs 800 USD·t⁻¹) result in a substantial increase in net income from −36.8∼−43.3 to 826.3 USD·t⁻¹ dry sludge. This makes P recovery from CPR sludge for LFP production a highly feasible and more profitable endeavor, contributing to both resource conservation and clean energy for global sustainability.

4. Conclusions

Fe-based CPR sludge from municipal wastewater was successfully converted to purified P–Fe oxides (Fe_2.1P_1.0O_5.6) through sintering and acid washing. The as-obtained P–Fe oxides were used to substitute a significant portion of the commercial FePO₄ reagent as the precursor for synthesizing LiFePO₄/C cathode material for LFP batteries. The initial discharge-specific capacities of the LiFePO₄/C cathodes prepared with different dosing ratios of the SS in terms of the P percentage were 154.8 (SP-0), 149.9 (SP-5), 134.1 (SP-15), 118.8 (SP-25), 114.9 (SP-35), and 88.5 (SP-50) mA·h·g⁻¹. The sludge-derived LiFePO₄/C cathode materials are suitable for large-scale ESSs with low power density requirements. The moderate incorporation of impurities (Ca²⁺, Na⁺, Mg²⁺, Al³⁺, and K⁺) derived from CPR sludge improved the cycling performance of as-prepared LiFePO₄/C cathode compared with the reagent-based LiFePO₄/C cathode, which was attributable to the improved stability of the crystal structure. Compared with conventional P recovery methods involving extraction from the sludge for P-fertilizer production, the proposed P recovery technology can convert 100% of the P in CPR sludge into the P–Fe precursor, producing a LiFePO₄/C cathode with a much higher market value than P-fertilizers. The amount of P recovered from municipal wastewater is projected to be sufficient to meet up to 35% of the P demand by the LIBs industry in China, enhancing the cost-effectiveness of P recovery and alleviating the global shortage of P resources to achieve both clean energy and sustainable development.

Acknowledgments

This research was financially supported by the National Natural Science Foundation of China (52100093, 52270128, and 52261135627), the Guangdong Basic and Applied Basic Research Foundation (2023A1515011734 and 2021B1515120068), the Municipal Science and Technology Innovation Council of the Shenzhen Government (KCXFZ20211020163556020 and SGDX20230116092359002), the Research Grants Council (17210219), and the Innovation and Technology Fund (ITS/242/20FP) of the Hong Kong SAR Government.

Conflicts of interests

Yijiao Chang, Xuan Wang, Bolin Zhao, Anjie Li, Yiru Wu, Bohua Wen, Bing Li, Xiao-Yan Li, and Lin Lin declare that they have no conflict of interest or financial conflicts to disclose.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.eng.2024.05.018.

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