A High‐Performance Zinc–Air Battery Cathode Catalyst from Recycling of Spent Lithium Iron Phosphate Batteries

A novel recycling process of the conductive agent in spent lithium iron phosphate batteries is demonstrated. Wet chemistry is applied in recovering lithium and iron phosphate, and the filter residue is calcined with a small amount of recovered iron phosphate in N2 at 900 °C to form a FeNP‐codoped carbon catalyst, which exhibits a low half‐wave potential and excellent durability for oxygen reduction. When applied in a rechargeable Zn–air battery, the power density can reach 80 mW cm−2.


Introduction
Recycling of the materials in spent lithium-ion batteries (LIBs) has attracted increasing attention for the upcoming retirement tide of power batteries. [1,2]LIBs in the electric vehicles can be simply classified into two categories based on the cathode materials, lithium nickel-cobalt-magnesium oxide (NCM), and lithium iron phosphate (LFP). [3]Methods to recover/reuse the valuable elements in NCM and LFP have been developed, [4,5] including pyrometallurgical, hydrometallurgical, and biometallurgical processes, among which hydrometallurgical recovery combined with separation technologies exhibits significant advantages of low operating temperature, low energy consumption, and high recovery rate. [6,7]Unfortunately, the high-cost auxiliary material of conductive agent (carbon black, carbon nanotube, or graphene) in spent LIBs is currently treated as solid waste or plastics filler, which has much better electronic conductivity, less contaminants, and larger specific surface area than the normal carbon black. [8]anocarbon materials exhibit extraordinary physiochemical properties due to their distinct dimensionalities and atomic structures, [5] and heteroatom-doped carbon catalysts are expected to replace the precious metals (Pt and Pd etc.) in oxygen reduction reaction (ORR). [9,10][18][19][20][21] Razmjooei et al also reported the codoping of Fe─P with functionalized graphene, which effectively enhanced ORR catalytic activity in both alkaline and acidic media. [22]Herein, we demonstrate a simple method to synthesize a Fe─N─P-codoped carbon catalyst using the recovered conducting agent of spent LFP batteries, which turns the solid waste to the affordable ORR catalyst for enhancing rechargeable zinc-air batteries.

Chemicals and Materials
The cathode material of spent LFP batteries was from Gotion High-Tech Co., Ltd.Sulfuric acid (H 2 SO 4 , 95-98%), hydrogen peroxide (H 2 O 2 , 30% solution), and zinc acetate (Zn(Ac) 2, ≥99%) were purchased from Sinopharm Chemical Reagent Co., Ltd., and potassium hydroxide (KOH, ≥90%) was bought from Adamas Chemical Co. Ltd.Carbon black (Vulcan XC-72) was obtained from Shanghai Macklin Biochemical Technology Co., Ltd.Water-proof breathable membrane and Ni foam from Changsha Spring New Energy Technology Co., Ltd were used as the cathode substrate of the Fe─N─P-codoped carbon catalyst.All the chemicals and materials were used directly without further purification.

Treatment of Spent LFP Batteries and Synthesis of Fe─N─P-Codoped Carbon Catalyst
Figure 1 shows the schematic diagram of the recycling route of LFP cathode materials.The spent LIB pack was discharged and disassembled to obtain the cathode material, which basically contained LFP powder, carbon black (CB, conducting agent), and polymer binder.10.0 g of the LFP cathode material was dispersed in 80 mL deionized water, and then 2.2 mL of H 2 SO 4 under magnetic stirring, and then 4.5 mL of H 2 O 2 were slowly added, where the reaction was allowed to continue for 3 h in a water bath at 80 °C.The solid (conductive agent) in the leaching solution was separated by filtration, which was used as a precursor after drying for catalyst synthesis.The filtrate was employed to recover lithium (Li) and iron phosphate (FePO 4 ). [4,6]00 mg of the precursor filtering residue were mixed and ground with 20 mg of recovered FePO 4 and then calcined at 800, 900, and 1000 °C for 3 h under N 2 atmosphere.Then, the obtained powders were stirred in 100 mL of 1 mol L À1 H 2 SO 4 to recover the soluble Fe-containing species.The remaining filtered solid was dried and marked as the C-FP800, C-FP900, and C-FP1000 catalysts, respectively.As reference, pristine Super P powders were calcined at 900 °C for 3 h under N 2 atmosphere.

Catalytic Performance of Fe─N─P-Codoped Carbon Catalysts
Electrochemical measurements were performed using an electrochemical workstation (CHI760E, Shanghai Chenhua Instrument Co., Ltd.) coupled with a rotating ring-disk electrode (RRDE, ALS RRDE-3A, BAS Company).A mercury/mercury oxide (vs.Hg/HgO) and a Pt wire were used as the reference and counter electrodes, respectively.The catalyst slurry was employed to modify the working electrodes, where 4 mg of the as-prepared catalysts was dispersed into a vial with 800 μL of deionized water, 200 μL of ethanol, and 100 μL of 5% Nafion solution by ultrasonic stirring for 30 min.In the rotating disk electrode (RDE) and cyclic voltametric (CV) analyses, a glassy carbon disk electrode (GCE, d = 3 mm, S = 0.07 cm 2 ) was modified with 6 μL of the catalyst slurry and dried under infrared light.In the RRDE analysis, a working electrode equipped with a glassy carbon disk (d = 4 mm, S GCE = 0.126 cm 2 ) and a Pt ring (S Pt = 0.189 cm 2 ) was used.In this case, the glassy carbon disk was modified with 8 μL of the catalyst slurry, the Pt ring was left free of catalyst, and then allowed to dry under infrared light prior to use.CV analysis was performed at a scan rate of 5 mV s À1 in O 2 -saturated 0.1 mol L À1 KOH solution from 0 V to 0.2 V (vs.Hg/HgO).
Linear sweep voltammograms (LSV) equipped with RDE were recorded at the same rate of 5 mV s À1 ranging from 0 V to 0.2 V.The electron transfer number in the ORR process can be estimated by the Koutecky-Levich (K─L) equation (1) where J is the measured current density, mA cm À2 ; J K is kinetic current density, mA cm À2 ; J L is the diffusion limiting current density, mA cm À2 ; ω is the rotational angular velocity of the disk (ω = 2πN, N is the rotational rate of the disk), rad s À1 ; F is Faraday constant, taken as 96 485 C mol À1 ; n is the electron transfer number per oxygen molecule ; C 0 is the volume concentration of oxygen, taken as 1.2 Â 10 À6 mol cm À2 ; D 0 is the diffusion coefficient of O 2 , taken as 1.9 Â 10 À5 cm 2 s À1 ; ν is the dynamic viscosity of electrolyte, taking 0.01 cm 2 s À1 ; B is the straight slope; k represents the apparent rate constant.
The rechargeable Zn-air battery was tested by a two-electrode cell, which included a polished zinc anode, an electrolyte solution containing 6.0 mol L À1 KOH and 0.2 mol L À1 Zn(Ac) 2 , and a heteroatom-doped carbon catalyst-modified cathode.The cathode substrate with water-proof breathable membrane and Ni foam was modified with the catalyst slurry and dried at 60 °C prior to use, with the mass loading of 1 mg cm À2 .Galvanostatic discharge-charge tests were conducted in the air using a Neware Battery Testing System (CT-4008W-5 V 10 mA-S4, ShenZhen Neware Instrument Company) at ambient temperature.

Characterizations
Scanning electron microscopy (SEM, FESEM SEU8010) and X-Ray diffraction (XRD, D/Max-RB at 12 kW using Cu Kα radiation) were used to investigate the morphology and structure of the samples.X-ray photoelectron spectroscopy (XPS, ESCALAB 250 Xi) and Raman spectroscopy (DXR2 532 nm Avantes of the Netherlands) were used to characterize the surface chemistry.

Structure Characterizations
Figure 1 shows the schematic illustration of the synthesis of the Fe─N─P-codoped carbon catalyst using the recovered conducting agent in the leaching residue, where the LFP cathode materials in spent LIBs were treated with a mixture of sulfuric acid and hydrogen peroxide.The leaching residue was separated by filtration, and the filtrate was allowed to recover Li and FePO 4 by neutralization.Figure S1a,b, Supporting Information, shows the XRD analyses of the LFP cathode material and the leaching residue, where the former is mainly LFP and the latter is assigned to FePO 4 .Table S1, Supporting Information, shows that about 99.37% of Li was recovered from the LFP cathode material.Figure S2a,b, Supporting Information, shows the SEM images of the filtering residue and the C-FP900 catalyst at 900 °C, illustrating the reduced amount of crystals after calcining in N 2 atmosphere.
Figure 2a shows the TEM image of the C-FP900 catalyst by calcining the leaching residue and recovered FePO 4 , where nanocrystals are seen on the recovered conductive agent carbon black.The crystal space is measured as 0.295 nm corresponding to the ( -201) facet of Fe 2 P 2 O 7 crystal in the high resolution electron microscope (HRTEM) image (inset).Figure 2b further illustrates the element distribution in the C-FP900 catalyst, in which the EDX analysis illustrates that nitrogen is uniformly distributed in carbon, while iron and phosphorus are dispersed in several areas attributed to the solid-state reaction between carbon in the conductive agent and FePO 4 .XRD analysis of the C-FP900 catalyst in Figure 2c indicates that Fe 2 P and Fe 2 P 2 O 7 are formed, suggesting the solid-state reaction between carbon and FePO 4.
Raman analysis in Figure 2d illustrates that typical D and G bands originated from disordered and graphitic phases, which are employed to investigate the effect of the solid-state reaction on the structure of the conductive agent.As a reference, the pristine Super P was introduced to show the doping effect of nitrogen.It is seen that the Super P and C-FP900 catalysts are close in the I D /I G ratios, showing that the solid-state reaction between carbon and FePO 4 does not influence the bulk carbon structure.The I D /I G ratio of the leaching residue is apparently larger with an extra band at 1015 cm À1 , which can be attributed to the symmetrical PO 4 3À stretching, in agreement with XRD analysis in Figure S2, Supporting Information.The specific surface area of the C-FP900 catalyst was tested by the N 2 adsorption/desorption analysis (Figure 2e), which exhibits a type IV isotherm with a specific surface area of 222.1 m 2 g À1 .
Figure S3a, Supporting Information, shows the total XPS spectrum of the C-PF900 catalyst, which contains the C, N, P, Fe, and O signals.In the C1s XPS spectrum, five typical carbon peaks are fitted at 284.5, 284.83, 285.2, 286.2, and 290.5 eV, referring to C═C, C─O, C─P, C─N, and C─N, respectively (Figure S3b, Supporting Information). [23,24]The C─N peak, serving as an oxygen reduction active site, can be split into three types: pyridinic-N (398.35eV), pyrrolic-N (399.71eV), and graphitic-N (401.19 eV). [25,26]Both pyridinic-N and graphitic-N are reported to be beneficial for ORR activity, [27,28] while pyridinic-N is beneficial to increase the onset potential, and Fe atom is prone to coordinating with pyridinic N to produce Fe─N structure and work with C─N to catalyze oxygen reduction. [29,30]In Figure S3c, Supporting Information, the fitting of the resolved N1s signal suggests the presence of pyridinic-N, pyrrolic N, and graphitic-N, together with the Fe─N (399.01 eV) in the C-FP900 catalyst, indicating the codoping of F─N─C in the sample. [25,26]oreover, Figure S3d, Supporting Information, illustrates the four types of P doping in the C-FP900 catalyst.In the fitted P2p spectrum, the C─P peak locates at 133.1 eV, suggesting the combination of C and P atoms, while the P─O peak appears at 134 eV, [31][32][33] attributed to the decomposed FePO 4 adjacent to the carbon framework.The peaks at 129.56 and 130.46 eV represent satellite peaks for C─P and P─O, respectively.Similar to N atoms, P atoms with stronger electronegativity have been proven to be beneficial for regulating the electronic structure of carbon. [34,35]The resolved Fe 2p spectrum in Figure S3d, Supporting Information, shows that the main valence of the Fe species is situated between þ2 and þ3, and no Fe 0 or iron carbides were observed. [36,37]This result indicates that most of Fe atoms were coordinated with N (or O) atoms in N-doped carbon fragments.The XPS results are listed in Table 1.

Electrocatalytic Performance
Cyclic voltammetry (CV) analysis was carried out in the O 2 -saturated 0.1 mol L À1 KOH electrolyte, in which the C-FP900 catalyst was used to modify the glassy carbon electrode, and the calcined Super P, C-FP800, and C-FP1000 were also analyzed as reference.As shown in Figure 3a, the calcined Super P, C-FP800, C-FP900, and C-FP1000 exhibit the oxygen reduction peaks at 0.603, 0.64, 0.651, and 0.646 V, and the corresponding current densities are 0.157, 0.156, 0.437, and 0.29 mA cm À2 , respectively.It is clear that the C-FP900 catalyst owns the highest catalytic activity.Figure S4, Supporting Information, presents the CV curves of the C-FP900 catalyst in both N 2 -and O 2 -saturated 0.1 mol L À1 KOH solutions, where the former has no peak, indicating that the peak is associated with the ORR process.Moreover, the analysis on the electric double-layer capacitance indicates (Figure S5, Supporting Information) that the electrochemical active area (ECSA) of the C-FP900 catalyst (9.79 mF cm À2 ) is also larger than the calcined Super P (7.09 mF cm À2 ).The linear sweep voltammetry (LSV) analysis using RDE at 1600 rpm in Figure 3b shows that the onset and half-wave potentials of the C-FP900 catalyst are 0.845 and 0.703 V versus reversible hydrogen electrode (RHE), respectively, with the diffusion current density of 5.45 mA cm À2 .In contrast, the onset and half-wave potentials of the calcined Super P are 0.81 and 0.659 V versus.RHE, respectively, with a diffusion current density of only 1.79 mA cm À2 .The other catalysts of the C-FP800 and C-FP1000 are also weaker in ORR catalytic activity than the C-FP900 catalyst.Moreover, the Tafel slope of the C-FP900 is 89 mV dec À1 , and the value of the calcined Super P is 111 mV dec À1 (see Figure 3c).RDE analysis of the C-FP900 catalyst was performed at a scan rate of 5 mV s À1 in a rotation speed of 400-1600 rpm (see Figure 3d).It can be seen that the current densities increase with the rotating speeds, while the onset potentials are stable.Figure 3e shows a linear relationship between J À1 and ω À1/2 , and according to the Koutecky-Levich (K-L) equation the electron transfer number of the C-FP900 catalyst in ORR is in the range of 3.5-3.8.Moreover, Figure 3f illustrates the RRDE analysis of the C-FP900 catalyst in the inset, where the disk current keeps growing with the increase of potential, and the electron transfer number at À0.8 V (vs.Hg/HgO) is 3.8.The i-t curve displays that the current attenuates to 73.5% of the initial value at 80000s in O 2 -saturated 0.1 mol L À1 KOH solution, iillustrating the high stability of the C-FP900 catalyst in ORR process.

Enhancement of Zinc-Air Batteries
In order to prove the ORR catalytic activity, the catalysts were sprayed on the cathode substrates of water-proof membrane and Ni foam.The air cathode and zinc anode were assembled in a zinc-air battery (see Figure 4a).The open-circuit voltage (OCP) for the zinc-air battery with the calcined Super P catalyst was tested as 1.37 V, and the OCP is measured as 1.45 V for the cell with the C-FP900 catalyst, which maintains 1.44 V after 4 h of testing (see Figure S6, Supporting Information).
The zinc-air batteries were cycled at a current density of 5 mA cm À2 .As shown in Figure 4b, the gap between the charge and discharge potentials remains above 1.04 V for the cell with the C-FP900 catalyst in 120 cycles (red), while the gap in the cell with the calcined Super P was only 1.0 V within 18 cycles (black), highlighting the effect of the C-FP900 catalyst on oxygen reduction.The polarization curves (solid lines) in Figure 3c illustrate the variation of cell voltage with the current density, where the cell with the C-FP900 catalyst (red) decreases slower than the one with the calcined Super P (black).Furthermore, the peak power density of the cell with the C-FP900 catalyst (red) reaches 80 mW cm À2 , and the value for the one with the calcined Super P (black) is only 11 mW cm À2 .Figure 4d displays the comparison of the specific capacities at the first discharge at 5 mA cm À2 , where the cell with the C-FP900 c (red) exhibits a higher voltage and specific capacity than the one with the calcined Super P (black), which are calculated as 581 mAh g Zn À1 and 460 mAh g Zn À1 based on the mass of zinc foil consumed, respectively.The above result demonstrates that the high catalytic activity and stability of the C-FP900 catalyst leads to the effective enhancement of zinc-air batteries.

Conclusion
In summary, the simple calcining of recovered conductive agent and iron phosphate from spent LFP batteries in nitrogen resulted in the formation of the low cost Fe-N-P-codoped catalyst.The Fe and P atoms provided by recovered FePO 4 and the N element from nitrogen atmosphere lead to the codoping of carbon, which synergistically promotes the ORR activity and stability.The Fe-N-P-codoped catalyst exhibits excellent ORR performance in alkaline media and presents a high peak power density in zinc-air batteries.More importantly, this work proves that FePO 4 can be used as both iron source and phosphorus source to prepare high-performance catalysts, which provide a new research direction for the high-value recovery of LiFePO 4 as cathode material for LIBs and other rechargeable batteries.

Figure 1 .
Figure 1.Recycling of conductive agent from spent LFP batteries.

Figure 2 .
Figure 2. a) TEM, b) EDX, and c) XRD analyses of the C-FP900 catalyst.d) Raman spectra of the calcined Super P, leaching residue, and C-FP900 catalyst, and e) the adsorption/desorption isotherm of the C-FP900 catalyst.

Figure 3 .
Figure 3. a) CV profiles, b) LSV curves, and c) Tafel slopes of the calcined Super P, C-FP800, C-FP900, and C-FP1000 in 0.1 M KOH solution.d) ORR polarization curves of C-FP at different rotating speeds in 0.1 mol L À1 KOH, e) K-L plots, f ) electron transfer number and H 2 O 2 % yield of the C-FP900 by RRDE analysis.

Figure 4 .
Figure 4. a) Illustration of zinc-air battery, b) charge-discharge cycling at 5 mA cm À2 , c) polarization (solid) and power density (dotted) curves, and d) specific capacities of the cells with the calcined Super P and the C-FP900 catalysts.

Table 1 .
XPS analysis of the C-FP900 catalyst.