Waste PET Plastic-Derived CoNi-Based Metal-Organic Framework as an Anode for Lithium-Ion Batteries.

Recycling waste PET plastics into metal-organic frameworks is conducive to both pollution alleviation and sustainable economic development. Herein, we have utilized waste PET plastic to synthesize CoNi-MOF applied to lithium battery anode materials via a low-temperature solvothermal method for the first time. The preparation process is effortless, and the sources' conversion rate can reach almost 100%. In addition, the anode performance of MOFs with various Co/Ni mole ratios was investigated. The as-synthesized Co0.8Ni-MOF exhibits excellent crystallinity, purity, and electrochemical performance. The initial discharge and charge capacities are 2496 and 1729 mAh g-1, respectively. Even after 200 cycles, the Co0.8Ni-MOF electrode can exhibit a high Coulombic efficiency of over 99%. Consequently, given the environmental and economic benefits, the Co0.8Ni-MOF derived from waste PET plastic is thought to be an appealing anode material for lithium-ion batteries.

Introduction

Environmental and energy issues are the two most serious problems of the 21st century.[1−3] Nowadays, demand for energy conversion and storage is increasing as a result of severe environmental pollution caused by the widespread use of fossil fuels and the need to power millions of portable electronics and electric vehicles. The rational use of clean and renewable energy and the development of sustainable energy storage systems (EES) have emerged as global goals for addressing energy and environmental crises.[4−9] Among various energy storage equipment, lithium-ion batteries (LIBs) are the first to be considered owing to qualities such as high energy density, long cycle life, low self-discharge, and environmental friendliness.[10−12] However, commercially available graphitic anodes in LIBs with limited theoretical capacity (370 mAh g–1) cannot meet the ever-increasing demands for high energy densities in electric vehicles and grid-scale energy storage systems.[13−16] As a result, the key to achieving high-energy LIBs is to look for promising anode candidates to replace conventional graphite.

Metal–organic frameworks (MOFs) have appeared as a burgeoning class of porous material consisting of metal nodes and organic linkers.[17,18] Since the synthesis of the first MOFs by Yaghi and Li in the 1990s,[19] they have attracted considerable attention in recent decades due to their intriguing structural diversity and a wide range of potential applications, such as catalysis,[20,21] gas storage and separation,[22,23] sensing,[24,25] drug delivery,[26,27] and a variety of other areas. Recently, many researchers have concentrated their efforts on the design and synthesis of MOFs for clean energy applications such as fuel cells,[28] lithium-ion rechargeable batteries,[10] supercapacitors,[29] and solar cells.[30] In 2009, Tarascon et al. first demonstrated that MOFs (MIL-53 (Fe)) were used as anode materials for lithium storage via a conversion reaction.[31] Since then, because of their structural diversity, customizable redox characteristics, simple synthesis procedures, and low cost, MOFs have emerged as novel electrode materials with significant potential in lithium-ion batteries.[10,32−34] Meanwhile, with regard to MOFs, the metal center has a significant impact on the electrode’s lithium storage behavior.[35] Co-based MOFs have been widely used as electrode materials for advanced energy storage applications due to their availability and ease of electron transfer between multiple oxidation states ranging from +1 to +4 during conversion reactions. However, most Co-based MOFs have low conductivity, and this problem has been addressed by incorporating insoluble conductive additives and combining MOFs with conductive matrices (e.g., carbon nanotubes, reduced graphene oxide, conductive polymers, etc.). Additionally, the synthesis of multi-metallic MOFs is also a great alternative for intensifying electronic conductivity. Doping metal ions such as lightweight Li, Mg, Al, Ni, and Mn into Co-based MOFs could help to conserve cobalt resources while also effectively facilitating electronic coupling between metal nodes to improve material conductivity.[32,36,37] Hu et al. reported a novel bimetallic ZnCo-MOF, and using soft X-ray absorption spectra (sXAS), they demonstrated that bimetallic MOFs could significantly improve electrode performance due to the synergistic effect of Zn2+ and Co2+.[38] Additionally, Zhou et al. reported a 2D BP/NiCo MOF via a simple solution reaction route at room temperature using Ni2+, Co2+, and benzene dicarboxylic acid (BDC) as precursors, which demonstrated high reversible capacity and excellent cycling durability as an anode in LIBs.[39] Although tremendous progress has been made in the development of MOF-based lithium electrode materials, there are still many challenges that hinder the practical application of these materials. First, many of the complex organic ligands used in the synthesis of MOFs are expensive and may require a degree of processing complexity. Therefore, large-scale production of MOF materials at a reasonable cost is critical for making them economically viable for industrial applications.

As an important ligand, 1,4-benzenedicarboxylic acid (BDC) has been used to design many MOF system structures.[21] More importantly, it is abundant as a starting material from the recycling of polyethylene terephthalate and the metabolites of aromatic hydrocarbon oxidation.[40] Poly(ethylene terephthalate) (PET) is widely used in packaging and textiles due to its good mechanical properties, low permeability to moisture and gas, and high heat resistance.[40−43] According to statistics, global PET consumption has surpassed 24 million tons (62.8 billion bottles) per year and is continuing to rise. Because of its poor biodegradability, the current common practice of PET waste land filling has caused serious environmental problems. As a result, the innovative use of BDC derived from waste PET for MOF synthesis will not only improve the economics of MOF fabrication but will also reduce the environmental threat associated with waste PET landfilling, ultimately creating a high-value end-use market for PET waste.[44−46]

Herein, we concentrate on a simple and integrated approach to addressing the environmental concern associated with waste PET bottles by recycling them into promising lithium anode materials in the form of CoNi-MOF. Moreover, we also reported the solvothermal synthesis of CoNi-MOF with various Co/Ni ratios (the molar ratios of Co/Ni are 0.5:1, 0.8:1, and 1:1) to select the optimal MOF as the LIB anode material. Among them, the as-synthesized Co0.8Ni-MOF exhibits excellent crystallinity, purity, and electrochemical performance with superior cyclic stability. To the best of our knowledge, there is no literature report that efficiently recycles waste PET into CoNi-MOF as an anode for LIBs. Thus, it is firmly believed that the waste PET plastic-derived Co0.8Ni-MOF has great potential as an electrode material and would be beneficial to the mitigation of global warming as well as the circular construction of the economy.

Experimental Section

Chemicals and Regents

Cobaltous chloride hexahydrate (CoCl2·6H2O, 99.5%), terephthalic acid (BDC, 98%), and nickel chloride hexahydrate (NiCl2·6H2O, 95%) were bought from Sinopharm. Ethylene glycol (EG, 99.8%), ethanol (95%), and N,N-dimethylformamide (DMF, 99.8%) were purchased from Aladdin. No further purification was performed on any of the reagents. The raw material was clear mineral water bottles (PET plastics) collected from domestic waste. All of the experiments in this work were conducted using deionized water.

Synthesis of BDC

The process schematic for the synthesis of terephthalic acid derived from PET according to reference is illustrated in Scheme S1.[47] Briefly, the clear mineral water bottles’ labels and caps were removed, and the bottles were cut into small pieces. The small flakes were washed with water and soap before being dried in an oven at 60 °C overnight. Then, the PET hydrolysis experiment was carried out in a stainless-steel autoclave lined with Teflon. PET flakes (2.5 g) were placed in the autoclave along with 5 mL of ethylene glycol (EG) and 50 mL of H2O, and the reactor was heated to 210 °C and kept there for 8 h. After the reaction, the product was centrifuged, washed twice with ethanol, and dried at 80 °C for 12 h.

Synthesis of CoNi-MOF

As shown in Scheme , the CoNi-MOF was synthesized via solvothermal methods in a Teflon-lined high-pressure reactor. First, 10 mL of DMF, 10 mL of ethanol, and 10 mL of deionized water were combined in a beaker. Second, 7.2 mmol of BDC (the metal’s molar ratio to BDC is 1.05) was added to the above mixture, and the solution was sonicated for 15 min to achieve a uniform suspension. Third, 3.02 mmol of NiCl2·6H2O and 3.78 mmol of CoCl2·6H2O were added to the solution and sonicated for 15 min until completely dissolved. Then, the above solution was transferred into a 50 mL Teflon-lined stainless steel autoclave and heated at 150 °C for 24 h. Finally, the product was obtained via centrifugation, washed with DMF and ethanol five times, and then dried in a vacuum oven overnight at 60 °C. In comparison, CoNi-MOF materials (Co0.5Ni-MOF and Co1.0Ni-MOF) were, respectively, synthesized by adding different Co/Ni molar ratios (such as 0.5:1 and 1:1) to the solution.

Scheme 1

(a) Preparation Process of CoNi-MOF. (b) Working Principle of the CoNi-MOF/Li Half-Cell. (c) CoNi-MOF with Possible Interaction of Li+ Ions

Material Characterization

X-ray powder diffraction (XRD) measurements were taken using a PAN analytical X-ray diffractometer (Cu Kα source, λ = 1.54178 Å). X-ray Rietveld refinement was performed with the FullProf Software package. The scanning electron microscopy images were obtained via a Carl Zeiss Ultrafield-emission scanning electron microscopy (FESEM) unit equipped with energy-dispersive spectroscopy (EDS). The materials’ morphology and microstructure were examined further using transmission electron microscopy (TEM; JEOL-JEM-2100F). Fourier transform infrared spectroscopy (FTIR) was performed with a PerkinElmer (Spectrum 2) spectrophotometer at a resolution of 4 cm–1 in the scanning range of 400–4000 cm–1. The thermal stabilities of the samples were determined using the TA Instruments SDT650 thermogravimetric analysis (TGA) method. N2 adsorption–desorption studies were performed using the Brunauer–Emmett–Teller (BET) method via a Quantachrome Nova 2000e BET analyzer. The Co/Ni ratio in the CoNi-MOF was analyzed by inductively coupled plasma-atomic emission spectrometry (ICP-AES) using IRIS Intrepid II XSP (Thermo Fisher Scientific). X-ray photoelectron spectroscopy (XPS) spectra were obtained using a Thermo Fisher Scientific ESCALAB250Xi spectrometer. 1H NMR and 13C NMR spectra were provided via a Bruker spectrometer (MSL 400).

Electrochemical Measurements

Electrochemical tests were performed on coin-type cells (size: 2025), which were made up of a working electrode and a lithium foil counter electrode separated by a Celgard 2400 microporous membrane. The active materials, carbon black (super P) and poly vinylidene difluoride (PVDF), were dissolved in N-methyl pyrrolidinone (NMP) in a weight ratio of 7:2:1. The resulting slurry was cast on Cu foil and dried in a vacuum at 100 °C for 10 h. The cell assembly was carried out in a glove box filled with argon (H2O, O2 < 0.5 ppm). The electrolyte used was 1 M LiPF6 in a 1:1 volume ratio of ethylene carbonate (EC) and dimethyl carbonate (DMC). The cells were galvanostatically charged and discharged at different rates using multichannel Neware battery testing equipment in a voltage cutoff range of 0.01–3 V for cycle and rate performance. An electrochemical workstation (CHI 660a) was used to do CV measurements at a scan rate of 0.2 mV s–1. Electrochemical impedance spectra (EIS) were also measured in the frequency range of 105 to 10–2 Hz on a CHI 660a electrochemical workstation.

Results and Discussion

Characterization of PET-Derived BDC

The XRD patterns (Figure a) ensure that the peaks of PET-derived BDC are positioned at 2θ = 17.4, 25.3, and 28°, which are characteristically identical to the commercial sample and reported literature.[48] From Figure b, both FTIR spectra show the same characteristic peaks. Aromatic ring characteristic peaks are observed at 730 and 689 cm–1, which are assigned to ringing-and-out-of-plane bending vibrations of aromatic rings,[49] whereas the peaks for carboxylic groups are located at 1288, 1420, 1689, 2542, and 2657 cm–1. The band at 1689 cm–1 denotes the C=O stretching vibration of carbonyl groups, and the peak at 1288 cm–1 is attributed to the C–O symmetric stretching, as per characteristic IR absorption frequencies of organic functional groups. This indicates that the obtained product was indeed BDC, as evidenced by the spectrum’s similarity to commercial BDC. The FESEM analysis (Figure c,d) reveals that both PET-derived BDC and commercially available BDC have a similar morphology. The1H NMR and 13C NMR spectra of the PET-derived BDC shown in Figure S1 confirm that the product does not require purification. The PET-derived BDC also exhibits an identical TGA curve to the commercial BDC (Figure S2).

Figure 1

(a) XRD patterns, (b) FTIR infrared spectra, and SEM images (c, d) of the PET-derived BDC and commercial BDC.

Characterization of CoNi-MOF

The crystallinity, phase analysis, and structural properties of MOF samples prepared at different molar ratios of Co/Ni were examined by powder X-ray diffraction (XRD), as shown in Figure a and Figure S3. All the CoNi-MOF XRD patterns are isostructural to the previously reported Ni-based MOFs (no. 985792).[50] Therefore, the atomic arrangement inside CoNi-MOF can be successfully defined (Figure S4 and Table S1). Four sharp peaks appearing at 2θ = 9.219, 14.302, 15.959, and 18.161° and matching well with the relevant crystal faces are (200), (001), (201), and (−201), respectively, indicating the formation of pure products. Furthermore, as the Co/Ni molar ratio increases from 0.5 to 1.0, the characteristic peak intensity is enhanced. This can be attributed to the crystalline enhancement and size growth of CoNi-MOF,[51] which will be discussed in the following. All the XRD patterns indicate that these frameworks have similar structures, as shown in Figure b. Six O atoms coordinate both the Co and Ni atoms, and these pseudo octahedra are further edge/corner-connected with each other in the (200) crystallographic plane along the [010]/[001] direction to form 2D bimetal layers separated by BDC molecules.[52] Furthermore, Figure c displays the FTIR spectra of all samples of CoNi-MOF synthesized from PET. It can be seen that there is a wide band at 3580–3400 cm–1, indicating the presence of hydroxyl and water molecules,[53] and for comparison, the corresponding FTIR spectrum for 1,4-H2BDC is also shown. The reaction of 1,4-H2BDC with Co2+ and Ni2+ results in complete deprotonation by the disappearance of the nonionized carboxyl group’s characteristic bands (vC=O, 1684 cm–1). New bands can be observed in the regions of 1672–1508 cm–1 ascribed to carboxylate group asymmetric stretching vibrations, whereas bands in the 1444–1375 cm–1 range can be attributed to symmetric stretching.[54] These new bands suggest that the Co2+ and Ni2+ ions have successfully interacted with the 1,4-H2BDC ligands. The TGA analysis determined the thermostability of all CoNi-MOF samples in a temperature range of 25–800 °C (Figure d and Figure S5). All samples exhibit good thermal stability and still maintain their structural integrity at a temperature of 375 °C. There is a significant decrease at around 375–525 °C, which is due to the thermal degradation of MOF.[51] In the case of Co0.8Ni-MOF, at temperatures ranging from 25 to 350 °C, an initial weight loss of 10% was observed due to the release of DMF and ethanol molecules. The next weight loss of approximately 54% may result from the decomposition of the main skeleton of Co0.8Ni-MOF and the gasification of ligands at temperatures ranging from 425 to 525 °C. The residual mass of the product after a complete pyrolysis reduction at 525 °C was 36%.

Figure 2

(a) PXRD patterns, (b) crystal structure of CoNi-MOF, (c) FTIR infrared spectra of CoNi-MOF synthesized from PET-derived BDC, and (d) TGA curves of Co0.8Ni-MOF.

The morphologies of MOFs synthesized at different Co/Ni molar ratios were analyzed by SEM at different magnifications, as presented in Figure a,b and Figure S6. The Co0.8Ni-MOF has a micrometer-sized dandelion-like morphology composed of bulks with a length of 6–10 μm, as shown in Figure a,b. The assembled bulk-like framework of the Co0.8Ni-MOF can also be apparently observed in the TEM image in Figure c, which could lead to improved electrolyte accessibility by forming an ion-buffering reservoir and thus shortening the Li+ diffusion length to the interior surfaces.[55] Furthermore, the HRTEM image in Figure d shows very clear crystalline planes with d-spacings of 1.115 nm, which can be ascribed to the (200) planes of Co0.8Ni-MOF. The chemical composition of Co0.8Ni-MOF was also investigated using energy-dispersive spectroscopy (EDS), as presented in Figure f–i. The EDS pattern of the Co0.8Ni-MOF shows that the elements C, O, Co, and Ni are uniformly distributed throughout the sample, indicating that the Co0.8Ni-MOF was successfully formed. Figure S6 also includes SEM images of the Co0.5Ni-MOF and Co1.0Ni-MOF samples. The morphology of the MOF samples changes from bowknot-like (Figure S6a) to dandelion-like (Figure a) and then to flower-like (Figure S6c). When the molar ratio of Co/Ni is 0.5:1, as shown in Figure S6b, relatively small bulk structures are formed first, and then, with the increase in Co/Ni molar ratio, the smaller bulks gradually develop into larger bulks. Figures S7 and S8 represent the SEM–EDX mapping images of Co0.5Ni-MOF and Co1.0Ni-MOF, respectively, which also clearly support that, in the sample, all elements are distributed uniformly. Inductively coupled plasma-atomic emission spectrometry (ICP-AES) elemental analysis demonstrated that the atomic ratios of Co/Ni in CoxNi-MOF (x = 0.5, 0.8, and 1.0) are calculated to be 0.46, 0.75, and 0.96, respectively (Figure S9 and Table S2). These observations are in accordance with the molar ratios of the initial feeding materials, further revealing the successful preparation of the CoxNi-MOF.

Figure 3

(a, b) SEM images, (c, e) TEM images, and (d) HRTEM image of Co0.8Ni-MOF; (f–i) TEM-EDS mapping images of Co0.8Ni-MOF: (f) C, (g) O, (h) Co, and (i) Ni elements.

As shown in Figure , the XPS technique was used to investigate the chemical composition and oxidation states of Co0.8Ni-MOF. Figure a indicates the existence of Ni, Co, C, and O elements. The high-resolution Ni 2p spectra (Figure b) display two primary peaks centered at 856.3 and 874.0 eV, respectively, which are ascribed to Ni 2p3/2 and Ni 2p1/2, as well as two matching satellite peaks positioned at 861.4 and 879.6 eV, demonstrating the presence of Ni2+ in the Co0.8Ni-MOF.[35] Further, the high-resolution Co 2p XPS spectrum of the as-synthesized Co0.8Ni-MOF can also be seen in Figure c. Two distinct peaks appear at binding energies of 781.54 eV for Co 2p3/2 and 797.49 eV for Co 2p1/2, with two pronounced satellite peaks (786.44 and 803.11 eV, respectively) beside them, illustrating that Co ions are predominantly in the Co2+ state.[33] Notably, the coexisting Ni2+ and Co2+ cations in the Co0.8Ni-MOF supply multiple active sites for lithium storage. In addition, C=C, C–C, and O=C–O are attributed to three peaks (Figure d) centered at 284.5, 285.3, and 288.5 eV, respectively. The O 1s spectrum (Figure e) could be separated into two peaks, positioned at 531.5 and 532.6 eV, respectively, owing to hydroxyl and chemisorbed water. Finally, by using the XPS technique, the atomic content ratio of Co and Ni in the Co0.8Ni-MOF could be identified to be 0.72:1.

Figure 4

(a) Wide-range XPS spectrum of Co0.8Ni-MOF. (b) XPS survey spectrum of Ni 2p and the corresponding XPS spectra of (c) Co 2p, (d) C 1s, and (e) O 1s.

Figure and Figure S10 present the N2 adsorption–desorption isotherms and the corresponding Barrett–Joyner–Halenda (BJH) pore size distribution profiles of the CoNi-MOF. The results are summarized in Table S3. All of the CoNi-MOF samples exhibit a typical type-II sorption isotherm with an H4-type hysteresis loop, indicating that they are mesoporous. As depicted in Figure , the as-calculated Brunauer–Emmett–Teller (BET) specific surface area and total pore volume for Co0.8Ni-MOF are 13.688 and 0.055 cm3 g–1, respectively, demonstrating that the Co0.8Ni-MOF has low porosity. Furthermore, previous research has also reported MOFs with low porosity.[39,56] The ultrahigh specific surface of the electrode materials frequently results in superfluous side reactions with the electrolyte, particularly at low potentials (vs Li/Li+), causing the LIB’s performance to deteriorate.[55] As a result, the as-synthesized low CoNi-MOF’s BET specific surface area may be advantageous.

Figure 5

N2 adsorption–desorption isotherm of Co0.8Ni-MOF. The inset shows the BJH pore size distribution curve.

To assess the electrochemical performance of the Co0.8Ni-MOF as-prepared for Li-ion storage, the first three successive CV scans of Co0.8Ni-MOF at a scan rate of 0.2 mV s–1 in the potential range of 0.01–3.0 V vs Li/Li+ are shown in Figure a. The initial cathodic scan illustrates two distinct cathodic peaks at 0.625 and 0.95 V, which belong to the Li-ion intercalation Co0.8Ni-MOF. A sloppy peak at 1.05 V (lithiation) disappears in the subsequent scans, which is attributed to the establishment of the SEI layer at the electrode/electrolyte interface.[57] The anodic peaks at 0.825 and 1.25 V are caused by the delithiation reaction of Co0.8Ni-MOF. In the following successive cycles, the weak peaks at 0.615 and 1.15 V may be related to the reversible insertion and deintercalation of Li ions into the benzene ring and carboxylic acid groups of the Co0.8Ni-MOF framework.[49] After the first electrochemical cycle, the CV curves of subsequent cycles almost overlap, indicating good cycling stability and reversibility of Co0.8Ni-MOF. The galvanostatic charge/discharge profiles of the Co0.8Ni-MOF for various cycles within the potential window of 3.0–0.01 V vs Li/Li+ and at a current density of 0.5 A g–1 are shown in Figure b. The initial discharge and charge capacities are 2496 and 1729 mAh g–1, respectively, corresponding to a Coulombic efficiency (CE) of 69.27%, indicating a significant initial irreversible reaction caused by the formation of an SEI (surface electrolyte interface) film on the electrode surface and electrolyte decomposition.[59] During the first discharge process, three plateaus can be seen around 1.25, 0.95, and 0.55 V, which are consistent with the CV results mentioned above, suggesting the same Li+ ion insertion mechanisms. Although there is a decrease in reversible capacity values for the first few cycles, a nearly invariable capacity of 1419 mAh g–1 is obtained in the following cycles. Moreover, the charge–discharge curves of Co0.8Ni-MOF are comparable from the 100th to the 200th cycle, which demonstrates its stable electrochemical performance during continuous intercalation and deintercalation of Li ions. Furthermore, Figure S11 shows that the galvanostatic discharge and charge profiles of Co0.5Ni-MOF and Co1.0Ni-MOF are similar for Co0.8Ni-MOF under 500 mA g–1 in the first cycling processes, indicating that the same redox reactions are occurring during the electrochemical processes. It was also found that both Co0.5Ni-MOF and Co1.0Ni-MOF presented smaller discharge (charge) capacities than Co0.8Ni-MOF. The discharge (charge) capacities of Co0.5Ni-MOF and Co1.0Ni-MOF are 1768 (1056) and 2036 (1254) mAh g–1, resulting in Coulombic efficiency (CE) values of 59.74 and 61.61%, respectively. Figure c and Figure S12a exhibit the cycling behavior of all CoNi-MOF samples. It is obvious that Co0.8Ni-MOF maintains its capacity advantage over other samples. A reversible capacity of 1200 mAh g–1 is retained after 200 cycles, which is much higher than those of Co0.5Ni-MOF (750 mAh g–1) and Co1.0Ni-MOF (900 mAh g–1). Even after 200 cycles, the Co0.8Ni-MOF electrode can exhibit a high CE of over 99%, as shown in Figure c. The improved Li+ diffusion kinetics during repeated lithiation and delithiation,[51] a gradual activation process could be attributed to the stable cycling performance. As summarized in Figure d and Figure S12b, Co0.8Ni-MOF also shows the most outstanding rate property among all CoNi-MOF samples. When cycled at 0.1, 0.2, 0.5, 1, and 2 mA g–1, the Co0.8Ni-MOF electrode delivers average charge capacities of 914, 830, 713, 620, and 516 mAh g–1, respectively. More importantly, the capacity is increased to 1113 mAh g–1 after lowering the current rate back to 0.1 A g–1. This may be due to the activation caused by higher-rate discharging/charging cycles, illustrating the MOF structure’s robustness in the face of abrupt current changes. Co0.5Ni-MOF and Co1.0Ni-MOF, on the other hand, only provide 531 and 401 mAh g–1 at 1 A g–1, respectively, and 276 and 198 mAh g–1 at 2 A g–1, respectively (Figure S12b). When the current density is restored to 0.1 A g–1, the capacities of Co0.5Ni-MOF and Co1.0Ni-MOF are restored to 945 and 825 mAh g–1, respectively, which are basically the same as the initial capacitance of the samples. This shows that, after repeated charging and discharging at different current densities, the overall structure of MOFs is not damaged and still has excellent lithium storage performance, which may be due to the inherent characteristics of CoNi-MOF. On the basis of the above discussion, we speculate that Co0.8Ni-MOF as an anode material for Li-ion batteries has great potential, even in the previously reported MOFs (Table S4).

Figure 6

Electrochemical performance of the as-synthesized Co0.8Ni-MOF: (a) Cyclic voltammograms (CVs) at a scan rate of 0.2 mV s–1. (b) Galvanostatic charge–discharge profiles at a current density of 500 mA g–1. (c) Cycling performance and Coulombic efficiency at a current density of 500 mA g–1. (d) Rate performance at different current densities from 0.1 to 2.0 A g–1.

Additionally, all CoNi-MOF samples were tested by electrochemical impedance spectroscopy (EIS) before and after 200 cycles at 0.5 A g–1 to further understand their electrochemical behaviors. An equivalent circuit model was used to fit the impedance data (Figure ), and the fitting parameters are summarized in Table S5. All cells exhibit a depressed semi-circle in the high-frequency region, which corresponds to the electrode’s charge transfer resistance (Rct), and an inclined line at the low frequency, which corresponds to the Warburg diffusion process (Zw). Internal resistance is correlated with the intersection of the semi-circle at the real axis in the high-frequency region (denoted as Rs). Both the charge-transfer resistance (Rct) in the high-frequency region and the Warburg impedance in the low-frequency region of Co0.8Ni-MOF are smaller than those of Co0.5Ni-MOF and Co1.0Ni-MOF, indicating that the Co0.8Ni-MOF hybrid has a large electrode/electrolyte contact area and accelerated charge transfer and Li+ diffusion, as shown in Figure a. The Nyquist plots of all samples after 200 cycles are shown in Figure b, which presents the same order as before cycling. Taking Co0.8Ni-MOF as an example, the solution resistance (Rs) increases slightly from 3.126 Ω in the pristine state to 10.72 Ω, signifying its great stability, and the Rct also decreases from 110.2 to 21.1 Ω after 200 cycles, showing the enriched wettability and better connectivity of the electrodes. Furthermore, the Li+ ion diffusion coefficient of the three electrodes is also compared to demonstrate the electrochemical behaviors mentioned above, as presented in Figure S13.

Figure 7

Nyquist plots of all samples (a) at a pristine state and (b) after 200 continuous cycles at 0.5 A g –1. Inset: the equivalent circuit models used to fit impedance data.

To better understand the possible reasons for the excellent performance, the reaction kinetics of Co0.8Ni-MOF (Figure a) at different scan rates ranging from 0.2 to 1.2 mV s–1 were analyzed using the following equation: log i = log a + b log v, where i is the peak current, v is the scan rate, a is a constant, and b is a function variable parameter. When b approaches 1, the nanofaradaic pseudocapacitance takes over. When b approaches 0.5, the system is said to be Li+ ion diffusion-controlled.[33,58] As shown in Figure b, the calculated values of b at the oxidation and reduction peaks are 0.610 and 0.699, respectively, indicating that the electrochemical reaction of Co0.8Ni-MOF was related to capacitive and diffusion behaviors. Furthermore, Trasatti’s equation Q(v) = Qs + Qb = Qs + k (v1/2) distinguishes the surface charge contribution (Qs) and the bulk diffusion contribution (Qb).[59] It can be seen from Figure c that the diffusion-controlled contribution is 40.5% at a scan rate of 0.6 mV s–1, and the capacitive contribution of Co0.8Ni-MOF increases with the scan rate, as shown in Figure d. When the sweep rate is increased from 0.2 to 1.2 mV s–1, the capacitive contribution also increases from 31.82 to 73.2%. Therefore, the excellent electrochemical performance of Co0.8Ni-MOF can be partly attributed to the pseudocapacitive behaviors, which occupied a significant portion of the redox processes, leading to high capacity and rate performance.

Figure 8

Kinetics of the electrochemical behavior toward Li+ for the Co0.8Ni-MOF electrode: (a) CV curves at various scan rates ranging from 0.2 to 1.2 mV s–1. (b) Relationship between peak currents and potential swing rates. (c) Separation of capacitive and diffusion currents to the total at 0.6 mV s–1 scanning speed. (d) Charge contribution ratios of capacitive and diffusion-controlled devices at various scan rates.

Conclusions

In this work, by using a one-pot solvothermal method, we successfully recovered PET plastic waste as a precursor to fabricate value-added CoNi-MOF and investigated lithium anodic performance for the first time. The as-synthesized Co0.8Ni-MOF exhibits excellent crystallinity, purity, and electrochemical performance. The initial discharge and charge capacities are 2496 and 1729 mAh g–1, respectively. Even after 200 cycles, the Co0.8Ni-MOF electrode can exhibit a high CE of over 99%. Furthermore, the Co0.8Ni-MOF shows excellent rate performance at various rates. The CV curves show excellent overlap and reproducibility, indicating great reversibility and cycling stability of the Co0.8Ni-MOF. Altogether, given the environmental and economic benefits, the Co0.8Ni-MOF derived from PET waste is thought to be an appealing material for a variety of potential practical applications. Furthermore, compared with the initial CE of most commercial anodes, the first efficiency of Co0.8Ni-MOF is still slightly lower. However, this problem has yet to be overcome. Surface coating and prelithiation may provide ideas to solve this problem and improve the overall performance of MOF anodes. Once this problem is solved, lithium anode materials can be easily produced on a large scale in the future and can be explored in other lithium-related fields.