Upcycling Waste Poly(Ethylene Terephthalate) into Polymer Electrolytes

Lithium-ion batteries (LiB) play an important role for energy storage in our increasingly-electrified modern world, with polymer electrolyte (PE) materials poised to revolutionise battery design by eliminating the most critical...

Plastics are ubiquitous materials in modern society due to their low cost, durability and versatility for diverse applications.With soaring demand worldwide, the quantity of post-consumer waste plastics produced is also increasing rapidly.Of the 460 million tons of plastics produced globally in 2019, only 9% is recycled, with the remainder being either incinerated, disposed of in landlls or irresponsibly discarded in the environment. 1olyethylene terephthalate (PET, SPI code 1) is amongst the most abundant plastics produced today (∼31 million tonnes in 2019). 2 Being mechanically tough, ductile and mouldable, PET is commonly used as bottles and jars, synthetic bres for clothing, carpets and ropes, protective packaging and even construction materials.Due to its relatively uncomplicated waste stream, 3,4 PET is also the most-recycled plastic worldwide. 5However, with rapid degradation of physical properties during repeated mechanical recycling due to polymer chain cleavage, only a minority of recycled PET is suitable for use in its original applications. 5While chemical recycling offers the possibility of reforming high molecular weight PET polymers aer rst breaking them down into monomers, 6 this process is disadvantaged by its high energy demands and need for monomer purication. 7Moreover, recycled PET can cost more than virgin polymers, 8,9 reducing the economic viability of recycling.
1][12][13][14][15] PET has been upcycled into small molecules which can be used as solvents and reagents for the chemical industry, 16 including the production of new polymers such as polyhydroxyalkanoates (PHA), 17 as well as porous materials for carbon dioxide (CO 2 ) capture 18 and utilisation. 19Recently, with growing interest in developing sustainable materials for energy storage applications to meet ever-increasing global energy demands, 20 upcycling of post-consumer plastics into energy storage materials is expected to be of growing importance.2][23] Thus far, other than Sardon's recent report of upcycling bisphenol A polycarbonate plastics into aliphatic polycarbonates useful as polymer electrolytes (PEs) for solid-state lithium ion batteries (LiBs), 24 upcycling of other readily-available, abundant waste commodity plastics for this lucrative application has been largely overlooked. 25Indeed, compared to conventional liquid electrolytes currently used in LiBs, PEs offer numerous practical advantages such as reducing the risks of electrolyte leakage, thermal runaway, uncontrolled volume expansion, dendrite growth, and unwanted electrolyte side reactions. 26erein, we report the rst examples of PEs derived from waste PET beverage bottles.PET offers numerous desirable ready-made features for upcycling to solid polymer electrolytes (SPEs): other than its susceptibility towards chemolysis into polymer building blocks, 6 the rigid aromatic terephthalate components which are responsible for PET's excellent mechanical properties 27 can also be exploited for enhancing the mechanical robustness of SPEsfacilitating device integration and fabrication, suppressing dendrite formation 28 and potentially, for use in structural power devices. 29Aer assessing the viability of our PET-derived polymers as solid polymer electrolytes, we further evaluated their ionic conductivity and cycling performance when used as gel polymer electrolytes for LiBs.
The design of our PET-derived polymer electrolytes is summarised in Fig. 1.The structural rigidity conferred by the "hard" aromatic units from PET is balanced by "so" poly(ethylene glycol) (PEG) segments which provide the requisite exibility for Li + transport by segmental chain motion. 281][32][33] Indeed, polyurethanes (PUs) are gaining popularity as polymer electrolyte materials due to their oen excellent mechanical strength, desirable thermal and electrochemical stability. 345][36] In addition, extensive hydrogen bonding from the abundant amide and urethane groups present can enhance polymer-polymer interactions for better mechanical robustness.These hydrogen bond donors also facilitate interphase compatibility with additives such as LiTFSI and ionic liquids. 34he synthesis of the polyurethane electrolytes from waste PET bottles is summarised in Scheme 1. Clean waste PET bottles were rst depolymerised by aminolysis using neat ethanolamine to form bis(2-hydroxyethyl)terephthalamide (BHETA) in the presence of the 1,5,7-triazabicyclo[4.4.0]dec-5ene (TBD) organocatalyst (Fig. S1 †). 37Compared with synthesising BHETA directly from terephthalic acid, production of terephthalamides from PET aminolysis avoids the usage of hazardous acid chloride reagents traditionally required, 38 potentially offering a more sustainable synthesis route. 14hereaer, the easily-puried BHETA monomer was mixed with PEG (M n 17.5 kDa) in different mass ratios, and reacted with hexamethylene diisocyanate (HMDI) to form a family of polyurethanes containing different ratios of hard (BHETA): so (PEG) blocks in generally excellent yields (Table 1).‡ To better appreciate the roles of the BHETA component in our polymer electrolyte design, we also synthesised a polyurethane control (named PEG-PU) made up solely of PEG and HMDI components (Fig. S2 †) for comparison.
1 H NMR spectroscopy showed unequivocal evidence of the formation of polymers containing PET-derived BHETA and PEG, evident from diagnostic resonances of the different components (Fig. 2).The presence of amide and urethane bonds on the polymer were further veried by their characteristic 13 C NMR signals at 165.8 and 156.1 ppm respectively (Fig. S3 †).In addition, the absence of 13 C NMR signals at ca. 120 ppm and FTIR absorbance peaks between 2250-2300 cm −1 further indicates that all isocyanate groups from the HMDI linker have been fully consumed to form urethanes. 39 FTIR spectroscopy provides further evidence of extensive hydrogen bonding between polymers: compared to free urethane C]O stretches with typical absorbance at 1710-1730 cm −1 , the lower-than-expected urethane C]O stretch of the 1BHETA-2PEG polymer at 1689 cm −1 (Fig. S4 †) is characteristic of hydrogen-bonded urethane groups in ordered hard polymeric domains. 40The compositions of the BHETA-PEG series of polymers were determined by integrating the 1 H NMR signals at 7.9, 3.5 and 1.9 ppm to obtain the percentage masses of the BHETA, PEG and HMDI components respectively for each polymer (Table 1).The interactions between 1BHETA:2PEG polymers were further probed by 1 H NMR dilution experiments.At gradually lower polymer concentrations in d 6 -DMSO, the polymer's aromatic and aliphatic NMR signals became increasingly sharper and more well-dened (Fig. S5 †).This suggested that at high polymer concentrations, considerable interactions between the polymers occurred, restricting their conformational and translational freedom in solution.Primarily, this could be due to the extensive hydrogen bonding from the amide and urethane groups present, with insignicant contributions from p-p stacking as observed from the lack of signicant chemical shi perturbations of the BHETA aromatic signals upon dilution.
The thermal properties of the PET-derived BHETA-PEG family of polymers were probed using TGA and DSC to determine their thermal stability, glass transition (T g ) and crystallisation temperatures (T c ). § TGA studies showed that all polymers were stable up to 250 °C (Fig. S6 †), well beyond the working temperatures for battery applications.Derivative thermogravimetry (DTG) (Fig. S7 †) reveals that the BHETAcontaining polymers show a rst thermal degradation peak centred around ∼300 °C, which likely resulted from decomposition of the BHETA component as this peak became less pronounced with polymers containing smaller percentages of BHETA.DSC studies (Table 1 and Fig. S8 †) showed that generally, lower T c was observed with the polymers containing the highest percentages of BHETA (1BHETA:2PEG and 1BHETA:4-PEG).These ndings suggested that BHETA was able to reduce the interchain interactions and crystallinity of the PEG components, potentially facilitating segmental chain motion and Li + ion transport.
The PET-derived BHETA-PEG polyurethanes were rst studied as SPEs.The polyurethane lms were cast as a 10 wt% solution in anhydrous DMSO at 120 °C with LiTFSI salt (10 mol% w.r.t.PEG repeating units) in an Ar-lled glovebox, before the DMSO was evaporated to obtain a solid lm (see Fig. 3A inset).As LiTFSI can act as a plasticiser, 41 only 1BHE-TA:2PEG and 1BHETA:4PEG samples were able to form freestanding lms under such conditions.Indeed, the presence of LiTFSI reduced the elastic modulus of the 1BHETA:2PEG lm from 61.7 ± 4.8 to 23.2 ± 3.6 MPa (Fig. S9 and S10 †).While increasing the solid weight percentage to >15 wt% (w.r.t.DMSO solvent) for 1BHETA:6PEG allowed the formation of a freestanding lm in the presence of LiTFSI, free standing lms could not be formed from the other polyurethanes containing lesser amounts of BHETA (i.e.1BHETA:7PEG and PUs without BHETA) using a similar approach.This demonstrated the importance of BHETA as a structural component of the polyurethane electrolytes in improving their mechanical integrity.The 1BHETA:2PEG polymer lm containing LiTFSI showed improved thermal stability, with onset of thermal decomposition at ∼350 °C (from TGA), compared to that of the polymer alone (onset ∼240 °C) (Fig. S11 †), likely arising from interactions between the PEG and LITFSI salt. 42he ionic conductivities of the BHETA:PEG SPEs were measured above their T g values between 20-80 °C.Linear Arrhenius-type behaviour was observed with expected   augmented conductivities at higher temperatures (Fig. 3A), suggesting that Li + ion conduction is occurring via a hopping mechanism within the polymer matrix decoupled from longchain motion of the polyurethane electrolytes. 43When the BHETA : PEG ratio increases from 1 : 2 to 1 : 4, the ionic conductivity at both 20 °C and 60 °C were increased by more than an order of magnitude (Table 2).This is expected from the higher proportions of PEG chains present, whose lower rigidity facilitated Li + ion transport.Using the Arrhenius equation: where T is the absolute temperature, k is the Boltzmann's constant and s 0 is the pre-exponential factor, the activation energy (E a ) for ion conduction was determined to be between 0.56 and 0.65 for the three polyurethanes tested (Table 2).However, further increasing the BHETA : PEG ratio to 1 : 6 did not further enhance ionic conductivity.Notably, these values are comparable to those previously reported for PEO-LiTFSI systems of 0.49 to 0.7 eV, [45][46][47] suggesting that the dissociation of the Li + and TFSI − was not impeded.Furthermore, the similar E a to neat PEO-LiTFSI systems would also suggest that the presence of hydrogen bonding between the amides and the urethanes, as well as the rigid aromatic groups which enhances the SPE lms' mechanical integrity, are decoupled from the ion conducting EO motifs.The decoupling of functionalities is an important strategy to achieve both facile ion transport and structural stability simultaneously without falling into the rule of mixtures. 48hile PEO has been explored extensively as an electrolyte for lithium polymer batteries, replacing part of the polymer electrolyte content with a waste plastic-derived alternative can offer more opportunities for sustainable battery design, as well as to enhance the electrolyte's mechanical robustness.To explore this possibility, we formulated mixed polymer electrolytes containing the PET-derived BHETA:PEG polymers with PEO (100 kDa) and measured their ionic conductivity.Compared with the pure BHETA:PEG polymers, an increase in ionic  a Amount of LiTFSI determined using the no. of mols of PEG (M n 17.5 kDa) based on the mass percentage of PEG from each polymer (Table 1).
b Determined from eqn (1).c Goodness-of-t when log(sT) was plotted against (1000/T).d Mass percentage of EMITFSI ionic liquid content calculated w.r.t. the total mass of polymer, LiTFSI and EMITFSI present (see ESI).
conductivity was observed at 60 °C in the presence of PEO (Table 2, Fig. S12 and S13 †).Notably, these conductivities are comparable to those of PEO/LiTFSI polymer electrolytes. 49For solid polymer electrolyte systems based solely on semicrystalline polymers such as PEO, an appreciable ionic conductivity can only be observed near the polymer's melting point (T m ∼65 °C). 50However, this is not ideal as the polymer will have negligible structural properties in the liquid phase and will not offer any improvements in battery safety.In contrast, the combination of BHETA:PEG/PEO could give linear Arrhenius behavior for ion transport between 20-80 °C without any signicant thermal phase transitions.This shows that despite the plasticizing effect of LiTFSI, the BHETA:PEG polymers were able to provide sufficient mechanical support to resist signicant phase changes.Indeed, maintaining the mechanical rigidity of the electrolyte without compromising on the ionic conductivity is important to the overall safety of the battery.This is especially true for next generation of energy storage devices like Li-metal and structural batteries.Compared to SPEs, GPEs are considered to be more practically applicable in lithium-ion batteries. 513][54] As GPEs matrices, polyurethanes can hydrogen bond to the liquid electrolytes, which enhances interaction and wettability with the liquid phase. 55Encouraged by the favourable aforementioned mechanical properties of the 1BHETA:2PEG SPE lms, we investigated its use as a polymer matrix material for GPEs to further enhance the lms' ionic conductivity.1BHETA:2PEG and LiTFSI were cast into homogeneous free-standing nonleaky gel electrolyte lms containing different quantities of EMI-TFSI ionic liquid, whose conductivities at different temperatures were then determined.
As shown in Fig. 3B and Table 2, the presence of the EMI-TFSI/LiTFSI liquid phase resulted in considerable enhancements to the ionic conductivity of the GPE of nearly 3 orders of magnitude compared with just 1BHETA:2PEG alone.Arrheniuslike behavior with the expected higher conductivity at elevated temperatures were observed.In addition, the lack of conspicuous phase changes (see Fig. S14 † for DSC) and sharp changes in the conductivity over the range of temperatures investigated is indicative of the amorphous nature of the 1BHETA-2PEG polymer electrolyte.Increasing EMI-TFSI ionic liquid content also reduced the activation energy for ion conduction, likely due to improved liquid phase percolation through the porous polymer matrix (Fig. 3C), generating conducting channels which facilitate ion conduction.Indeed, the ionic conductivity (∼10 −4 S cm −1 ) observed in the presence of 38% EMI-TFSI loading at ambient temperature is comparable to those of liquid electrolytes. 56he lithium transference number of the 1BHETA:2PEG GPE was estimated to be 0.48 (Fig. S15 †) using the Bruce and Vincent method, 57 which indicated that Li + and TFSI − contributed almost equally to ionic conductivity.These values are comparable with similar GPE systems based on PEO-ionic liquids 58 and shows that the diffusion of Li + under polarisation was unhindered by the PU structural framework.Measurements of the oxidative stability of this GPE system revealed an onset of oxidation at ∼3.3 V (Fig. S16 †).While this precluded usage with high-voltage cathodes, our GPE system was still compatible with the widely-used LiFePO 4 (LFP) cathode material.
Thus, we next evaluated the feasibility of BHETA:PEG gelpolymer as a electrolyte by assembling proof-of-concept Li-ion cells with Li 4 Ti 5 O 12 (LTO) as anode and LFP as cathode.The cycling performance was investigated in the voltage range of 1.5 V to 2.5 V at room temperature and at 60 °C at constant current density of 0.1C.Fig. 4A and B show the room temperature charge/discharge voltage plateaus and coulombic efficiency of LTO:BHETA:PEG gel-polymer: LFP for 150 cycles, respectively.
The cells can be successfully charged and discharged both at room temperature and 60 °C (Fig. S17 †).The charge stored and delivered at room temperature stabilised aer the initial 12 cycles, with coulombic efficiency of >98% throughout the 150 cycles.For the cells running at elevated temperature of 60 °C, higher capacities (close to theoretical capacity of 170 mA h g −1 ) was achieved.The more dramatic capacity loss observed for these cells may be attributed to the increase of cell resistance and loss of effective contact due to the gradual extraction of ionic liquid while cycling at higher temperature.This may be due to the limited absorption capacity of the BHETA:PEG polymer. 59n conclusion, we have demonstrated successfully for the rst time, the upcycling of PET from waste beverage bottles into polymer electrolytes.PET was rst organocatalytically depolymerised into BHETA, which was then re-polymerised with PEG segments to form polyurethanes.The rigid aromatic BHETA components were essential for providing mechanical support for the formation of free-standing conductive lms in the presence of LiTFSI, while the PEG segments provided the exibility for ion conduction.Overall, our ndings clearly conrm the potential of these waste PET-derived polyurethanes as polymer electrolytes for Li-ion batteries, achieving a room temperature conductivity of 10 −4 S cm −1 as a GPE.To the best of our knowledge, this is the rst report of a working Li-ion battery assembled using upcycled waste plastic-derived polymers as the electrolyte.The promising performance of our BHETA:PEG electrolyte opens new pathways and possibilities towards application of upcycling high-volume waste commodity plastics as battery materials, contributing to a sustainable energy future.

Fig. 1 Scheme 1
Fig.1Design of polyurethane polymer electrolytes derived from waste PET with hard (aromatic) and soft (PEG) components.

a
Polymers are named according to their approximate BHETA:PEG mass percentage compositions.b Determined by integration of the aromatic BHETA (7.92 ppm), PEG (3.51 ppm) and HMDI (2.93 ppm) signals in the polymers 1 H NMR spectra (d 6 -DMSO).c Determined from DSC measurements during the second heating scan (heating rate = 20 °C min −1 ; cooling rate = 20 °C min −1 ).

Fig. 3
Fig. 3 Effects of (A) modifying the BHETA:PEG ratio in the solid polymer electrolyte; inset: appearance of the 1BHETA:2PEG/LiTFSI solid polymer electrolyte film; and (B) increasing EMI-TFSI ionic liquid content in 1BHETA:2PEG gel polymer electrolytes on the ionic conductivity of the polyurethane; (C) SEM surface morphology of BHETA:PEG electrolyte after hexane rinsing to remove EMI-TFSI at 1000× magnification (inset: 3000× magnification).

Table 1
Compositions and thermal properties of the BHETA-PEG polyurethane polymer electrolyte candidates

Table 2
Electrochemical performance of polyurethane polymer electrolytes