image: This work introduces a sustainable co-recycling strategy that transforms spent LMO cathodes into highly active photothermal catalysts for efficient polyester depolymerization. Lithium deficiency in spent LMO enhances Mn3+/Mn4+ redox properties, boosting adsorption and catalytic activity—achieving up to 17.5-fold higher efficiency than pristine LMO. This work establishes a sustainable pathway for upcycling both LIBs and plastic waste, demonstrating remarkable scalability and sustainability implications.
Credit: ©Science China Press
Co-upgrading of spent LMO and waste PET
Spent lithium manganate oxides (LMO) exhibit variations in lithium content, which may affect their catalytic performance when upgraded to catalysts. To address this issue, the researchers employed a controlled chemical delithiation method to investigate how the gradient lithium content in cathode materials influences the photothermal catalytic performance of PET glycolysis. The study revealed that Li0.51Mn2O4 demonstrated the highest photothermal catalytic efficiency, achieving a PET conversion rate of 56.9% and a BHET yield of 32.2%. These results were 17.5 times and 87 times higher, respectively, than those of the P-LMO catalysts.
"Through controlled chemical delithiation, we identified Li0.51Mn2O4 as the optimal solution, enabling the transformation of low-value spent materials into highly efficient photothermal catalysts," said Prof. PanPan Xu, co-corresponding author of the study.
Mechanism of PET Glycolysis
By incorporating the photothermal effect into the catalytic system, the researchers proposed the reaction mechanism of photothermal catalytic glycolysis of PET using LMO. Initially, the DL-LMO catalyst converts sunlight into thermal energy, raising the temperature of the entire reaction system. The PET chains are gradually dissolved by EG and migrate to the catalyst surface for subsequent depolymerization. During the catalytic process, the carbonyl oxygen of the PET chain interacts with Mnδ⁺. The enhanced-polarized carbonyl group facilitate the formation of active C+ site, while the electron-rich oxygen atoms in the EG molecule simultaneously attack the C+ site, leading to the cleavage of the macromolecular chain via a nucleophilic substitution reaction. By repeating this process, the PET chain is gradually depolymerized into the monomer BHET.
"We have combined photothermal conversion with catalytic reactions, addressing the 'high energy consumption' issue of traditional thermal catalysis and the 'low efficiency' problem of pure photocatalysis," said Prof. Muhan Cao. "Meanwhile, this has endowed spent LMO – a low-value waste material – with two functions: 'energy conversion' and 'catalysis', realizing the high-value utilization of resources."
Degradation of real-world Polyester Plastics
"The real application should not be confined to single materials or ideal conditions in the laboratory. Instead, it should target the most common polyester wastes in daily life, even complex mixed plastics," Prof. Chen Jinxing emphasized.
Accordingly, the researchers evaluated the photothermal depolymerization performance of spent LMO powder on commercial polyester plastics, covering common daily plastic wastes such as PET carpets, PET towels, PET films, colored PET bottles, and black PET food containers. Furthermore, they tested the universality of the LMO catalyst for mixed waste plastics, including blends of PET with polycarbonate (PC), polylactic acid (PLA), polypropylene (PP), polystyrene (PS), and polyethylene (PE). The glycolysis reaction of PET maintained high activity even in these mixed systems.
In addition, the researchers conducted outdoor experiments using natural sunlight as the energy source. After 50 minutes of reaction, the PET conversion rate reached 97.8%. These efforts demonstrate the researchers’ commitment to enhancing the universality and real-world applicability of this technology.
From Technical Concept to Commercial Feasibility
To evaluate economic feasibility, the researchers conducted a Techno-Economic Analysis (TEA) and Life Cycle Assessment (LCA) focusing on photothermal glycolysis for waste PET recycling.
Based on an annual processing capacity of 80,000 tons of waste PET, the Minimum Selling Price (MSP) of BHET produced via the photothermal route was $1.037 kg–1. Waste PET raw material accounted for 53% of the total cost, making it the primary cost driver. The price of waste PET had the most significant impact on the MSP. When the factory scale reached an annual processing capacity of 10,000 tons, the MSP dropped to $1.36 kg–1, approaching the commercial price of $1.35 kg–1.
Comparing three routes—photothermal (including two schemes: PT-A and PT-B), traditional thermal catalysis, and industrial dimethyl terephthalate (DMT)—with the functional unit of producing 1 kg of BHET, the photothermal route showed significant advantages. Under the European scenario, the Global Warming Potential (GWP) of PT-A was only 0.67 kg CO2 eq/kg BHET. Under the Chinese scenario, the photothermal route reduced fossil resource consumption by 70%–77% compared to the DMT route.
The co-upgrading of spent LMO and waste PET combines economic feasibility with environmental benefits. When the factory scale meets the required standards, the product cost approaches the commercial level, and the technology achieves substantial emissions reduction by relying on solar energy.