Role of Cyclopentyl methyl ether co-solvent in Improving SEI layer stability in Hard Carbon Anode for Sodium-ion Batteries

The conversion of polyethylene terephthalate (PET) municipal waste via the carbonization process into hard carbon (WPET-HC) delivers a high-performing, low-cost, sustainable anode material for sodium-ion batteries (SIBs). To further optimize the anode, electrolytes and interfacial chemistry are critical in improving cycling stability and rate capability. Herein, cyclopentyl methyl ether (CPME), a weakly solvating and a wide temperature solvent, is used as an alternative co-solvent to ethylene carbonate (EC) to deliver a high initial coulombic efficiency (ICE) up to 75%. The larger interlayer spacing, low surface area, and slit-shaped micro and mesoporous presence in the WPET-HC structure enhance the low potential plateau capacity to 68%, showing a more battery-type anode material from plastic trash. The WPET-HC delivered the excellent reversible capacity of 356 mAh g-1 at the current density of 30 mA g-1 with superior cycling of 91% after 100 cycles using CPME-PC-based electrolyte. The reduction of CPME co-solvent forms a more inorganic SEI than EC-generated SEI, providing a stable and thin SEI layer boosting the ICE and cycling stability of the anode. The low-temperature battery metric for CPME-PC-based electrolytes showed ~30% added capacity and improved ICE value compared to EC-PC-based electrolytes. The CPME-PC-based electrolyte maintained the higher capacity retention of 88% and 74% at 10℃ and 0℃, respectively, with a coulombic efficiency of 100%, revealing the excellent stability of the electrolyte with the HC anode. The work provides an eco-friendly approach to developing hard carbons from plastic trash and reports for the first time the use of greener, low-solvating CPME in improving the reversible capacity and ICE for low-temperature applications of SIBs.