A biodegradable gel polymer membrane derived from poly-gamma-glutamic acid for high-rate and long-lifespan sodium metal batteries

Sodium metal battery (SMB) technology is gaining increasing popularity due to its potential for cost-effective and sustainable energy storage solutions. However, a critical challenge hindering its commercialization is the development of safe, eco-friendly and high-performance electrolyte systems. In this study, poly-gamma-glutamic acid (γ-PGA)-based gel polymer membrane (GPM) for SMBs (denoted as γ-PP64) is obtained using a mixed solvent method. The resulting γ-PP64 membrane with the thickness of about 30 μm exhibits a porous structure that significantly enhances its electrolytes absorption to 144.4 % and excellent thermal stability to 150 °C. This structural innovation also facilitates the migration of Na ⁺ ions (up to 0.11 mS cm⁻¹) and high transference number of 0.536, which promotes reversible and uniform sodium deposition and dissolution. During cycling tests at 0.5 C, the corresponding Na/γ-PP64 GPM/Na₃V₂(PO₄)₃ full cell demonstrates an initial capacity of 103 mAh g⁻¹ and a 92.8 % capacity retention. Additionally, the γ-PP64 GPM cell records a capacity of 87.3 mAh g⁻¹ after 1100 cycles at a high rate of 5 C. The findings highlight the potential of biodegradable material-based GPM to enhance the performance and longevity of SMBs, thereby paving the way for their commercialization and contributing to environmental sustainability goals in energy storage applications.