Ion-flux-boosted polyimide separators via synergistic pore and surface charge engineering for highly stable Li- and Na-metal batteries

초록

Polyolefin separators are widely used in Li-ion batteries due to their excellent mechanical and chemical stability. However, their low thermal stability and poor electrolyte wettability necessitate the development of alternative separator materials for high-performance energy storage systems. In this study, a porosity-tailored polyimide (PPI) separator was designed to improve electrochemical stability and ion transport kinetics in Li-and Na-metal batteries. The physical, electrochemical, and surface charge of the separator were optimized by adjusting the relative ratio of porogen to poly(amic acid) film. The PPI separator exhibited excellent cycling stability in Li||Cu cells compared to PP separators, due to its synergistic combination of homogeneous Li-ion diffusion between electrodes, high surface polarity, and robust mechanical strength. Furthermore, Li||LFP full-cell tests confirmed its superior thermal stability and electrochemical durability, even after thermal shock. Building on these findings, the potential application of PPI separators in Na-metal batteries was further investigated. The symmetric Na||Na cell with a PPI separator exhibited a significantly smaller voltage gap and greater cycling stability than that with a PP separator across various current densities. In addition, Na||NaNi1/3Fe1/3Mn1/3O2 full-cell tests demonstrated that the PPI separator also outperformed the PP separator, attributed to its higher Na+ transference number and enhanced reaction kinetics enabled by polar imide groups and optimal porosity. These results highlight PPI separators as a promising alternative to polyolefin separators, addressing key challenges in alkali-metal batteries by offering enhanced thermal stability, superior ion transport, and long-term cycling performance.

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