Thermally stable polyamide-imide (PAI) hierarchical porous separator for lithium metal batteries

Although the structural design and physicochemical characteristics of separators are critical determinants of lithium metal batteries safety and overall performance, commercial polyolefin separators exhibit inherent limitations in thermal stability, porosity and electrolyte wettability. These deficiencies not only elevate the propensity for battery safety incidents but also severely compromise ionic transport capability and cycling performance. Herein, we have designed and synthesized a high molecular weight polyamide-imide (PAI) polymer by one-step polymerization, and subsequently prepared a PAI separator with high thermal stability, high porosity and excellent electrolyte wettability using vapor-induced phase separation. Specifically, thermal stability testing has confirmed that PAI separators maintain structural integrity at temperature as high as 240 ​°C. Additionally, a bi-continuous sponge-like structure with high porosity (86 ​%) has been achieved by controlling the phase inversion parameters, and the abundant porous structure has facilitated ion transport. Thus, the PAI separators exhibit an ionic conductivity of 1.67 ​mS ​cm⁻¹ and a lithium ions transference number of 0.64. Importantly, the polar amide and imide groups in the PAI molecular structure have further ensured excellent electrolyte wettability of the PAI separator, which has resulted in high electrolyte uptake (326 ​%). As a result, LFP/Li batteries with PAI separators exhibit superior rate performance (147 mAh g⁻¹ at 1 ​C) and excellent capacity retention (78.6 ​% at 200 cycles) which are better than that of the commercial PE separator. This investigation reveals that the organic soluble and thermal stable PAI material could be excellent lithium metal batteries separator candidates to improve the safety and comprehensive performance.