Unified throughout-pore microstructure enables ultrahigh separator porosity for robust high-flux lithium batteries

Abstract

With small thickness, commercial polyolefin separators own low porosity to ensure sufficient thermomechanical properties, resulting in tortuous and enlarged Li⁺ diffusion pathways that induce large overpotentials and detrimental dendrite growth. As a dilemma, the exploration of highly porous separators has been challenged by their large thickness, impairing the applicability of such pursuits. Herein, an ultraporous architecture is designed to shorten Li⁺ transfer pathways by impregnating electrolyte-affinitive poly (vinylidene fluoride-co-hexafluoropropylene) into ultralight ∼3 μm 3D-polytetrafluoroethylene scaffold (abbreviated as UP3D). The UP3D separator with a porosity of 74% gives rise to 70% enhancement in Li⁺ transference and 77% reduction in Li⁺ transfer resistance (2.67 mΩ mm⁻¹) and thus enables an ultrahigh Li⁺ flux of 22.7 mA cm⁻², effectively alleviating Li⁺ concentration gradient across the separator. With the separator, the LiFePO₄ half cell delivers a capacity of 118 mAh g⁻¹ with an unparalleled capacity retention of 90% after 1000 cycles at 2 C, and a graphite || LiNi_0.6Co_0.2Mn_0.2O₂ pouch full cell delivers an areal energy density of 6.8 mWh cm⁻² at 8.848 mA (1.4 mA cm⁻²) with a high cathode loading of 134.9 mg. Such results, together with the scalable production of the separator, reflect its promising potential in high-flux battery applications of separators that require both ultrahigh porosity and reliability.