Tunable Large Mesopores in Carbon Nanofiber Interlayers for High-Rate Lithium Sulfur Batteries

Abstract

Lithium sulfur battery interlayers were made from mesoporous carbon nanofibers with large tunable mesopores (17–50+ nm). The nanofibers were prepared by blending and electrospinning two immiscible polymers, polyacrylonitrile (PAN) and cellulose diacetate (CDA) with dimethylformamide as the solvent. Humidity control during the electrospinning process made adjustments to the size of the meso-scaled phase separation domains of the sacrificial CDA. The resultant fiber mat was carbonized and activated in air to create micropores in mesoporous carbon nanofibers (MPCNF). The pore characteristics of the activated MPCNFs were examined and then tested as interlayers in Li-sulfur batteries. First, a fundamental study was conducted regarding the effectiveness of the interlayer versus its own weight and the overall sulfur composition along with a characterization of the pore filling and impedance. Secondly, the effect of micropores and three different pore widths of large mesopores in interlayers on battery performance was studied which revealed that the largest mesopores played a critical role in improving rate capability. Cells with MPCNF interlayers with 50+ nm pores and 70:30 sulfur:Super P composite cathodes had a discharge capacity of over ∼1100 mAh g −1 sulfur for 100 cycles at 0.5C and exhibited improved high charge rate capabilities with 850mAh g −1 at 3C. Mesoporous (2–50 nm) carbons have been heavily studied for many applications including water purification, catalysis, molecular sieves, biomolecule adsorption, and energy storage because they can provide a relatively chemically inert, robust, conductive support with a balance of high surface and pores wide enough for quick diffusion. 1–3 For battery applications, a particular pore distribution may be desired over another to fit the exact diffusion needs of a particular electrolyte or the storage of a particular reaction product. 4–6 In a lithium sulfur battery, unreacted sulfur is stored on a carbon cathode and is lithiated into lithium polysulfides (Li 2 S x, 8 ≥ x ≥ 2) and then, ideally, fully lithiated into lithium sulfide (Li 2 S) on the cathode. 7–15 Lithium polysulfides greater than Li 2 S 2 are well known to be soluble in the commonly used electrolytes in lithium sulfur batteries. 7–15 Micropores or small mesopores are essential to carbon-based sulfur cathodes in organic electrolyte to resist and adsorb the dissolved lithium polysulfides from diffusing out of the cathode and to the anode. 7,8,10,11,13–18 At the anode, migrating polysulfides form an insoluble insulating lithium sulfide layer and contribute to the lithium polysulfide shuttle mechanism causing polarization and self-discharge throughout the cell described in more detail elsewhere. 7,13–15,18–21 Pre-vious works have found that ordered mesoporous carbons have been effective as cathodes as the mesopores offer a balance of surface area to provide diffusion retarding intermolecular forces and short electron diffusion distances to the polysulfides as well as large pore volumes for product storage and electrolyte diffusion. 6,8–10,13,15,18,22–24