Graphene Foam Current Collector for High-Areal-Capacity Lithium–Sulfur Batteries

Abstract

Extending lithium–sulfur battery (LSB) electrode architecture into three dimensions (3D) has been proposed for more than a decade. A 3D lightweight and porous current collector that is capable of holding high amounts of sulfur (S) without any significant decrease in performance has been elusive. Although many material solutions (such as sulfurized polyacrylonitrile or SPAN) have been identified for alleviating polysulfide formation and the so-called shuttle effect, their incorporation into 3D current collectors with high capacity at the electrode level has not yet been realized. Here, we show that graphene foams (GFs) are ideally suited as 3D lightweight current collectors for LSBs and outperform the conventional carbon-coated Al (Al/C) foils at the electrode level. Specifically, we demonstrate that the open framework of GFs facilitates high mass loading of SPAN without any deterioration in capacity at the active material level even at high S loading. At the electrode level, GF-SPAN cathodes exhibited capacities of ∼200 mAh gelectrode –1 at 0.1C even with low S loadings (∼1.1 mg cm–2), which is at least 3 times higher than conventional Al/C electrodes. More importantly, we fabricated cells with a high mass loading of 26.5 mg cm–2 S by stacking multiple GFs to achieve an areal capacity as high as ∼20 mAh cm–2 (at a current density of 3.0 mA cm–2 up to 50 cycles), which is at least 3 times higher than LSB areal capacity (6 mAh cm–2) needed to displace LIBs.

Defect Engineering in a Multiple Confined Geometry for Robust Lithium–Sulfur Batteries

“…Carbon-based materials and MOF/COF can enhance the utilization of active sulfur because of their high conductivity and sufficient pores to accommodate sulfur. [8,9] Hollow porous carbon, [10][11][12] carbon nanofibers (CNFs), carbon nanotubes (CNTs), [13,14] modified graphene, [15] and 3D carbon-based materials [16] have been developed to host sulfur with capacities over 1000 mAh g −1 . However, nonpolar fixation of sulfur in carbon materials shows poor immobilization of LiPSs due to weakThe decay of lithium-sulfur (Li-S) batteries is mainly due to the shuttle effect caused by intermediate polysulfides (LiPSs).…”

A Lithium–Sulfur Battery Using Binder-Free Graphene-Coated Aluminum Current Collector

“… 26 , 27 Furthermore, the characteristic electrochemical process involving the electro-deposition/dissolution of soluble species at the cathode side focused the attention on the nature of the current collector. 28 − 30 Aluminum is typically used as the cathode support in lithium batteries for either insertion or sulfur-based electrodes due to its relevant oxidative stability, promoted by the presence of an Al 2 O 3 nanometric passivating layer which remarkably protects the metal surface from further reactions and enhances the safety content of the system. 4 , 31 However, flat and thin metal supports ( e.g ., bare Al current collector) may lead to poor performances due to high overall impedance of the cell and modest ability in allowing the complex multi-step reaction pathway, while thicker porous supports ( e.g ., gas diffusion layer, GDL) can enhance the cell response, reduce the impedance, and actually boost the kinetics of the Li/S process.…”

“…, 450 vs 18 μm). Carbon coatings on thin metallic substrates can actually reduce the thickness, while holding acceptable gravimetric capacity, 30 and may involve the use of a polymeric binder. 35 , 36 However, excessively porous carbon blends with binder can increase again the thickness, thus vanishing the advantages of the metal support in terms of volumetric energy density.…”

“…Despite the fact that GDL can enhance the Li/S cell performances, it may pose concerns linked with the relevant reduction of the volumetric energy density due to the higher thickness compared to bare Al at the cathode side (i.e., 450 vs 18 μm). Carbon coatings on thin metallic substrates can actually reduce the thickness, while holding acceptable gravimetric capacity, 30 and may involve the use of a polymeric binder. 35,36 However, excessively porous carbon blends with binder can increase again the thickness, thus vanishing the advantages of the metal support in terms of volumetric energy density.…”

“…In addition , sulfur holds various bonuses including the large abundance on the earth’s crust, low price, and environmental compatibility . Sulfur (S 8 ) can reversibly operate in a lithium cell through a multi-step electrochemical process, leading to the formation of various polysulfide intermediates, which can be highly soluble (Li 2 S x , 6 ≤ x ≤ 8) or almost insoluble (Li 2 S 2 and Li 2 S) into the electrolyte media according to the overall reaction: S 8 + 16Li + + 16e – ⇄ 8Li 2 S. , Unfortunately, soluble polysulfides can migrate and directly react with the lithium anode or shuttle between the anode and cathode throughout a continuous process without any charge accumulation. , This leads to an efficiency decrease, active material loss, or even to short circuits and cell failure, while insoluble polysulfides can precipitate into the cell and cause resistance increase and capacity fading. , It is worth mentioning that the low electronic and ionic conductivity of elemental sulfur triggered its use as composite mainly with carbons, − metals, − metal oxides, − and conductive polymers. , Furthermore, the characteristic electrochemical process involving the electro-deposition/dissolution of soluble species at the cathode side focused the attention on the nature of the current collector. − Aluminum is typically used as the cathode support in lithium batteries for either insertion or sulfur-based electrodes due to its relevant oxidative stability, promoted by the presence of an Al 2 O 3 nanometric passivating layer which remarkably protects the metal surface from further reactions and enhances the safety content of the system. , However, flat and thin metal supports ( e.g ., bare Al current collector) may lead to poor performances due to high overall impedance of the cell and modest ability in allowing the complex multi-step reaction pathway, while thicker porous supports ( e.g ., gas diffusion layer, GDL) can enhance the cell response, reduce the impedance, and actually boost the kinetics of the Li/S process. − In fact, the rough microporous surface of the GDL (micropore area of 0.3 m 2 g –1 with pore volume of 0.04 cm 3 g –1 ) can host the active material, allow a continuous contact of sulfur with the current collector, and facilitate the electrochemical reaction of dissolved intermediates . As a result, GDL shows a higher specific capacity compared to bare Al (1060 vs 770 mAh g –1 at C/5 using a graphene-based sulfur composite), which is typically used as support in LIBs.…”