Wood-Based Cathodes for High-Performance Lithium-Sulfur Batteries
Authors: Sapkota Nawraj , Sabet Morteza , Zheng Ting , Chiluwal Shailendra , Clemons Craig M. , Rao Apparao M. , Pilla Srikanth 
Published: November 2022

Lithium-ion batteries (LIBs) are dominant electrochemical energy storage systems in rapidly growing electronic markets. However, the conventional anode and cathode materials have limited specific capacity and quickly approach their theoretical limit. Hence, alternate battery chemistry beyond LIBs is required. Lithium-sulfur (Li-S) batteries are noteworthy energy storage systems due to their high theoretical capacity (1672 mAh/g), high theoretical energy density (2600 Wh/kg), and low cost of raw materials. Unfortunately, commercialization of Li-S batteries is hindered by the low electronic conductivity of sulfur and intermediate polysulfides, polysulfide dissolution and shuttle effect, volume expansion of elemental sulfur during the charge/discharge process, and lithium dendrite growth at the anode side. To address these challenges, various strategies such as modification of sulfur hosts, electrolyte optimization, and separator modifications have been employed. Initially, researchers encapsulated sulfur in various carbonaceous materials to mitigate the polysulfide shuttle. Later, a new active material, called sulfurized polyacrylonitrile (SPAN), was developed to mitigate the polysulfide shuttle and extend the life cycle of Li-S batteries. The main advantage of the SPAN over elemental sulfur is that it is more effective in mitigating polysulfide dissolution and shuttling due to the presence of the C−S bonds, consequently preventing the loss of active material. On the other hand, due to the limited number of binding sites in the SPAN structure, the sulfur content of SPAN materials is limited to 40 – 44 wt.%. To compete with the state-of-the-art LIBs, the Li-S batteries should provide an areal capacity of >5 mAh/cm2, corresponding to a sulfur loading of >5 mg/cm2. Realizing such a high S loading using 2D planar current collectors (e.g., aluminum foil) without compromising specific capacity and cycling stability is practically unrealistic. Using synthetic materials (e.g., graphene foam, graphene aerogel, metal-organic framework), three-dimensional (3D) electrode architectures with high SPAN loading and efficient electron and ion conductive pathways have been constructed. However, preparing these 3D electrodes mainly involves synthetic raw materials, harsh chemicals, and costly fabrication processes. Porous carbon frameworks derived from biomass, which are plant-based materials derived from nature, are alternative candidates. Biomass structures with interconnected channels over multiple length scales can provide a natural 3D template for electrochemical energy storage systems.
In this study, we took advantage of the inherent porous structure of natural wood and engineered 3D frameworks using a delignification/low-temperature pyrolysis approach. A block of Balsa wood was sliced into thin sheets parallel to the tree growth direction. To facilitate the permeation of active material into the cellular structure, we first subjected the wood to a facile chemical treatment to remove lignin and hemicellulose. The delignified wood (DW) was then converted to an ultra-lightweight carbonized DW (CDW) scaffold using a low-temperature pyrolysis approach. To prepare CDW scaffolds, the DW sheets were pyrolyzed at 500, 600, 700, and 1000 °C. Continuous electron transport through the CDW conductive network and ion transport along the highly porous cellular body makes intrinsic and effective charge transfer through the CDW scaffold. SPAN active material was synthesized by pyrolyzing a mixture of elemental S and polyacrylonitrile under nitrogen flow and then using it to prepare an aqueous slurry of SPAN, CMC, and carbon black with a mass ratio of 70:15:15. To prepare the cathodes, CDWs were cut into squares with an area of 1.00 ± 0.05 cm2, dipped into a beaker of SPAN slurry, and kept under a vacuum. SPAN-impregnated free-standing electrodes (SPAN@CDW) were dried and utilized for electrochemical studies. The unique design of the SPAN@CDW electrode, wherein the interconnected pathways run in three-dimension, ensured effective electron and ion transport within the cathode. Thus, SPAN@CDW cathodes exhibited an excellent rate capability, with discharge capacities >1000 mAh/gs at 1C (1672 mA/g). The electrode with CDW pyrolyzed at 600 °C, and S loading of ~2 mg/cm2 exhibited a high specific capacity of ~1350 mAh/gs after 500 cycles at 0.1C, indicating good cycling stability. We also demonstrated the fabrication of SPAN@CDW electrodes with a high S loading of ~12.3 mg/cm2, which could deliver a high areal capacity of ~15.1 mAh/cm2 at 0.1C.