Biomass-derived tube-like nitrogen and oxygen dual-doped porous carbon in the sulfur cathode for lithium sulfur battery

Highlights

• Tube-like porous biomass waste derived carbon has enough space to store sulfur.

• A high sulfur content of 82.5 wt%.

• Nitrogen and oxygen dual-doped ameliorating the terrible polysulfide shuttle effect.

Abstract

The high theoretical capacity around 1672 mA h g⁻¹ and the energy density of 2567 Wh kg⁻¹and addition advantages of the low cost and environmental benignity, make lithium-sulfur batteries (LiSBs) next generation battery. However, due to restrictions on the low conductivity of sulfur and soluble polysulfides during discharge, electrochemical performance of the batteries is deteriorated. Heteroatom doped carbon as an effective strategy can improve the poor electrochemical performance caused by polysulfide shuttle. Hence, the nitrogen and oxygen dual-doped tube-like porous biomass-derived carbon from the fluffy catkins is successfully fabricated for the LiSBs. After carbonization and activation, the tube-like and unique mesoporous structure are obtained and maintained, and a high sulfur content of 82.5 wt% is achieved in tube-like activated carbon material/sulfur composites (TACM/S). The N, O dual-doping introduces more active sites and strong chemical adsorption to anchor the polysulfides, therefore remarkably ameliorating the intractable polysulfide shuttle effect and enhancing the utilization of sulfur. The electrochemical results show that the LiSBs with the TACM/S cathode exhibit high initial discharge capacities as high as 1041.7 mAh g⁻¹ at 0.1 C, and outstanding capacity retention of about 77.5% after 500 cycles at 0.5 C with an ultralow capacity fading rate of 0.043% per cycle.

Introduction

In recent years, the unprecedented growth in increasing for requirements of electrical vehicles and large-scale smart grids, lithium-sulfur batteries (LiSBs) may be the next generation of the most potential candidates due to multielectron conversion electrochemistry between elemental sulfur and lithium endowing the with the high theoretical capacity of 1672 mAh g⁻¹ and the energy density of 2567 Wh kg⁻¹, which is six fold higher than those of conventional lithium-ion batteries [[1], [2], [3]]. In addition, the low cost, nontoxicity, environmental benignity and abundance in nature of the sulfur also attract good graces of the scholars. Nonetheless, LiSBs still is up against several intractable issues, containing inherent insolubility and insulativity of sulfur and discharge product (Li₂S₂ and Li₂S), formation of lithium dendrites during the cycle process, 80% volume expansion/contraction of active materials during discharge/charge process and shuttle effect by dissolution of the polysulfide intermediates, which caused the loss of the active materials, short circuit, rapid capacity fading and high self-discharge rate [[4], [5], [6], [7]]. Thus, it is pivotal to ameliorate above-mentioned dilemmas to the development of high-performance LiSBs. An effective strategy is to build a porous barrier, which can not only restrain the diffusion of the polysulfides, but also possess sufficient space to withstand volume changes of the active materials [8,9]. In recent years, the carbon matrixes, containing graphene [10,11], carbon nanotubes (CNTs) [12,13], carbon nanofibers (CNFs) [14,15] and micro/mesoporous carbon [16,17], have been extensively used as a sulfur host. On the one hand, the carbon skeleton with high conductivity efficiently transports electrons for the redox reaction. On the other hand, the abundant porous structure of carbon matrixes paves the way for lithium ion transporting [18,19]. However, the exorbitant cost of processes hinders commercialization application of such carbon materials in lithium sulfur batteries.

Recently, biomass-based porous carbon materials, arriving from animals and plants, have attracted the much attention due to its sustainability, environment-friendly, low cost and abundant in nature, especially, biomass waste material [20,21]. It is worth to be noted that most biomass derived carbon need to be activated to create more mesoporous or microporous structures. In addition, heteroatom-doped (O, N) carbon materials, affording an active site to form strong chemical bonds between carbon and polysulfides to inhibit the dissolution and diffusion of polysulfides due to Lewis acid-base interaction, will be formed during the heat treatment process [22]. Gu et al. [23] reported porous bamboo biochar via carbonization and subsequent activation. The bamboo biochar/sulfur nanocomposite with 50 wt% sulfur content exhibited a high initial capacity of 1295 mAh g⁻¹ at a low discharge rate of 0.1 C and showed cycling stability with 0.285% capacity fading per cycle at 0.5 C. Zhao et al. [24] designed and fabricated waste coffee grounds based porous (micropores and mesopores) carbon with doped N/O heteroatoms, which exhibited a reversible capacity of 613 mAh g⁻¹ after the 100th cycle at 0.2 C as well as a discharge capacity of 331 mAh g⁻¹ at 1 C after 10 cycles via the immobilization of the polysulfides through strong chemical binding.

Herein, a kind of biomass waste, fluffy catkins, which not only was prone to respiratory diseases for human, but also raised safety-induced hazards, was utilized as a carbon source for LiSBs. The tube-like structure of the carbon material exhibits interconnected framework and is favorable for ion diffusion. Simultaneously, a tube-like activated carbon material (TACM) also possessed a large specific surface area to provide more redox reactive sites. In addition, the abundant nitrogen and oxygen of TACM affords plenty of anchor points for polysulfide, and the electrochemical result proves that outstanding cycle stability and high coulombic efficiency. Up to 82.5% of sulfur was loaded into the tube of carbon materials, the high discharge capacity with 540 mAh g⁻¹ and capacity retention with 77.5% were still achieved at 0.5 C after 500 cycles.