Chemical synthesis of dendritic interlaced network graphene quantum dots/sulfur composite for lithium-sulfur batteries

Highlights

• GQDs with polar groups can absorb the polysulfides proved by absorption tests and DFT calculations.

• Well-dispersed GQDs serves as nucleation sites and dendritic interlaced sulfur framework was obtained.

• Dendritic network structure of GQDs/S is beneficial to improve the utilization of sulfur and accelerate the ion transfer.

Abstract

In this work, graphene quantum dots/sulfur (GQDs/S) composite was prepared as cathode material for lithium sulfur (Li-S) batteries by a feasible chemical method. Multiple polar functional groups in GQDs can provide more active absorption sites to significantly strengthen the chemical absorption with polysulfides, suppressing the ‘shuttle effect’. In addition, the highly dispersed GQDs particles in the solution served as nucleation sites, the dissolved sulfur are reprecipitated around the GQDs, forming a dendritic interlaced network structure to prevent the aggregation of the sulfur, and further enhance the utilization of active substances. A high initial discharge capacity of 1125 mA h g⁻¹ at 0.1C has been obtained in the GQDs/S cell, which is much higher than the initial discharge capacity of 658 mA h g⁻¹ of GQDs/S_R prepared by physical melting method. The superior performance of GQDs/S is attributed to the unique dendritic interlaced framework and multiple polar functional groups. The strong chemical effects between polar functional groups and polysulfides have been confirmed by the absorption experiment and DFT calculation. This work develops a new strategy in cathode material design for Li-S batteries.

Introduction

Lithium sulfur batteries with theoretical higher specific capacity (1675 mA h g⁻¹) and energy density (2600 Wh kg⁻¹) are one of the most promising alternatives for the next generation secondary batteries [1,2]. However, the severe “shuttle effect” results from the dissolved long-chain polysulfides (S_x²⁻, 4 ≤ x ≤ 8), leading to the irreversible loss of active material, poor coulombic efficiency and cycling performance [3,4]. To solve this issue, many strategies have been proposed, such as optimizing the cathode design [[5], [6], [7]], modifying the separator [8,9], and developing the electrolyte [10,11]. Among these possible approaches, carbon-based materials [[12], [13], [14], [15]] as sulfur host play a significant role in immobilizing polysulfides due to the high conductivity, excellent mechanical stability, large surface area and adjustable pore structure. Nevertheless, the relatively weak interaction between nonpolar carbon structure and polar polysulfides can result in the poor electrochemical performance [16]. As previous literatures [17,18], the heteroatoms, such as O, N, B, in the carbon skeleton can strengthen chemical anchoring to polysulfides because the lone electron pair on the exotic atoms exits electrostatic attraction. For example, Shao [19]. et al. synthesized mesoporous Co, N-doped carbon nanosheets, which possessed high specific surface area and affluent defects. The results of DFT calculation and in-situ Raman spectra demonstrate that the Co-N-C catalytic sites as the effective host not only can confine the polysulfides, but also facilitate the conversation from long-chain polysulfides to Li₂S₂/Li₂S, increasing the specific capacity and promoting the reaction kinetics. A high initial specific capacity of 1171 mA h g⁻¹ was derived and maintained at 907.5 mA h g⁻¹ after 100 cycles at 0.2C with coulombic efficiency around 100%.

Graphene quantum dots (GQDs), a new kind of zero-dimensional honeycomb sp² carbon nanomaterials, possess the favorable electric conductivity, high specific area, excellent solubility and quantum confinement effect, which have been used as electrode material or additive in the lithium-ion batteries [20,21], supercapacitors [22,23], and fuel cells [24,25].The oxygen-containing groups can be easily introduced into the edge and surface of GQDs through top-down or bottom-up strategies, which can be beneficial to absorb the polysulfides in Li-S batteries. In 2016, Park [26] et al. synthesized GQDs-S/carbon black (CB) cathode material for Li-S batteries. The GQDs ensure the structural integrity via charge interaction with S and CB by oxygen-rich functional groups. The C-S bonds generated during cycling due to the oxygen-rich groups at the edge of GQDs, which reduce the loss of polysulfides.

In this work, GQDs with oxygen-containing functional groups was prepared as cathode material for Li-S batteries by a top-down method. Flake graphite as the starting material was treated by strong acid, the GQDs with abundant oxygen-containing functional groups and a few pyrrole-N was obtained finally. GQDs/S composite was synthesized by a chemical method, where the GQDs served as nucleation sites and surrounded by the reprecipitated sulfur, forming a dendritic interlaced network structure. The branch-liked construction prevents the aggregation of sulfur and improves the utilization of sulfur, further improving the electrochemical performance of Li-S batteries. During the cycling processes, the polar functional groups of GQDs can help to preserve dissolved polysulfides by chemical effect, which can effectively relieve shuttle effect. The results have been confirmed by Ultraviolet-visible (UV-vis) absorption spectra and density functional theory (DFT) calculations.