Synthesis of nickel-porous carbon with controllable morphology and its electrochemical performance for lithium sulfur batteries

Highlights

• A series of nickel porous carbon with controllable morphology are synthesized.

• Ni-PC-160 with honeycomb structure forms highly porous 3D interconnected structure.

• The honeycomb structure provides the shot pathways to fast transportation of Li+.

• Ni nanoparticles accelerate the polysulfides conversion by catalytic effect.

Abstract

A series of nickel porous carbon (Ni-PC-x, x represents solvothermal temperature) materials with controllable morphology, which has the different catalytic effect of the metallic Ni catalyst derived from Ni-MOF and Li+ transfer efficiency, are designed and synthesized through the simple solvothermal method and pyrolysis. Among the composites, the S/Ni-PC-160 cathode exhibits excellent long-cycle stability (an average capacity fading rate of only 0.14% per cycle for 300 cycles at 1C), and wonderful rate performance with a capacity of 815.5 mAh·g−1 at 1C. The improved performance of S/Ni-PC-160 is attributed to the catalysis derived from Ni nanoparticles and rapid Li+ diffusion from a 3D honeycomb structure. This work provides an important reference value for the potential application of the relationship between morphology and electrochemical performance in LiS batteries.

Introduction

With the depletion of fossil fuels and environmental degradation, it is urgent to develop replaceable and high-efficiency energy devices. Among various energy conversion devices, lithium‑sulfur batteries (LSB) are extensively explored and stimulate widespread interest due to their high theoretical specific capacity (1675 mAh·g⁻¹) and theoretical specific energy (2500 Wh·kg⁻¹), low cost, and environmentally friendly [[1], [2], [3]]. Despite these merits, this system confronts several challenges, caused by: (1) the poor ionic and electronic conductivity of the elemental sulfur and its final reduction product (Li₂S₂ and Li₂S) leads to a low sulfur utilization and limited rate capability [4,5]; (2) there is about 80% volume expansion during the discharge process [6]; (3) the shuttle effect of the soluble intermediate product Li₂S_n (4 ≤ n ≤ 8) results in rapid capacity decay and low coulombic efficiency [4,5,[7], [8], [9]].

Tremendous efforts have been made to solve the abovementioned problems. Up to now, porous carbon materials, considering their excellent electrical conductivity, are still regarded as the most efficient host materials [[10], [11], [12]]. Wang et al. [10] had successfully prepared a nano-sized microporous carbon with a sulfur content of 74.2%, showing a remarkable initial capacity of 865 mAh·g⁻¹ at 0.2 A·g⁻¹ and maintained at 689 mAh·g⁻¹ over 100 cycles. Unfortunately, the weak interactions between the nonpolar sp² carbons and the polar polysulfides cannot restrain the soluble polysulfides effectively, resulting in poor electrochemical performances [4,[13], [14], [15]]. Therefore, polar materials have been introduced to porous carbon to improve the interactions between carbons and polysulfides and trap the polar sulfur species efficiently [[16], [17], [18], [19], [20]].

In recent years, metal-organic framework materials (MOFs) have been used as precursors or templates for preparing metals, metal oxides and carbon-based materials by pyrolysis [[21], [22], [23], [24], [25], [26]]. This is mainly because: compared with other templates, (1) MOFs as templates can prepare high-quality materials, which have more uniform size distribution, better dispersibility, exceptional specific surface area, and can maintain the porosity and morphology [21]; (2) The pore/particle size of MOF and MOF-derived materials can be adjusted by changing the synthesis and pyrolysis conditions [27,28]; (3) The existence of MOFs-derived metal/metal compound species can promote the fundamental interactions between polysulfides and the host framework during the electrochemical process, thus providing a unique opportunity to develop a novel kind of highly tailorable carbon materials with polar sites [[29], [30], [31]]. Due to these unique advantages, MOFs can be used as sacrificial templates to prepare various micro/nanostructures through different processing methods. Indeed, some composites derived from MOFs structures have demonstrated impressive lithium storage performance as electrode materials.

However, it is difficult for us to predict the intermolecular forces in MOFs. Therefore, it is necessary to constantly try and explore the synthesis conditions to make the bound organic ligands in the metal work in an expected way. Tiny changes in the reaction temperature, pH value, reactant concentration, or other factors may cause the change of coordination mode between the metal center and organic ligand, resulting in a new skeleton structure, or affecting the crystal quality and yield. Therefore, how to directionally build functional MOFs is one of the current research hotspots.

Herein, a series of Ni-MOFs with different crystal phases and morphologies are designed and synthesized under different solvothermal temperature conditions. Then, a novel highly conductive nickel embedded dual porous carbon is rationally designed and constructed by carbonization treatment (denoted as Ni-PC). The porous carbon can not only be employed as a carbon source to improve the conductivity of composite material but also as a template to encapsulate sulfur. The polar Ni nanoparticles play an important role in adsorbing polysulfides and accelerating their electrochemical reaction by supplying some active sites. With these substantial merits, the as-fabricated S/Ni-PC-160 cathodes deliver a high initial capacity of 958.3 mAh·g⁻¹ at 0.2C, impressive rate performance with 815.5 mAh·g⁻¹ at 1.0C, and outstanding cycling stability with retained 59% of its capacity after 200 cycles at 0.5C.