TCNQ-derived N/S dual-doped carbon cube electrocatalysts with built-in CoS₂ nanoparticles for high-rate lithium-oxygen batteries

Highlights

• CoS₂@NS-C cubes were synthesized using TCNQ coordination polymer and CoI_2.

• CoS₂@NS-C cube is composed of N/S dual-doped carbon cube with embedded CoS₂.

• CoS₂ nanoparticles (D ≈ 5 nm) are uniformly distributed in N/S dual-doped carbon sheath.

• CoS₂@NS-C cubes provide high electronic conductivity and ORR/OER activity.

• CoS₂@NS-C cubes are achieved to high-rate performance and quick-charging capability.

Abstract

Nitrogen/Sulfur dual-doped carbon cubes with built-in CoS₂ nanoparticles (CoS₂@NS-C cubes) as an electrocatalyst are developed to improve the high-rate and long-term performance of lithium-oxygen batteries. The CoS₂@NS-C cubes were synthesized by coprecipitation between a cobalt salt and 7,7,8,8-tetracyanoquinodimethane (TCNQ) and subsequent thermal sulfidation. TCNQ acted as the coordination polymer for a cube shape and as the nitrogen source for nitrogen-dopants. The CoS₂@NS-C cubes comprise uniformly distributed CoS₂ nanoparticles in a nitrogen/sulfur dual-doped carbon matrix, providing fast catalytic activity for oxygen reduction/evolution reactions. The CoS₂@NS-C cubes exhibited a high-rate performance of 5000 mA g⁻¹ as well as long-term performance of > 100 cycles with a current density of 2000 mA g⁻¹. These results confirmed the enhanced performance of the lithium-oxygen battery, with fast charging capability.

Introduction

One of the most promising candidates for next-generation energy storage systems, lithium-oxygen batteries (LOBs) have a high theoretical energy density (~3500 Wh kg⁻¹). Ideal LOBs operate with the reversible formation and decomposition of Li₂O₂ [1], [2]. The electrochemical cell net reaction can be expressed as 2Li + O₂ ↔ Li₂O₂ (E^o = 2.96 V vs. Li/Li⁺), with the discharge process corresponding to the forward oxygen reduction reaction (ORR) and charge process corresponding to the reverse oxygen evolution reaction (OER) [3], [4]. However, LOBs have noticeable problems such as electrolyte decomposition, low cycle stability, and low energy efficiency due to slow reaction kinetics [5], [6]. To solve these issues, it is necessary to accelerate reaction kinetics by utilizing efficient catalysts within the cathodes [7].

Carbon materials are widely studied in regard to their application in LOBs due to high electrical conductivity, low cost, their porosity properties, and large energy density storage capacity [8], [9], [10]. Heteroatom-doped carbon materials can show promising catalytic performance for electrochemical reactions [11], [12], [13]. However, carbon catalysts generally have poor OER activity and side reactions. To improve the catalytic performance, the structural design of carbon-based hybrids including transition metal compounds have been extensively studied [14], [15], [16], [17], [18], [19]. Carbon-based hybrid structures not only modify the electronic composition and structural properties of the carbon material, but also stabilize the metal species [17], [18]. More importantly, the interaction of metal species, heteroatom doping, and the carbon lattice alters the charge distribution on the carbon surface through enhanced electron transfer effects to create active sites. As a result, it alters O₂ adsorption and consequently promotes both the ORR and OER [18], [19].

For the structural design of carbon-based hybrids, the use of coordination polymers (CPs) are a useful method because CPs are formed by the coordination of organic ligands and metal ions [20], [21], [22], [23]. 7,7,8,8-Tetracyanoquinodimethane (TCNQ) acts as the multi-redox active ligand, which can be readily formed into a TCNQ*^– radical by electrochemical or chemical reduction methods [24], [25], [26]. Thus, TCNQ acts as an electron donor, which reacts with a metal ion to form a charge-transfer metal complex participating in form of close π –π bond. As a result, TCNQ transition metal complexes [M(II)(TCNQ)bpy] (M = Zn, Fe, Cd, Co, and Mn, etc., bpy = 4,40 -bipyridyl) form 3-dimenesional (D) network structures.

Herein, we report nitrogen/sulfur dual-doped carbon cubes with built-in CoS₂ nanoparticles (CoS₂@NS-C cubes) as an electrocatalyst for high-rate performance in LOBs. The CoS₂@NS-C cubes were synthesized by a TCNQ redox process with CoI₂ and subsequent thermal sulfidation. TCNQ has a high nitrogen content (27.5 wt%), which makes it an advantageous organic ligand for producing nitrogen-rich carbon-based hybrids [27]. The CoS₂@NS-C cubes have the CoS₂ nanoparticles uniformly distributed in the nitrogen/sulfur dual-doped carbon matrix, which can effectively enhance catalytic activity for ORR/OER. The CoS₂@NS-C cubes show high-rate performance (5000 mA g⁻¹) as well as long-term performance (>100 cycles with a current density of 2000 mA g⁻¹). Furthermore, to our best knowledge, this is the first paper on the performance of the LOBs with fast charging capability (a 30 min charging time after discharging capacity of 1000 mA h g⁻¹).