Constructing Double Heterojunctions on 1T/2H-MoS2@Co3S4 Electrocatalysts for Regulating Li2O2 Formation in Lithium-Oxygen Batteries

Abstract

Co3S4 electrocatalysts with mixed valences of Co ions and excellent structural stability possess favorable oxygen evolution reaction (OER) activity, yet challenges remain in fabricating rechargeable lithium-oxygen batteries (LOBs) due to their poor OER performance, resulting from poor electrical conductivity and overly strong intermediate adsorption. In this work, fancy double heterojunctions on 1T/2H-MoS2@Co3S4 (1T/2H-MCS) were constructed derived from the charge donation from Co to Mo ions, thus inducing the phase transformation of MoS2 from 2H to 1T. The unique features of these double heterojunctions endow the 1T/2H-MCS with complementary catalysis during charging and discharging processes. It is worth noting that 1T-MoS2@Co3S4 could provide fast Co–S–Mo electron transport channels to promote ORR/OER kinetics, and 2H-MoS2@Co3S4 contributed to enabling moderate eg orbital occupancy when adsorbed with oxygen-containing intermediates. On the basis, the Li2O2 nucleation route was changed to solution and surface dual pathways, improving reversible deposition and decomposition kinetics. As a result, 1T/2H-MCS cathodes exhibit an improved electrocatalytic performance compared with those of Co3S4 and MoS2 cathodes. This innovative heterostructure design provides a reliable strategy to construct efficient transition metal sulfide catalysts by improving electrical conductivity and modulating adsorption toward oxygenated intermediates for LOBs.

1. Introduction

Aprotic lithium-oxygen batteries (LOBs) are one of the most promising candidates to replace lithium-ion batteries (LIBs) for electric vehicles, robots, and large-scale grids, because of their ultrahigh theoretical energy density (3,500 W h kg−1). Generally, the reaction mechanisms during cycling are based on Li+ + O2 ↔ Li2O2 (E0 = 2.96 V vs. Li/Li+), which involves oxygen reduction reaction (ORR) for discharging and oxygen evolution reaction (OER) for charging [1, 2]. However, such a powerful system is greatly hindered for further applications by poor rate performance, limited cyclic life, and high overpotentials. These problems mainly result from their sluggish redox kinetics, triggering the accumulation of inactive discharge product (Li2O2) and even insulated side reaction product (LiOH and Li2CO3) on the cathode [3]. Considering that the cathodes should exposure the triple-phase contact interface regions (cathode/electrolyte/O2) and enable discharge product storage, employing ideal cathode catalysts for fast electron transfer rate and reversible discharge product formation would greatly enhance the electrocatalytic performance and accelerate the commercial applications of LOBs.

Noble metals and their alloys exhibit high-efficiency catalytic properties in LOBs, while the high cost and scarcity on the Earth’s crust seriously restrict their large-scale applications [4, 5]. Transition metal chalcogenides (TMCs) have been attracting extensive attention worldwide on account of their low cost, excellent chemically active electrocatalytic property, and diversity of structure [6]. Among them, Co3S4 possesses a typical spinel structure with a mixture valence state of Co2+ and Co3+, and this unique arrangement endows Co3S4 eye-catching recognition as the OER electrocatalyst, proved by density functional theory (DFT) calculations [7, 8]. Liu et al. [9] synthesized ultrathin Co3S4 nanosheets via a sonicated treatment, delivering high OER activities and reduced overpotentials in overall water splitting reactions. Wang et al. [10] present superior manner in OER performance, which can be realized by inducing sulfur vacancies in Co3S4. Furthermore, Yang et al. [11] demonstrate that the adsorption states of oxygen-containing intermediates could be tailored by Ni incorporation to largely influence the electrocatalytic activity.