Sulfur-rich CuS_1+x cathode for lithium batteries

Highlights

• Sulfur-rich CuS_1+x (0 ≤ x ≤ 0.61) was successfully prepared by spray pyrolysis.

• CuS1.58 exhibits a high discharge capacity of about 900 mAh g⁻¹ at 0.1C.

• CuS_1.58 WBM with 5 wt% carbon exhibits discharge capacity of 1700 mAh g⁻¹ at 0.1C.

Abstract

Copper sulfide is one of the promising cathode materials for the next-generation secondary batteries due to its high electrical conductivity and relatively high capacity. The outstanding cycling performance of sulfur-rich CuS_1+x (0.58 ≤ x ≤ 0.61) cathode material obtained by one-step spray pyrolysis was demonstrated in this work. A simple approach of increasing the sulfur content up to CuS_1.58 achieved ~900 mAh g⁻¹ for 20 cycles at 0.1C. Furthermore, a small addition of carbon (5 wt%) by wet ball milling extremely enhanced discharge capacity to 1200 mAh g⁻¹.

Introduction

Lithium-ion battery (LIB) is the most realistic energy storage technology to satisfy the needs of today’s society [1], [2]. Although nowadays, the research and efforts to build the most applicable system of LIBs to use in electric vehicles (EVs) are intense, they yet do not have enough energy and life to meet the requirements. As an alternative, metal sulfides are recently getting more attention due to their electrochemical superiority from the capacity and rate capability points of view. Additionally, metal sulfides are abundant in the Earth’s crust (for instance, chalcocite); thus, they and their composites are cheap, which means their application can be utilized widely. Among transition metal sulfides such as copper sulfide (CuS) has potential for consideration as a promising cathode material due to its high conductivity (10⁻³ S cm⁻¹) and a high theoretical capacity of 560 mAh g⁻¹ with flat charge/discharge plateaus [3], [4]. However, the issue hindering its commercial realization lies in the severe capacity decay due to the dissolution of lithium polysulfides (LPS) in the electrolyte [5]. An ineffective discharge reaction activity between Cu and Li₂S has been believed as one of the main problems causing cell deterioration. The nanostructuring approaches, surface modification of CuS, carbon encapsulation, and interlayer insertion have been recently demonstrated to suppress LPS dissolution [6], [7], [8]. In this work, we aim to study the CuS electrode via increasing its sulfur content to enhance its electrochemical properties.

As was mentioned, the high electronic conductivity is one of the important features of the CuS electrode. This property allows the rechargeable battery to exhibit high rate capability or excellent cyclability without conductive additives. We investigated the sulfur-rich CuS_1+x cathodes without either complicated synthesis or surface modification methods by mostly relying on the material’s high conductivity. Since we aim to increase the S content in CuS_1+x cathodes, we can also highlight that in terms of theoretical specific capacity and energy density, sulfur is one of the champion cathode materials that has been intensively studied. It has a theoretical specific capacity of 1675 mAh g⁻¹ and a high energy density of around 2600 Wh kg⁻¹ on a weight basis, assuming a complete reaction with lithium [9], [10], [11]. Even though Li-S batteries are being under substantial research, commercialization still impedes due to its low conductivity, short cycling life, and high self-discharge rate. The main challenges faced by Li-S batteries that deteriorate their electrochemical performances lay in the vast volume change coming from conversion reactions, the insulating nature of sulfur, the shuttle reactions related to the dissolved LPS (Li₂S_x, x  > 2) in the electrolyte. As an alternative to widely applied methods on the Li-S battery improvements [12], [13], [14], [15], we are investigating for the first time the enhancement of the electrochemical properties of CuS_1+x electrode by increasing its sulfur content by the one-step preparation method of spray pyrolysis (SP).