Long-Life Lithium-Sulfur Batteries with a Bifunctional Cathode Substrate Configured with Boron Carbide Nanowires

Abstract

Developing high-energy-density lithium-sulfur (Li-S) batteries relies on the design of electrode substrates that can host a high sulfur loading and still attain high polysulfide-retention capability. Recently, the application of transition-metal oxides, sulfides, carbides, and nitrides as sulfur cathode substrates has witnessed greatly improved polysulfide-retention capability by introducing Lewis acid–base interaction, forming strong chemical bonding, or exerting a catalytic effect on the diffusing polysulfides. Among them, transition-metal carbides such as TiC, Ti2C, Mo2C, and WC have been applied to sulfur cathode configurations and proved to be effective in improving the electrochemical functionality of Li-S batteries. However, most of them suffer from a critical drawback of low sulfur content (< 56 wt.%) and low sulfur loading (< 2 mg cm-2). Besides, most carbides have much higher density than sulfur, which also limits the sulfur content achievable. It is thus essential to explore lightweight, yet efficient, materials with a binder/current collector-free fabrication strategy to improve polysulfide-retention ability and realize acceptable electrochemical performance with Li-S batteries. Boron carbide (B4C), due to its low density (~ 2.5 g cm-3), good conductivity (1.25 ~ 3.33 S cm-1), and superior catalytic effect, is a promising candidate for battery applications. Herein, for the first time, the lightweight B4C substrate is successfully applied in the Li-S system by a facile catalyst-assisted fabrication process and in-situ grown on carbon-nanofibers (CNF) to form a free-standing bifunctional cathode substrate (B4C@CNF). The B4C nanowires acting as chemical-anchoring centers provide strong polysulfide adsorptivity, as validated by experimental adsorption test and first-principle calculations. Meanwhile, the catalytic effect of B4C also accelerates the redox kinetics of polysulfide conversion, contributing to improved active-material utilization and enhanced rate capability. As a result, a remarkable capacity retention of 80% after 500 cycles, a low capacity-fading rate of 0.04% per cycle, and good cyclability at varying cycling rates (C/20 – 4C) are accomplished with the cells employing B4C@CNF as a cathode substrate for sulfur. Moreover, the B4C@CNF substrate enables the cathode to achieve both high sulfur content (70 wt.%) and sulfur loading (10.3 mg cm-2) in coin cells, delivering a superb areal capacity of 9 mA h cm-2. Additionally, the B4C@CNF substrate is further applied to pouch cells with a high sulfur mass (40 – 200 mg per cathode), delivering high discharge capacities (40 – 160 mA h) with good cyclability (50 – 100 cycles). This work presents a strategy to construct high-loading cathodes for practically viable, high-performance Li-S batteries. Figure 1