Abstract
Lithium-sulfur (Li-S) batteries are considered one of the most promising energy storage technologies, possibly replacing the state-of-the-art lithium-ion (Li-ion) batteries owing to their high energy density, low cost, and eco-compatibility. However, the migration of high-order lithium polysulfides (LiPs) to the lithium surface and the sluggish electrochemical kinetics pose challenges to their commercialization. The interactions between the cathode and LiPs can be enhanced by the doping of the carbon host with heteroatoms, however with relatively low doping content (<10%) in the bulk of the carbon, which can hardly interact with LiPs at the host surface. In this study, the grafting of versatile functional groups with designable properties (e.g., catalytic effects) directly on the surface of the carbon host is proposed to enhance interactions with LiPs. As model systems, benzene groups containing N/O and S/O atoms are vertically grafted and uniformly distributed on the surface of expanded reduced graphene oxide, fostering a stable interface between the cathode and LiPs. The combination of experiments and density functional theory calculations demonstrate improvements in chemical interactions between graphene and LiPs, with an enhancement in the electrochemical kinetics, power, and energy densities.
Highlights
New electrochemical energy storage systems are urgently needed to meet the ever-growing needs of the portable electronics market
We investigated the effect of the nitro group and sulfonate group on the performance of Li-S cells because 1) nitrogen doping has been demonstrated to effectively enhance the interactions between carbon and sulfur upon battery cycling; and 2) sulfonate group has an affinity for LiPs species formed during the electrochemical process
A new pathway to improve the specific energy and power densities of Li/S cells was proposed as an alternative to doping by introducing versatile functional groups on the surface of highly conductive graphene-based substrates
Summary
New electrochemical energy storage systems are urgently needed to meet the ever-growing needs of the portable electronics market. Direct grafting of small and polar functional groups on the surface of the host would be more favorable for building the Li-S cell at the practical energy density level. Understanding the interactions of LiPs with a specific functional group at the molecular level is important for selecting suitable molecules that can improve both the energy density and cycling stability of Li-S batteries. The results demonstrate how the presence of vertical nitro- or sulfonate benzene molecules (≈1 nm high) on the surface of graphene facilitates the binding with LiPs and stabilizes the performance of Li-S cells reaching practical energy and power densities
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