Carbon Inverse Opals As a Sulfur Host for Advanced Lithium–Sulfur Batteries
Authors: McNulty David , Trabesinger Sigita 
Published: February 2020

Lithium Sulfur (Li–S) batteries are one of the most promising “beyond-lithium-ion” rechargeable battery systems in terms of both cost and specific energy density. (1, 2) Sulfur is one of the most abundant elements in the Earth’s crust and offers a high theoretical specific charge of 1672 mAh/g. (3) Furthermore, Li–S batteries can deliver a practical specific energy density of 400-600 Wh/kg, which is more than double the values offered by state-of-the-art lithium-ion batteries. (4, 5) However, issues related to the inherent insulating nature of S and the polysulfide shuttle continue to hinder the widespread commercialization of Li–S batteries. Consequently, there has been a significant research effort to identify S hosts that can assist in mitigating these limitations. Porous carbon materials have attracted a great deal of attention due to their excellent conductivity, high surface area and light weight. (6) Activated carbons and hollow carbon spheres (HCSs) have been shown to significantly improve the electrochemical performance of Li–S batteries in terms of specific capacity and its retention. (7-10) Inverse opal (IO) structured electrodes offer many potential benefits as a S host material. The spherical voids of an inverse opal can be filled with S and ensure good access of the electrolyte to the electrode surface. The carbon IOs may act as a highly ordered, three-dimensional, conductive scaffold to reduce lithium polysulfide dissolution into the electrolyte during cycling, thus reducing the effects of the detrimental polysulfide shuttle. Additionally, during discharge S is reduced to form Li2S and with prolonged cycling this process leads to structural instability of electrodes due to the large volume expansion (~80%) caused by S conversion into Li2S. The spherical pores of a carbon IO scaffold may constrain the volume expansion due to the formation of Li2S and therefore increase capacity retention.

IOs and HCSs are typically synthesised using silica nanospheres, which require treatment in hydrofluoric acid or long-term exposure to highly basic solutions to etch away the hard template. (11, 12) In this work we detail the facile synthesis of carbon IO structured samples using polystyrene nanospheres, which are easily removed via thermal treatment, and their application as a S host material for Li–S batteries. We present a structural characterization of our carbon IO samples via analysis of Raman spectra, Fourier-transform infrared spectra and X-ray diffraction patterns. The electrochemical performance of the carbon IO S-hosts is evaluated via cyclic voltammetry, rate capability testing and long-term galvanostatic cycling tests. We demonstrate that our S infilled carbon IO cathodes are capable of delivering high specific charges with stable capacity retention, achieving a reversible capacity of ~ 750 mAh/g after the 100 cycles at a C/5 rate. The morphology of the carbon IOs after cycling will also be shown via ex-situ scanning electron microscopy. We demonstrate that by preparing a highly ordered, conductive, three dimensionally interconnected network in the form of a carbon IO and then infilling this porous scaffold with S, we can achieve specific charge values which are greater than standard S/C composite slurry electrodes.

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