(Invited) Microstructural Design Strategies of Sulfur Electrodes for High Specific Energy Lithium/Sulfur Cells
Authors: Hwa Yoon 
Published: July 2021

Lithium (Li) ion batteries have been regarded as major contributors to the energy solution and they have been intensively developed in terms of performance, cost and lifetime to meet the demands of emerging applications such as electric vehicle and energy storage systems for power grids. One of major goals of Li rechargeable battery development is to enhance its energy storage capability while reducing its weight or volume. In this regard, a lithium/sulfur (Li/S) cell has attracted great attentions of battery researchers because the Li/S cell offers large theoretical specific energy of 2600 Wh/kg that is ~ 5 times larger than that of conventional Li ion cells (500~600 Wh/kg). With consideration of the current state-of-the art of the Li ion cells that delivers specific energy of 150~250 Wh/kg, the Li/S cell has obvious potential to compete with the conventional Li ion cells for applications that require high specific energy storage capability.
High specific energy of the Li/S cell is mainly attributed to high theoretical capacity of S electrode (Li2S: 1675 mAh/gS) where two Li ions can be stored per one S atom. To achieve near theoretical capacity of the S electrode, several challenges associated with the S electrode need to be overcome, that are (1) poor electrical conductivity of S (and Li2S) (2) lithium polysulfide (Li-PS) shuttle and irreversible deposition of Li2S onto Li electrode[1] (3) structural changes of the S electrode due to the large volume change of S particles, and the formation and deposition process of Li-PS species.[2] In recent years, dramatic enhancement of electrochemical utilization of S has been reported by developing advanced S-based active materials that significantly improve electrical conductivity and durability of the S electrodes. In the most of those design, the confined S particles are encapsulated within the electronically conductive porous matrix or coated onto S-philic functional materials so that electronic conductivity of the S electrode is improved while S loss by Li-PS shuttle is suppressed during the cell operation.
Despite the advancement in the S-based active materials, current state-of-the art of the Li/S cell is still not satisfactory, thus further development strategies of S electrodes are required for the advanced Li/S cells demanding long life and high performance. Complex and simultaneous electrochemical processes of S electrode that involve formation and deposition of Li-PS intermediates occur during Li/S cell operation, and kinetic of the electrochemical process are strongly depend on structure of liquid electrolyte/solid S electrode interface. Therefore, rational microstructural design of the S electrode and maintaining the microstructure of the S electrode during the Li/S cell operation will play a key role to achieve high performance and durable Li/S cells. Implementation of specially treated membrane[3] or interlayer[4] are known to be very efficient methods for this purpose, but we still need to develop structural control methodologies to design microstructural of S electrodes.
Amongst various S/carbonaceous composites, a S/graphene oxide (S/GO) composite has been investigated as one of promising composite materials for S electrode.[5] The major benefit of the use of the GO in the S electrode is S-philic property of the GO that comes from oxygen-containing functional groups existing on large surface area of the GO. The key strategies for the efficient use of GO’s functionality to accommodate S species within the S/GO electrode is to maintain large surface area of the GO by minimizing restacking of individual GO sheets during entire material synthesis and electrode manufacturing processes. In this presentation, multifaceted approaches to develop rational design of the S/GO composites and electrode, from nano- to macro-scale, will be introduced. These approaches seek to identify the most effective solutions to mitigate current major challenges of S electrodes, such a porous electrode design strategies for the advanced Li/S cells, in particular, the new discoveries of (1) a three-dimensionally aligned structure of battery electrodes consisting of few-microns-thick material arrays with 10−20 μm interlayer spacings[6] (2) reticulated structure of S-nitrogen-doped graphene oxide (NrGO) composite electrode[7] will mainly be discussed.

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