Low-Porosity Sulfur Cathodes for High-Energy-Density Lithium-Sulfur Batteries
Authors: Chen Shuru , Dai Fang , Cai Mei 
Published: February 2020

Vehicle electrification has been adopted by General Motors (GM) as a technical strategy to meet the target of Zero Emission. It is thus critical to develop rechargeable batteries that are light and durable with high power and energy density, and low cost. Lithium-sulfur (Li-S) battery has been recognized as one promising system beyond conventional Li-ion systems thanks to its significantly higher theoretical specific energy of ~2600 Wh/kg and potentially lower cost.
Despite tremendous efforts since 2000s, [1] Li-S still confront great challenges, which typically include low practical energy density, poor cycle life, low cycling efficiency, and severe self-discharge. These challenges essentially arise from the insulating nature of sulfur and its discharged intermediates/products, dissolution/crossover of the polysulfide intermediates in organic electrolytes, and instability of Li/electrolyte interphase.
To enable the electrochemical reactions of insulating sulfur and its reduction products, large amount of porous conductive backbone, e.g. porous carbon, is typically required for making sulfur cathode electrodes.[2-3] Therefore, the laminated sulfur cathodes exhibit high surface area and high porosity, which come from both intra-particle voids and remaining pores of the active materials. A few examples calculated by the sulfur loading and thickness from published studies show much higher porosity (~ 60% - 70%) than those of electrodes in Li-ion batteries (typically < 30%).[4] The electrolyte needs to fulfill the pores in the electrodes to provide sufficient ionic conducting path. Higher cathode porosity always means more weight contribution from the electrolyte filling in the pores, leading to lower practical energy density of the cell, as shown in Figure 1.
In this presentation, we will discuss some technical challenges, from both material- and cell-level, to develop high-energy Li-S batteries. Moreover, we will introduce some strategies to tailor electrode porosity and control of electrolyte/sulfur (E/S) mass ratio, to achieve high practical cell energy density of Li-S batteries. We are working to reach 500 Wh/kg as targeted by Battery 500 project supported by DOE. Of course, cycle life is another big challenge for Li-S, especially under lean electrolyte condition. So finally we will also briefly discuss some insight of the tradeoffs on energy density and cycle life.

Refence

Marmorstein, T.. Yu, K.. Striebel, F.. McLarnon, J. Hou, E.. Cairns, Electrochemical performance of lithium/sulfur cells with three different polymer electrolytes, J. Power Sources. 89 (2000) 219–226.

Manthiram, Y. Fu, Y.S. Su, Challenges and prospects of lithium-sulfur batteries, Acc. Chem. Res. 46 (2013) 1125–1134.

Manthiram, Y. Fu, S.H. Chung, C. Zu, Y.S. Su, Rechargeable lithium-sulfur batteries, Chem. Rev. 114 (2014) 11751–11787.

Hagen, D. Hanselmann, K. Ahlbrecht, R. Maça, D. Gerber, J. Tübke, Lithium–Sulfur Cells: The Gap between the State-of-the-Art and the Requirements for High Energy Battery Cells, Adv. Energy Mater. 5 (2015) 1401986.

Figure 1