The Li/S cell has attracted great attention because of the need of the electrical vehicle (EV) market for high specific energy batteries (∼350 W·h kg⁻¹ at C/3 discharge rate), which greatly exceeds the practical specific energy of current Li ion cells (100−200 W·h kg⁻¹). However, despite the great advantages of Li/S cells, the early S-based cathodes in organic electrolytes showed a low utilization and a poor cycle life owing to several major problems: (i) The insulating nature of Li₂S and S that are the final products of the S electrode at the fully discharged and charged states, respectively. (ii) A large volume change of the S particles during cycling (∼80%) resulting in mechanical degradation of the electrode. (iii) Highly soluble intermediate species (polysulfides, Li₂S_n, n = 4−8) in most organic liquid electrolytes, which causes the loss of active material from the cathode and the polysulfide shuttle effect. When the polysulfides are dissolved into the liquid electrolyte, they can diffuse back and forth between electrodes and can form insoluble Li₂S (or Li₂S₂) on the surface of the Li metal electrode, which leads to lower Coulombic efficiency.

—Hwa et al.

Key factors to improve the electrochemical performance of Li/S cells are to increase the electronic conductivity of the cathode and to suppress the polysulfide dissolution as well as the mechanical stress caused by the volume change during cycling. Numerous approaches have been proposed: nanofabrication of S (or S-based composites); chemically (or mechanically) protective materials on the S particles; and composites with mesoporous carbon or graphene oxide (GO) that can act as S immobilizers.

GO is very attractive for stabilizing the cycling performance of S-based cathodes because the reactive functional groups on the surface of GO can form bonds with S, indicating that S (or polysulfides) can be captured by those functional groups, the Berkeley team noted.

Recent work has begun exploring the use of lithium sulfide (Li₂S, theoretical specific capacity: 1166 mA·h g⁻¹) as the initial cathode material instead of S (e.g., earlier post, earlier post). With Li₂S as the cathode material, the mechanical damage of the cathode due to the volume expansion of S particles (up to 80%) caused by the lithiation process during discharge can be reduced because Li₂S particles already occupy the maximum volume relative to S.

In addition, the prelithiated state of the Li₂S cathode can be coupled with Li-free anodes such as silicon (Si) and tin (Sn), thereby avoiding the problems currently associated with Li-metal anodes such as dendritic growth.

However Li₂S suffers from very poor electronic conductivity, polysulfide dissolution and the shuttle effect, which cause low S utilization, low Coulombic efficiency, and rapid degradation during cycling.

To address those issues, the Berkeley researchers used Li₂S/GO nanospheres with a conformal carbon coating on the surface (Li₂S/GO@C). Their material offers a number of benefits:

• The conformal carbon coating not only prohibits polysulfide dissolution into the electrolyte by preventing direct contact between Li₂S and the liquid electrolyte, but also acts as an electrical pathway resulting in the reduction of the electrode resistance.

• The spherical shape of the submicron size particles can provide a short solid-state Li diffusion pathway and better structural stability of the carbon shell during cycling.

• Void space is created within the carbon shell during charge, and it will provide enough space to accommodate the volume expansion of up to 80% during discharge. As a result, better structural stability of the carbon shell can be secured because the carbon shell will not need to expand during cycling.

• Even if some percentage of the carbon shells is broken due to physical imperfections, the GO in the particles can act as a second inhibitor for polysulfide dissolution due to its S immobilizing nature.

Cycling performance of the electrodes cycled at various rates. Credit: ACS, Hwa et al. Click to enlarge.

The Li2S/GO@C nanosphere cathode demonstrated promising electrochemical performance:

• Prolonged cycle life (1500 cycles) at the 2.0 C discharge rate (1.0 C = 1.163 A g⁻¹ of Li₂S) with a high initial capacity of 650 mA·h g⁻¹ of Li₂S (corresponding to 942 mA·h g⁻¹ of S) and 699 mA·h g⁻¹ of Li2S (1012 mA·h g⁻¹ of S) at 0.05 C after 400 cycles at 2.0 C discharge; and

• excellent capacity retention of more than 84% with a high Coulombic efficiency of up to 99.7% after 150 cycles at various discharge C-rates (2.0, 3.0, 4.0, and 6.0 C discharge rates).

Resources

• Yoon Hwa, Juan Zhao, and Elton J. Cairns (2015) “Lithium Sulfide (Li₂S)/Graphene Oxide Nanospheres with Conformal Carbon Coating as a High-Rate, Long-Life Cathode for Li/S Cells” Nano Letters doi: 10.1021/acs.nanolett.5b00820