The graphene-modified Li anode with the SEI—which was induced by the polysulfide-containing electrolyte—improved the Coulombic efficiency to ∼97% for more than 100 cycles, while the control sample with Cu foil as the current collector exhibited huge fluctuations in Coulombic efficiency. Coulombic efficiency was 93% at a higher current density of 2.0 and 4.0 C.

Schematic diagrams of a Li metal anode with different structures during Li deposition and dissolution. (a) Without a graphene framework, Li dendrites appear during Li deposition, thus leading to a large amount of dead Li during Li dissolution. (b) The SCG-structured anode depicts a stable and uniform Li deposition and dissolution with high efficiency and low resistance. Credit: ACS, Cheng et al. Click to enlarge.

The unblocked ion pathways and high electron conductivities of frameworks in the modified metal anode led to the rapid transfer of Li ions through the SEI and endowed the anode framework with an ion conductivity of 7.81 × 10^–2 mS cm–¹, nearly quintuple that of the Cu foil-based Li metal anode. Additionally, the polarization in the charge–discharge process was halved to 30 mV. The stable and efficient Li deposition was maintained after 2000 cycles.

Among various promising battery candidates with high energy densities, lithium!sulfur (Li-S) batteries, with a high theoretical capacity of 1675 mAh g^-1 (based on sulfur) and an energy density of 2600 Wh kg^-1 (based on the lithium-sulfur redox couple), are highly considered. Despite these advantages, many obstacles still need to be overcome for practical applications of Li-S batteries, such as the low conductivity of sulfur and the shuttle of long-chain polysulfide intermediates during discharge/charge cycling. During the past 10 years, various strategies have been proposed to address these issues in Li-S cells, such as nanostructured hosts, conductive polymers, and a bifunctional separator, which improved the cycling performance to more than 1000 cycles with a Coulombic efficiency of 90-99% and discharge capacity of 500-800 mAh g^-1.

However, the superior cycling performance was mostly achieved by a 2000% lithium excess, which hindered the problem of the Li metal anode. Generally, the formation of Li dendrites is a primary issue for Li metal batteries (LMBs) including Li-S batteries, which always leads to serious safety concerns and low Coulombic efficiency.

Dendrite formation on a metal lithium anode in a Li-S cell is an even more severe problem than it is for routine lithium ion cells. Unlike lithium ion cells, intermediate products (lithium polysulfides) are formed in the cathode and are soluble in the electrolyte, which is in direct contact with the metallic anode and therefore leads to a much more complicated system with the coexistence of Li dendrites and polysulfide intermediates.

… Before the dendrite-induced short circuit, the impedance of the battery escalated sharply and the service life was terminated early. In a Li-S cell, this phenomenon is more frequent and serious, because sulfur and lithium sulfide products are both ion- and electron-insulating and the cross-coupling effect will lead to a sharp decrease in the voltage and energy density. Consequently, it is critically important to design an anode structure with desirable electron and ion channels to improve transfer properties and recycle dead Li in a Li-S cell.

—Cheng et al.

Lithium dendrites easily grow and detach on conventional 2D substrates, such as the copper foil, forming dead lithium. The Tsinghua team posited that a pre-existing conductive framework—e.g., self-supported graphene foam—would accommodate the deposited Li. Graphene foam offers a number of attractive features as an anode framework, they said, including:

• A larger surface area than 2D substrates to lower the real specific surface current density and the possibility of dendrite growth;

• An interconnected framework to support and recycle dead Li; and

• Good flexibility to sustain the volume fluctuations during repeated incorporation/extraction of Li.

Coulombic efficiency of SCG and Cu foil based anode with a lithiation capacity of 0.5 mA h cm-2 at a current of 0.5 mA cm-2 (1.0 C). Credit: ACS< Cheng et al. Click to enlarge.

To synthesize the framework, the researchers first obtained graphene oxide (GO), then reduced and assembled it through a hydrothermal treatment into a three-dimensional (3D) self-supported graphene framework (GF). The resulting macroscopic graphene foam was then directly used as support electrode to store metallic Li.

The foam, consisting of highly crumbled graphene nanoflakes, consists of an interconnected network with hierarchical pores. The pore volume was 1.6 mL g^-1, and the average pore size was 10 nm.

To build the working environment of Li-S cells, 0.1 M Li₂S₈ is added into the presented electrolyte. Several demonstrations on the Li anode reinforced by Li₂S_x (x = 1-8) enlightened a new prototype of a robust and stable SEI. Consequently, polysulfides can be presented as the SEI stabilizer of the anode. The smooth layer covering the surface of the graphene nanosheets was presumably assigned to in situ formed SEI and was identified in a subsequent demonstration by X-ray photoelectron microscopy (XPS). The dual-phase layered structure composed of an in situ formed SEI and a nanostructured graphene framework and denoted as SEI-coated graphene (SCG) ensures the requirements on the Li metal from the two sides of the electrode/electrolyte interface and, thereby, enables highly efficient and stable utilization of Li-S batteries.

—Cheng et al.

The team is working to improve further the Coulombic efficiency and ion conductivity of the SCG Li metal anodes and investigating the diffusion of Li ions before and after crossing the SEI.

Resources

• Xin-Bing Cheng, Hong-Jie Peng, Jia-Qi Huang, Rui Zhang, Chen-Zi Zhao, and Qiang Zhang (2015) “Dual-Phase Lithium Metal Anode Containing a Polysulfide-Induced Solid Electrolyte Interphase and Nanostructured Graphene Framework for Lithium–Sulfur Batteries” ACS Nano doi: 10.1021/acsnano.5b01990