(a) Schematic illustration of the porous ant-nest structure Li−S electrode (CNT-nest-S) fabrication procedure. SEM cross-section (b) and top (c) morphology of CNT-nest-S. (d) The TEM morphology of the pore in the fully discharged CNT-nest-S. Credit: ACS< Ai et al. Click to enlarge.

Lithium-sulfur batteries are extremely attractive as a next-generation energy storage solution due to their high theoretical energy density (2600 W·h·kg⁻¹), environmental friendliness, and low cost due to the earth-abundant resource of elemental sulfur—also a byproduct from the petroleum industry. However, the current shortcomings of the chemistry are also well known: poor cycling stability, low Coulombic efficiency, and practical low energy density at the cell level.

These challenging attributes result mainly from the dissolution of polysulfide in the electrolyte as an intermediate species during both charge and discharge processes. A large body of research has explored and continues to explore solutions to these issues.

Another issue that limits the performance of Li−S batteries is the poor electrical conductivity of the reaction products (S₈ in the charged state and Li₂S/Li₂S₂ in the discharged state), which is also an obstacle to the high utilization of the active material. In most research works, conductive additives with a high weight ratio (40−50%) are incorporated into the electrode to solve this problem, and a complicated design of sulfur/carbon composites needs to be performed. Therefore, the active material (sulfur) content is low at the electrode level, and the practical energy density of the Li−S batteries suffers.

The next step in advancing Li−S technology is to design a high-efficiency Li−S electrode that can achieve high practical energy density, have good cycling stability, and minimize the composition of the inactive component (e.g., conductive additive, binder, current collector, etc.). To achieve these application goals, additional high-loading Li−S battery research is critical as well as intriguing to the research community. … to achieve a high-efficiency, high-loading Li−S battery, several properties of the Li−S design need to be addressed: first, enough interconnected and continuous open channels to facilitate fast Li-ion and polysulfide transportation; second, a large and highly conductive surface for polysulfide reaction and S₈ or Li₂S/Li₂S₂ precipitation; third, the capability of the electrode to retain polysulfide and mitigate their diffusion into the electrolyte.

—Ai et al.

The structure developed by the Berkeley Lab team to address these issues was inspired by the ant-nest network. The ant-nest network features abundant storage spaces and multi interconnected channels between these storage sites enabling the efficient and fast transportation of food.

The Berkeley Lab ant-nest electrode is fabricated using three cost-effective industry-scale processing methods:

• ball-milling for slurry mixing
• the doctor-blade method for laminate casting
• the sacrificial method (using table salt) for porous creation.

The researchers mixed table salt (NaCl) with sulfur particles, binder, and CNT (as the conductive additive) to form the slurry using the ball-milling method. After the laminate is casted via the doctor-blade method, the NaCl microparticles are removed from the composite electrode with a simple water washing step. The result is a perfect ant-nest structure.

The ant-nest structure meets all the desired requirements:

1. the interconnected channels between storage sites enable fast ion and polysulfide transport and can prohibit channel blocking;

2. the nest structure features ample storage to efficiently sustain polysulfide as well as to accommodate sulfur volume change;

3. the maximized inner surface via the nest structure and the CNT facilitates efficient surface reaction for the transition among different sulfur species;

4. the functionalized conductive binder can help to further sustain the polysulfide inside storage pores with strong affinity between the functional binder and the polysulfide;

5. the conductive binder assists the interconnecting CNT framework in providing super conductivity and a long-distance charge transport pathway for the Li−S electrode; and

6. the strong mechanical properties of CNT and the binder enable the structure the mechanical strength to sustain the volume change during phase transformation of sulfur species.

(a) Voltage profiles for the 1.78 mg·cm−2 and 3.07 mg·cm−2 CNT-nest-S cell at C/10 and C/3) and a photo image of the separator of the 3.07 mg·cm−2 cell is shown in panel a, disassembled after two C/10 cycles and discharge at C/3. The comparison of rate performance (b) and cycling performance (c) between CNT-S and CNT-nest-S cell, with a sulfur loading of 1.78 mg·cm−2. Credit: ACS, AI et al. Click to enlarge.

… this research shows the viability of an electrode-scale nature-inspired ant-nest structure electrode for the application of Li−S batteries. It processes the unique properties of the ant-nest structure of the large polysulfide storage capability and highly efficient, multi interconnected ion transportation pathway design and extended reaction interface to achieve unique direction crystalline sulfur species precipitation on the surface of the CNT.

—Ai et al.

Funding was provided through the Advanced Battery Materials Research (BMR) program from US Department of Energy.

Resources

• Guo Ai, Yiling Dai, Wenfeng Mao, Hui Zhao, Yanbao Fu, Xiangyun Song, Yunfei En, Vincent S. Battaglia, Venkat Srinivasan, and Gao Liu (2016) “Biomimetic Ant-Nest Electrode Structures for High Sulfur Ratio Lithium–Sulfur Batteries” Nano Letters doi: 10.1021/acs.nanolett.6b01434