Researchers at Changchun University of Science and Technology in China have developed a nanobox strategy to improve the performance of lithium-sulfur batteries. A paper on their work appears in the Journal of Power Sources.

Li–S batteries theoretically offer a specific energy density of 2600 Wh kg⁻¹—five times than the commercial Lithium-ion batteries—and practical energy densities of beyond 500 Wh kg⁻¹. The system typically uses a lithium-metal anode and sulfur cathode.

However, Li-S batteries are still plagued with numerous challenges in practical applications—e.g., the cathode active materials of sulfur and lithium sulfide with poor electronic and ionic conductivities, volume change of about 80% during charge–discharge, and polysulfide shuttle. The challenges lead to severe devolution of the active materials, diminished specific capacity, inferior cycle stability, and low Coulombic efficiency of the lithium sulfur battery.

One approach to addressing these issues has been to use transitional metal sulfides as a cathode host. Compared with the carbon-based hosts that prevent the shuttle of soluble polysulfides through physical confinement, transitional metal sulfides offer strong interaction with the polysulfides. Researchers have shown that stronger interactions between metal sulfides and the polysulfide Li₂S₆ generated higher energy density and long-term cycling stability.

… transition metal sulfides have been regarded as a promising class of host materials for Li−S battery due to binding polysulfides and chemical interactions. Targeted at the issue of the incomplete sulfur utilization, numerous impressive efforts have been devoted to encapsulating active sulfur. Thereinto, the most popular strategy is to construct a host with hollow and porous structure. The porous microchamber inside affords active sulfur sufficient interspace and guarantees the effective contact area between the electrode material and the electrolyte, which results in a rapid diffusion of ion and electron and high-efficiency utilization of active materials.

Metal organic framework as a class of original porous materials has recently attracted considerable attention, and it provides remarkably high surface area and steerable pore size and framework structure … The porous hollow structure with large cavity volume and abundant interior mesopores will provide plentiful space for sulfur cathode materials, which can stimulate the development of high sulfur utilization of Li–S battery.

… as a superior coating layer, PPy distinctly possesses preponderance for remarkably improving Li−S battery performance.

Based on above consideration, we design heterogeneous porous hollow structure integrating CoS nanobox and PPy to encapsulate active sulfur cathode materials.… the CoS@PPy nanoboxes loading 60 wt% sulfur exhibit higher specific capability and good cycling stability. We believe that this design concept and synthesizing technique of heterogeneous hollow structure will provide valuable insight to develop high energy density electrode materials and be applied to energy conversion and storage systems.

—Song et al.

To prepare the CoS nanoboxes, the team synthesized ZIF-67 nanocubes as the precursor. The ZIF-67 nanocubes were dispersed in a mixed solution containing ethanol (20 mL) and deionized water (5 mL). Subsequently, 5 mL of deionized water containing 200mg of Na₂S was poured quickly into the aqueous solution. Finally, the resulting black products were collected through centrifuging, washed with deionized water and ethanol, and dried in a vacuum at 60°C overnight.

CoS@PPy nanoboxes were fabricated based on a facile vapor phase polymerization approach.

The CoS@PPy/S electrode with a sulfur content of 60% exhibited good sulfur utilization (initial discharge capacity of 1165 mAh g⁻¹ at 0.2 C) and a capacity retention of above 80% with a capacity fading rate of 0.10% per cycle after 200 cycles at 0.5 C.

Resources

• Yan Song, He Wang, Wensheng Yu, Jinxian Wang, Guixia Liu, Dan Li, Tingting Wang, Ying Yang, Xiangting Dong, Qianli Ma (2018) “Synergistic stabilizing lithium sulfur battery via nanocoating polypyrrole on cobalt sulfide nanobox,” Journal of Power Sources, Volume 405, Pages 51-60 doi: 10.1016/j.jpowsour.2018.10.032