Novel Protection Strategies for Sulfur and Silicon in Advanced Lithium-Sulfur Batteries
Authors: Zubair Usman , Alidoost Mojtaba , Versaci Daniele , Amici Julia , Vankova Svetoslava , Francia Carlotta , Bodoardo Silvia , Penazzi Nerino 
Published: February 2020

In our daily life the demand for electrochemical power sources with high energy density is becoming increasingly important. This kind of resources is fundamental for our future as it may represent a viable alternative to fossil fuels and gases offering a solution to environmental issues as well as energy crises. Until now Lithium-ion batteries (LIBs) have been the dominant power sources in the commercial energy storage devices field. Though representing a tremendous improvement with respect to previous systems like Lead-acid batteries, However, LIBs still cannot represent a feasible alternative where  gravimetric energy density, lower than 300 Wh.kg-1, is at stake. In the transportation field, f. i., it is not yet possible to buy an electric vehicle having performance comparable to that of an internal combustion vehicle of the same cost.

Among various types of new “beyond Lithium-ion” batteries, the Lithium-sulfur battery holds great potential due to its high theoretical specific capacity of 1672 mAh.g-1 and specific energy of 2600 Wh. Kg-1.  Nevertheless, the development of Lithium-sulfur battery has met several challenges for its typical low sulfur utilization, low rate capacity and poor cycle life, mainly due to the electronic and ionic insulating nature of sulfur, the solubility of reductive polysulfides in organic solvents based electrolytes and the large expansion of sulfur upon cycling. Therefore different approaches need to be explored to solve these issues.

Short lifetime of Lithium-sulfur batteries is linked, upon other factors, to the use of Li metal anode, which is known to cause safety hazard and internal short-circuit due to the formation of lithium dendrites, which may as well result in thermal runaway. Moreover this material is very expensive. One of the promising solutions is to use lithiated Si instead of Li metal. The theoretical capacity (4200 mAh.g-1) of silicon as anode is higher than the one of Li metal (3860 mAh.g-1), making it ideal to couple with the high capacity sulfur cathode in order to maintain a comparable energy density despite the lower discharge voltage obtained with silicon. However this approach raises another issue, silicon undergoes large volume expansion during lithiation risking compromising the anode structure. Therefore a silicon core is enclosed in a silica shell which is then partially removed by acidic treatment, thus creating empty spaces around the core to allow an effective lithiation. In order to make the whole structure conductive, an external shell is then created around it (carbon, metal,…).

In a parallel study, again regarding the anode, the synthesis of nanostructured materials such as, Graphene-Silicon composites to make cages that accommodate silicon’s volume changes during charging and discharging, is being carried out. Silicon nanoparticles are treated with acid to achieve fresh Si surface. Then, Si nanoparticles are reacted with amine group terminated long chain alkenes through addition reaction. Si nanoparticles bearing amine functional groups are dispersed with graphene oxide in mild acidic conditions. In this strategy, the interaction between positively charged nanoparticles and graphene oxide allow the uniform wrapping of the particles and long carbon chains will provide the room for large volume and morphological changes. The graphene oxides are further reduced to graphene to provide conductive pathways for electronic and ionic conductivities.

Regarding the cathode, a sustainable solution is suggested to prepare conductive carbon matrices from bio-based materials. Template-assisted hydrothermal carbonization of dextrose and/or β-cyclodextrin is carried out to prepare carbon nanospheres or nanosponges to host Sulfur (S). S is incorporated inside carbon nanosponges via solvent impregnation and thermal diffusion. Then, as prepared carbon-sulfur composite is encapsulated with conductive polymers like polyaniline/polypyrrole. This dual protection strategy in conductive matrices provide conductive network for S and effectively help to suppress the polysulfide dissolution.