Sulfur is an attractive cathode material due to its theoretical capacity of 1675 mAh/g. However, implementation of sulfur has been slow due to its inherent problems including polysulfide shuttling, volumetric expansion, and poor conductivity. … Fortunately, researchers have discovered methods to alleviate these issues ranging from mechanical barriers, to porous carbon networks, to other chemical methods. Promising performance from these solutions have resulted in much fervor surrounding sulfur.

The current anode of choice is silicon for its high theoretical capacity of 4200 mAh/g. Silicon faces two challenges—poor conductivity, and volumetric expansion. … To alleviate these issues, researchers utilize novel methods including nano silicon structures, conductive additives, and binders. Ultimately, the immense focus on solving each electrode’s issues has resulted in less research effort on combining a sulfur cathode and silicon anode in a full-cell configuration.

A full cell using sulfur and silicon electrodes is attractive for several reasons. Sulfur and silicon are environmentally benign and abundant. Furthermore, theoretical energy density of a sulfur silicon full-cell (SSFCs) is 1982 Wh/kg, far exceeding the theoretical energy density of current LiBs while only potentially costing $13/kWh. However, a major restriction for SSFCs is the lithium source. Currently, researchers utilize pre-lithiated materials such as lithium sulfide or lithium silicide, allowing for energy densities up to 600 Wh/kg. However,these full cells suffer from short cycles lives, typically less than 50 cycles, while the material used require specialized equipment and face restrictions in processing.

—Ye et al.

To create the sulfur-silicon full cells (SSFC) with the new architecture, the team added a piece of lithium foil into the traditional full-cell architecture, while enabling contact between the lithium foil and the current collector. This allows the lithium foil to integrate into the system while the battery is being cycled, allowing control over the amount of lithium inserted.

(A) SSFC battery architecture set up. (B) Assembled SSFC coin cell schematic. (C) SSFC Cross sectional discharge schematic. Ye et al. Click to enlarge.

In half cells, explained lead author Rachel Ye, pure lithium is used as the anode, which raises safety concerns such as dendrite formation and lithium corrosion. In a full cell, silicon is used as the anode instead, which mitigates the safety issues created by pure lithium anodes, while maintaining the desired high-battery capacity.

In addition to bypassing the complications of prelithiated cells, this method allows for the controlled loading of lithium to compensate for SEI formation and lithium degradation, prolonging the cycle life of the full cell. … Furthermore, this is the first time, to the best of our knowledge, a sulfur silicon full cell has been fully characterized using EIS, CV and GITT. The results presented will pave the way for new research into sulfur and silicon full cells.

—Ye et al.

This research is the latest in a series of projects led by Cengiz Ozkan, professor of mechanical engineering, and Mihri Ozkan, professor of electrical and computer engineering, to create lithium-ion battery materials and architectures from abundant resources and environmentally friendly materials.

Previous research has focused on developing and testing anodes from glass bottles, portabella mushrooms, sand, and diatomaceous (fossil-rich) earth.

Funding came from UCR and Vantage Advanced Technologies. The university’s Office of Technology Commercialization has filed a patent application for the inventions.

Resources

• Rachel Ye, Jeffrey Bell, Daisy Patino, Kazi Ahmed, Mihri Ozkan & Cengiz S. Ozkan (2017) “Advanced Sulfur-Silicon Full Cell Architecture for Lithium Ion Batteries” Scientific Reports 7, Article number: 17264 doi: 10.1038/s41598-017-17363-5