While silicon is targeted as one of the more promising anode materials for next-generation lithium-ion batteries due to its relatively low working potential, abundance in nature, and theoretical gravimetric (specific) capacity of 3,579 mAh g⁻¹ for Li₁₅Si₄, it also suffers from the well-known and well-researched problem of volumetric change (more than 300%) during the lithiation and delithiation processes (e.g., earlier post).

This results in pulverization of the material and loss of electrical contact, as well as formation and propagation of an unstable solid electrolyte interphase (SEI) on its surface—both of which result in rapid capacity fading of the battery.

Silicon has long been sought as a way to improve the performance of lithium-ion batteries, but silicon swells so much when it is charged that it can break apart, making a silicon electrode inoperable. The porous, sponge-like material we’ve developed gives silicon the room it needs to expand without breaking.

—Ji-Guang Zhang, PNNL Fellow and co-author

Zhang and his PNNL colleagues wondered if a sponge-like silicon electrode would address the volumetric expansion problem. Others had etched pores into a silicon electrode’s surface, but hadn’t succeeded in creating holes throughout the material.

The PNNL team approached Michael Sailor, a University of California, San Diego chemist whose research includes using porous silicon to detect pollutants and deliver drugs, for help. PNNL used Sailor’s method to create porous silicon—placing thin sheets in a chemical bath to etch out tiny holes throughout the material—and then coated the result with a thin layer of conductive carbon to make their electrodes.

Next, the team collaborated with materials scientist Chongmin Wang, who specializes in using in-situ transmission electron microscopes at DOE’s EMSL, the Environmental Molecular Sciences Laboratory at PNNL. Wang uses powerful microscopes to record close-up videos of tiny batteries, allowing researchers to better understand the physical and chemical changes that batteries undergo as they operate. Wang put the team’s sponge-like, carbon-coated silicon electrode through a series of charges and discharges under the microscope.

The team observed that while being charged, the new electrode mostly expanded into the empty spaces created by the material’s porous structure. The outside shape of the electrode only expanded by 30%—much less than the 300 % usually seen in silicon electrodes—and the electrode did not pulverize.

Next, Zhang and his colleagues plan to develop a larger prototype battery with their silicon sponge electrode. Part of that effort will involve creating a more streamlined production process so their new electrode can be made at a reasonable cost.

The insight obtained from this work also provides guidance for the design of other materials that may experience large volume variation during operations.Li et al.

This research was funded by the Department of Energy’s Office of Energy Efficiency and Renewable Energy.

Resources

• Xiaolin Li, Meng Gu, Shenyang Hu, Rhiannon Kennard, Pengfei Yan, Xilin Chen, Chongmin Wang, Michael J. Sailor, Ji-Guang Zhang and Jun Liu (2014) “Mesoporous Silicon Sponge as an Anti-Pulverization Structure for High-Performance Lithium-Ion Battery Anodes,” Nature Communications doi: 10.1038/ncomms5105