A sponge solution
Zhang and his PNNL colleagues wondered if a sponge-like silicon electrode would do the trick. Others had etched pores into a silicon electrode's surface, but hadn't succeeded in creating holes throughout the material. So they 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's careful eye.
Space to grow
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 percent much less than the 300 percent usually seen in silicon electrodes. And the new electrode didn't break down. After more than 1,000 charge-and-discharge cycles, the electrode maintained more than 80 percent of its initial energy storage capacity.
Next, Zhang and his colleagues plan
|Contact: Franny White|
DOE/Pacific Northwest National Laboratory