Stanford Researchers Create Technique to Boost Li-ion Battery Storage Center

The performance of lithium-ion batteries gets a boost as Stanford University scientists create novel electrodes made of silicon and conducting polymer hydrogel, a spongy material similar to that used in contact lenses and other consumer products.

Zhenan Bao, a leading Stanford chemical engineering professor, says the new technique produces low-cost, silicon-based batteries with potential applications for a wide range of electrical devices. The batteries retain a high storage capacity through 5,000 cycles of charging and discharging.

“Developing rechargeable lithium-ion batteries with high energy density and long cycle life is of critical importance to address the ever-increasing energy storage needs for portable electronics, electric vehicles and other technologies,” Bao says in a statement.

In a research report published in the journal Nature Communications, Bao says that to find a practical, inexpensive material that increases the storage capacity of Li-ion batteries, she and her team turned to silicon, an abundant, environmentally benign element with promising electronic properties.

Yi Cui, associate professor of materials science and engineering, says the team spent several years working to develop silicon-based electrodes for high-capacity Li-ion batteries because silicon has 10 times the charge storage capacity of carbon, the conventional material used in Li-ion electrodes.

The researchers knew silicon particles can undergo a 400% volume expansion when combined with lithium. When the battery is charged or discharged, the bloated particles tend to fracture and lose electrical contact.

To overcome this, the Stanford team uses a fabrication technique called “in situ” synthesis polymerization that coats the silicon nanoparticles within the conducting hydrogel. This allows them to create a stable Li-ion battery that retained a high storage capacity through 5,000 cycles of charging and discharging.

“We attribute the exceptional electrochemical stability of the battery to the unique nanoscale architecture of the silicon-composite electrode,” Bao says.

Using a scanning electron microscope, the team saw the porous hydrogel matrix is riddled with empty spaces that allow the silicon nanoparticles to expand when lithium is inserted. This matrix also forms a three-dimensional network that creates an electronically conducting pathway during charging and discharging.

“It turns out that hydrogel has binding sites that latch onto silicon particles really well and at the same time provide channels for the fast transport of electrons and lithium ions,” says Cui, a principal investigator with the Stanford Institute for Materials and Energy Sciences at the SLAC National Accelerator Laboratory.

“That makes a very powerful combination.”

A simple mixture of hydrogel and silicon proved far less effective than the in situ synthesis polymerization technique. “Making the hydrogel first and then mixing it with the silicon particles did not work well,” Boa says.

“It required an additional step that actually reduced the battery's performance,” Bao says. “With our technique, each silicon nanoparticle is encapsulated within a conductive polymer surface coating and is connected to the hydrogel framework. That improves the battery's overall stability.”

Hydrogel primarily consists of water, which can cause Li-ion batteries to ignite – a potential problem that had to be addressed.

“We utilized the 3-dimensional network property of the hydrogel in the electrode, but in the final production phase, the water was removed,” Bao says. “You don't want water inside a lithium-ion battery.”

Although a number of technical issues remain, Cui is optimistic about potential commercial applications of the new technique. “The electrode-fabrication process used in the study is compatible with existing battery-manufacturing technology,” he says.

Silicon and hydrogel are also inexpensive and widely available. “These factors could allow high-performance silicon-composite electrodes to be scaled up for manufacturing the next generation of lithium-ion batteries,” Cui says. “It's a very simple approach that's led to a very powerful result.”