To build graphene cages around silicon particles, researchers coated the particles with nickel;
grew layers of graphene on top of the nickel; and used acid to dissolve the nickel away, leaving
enough space for the silicon to expand inside the cage. (Y. Li et al., Nature Energy)
(January 28, 2016) Approach Could Remove Major Obstacles to Increasing the Capacity of Lithium-ion Batteries
Scientists have been trying for years to make a practical lithium-ion battery anode out of silicon, which could store 10 times more energy per charge than today’s commercial anodes and make high-performance batteries a lot smaller and lighter. But two major problems have stood in the way: Silicon particles swell, crack and shatter during battery charging, and they react with the battery electrolyte to form a coating that saps their performance.
Now, a team from Stanford University and the Department of Energy’s SLAC National Accelerator Laboratory has come up with a possible solution: Wrap each and every silicon anode particle in a custom-fit cage made of graphene, a pure form of carbon that is the thinnest and strongest material known and a great conductor of electricity.
In a report published Jan. 25 in Nature Energy, they describe a simple, three-step method for building microscopic graphene cages of just the right size: roomy enough to let the silicon particle expand as the battery charges, yet tight enough to hold all the pieces together when the particle falls apart, so it can continue to function at high capacity. The strong, flexible cages also block destructive chemical reactions with the electrolyte.
This time-lapse movie from an electron microscope shows the new battery material in action:
a silicon particle expanding and cracking inside a graphene cage while being charged.
The cage holds the pieces of the particle together and preserves its electrical conductivity
and performance. (Hyun-Wook Lee/Stanford University)
“In testing, the graphene cages actually enhanced the electrical conductivity of the particles and provided high charge capacity, chemical stability and efficiency," said Yi Cui, an associate professor at SLAC and Stanford who led the research. “The method can be applied to other electrode materials, too, making energy-dense, low-cost battery materials a realistic possibility.”
The Quest for Silicon Anodes
Lithium-ion batteries work by moving lithium ions back and forth through an electrolyte solution between two electrodes, the cathode and the anode. Charging the battery forces the ions into the anode; using the battery to do work moves the ions back to the cathode.