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.
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.