It turns out you can be too thin—especially if you’re a
nanoscale battery. Researchers from NIST; the University of Maryland, College
Park; and Sandia National Laboratories built a series of nanowire batteries to
demonstrate that the thickness of the electrolyte layer can dramatically affect
the performance of the battery, effectively setting a lower limit to the size
of the tiny power sources. The results are important because battery size and
performance are key to the development of autonomous
MEMS—microelectromechanical machines—which have potentially revolutionary
applications in a wide range of fields.
MEMS devices, which can be as small as tens of micrometers,
have been proposed for many applications in medicine and industrial monitoring,
but they generally need a small, long-lived, fast-charging battery for a power
source. Present battery technology makes it impossible to build these machines
much smaller than a millimeter—most of which is the battery itself—which makes
the devices terribly inefficient.
NIST researcher Alec Talin and his colleagues created a
veritable forest of tiny—about 7 um tall and 800 nm wide—solid-state lithium
ion batteries to see just how small they could be made with existing materials
and to test their performance.
Starting with silicon nanowires, the researchers deposited
layers of metal (for a contact), cathode material, electrolyte, and anode
materials with various thicknesses to form the miniature batteries. They used a
transmission electron microscope (TEM) to observe the flow of current
throughout the batteries and watch the materials inside them change as they
charged and discharged.
The team found that when the thickness of the electrolyte
film falls below a threshold of about 200 nm, the electrons can jump the
electrolyte border instead of flowing through the wire to the device and on to
the cathode. Electrons taking the short way through the electrolyte—a short
circuit—cause the electrolyte to break down and the battery to quickly discharge.
“What isn’t clear is exactly why the electrolyte breaks
down,” says Talin. “But what is clear is that we need to develop a new
electrolyte if we are going to construct smaller batteries. The predominant
material, LiPON, just won’t work at the thicknesses necessary to make practical
high-energy-density rechargeable batteries for autonomous MEMS.”
