How Electrolyte Thinness Can Affect Li-Ion Battery Performance

Researchers from the National Institute of Standards and Technology (NIST: Gaithersburg, MD), the University of Maryland (College Park), and Sandia National Laboratories (Albuquerque, NM) have built nanowire batteries to demonstrate that lithium-ion (Li-ion) battery performance is impaired when the battery's electrolyte layer becomes too thin. This discovery highlights that there is effectively a lower limit to the size of tiny power sources.

These findings are important for a range of electro devices because battery size and performance are key to the development of autonomous microelectromechanical systems (MEMS). MEMS devices, which can be tens of micrometers in size, are potentially suitable for many medical device applications because they generally rely on batteries that can last for long periods of time and be recharged quickly. However, because of current battery technology, MEMS devices much smaller than a millimeter in size cannot be fabricated.

To determine how small they could make solid-state Li-ion batteries using existing materials, NIST researcher Alec Talin and his colleagues created a forest of tiny batteries measuring approximately 7 µm tall x 800 nm wide. Starting with silicon nanowires, they created a contact out of layers of metal, cathode material, electrolyte, and anode materials with various thicknesses to form the miniature batteries. Then, they employed transmission electron microscopy to observe the flow of current through the batteries and study the materials as they charged and discharged.

The team found that when the thickness of the electrolyte film falls below approximately 200 nm, the electrons can jump the electrolyte border instead of flow through the wire to the device and on to the cathode. This short circuiting causes the electrolyte to break down and the battery to quickly discharge. "What isn't clear is exactly why the electrolyte breaks down," Talin remarks. "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."