Researchers at Pacific Northwest National Laboratory’s (PNNL) Department of Energy have discovered by mixing the right amount of salt with a small lithium-metal battery it can re-charge about seven times more than batteries with conventional electrolytes.
A battery’s electrolyte solution shuttles charged atoms between electrodes to generate electricity, however this corrodes the electrodes in a lithium-metal battery. The PNNL approach successfully creates a protective layer around the electrodes and significantly increases the charge/discharge cycle.
Conventional electrolytes used in lithium-ion batteries power electronics such as computers and smartphones but these devices are not suitable for lithium-metal batteries. Lithium-metal batteries that replace a graphite electrode with a lithium electrode are ideal for energy storage systems because lithium has a greater storage capacity. A lithium-metal battery has double or triple the storage capacity of lithium-ion batteries.
If these batteries could be used in electric vehicles, the driving range could be more than two times longer between charges.
Adding more lithium-based salt to the liquid electrolyte mix creates a more stable interface between the electrolyte and the electrodes which affects the life of the battery. That high concentration comes with a downside -- the high cost of lithium salt. This concentration also increases viscosity and lower conductivity of the ions through the electrolyte.
"We were trying to preserve the advantage of the high concentration of salt, but offset the disadvantages," said Ji-Guang "Jason" Zhang, a senior battery researcher at PNNL. "By combining a fluorine-based solvent to dilute the high concentration electrolyte, our team was able to significantly lower the total lithium salt concentration yet keep its benefits."
PNNL was able to localize the high concentrations of lithium-based salt into “clusters” that form protective barriers on the electrode and prevent the growth of dendrites, microscopic, pin-like fibers, that cause rechargeable batteries to short circuit and limit their life span.
The PNNL electrolyte was tested on an experimental battery cell similar in size to a watch battery. It was able to retain 80 percent of its initial charge after 700 cycles of discharging and recharging. A battery using a standard electrolyte can only maintain its charge for about 100 cycles.
Researchers plan to test this localized high concentration electrolyte on “pouch” batteries developed at the lab, which are the size and power of a cell phone battery. This will test how it performs at scale. Researchers say the concept of using fluorine-based diluent to manipulate salt concentrations also works well for sodium-metal batteries and other metal batteries.