Conventional electrolytes used in lithium-ion batteries that power household electronics like computers and cellphones are not suitable for lithium-metal batteries. Lithium-metal batteries that replace a graphite electrode with a lithium electrode are the holy grail of energy storage systems because lithium has a greater storage capacity and, therefore, a lithium-metal battery has double or triple the storage capacity. That extra power enables electric vehicles to drive 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 that, in turn, affects the life of the battery. But that high concentration of salt comes with distinct downsides including the high cost of lithium salt. The high concentration also increases viscosity and lowers conductivity of the ions through the electrolyte. A battery's electrolyte solution shuttles charged atoms between electrodes to generate electricity. Finding an electrolyte solution that doesn't corrode the electrodes in a lithium-metal battery has proven challenging.
A new approach adds a protective layer around the electrodes and achieves significantly increased charge/discharge cycles. Using the right amount of salt, a small lithium-metal battery can recharge about seven times more than batteries with conventional electrolytes. By combining a fluorine-based solvent to dilute the high-concentration electrolyte, the approach significantly lowers the total lithium salt concentration, yet keeps its benefits.
In this process, the high concentrations of lithium-based salt were localized into “clusters” that are able to 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 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. The localized high-concentration electrolyte will be tested on “pouch” batteries the size and power of a cellphone battery to see how it performs at that scale. The concept of using this fluorine-based diluent to manipulate salt concentration also works well for sodium-metal batteries and other metal batteries.