In a lithium-ion battery, the cathode is a lithium metal oxide while the anode is graphite. Researchers are currently looking for ways to replace graphite with lithium metal as the anode to boost the battery’s energy density.
Since the packing density of lithium atoms is the highest in its metallic form, batteries that use metallic lithium anodes can pack more energy per weight or volume than graphite-based anodes, Rensselaer explains. However, lithium metal anodes are plagued by ‘dendrite’ build-up that takes place over repeated cycles of charging and discharging.
Dendrites are protrusions that emanate out of the lithium metal surface and often, grow long enough to create a short circuit between the electrodes, leading to a fire hazard.
Rensselaer says it has discovered a way to use internal battery heat to diffuse the dendrites into a smooth layer. “We have found that lithium metal dendrites can be healed in situ by the 'self-heating' of the dendritic particles,” says Professor Nikhil Koratkar of Rensselaer.
So far, the lithium dendrite challenge has been tackled by using carbon, typically graphite, anodes. In this approach, Rensselaer explains that lithium ions diffuse into and are stored within the carbon matrix, isolating each lithium atom and preventing dendrite build-up. Usually, one lithium atom is stored for every 6 carbon atoms, with the excess carbon material serving little more than deadweight.
“Lithium-ion batteries with carbon-based anodes are the best available option, but they can no longer keep up with the storage-capacity demand,” Prof. Koratkar contends. “For any significant new improvements, we must look elsewhere. The best option would be a lithium metal system.”
By taking advantage of the battery’s internal resistive heating to eliminate the dendrite build-up, Rensselaer believes it has a solution.
Resistive heating (also known as Joule heating) is a process in which a metallic material resists current flow and, as a result, produces heat. This self-heating occurs through the charging and discharging process.
In their work, the team ramped up the self-heating effect by increasing the current density (charge-discharge rate) of the battery. According to Rensselaer, the process triggered extensive surface diffusion of lithium, spreading the dendrites into an even layer.
The researchers say they first demonstrated this smoothening of the dendrites in a lithium-lithium symmetrical cell, before showing the process with the same results in a proof-of-concept demonstration using a lithium-sulfur battery.
Dendrite healing would be carried out by battery management system software, Rensselaer explains, which would provide doses of self-healing treatment by running a few cycles at a high rate of charge and discharge when an electronic device is not in use.
“A limited amount of cycles at high current density would occur to heal the dendrites, and then normal operations can be resumed,” Prof. Koratkar says. “Self-healing would occur as a maintenance strategy, long before the dendrites become a safety hazard.”
“High-density energy storage remains a critical hurdle between renewable-energy harvesting and its widespread use in everything from electrical vehicles to solar powered homes,” adds Dean of Engineering Shekhar Garde. “Results from Prof. Koratkar’s lab show how the fundamental understanding of materials at the nanoscale can be employed to not only increase energy density of batteries, but also increase their life and make them safer.”
