Boosting battery performance by adding ‘something new’

Early stage research is said to show that a small amount of lithium hexafluorophosphate (LiPF₆)in a dual salt, carbonate solvent-based electrolyte can make rechargeable lithium-metal batteries stable, charge quickly and have a high voltage.

“A good lithium-metal battery will have the same lifespan as the lithium-ion batteries that power today’s electric cars and consumer electric devices, but also store more energy,” said PNNL chemist Wu Xu.

When lithium-based rechargeable batteries were first developed in the 1970s, researchers used metallic lithium for the anode because it could store ten times more energy storage capacity than graphite. But problems with dendrites saw early batteries fail. Researchers continue to address this issue by using other materials, using protective layers or electrolyte additives.

PNNL is revisiting an older design, with lithium metal as an anode, looking for a better performing electrolyte. However, researchers found either they could make ‘super-efficient’ batteries which charged very slowly and couldn't work at higher voltages, or a faster-charging battery that was unstable and had low voltages.

But, by adding small amounts of LiPF₆ – 0.6% by weight – they created a 4.3V battery that retained more than 97% of its initial charge after 500 repeated charges and discharges, while carrying 1.75mA/cm². According to the team, the battery takes about one hour to fully charge.

The battery performed well largely because the additive helps create a robust protective layer of carbonate polymers on the battery's lithium anode. This thin layer prevents lithium from being used up in unwanted side reactions, which can kill a battery.

Xu and his team are now working on ways to make rechargeable lithium-metal batteries viable, including improving electrodes, separators and electrolytes. Amongst the next steps are making and testing larger quantities of the electrolyte, improving the efficiency and capacity retention of a lithium-metal battery using the electrolyte, increasing the material loading on the cathode and trying a thinner anode.