“It’s a big step forward for a promising technology, but lithium-metal technology is not yet ready for prime time. While the lithium-ion batteries used in electric vehicles today hold less energy, they last longer, typically at least 1,000 cycles,” according to the lab.
The cell, a 2Ah 350Wh/kg Li:LiNi0.6Mn0.2Co0.2O2 pouch type, had 76% capacity retention after 600 cycles without sudden cell death – pouch cells were chosen as more representative of future use compared to the coil cells commonly used in research.
One of the advances inside the experimental cell was a move to thin – 20μm – lithium strips for anodes.
“Many people have thought that thicker lithium would enable the battery to cycle longer,’ said PNNL researcher Jie Xiao. “But that is not always true. There is an optimised thickness for each lithium-metal battery depending on its cell energy and design.”
The issue, according to the lab, is that electrolyte has to reach deeper into porous structures to reach the heart of a thick electrode, and that it gets more depleted the further it has to travel along pores.
This affects the complex reactions around a film on the anode known as the solid electrolyte interphase (SEI), which is by-product of side reactions between lithium and the electrolyte.
“It acts as an important gatekeeper that allows certain molecules to go from the anode to the electrolyte and back again, while keeping other molecules at bay,” according to the lab.
When it works well, SEI allows certain lithium ions to pass through, while limiting chemical reactions that reduce battery performance and accelerate cell failure – one of the goals of lithium-metal battery researchers is to tip the balance away from unwanted electrolyte-lithium reactions and towards the essential ones.
Thicker strips contribute to what the team has dubbed ‘dry SEI’ where the liquid electrolyte doesn’t reach all of the lithium.
“Simply, because the lithium strips are thicker, the electrolyte needs to flow into deeper pockets of the lithium and, as it does so, it leaves other portions of the lithium dry,” said the lab. “This stops important reactions from occurring, effectively smothering necessary electrochemical reactions, and contributes directly to the early death of the battery.”
600 cycles is a step up from 200 cycles achieved at PNLL two years ago, and 50 cycles two years before that.
PNNL is leading a multi-institution group called the Battery500 Consortium, which aims to develop electric vehicle batteries that are lighter, more energy intensive and less expensive than those currently used. It is aiming at 500Wh/kg, about double that in cars today.
The consortium is also tacking another big issue with Li-metal batteries: safety – they are far more prone to thermal run-away than Li-ion cells.
The 600 cycle battery is described in Nature Energy paper ‘Balancing interfacial reactions to achieve long cycle life in high-energy lithium metal batteries‘.