Simple solution found to lithium metal battery problem

Rechargeable lithium metal batteries, which offer high-energy density and longer shelf life, could power an EV for 500 to 700 miles on a single charge, twice the range of lithium-ion being used today.

The problem with lithium metal however, is that such batteries lose their capacity to store energy after relatively few cycles of charging and discharging, which is a problem that a study published today in the journal Nature, claims to have solved.

Researchers at Stanford University have been testing a variety of new materials and techniques to improve the battery’s cycle life and claim they have found a simple and low-cost solution: just drain the battery and let it rest for several hours. Doing this restored battery capacity and boosted overall performance.

Co-lead author Wenbo Zhang, a Stanford PhD student in materials science and engineering explained: ‘We were looking for the easiest, cheapest, and fastest way to improve lithium metal cycling life. We discovered that by resting the battery in the discharged state, lost capacity can be recovered and cycle life increased. These improvements can be realised just by reprogramming the battery management software, with no additional cost or changes needed for equipment, materials, or production flow.’

The results of the study could provide EV manufacturers practical insights on adapting lithium metal technology to real-world driving conditions.

Senior author Yi Cui, the Fortinet Founders Professor of Materials Science and Engineering in the School of Engineering, and professor of energy and engineering in the Stanford Doerr School of Sustainability said: ‘Lithium metal batteries have been the subject of a lot of research. Our findings can help guide future studies that will aid in the advancement of lithium metal batteries towards widespread commercial adaptation.’

A conventional lithium-ion battery consists of two electrodes – a graphite anode and a lithium metal oxide cathode – separated by an electrolyte that shuttles lithium ions back and forth.

In a lithium metal battery, the graphite anode is replaced with electroplated lithium metal, which enables it to store twice the energy of a lithium-ion battery in the same amount of space. The lithium metal anode also weighs less than the graphite anode, which is important for EVs. Lithium metal batteries can hold at least a third more energy per pound.

Co-lead author Philaphon Sayavong, a PhD student in chemistry added: ‘A car equipped with a lithium metal battery would have twice the range of a lithium-ion vehicle of equal size – 600 miles per charge versus 300 miles, for example,. In EVs, the goal is to keep the battery as lightweight as possible while extending the vehicle range.’

The problem is that repeated charging and discharging of the battery causes lithium metal batteries to degrade quickly and the build up of what’s known as ‘dead lithium’ causes the battery to rapidly lose capacity.

As Zhang says.’An EV with a state-of-the-art lithium metal battery would lose range at a much faster rate than an EV powered by a lithium-ion battery.’

Drawing on previous work and avoiding the technical details of the process, the team decided to test a theory they thought might solve this problem.

Zhang again: ‘The first step was to completely discharge the battery so there is zero current running through it. Discharging strips all the metallic lithium from the anode.’

Then they let the battery sit idle.

They found that if the battery rests in the discharged state for just one hour, some of the decomposed electrolyte that surrounds the dead lithium dissolves away. Subsequently, when the battery was recharged the dead lithium reconnects with the anode, ‘because there’s less solid mass getting in the way.’

This  brings dead lithium back to life, enabling the battery to generate more energy and extend its cycle life.

Cui said: ‘Previously, we thought that this energy loss was irreversible, but our study showed that we can recover lost capacity simply by resting the discharged battery.’

Significantly, this does not require expensive, new manufacturing techniques or materials.  As Zhang explains:  ‘You can implement our protocol as fast as it takes you to write the battery management system code. We believe that in certain types of lithium metal batteries, discharged-state resting alone can increase EV cycle life significantly.’