In the diverse world of rechargeable batteries, lithium metal batteries are attracting growing interest. They differ from lithium-ion batteries in that they use metallic lithium for the anode (in lithium-ion batteries, the anode is made of graphite or silicon).
This solution means that lithium-ion batteries perform better in terms of energy density, but there is an inherent problem in that their lifespan is too short.
Recently, however, a number of companies have considerably improved the technology, so lithium metal batteries are slowly making a comeback. More and more companies are entering the sector, and more and more car manufacturers are investing in research and development.
High density, high instability
Let's start with a trip down memory lane: lithium metal batteries were developed over half a century ago. Initially, this chemistry was used for disposable products, precisely because the large amount of energy they could store meant they could be used for longer. They were more expensive than average and also more complicated to produce, but were better suited to 'energy-intensive' applications.
In the 1980s, the first rechargeable lithium-metal battery appeared on the market. It was greeted with great interest, but because of serious reliability problems, it was withdrawn from the market. Due to the low Faraday efficiency from which all lithium metal batteries suffer, it was prone to undesirable secondary chemical reactions during charging and discharging, as well as the formation of dendrites. This often led to short circuits, with the associated risk of fire.
The right electrolyte is needed
Now back to the present day. Lithium metal batteries are even more expensive to produce than lithium ion batteries, but their performance justifies their higher price. In addition to their higher density, lithium-metal batteries can be recharged much more quickly. But how can this technology be made definitively mature? The secret lies in the electrolyte.
Solid electrolyte for lithium metal batteries
As we mentioned earlier, lithium metal batteries work in a similar way to lithium ion batteries. They consist of a negative electrode (anode) and a positive electrode (cathode), a separator that separates the two poles and an electrolyte that allows ions to pass back and forth. It is precisely by using the right electrolyte that lithium metal batteries can become safer and more durable. Research has also shown that there is no single method for finding the right chemical formula.
Liquid electrolyte is perfect
Researchers at Vilas Pol Energy Research at Purdue University in Lafayette, USA, have opted for a liquid electrolyte, for example. More specifically, they used a highly apolar electrolyte solvent that proved to be particularly effective in enabling the battery to have stable behaviour and constant performance over time. Compared with the average lithium-ion battery on the market, the US research institute's lithium-metal battery has a density 40% higher and shows virtually no dendrite formation.
Scientists at the University of Chicago's Pritzker School of Molecular Engineering have also developed a non-flammable, non-volatile liquid electrolyte: it is safe and could even double the density of the battery, but for the moment it is a product that is still being tested in the laboratory.
Some companies and universities believe that the lithium metal battery is most promising when combined with a solid electrolyte. This is the case of Factorial, a company in which numerous automotive groups such as Stellantis, Mercedes and Hyundai-Kia have invested. The American company (based in Massachusetts) has presented 100 amp-hour cells that should be able to be used in cars.
Factorial's 100 Ah solid-state battery
They are ready, have received United Nations safety certification and are currently being tested by the car manufacturers that have financially supported the company. They promise a density of 50% compared with lithium-ion batteries.
Still in the field of solid electrolytes, American and Canadian researchers have collaborated to develop a lithium-based electrolyte that effectively resists the formation of dendrites and guarantees acceptable performance for 1,500 cycles.
Finally, one of the most promising companies in the field of lithium-metal batteries is Sion Power, a US company that has been working for years on batteries using this technology and has developed a reliable and durable battery with a density of 420 kWh per kilo. A product that, thanks to its density, would also be suitable for electric aircraft. And if even NASA is interested in this product, it's because it clearly offers something interesting.