Constructing a reliable lithium-metal battery has long been a major interest of scientists around the world, as it is safe to assume that one day they may play an important role in next-generation battery technology. Lithium-metal batteries have a higher energy capacity than lithium-ion batteries and could potentially last longer and weigh less. However, a major problem concerning lithium-metal batteries is the low Coulombic efficiency, which is responsible for their going through only a limited number of cycles before failing. Also, safety problems like fires and explosions have impeded their widespread commercialisation. Several hypotheses have been voiced as to why they fail, but so far definite evidence has eluded scientists.
Now (2021), scientists at Sandia National Laboratories have managed to produce the first nanoscale images from the inside of intact lithium-metal coin batteries which will hopefully enable the production of more powerful as well as longer-lasting and safer high-performance batteries, as are needed, for example, in electric vehicles. The scientists repeatedly charged and discharged lithium coin cells with the same high-intensity electric current usually used for electric vehicles and found that some cells underwent only a few cycles, while others underwent more than a hundred cycles. Their hypothesis was that needle-shaped deposits of lithium would be found throughout the battery which had formed after the repetitive cycling and that these spikes had perforated the plastic separator between the anode and the cathode, creating a bridge and causing a short. However, to their surprise the team discovered a second culprit in the form of a hard build-up which formed a by-product of the internal chemical reactions of the battery, called solid electrolyte interphase. When capping the lithium it made holes in the separator, creating openings for metal deposits to spread and create a short.
Another problem in determining the cause of why lithium-metal batteries fail is related to the stainless-steel casing because the metal prevents accurate diagnostics, when using, for example, X-rays. Opening the battery, on the other hand, destroys its layers and distorts the results too. Therefore, the scientists used a microscope that had a laser attached to get a look inside and also combined it with a sample holder that kept the liquid electrolyte in a frozen state at temperatures between minus 100 and minus 120 degrees Celsius. Through the laser a small opening large enough for a narrow electron beam was created which entered it, then bounced back onto a detector and subsequently delivered a high-resolution image of the internal cross section of the battery. The image was detailed enough to make out the different materials.
Scientists have long tried to improve the performance of lithium-metal batteries. In 2020, the reactivity of metallic lithium was reduced by putting it into a substrate of silicon coated with thin films and etched with millions of tiny cells. The 3D substrate greatly enhanced the surface area of the anode compared with a traditional two-dimensional anode of lithium ion. If metallic lithium instead of a compound was used, the new anode even reached up to 10 times the capacity of a traditional intercalated graphite-lithium anode.
In 2019, scientists looked into the causes of lithium-metal battery failure and discovered that lithium metal deposits which broke off the anode during discharging and then got trapped in the solid electrolyte interphase layer were responsible for this, as they became inactive and could no longer be cycled through the battery. They created a method to analyse how much unreacted lithium metal was trapped as inactive lithium. For this purpose, water was put into a sealed flask containing a sample of inactive lithium that formed on a cycled half-cell so that the bits of unreacted lithium metal would chemically react with water and produce hydrogen gas. By measuring how much gas was produced, researchers could calculate the amount of trapped lithium metal; the more of it that formed, the lower the Coulombic efficiency.
The results of the study may lead to several improvements in battery design: the scientists learnt that separator materials need to be adapted for lithium metal in order to increase efficiencies, as the experiments had shown that degradation of the battery cycling performance was caused by the destruction of ion-transport pathways between the stacked electrodes and consumption of Li metal by excessive solid electrolyte interphase formation. Also, the new analysis method enabled a structural and chemical analysis of intact battery layers without harming the interfaces between the solid/liquid/polymer composite stack, thus reducing air or water vapor exposure that would otherwise alter the sensitive battery materials. Future research efforts will focus on minimising solid electrolyte interphase formation and improving the separator for high-rate cycling Li-metal batteries.
Until lithium-metal batteries can be successfully launched onto the market, more research will have to be carried out. The research conducted at Sandia National Laboratories is a valuable contribution to transforming lithium-metal batteries into marketable products.