09-12-2020 | | By Liam Critchey
Lithium-ion (Li-ion) batteries have become the most common rechargeable battery in modern-day society. They are used in a lot of consumer goods, as well as more advanced technologies. Li-ion batteries have become the norm thanks to their high stability, not necessarily their high-energy-density.
While they have a respectable energy density, there are better options out there—energy density wise—but they are either not stable enough for consumer goods, or they are unfeasible for large scale commercialisation. Even within the Li-ion space, silicon electrodes offer better energy densities than graphite, but the electrodes tend to destabilise and break down easily. Hence they are not used (although work is being done in this area).
Outside of Li-ion space, and within the general Lithium-metal (Li-metal) battery space, there is a lot of potential for high-density rechargeable batteries. However, there are still some issues that present themselves for high energy density lithium batteries, especially when the batteries are used for prolonged periods. One of the key issues in these scenarios is the formation of dendrites from the anode.
Dendrites can occur in all types of anodes, but Li-metal anodes tend to be susceptible. This is because they have one of the highest theoretical energy density capacities (and lowest electrochemical potentials) of all lithium-based anodes. When they are coupled with high-capacity cathode material, they can produce Li- batteries (Li-Sulphur, Li-oxygen, Li-air, and Li-metal) with high-energy densities—but this can hasten dendrite formations.
Over time, many of these high-energy-density batteries suffer from dendrite growth, and this affects the long-term performance and stability of the battery, as well as posing a fire safety hazard. So, there is a big drive to find ways where this growth can be minimised, and ideally, completely prevented in Li-metal batteries so that their energy density potential can be harnessed on a large scale.
Dendrites form because of the chemical reactions that occur within the battery. Unlike the graphite electrodes found in Li-ion batteries, the lithium-metal anodes rely on lithium stripping and plating mechanisms, which is ultimately the cause of lithium nucleation, and in turn, dendrite growth.
Dendrite growth can form by two different mechanisms. At low current densities, ‘mossy’ looking lithium (larger chunks of lithium) grows from the roots of the anode. From here, and when the current density is increased, the lithium dendrites to grow at the tips and form ‘wispy’ like dendrites that spread out in many directions.
Out of both dendrite stages, wispy dendrites pose a big issue because the formation process is self-amplificated, where the enhanced electric field at the tip of the dendrite attracts more lithium ions to the dendrite, causing them to grow continually. Moreover, the tips of the dendrites tend to be hemispherical, which promotes the growth of the dendrites in all three dimensions.
Because wispy dendrites pose the most risk and form from the larger mossy dendrite mass, the prevention of dendrites needs to start at the initial stages. Inactivating the mossy lithium has become a key target area as this is a way of preventing them from growing into wispy dendrites.
There have been several different strategies proposed, but the addition of additives into the electrolyte has offered the most potential for suppressing the formation of lithium dendrites. Some of these are utilised in the formation of the solid electrolyte interphase (SEI). In contrast, others absorb on to the surface of the lithium buds (that protrude from the electrode) to prevent the formation of dendrites. Both strategies have their own merits and work is being done in both areas to try and completely remove dendrite growth in Li-metal batteries.
Researchers have now found a novel way of reducing the growth of dendrites in Li-metal anodes. Researchers are continually finding new ways of tackling dendrite growth, from utilising solid-state electrolytes to inorganic shells around the electrode. Still, now a team has chosen to go via a more natural and organic approach and used proteins to try and prevent the wispy dendrites from forming before they become too large and uncontrollable.
By adding fibroin molecules into the battery’s electrolyte, a ‘self-defence’ agent was formed that mimics the natural immunisation mechanisms found within the body, i.e. an early response to the formation of mossy lithium is much like an early immune response (pathogen localisation and antibody formation) by the body to a pathogen. The protein molecules within the electrolyte absorb onto the surface of the lithium-metal anodes and locate themselves on the tips of any lithium dendrite buds on the electrodes. Once here, the proteins prevent the growth and nucleation of dendrites by blocking the evolution of the lithium buds at the initial growth stage.
This adsorption onto the lithium buds is a preferential and spatial conformation process. The adsorption of the proteins onto the lithium buds transforms their secondary structures from α-helixes to β-sheets and is a key mechanism in their success. This conformational change changes the electric field distribution around the lithium buds, which results in a homogenous plating and stripping of lithium ions from the electrode during use. In practical terms, this means that any lithium nuclei will deposit further away from any of the dendrite buds, so the lithium deposition is levelled out across the whole battery and dendrites do not form.
The research team also utilised fibroin interlayers to overcome the limited dispersibility of the protein with the electrolyte, which enables a slow and sustained release of the protein into the electrolyte over time. By doing this, they produced a dendrite-free Li-metal battery with high cycling performance.
While this is one of several different strategies that have come to the fore recently, it’s certainly an interesting one as it mimics natural immunisation mechanisms. Whether it is this specific method or one of the many others being trialled, battery scientists and engineers are getting ever closer to realising dendrite-free batteries that could have high energy densities. So, it may only be a matter of time before we see the various lithium-metal batteries being used more and more in modern-day technologies.