Innovative battery design: more energy, less environmental impact

Researchers at ETH Zurich have developed a new method that drastically reduces the amount of fluorine required to stabilise these high-energy batteries.

Lithium metal batteries: the future of energy storage

Lithium metal batteries are widely considered the next generation of high-energy batteries. They can store at least twice as much energy per unit of volume as the lithium-ion batteries that are commonly used today. This would enable electric cars to travel twice as far on a single charge and reduce the frequency of recharging for smartphones.

However, there has been a significant challenge with lithium metal batteries: the need for large amounts of fluorine in the liquid electrolyte to ensure stability. Without fluorine, these batteries would quickly become unstable, fail after a few charging cycles, and be prone to short circuits, overheating, and even igniting.

The role of fluorine in battery stability

In lithium metal batteries, the electrolyte must prevent the formation of lithium metal whiskers, known as dendrites, during the recharging process. These dendrites can cause short circuits if they touch the positive electrode, posing a significant safety risk. Fluorine compounds in the electrolyte help form a protective layer around the lithium metal at the anode, similar to tooth enamel, which protects the lithium from continuous reactions with the electrolyte. This layer is crucial for battery efficiency, safety, and longevity.

Reducing the fluorine content

The challenge was to reduce the amount of fluorine without compromising the stability of the protective layer. The research team at ETH Zurich, led by Professor Maria Lukatskaya, developed a method using electrically charged fluorinated molecules to transport fluorine to the protective layer. This approach reduces the fluorine content in the liquid electrolyte to just 0.1% by weight, which is at least twenty times lower than in previous studies.

This method employs fluorinated cations, which serve as vehicles to transport fluorine to the protective layer. As a result, the layer remains stable while minimising fluorine use, reducing production costs, and enhancing the sustainability of the battery.

Addressing environmental and cost challenges

The environmental and economic impact of fluorine in battery production has been a significant concern. The high cost of lithium salts and the increased solution viscosity at high salt concentrations make it challenging to implement highly concentrated electrolytes in commercial batteries. Additionally, using heavily fluorinated solvents can substantially increase battery costs and environmental footprint.

The new method developed by the ETH Zurich team addresses these challenges by leveraging electrostatic attraction between positively charged fluorinated cations and the negatively charged lithium metal anode. This technique ensures a significant population of fluorinated species reaches the electrode surface, even when the overall additive concentration in the bulk electrolyte is very low.

Improved battery performance

The research shows that adding fluorinated cations to the electrolyte improves the average coulombic efficiency of lithium plating/stripping from 96.4% to 99.6%. This efficiency is comparable to the best-performing electrolytes containing high concentrations of fluorinated solvents or salts. The addition of these cations also enhances the stability of the battery, with the overpotential remaining almost unchanged for at least 3,000 hours of cycling.

Enhanced safety and efficiency

One of the key advantages of this new method is the formation of a stable and robust solid electrolyte interphase (SEI) that prevents the growth of dendrites and maintains the integrity of the battery during cycling. The SEI formed in the presence of fluorinated cations is enriched with lithium fluoride (LiF), which provides excellent electronic insulation and structural stability.

X-ray photoelectron spectroscopy (XPS) and electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D) studies confirmed that the SEI formed in the presence of fluorinated cations is highly rigid and passivating, indicating a robust and protective nature. This contrasts with the viscoelastic behaviour of SEI formed in additive-free electrolytes, which leads to uncontrolled growth and porous structures.

Future implications for battery technology

The implications of this research are significant for the future of battery technology. The ability to reduce the environmental footprint and production costs while enhancing the efficiency and safety of lithium metal batteries paves the way for their widespread adoption in electric vehicles and other high-energy applications.

By minimising the amount of fluorine required and using readily reducible fluorinated cations, the new method not only addresses environmental concerns but also improves the overall performance and longevity of the batteries. This innovation represents a crucial step towards the development of sustainable and cost-effective high-energy batteries.

The innovative electrolyte design developed by ETH Zurich researchers marks a significant advancement in battery technology. By reducing the fluorine content to a minimal level and leveraging electrostatic attraction, the new method enhances battery efficiency, safety, and sustainability. This breakthrough has the potential to overhaul the production and performance of lithium metal batteries, contributing to the development of more efficient and environmentally friendly energy storage solutions.