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New electrolyte boosts lithium-air batteries

Scientists in the UK reported a breakthrough in lithium-oxygen batteries, achieved by experimenting with different electrolyte compositions. Working with ionic liquids, they were able to tweak the electrolyte recipe to minimize unwanted reactions during battery cycling, and greatly improve on both performance and stability.

It’s a common story in the world of battery research – new materials and designs that promise much higher performance face challenges and limitations in terms of reaching their potential in a practical device and demonstrating long-term stability.

And this is particularly true for batteries incorporating lithium or other transition metal anode materials, which tend to quickly lose performance due to the formation of dendrites – branch-like structures that grow into the electrolyte and even across to cathode side, causing short-circuiting. Solid-state batteries currently appear the most promising commercial route to solving this problem, however, several other approaches have also caught the attention of researchers, among them lithium-oxygen batteries.

A group of scientists led by the UK’s University of Liverpool (UoL) has found that careful control of the electrolyte composition can effectively “switch off” the reactivity of certain components and minimize the appearance of unwanted side reactions that happen during battery cycling, including the growth of dendrites.

“The ability to precisely formulate the electrolyte using readily-available, low volatility components enabled us to specially tailor an electrolyte for the needs of metal-air battery technology that delivered greatly improved cycle stability and functionality,” explained UoL research associate Alex Neale. “The outcomes from our study really show that by understanding the precise coordination environment of the lithium-ion within our electrolytes, we can link this directly to achieving significant gains in electrolyte stability at the Li metal electrode interface and, consequently, enhancements in actual cell performance.”

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All in the electrolyte

The group worked with varied formulations of solvent, salt, and ionic liquid, and found that adding an ionic liquid allowed them to achieve much better stability results. “It was exciting to see through the use of both calculations and experimental data we were able to identify the key physical parameters that enabled the formulations to become stable against the lithium metal electrode interface,” said Pooja Goddard of Loughborough University, who collaborated in the research.

Their optimized electrolyte solutions are described in full in the paper Design Parameters for Ionic Liquid–Molecular Solvent Blend Electrolytes to Enable Stable Li Metal Cycling Within Li–O2 Batteries, published in Advanced Functional Materials. The optimizations allowed them to demonstrate battery cells with 94% Coulombic efficiency as well as more than 900 hours of cycling with no increase in overpotential. “This work exemplifies a useful electrolyte design strategy for Li-air batteries underpinned with excellent science within a great collaboration,” added Enrico Petrucco of Johnson Matthey PLC – a London-based chemicals company that partnered with UoL on this research. “This moves us another step closer towards practical routes to overcome complex Li-air challenges.”

The group says that the electrolytes could be even further improved through specific tailoring of the lithium salts and ionic liquids to achieve lower viscosity and that the careful introduction of a fourth element, a non-solvating thinning additive, to the electrolyte mix would also be worth investigating.

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Source: pv magazine