In recent years, researchers have been trying to develop increasingly advanced battery technologies that can be charged faster and store more energy, while also remaining safe and stable over time. Lithium-metal batteries (LMBs), which contain a lithium-metal-based anode, have been found to be promising alternatives to lithium-ion batteries (LiBs), which are currently the most widely used rechargeable batteries.

A key advantage of LMBs is that they can store significantly more energy than LiBs, which could be advantageous for electric vehicles and other large or advanced electronics. Despite their potential, these batteries have so far proved to be less stable and safe than LiBs, while also charging relatively slowly; limitations that have so far prevented their widespread adoption.

A research team at the Korea Advanced Institute of Science and Technology (KAIST) and other institutes recently designed new electrolytes based on symmetric organic salts, which could help to boost the performance of LMBs. Their newly designed electrolytes, introduced in a paper in Nature Energy, were found to improve the stability and charging speed of LMBs, preventing the formation of dendrites (lithium deposits that cause a battery’s performance to decline over time).

“The realization of LMBs requires an electrolyte that combines non-flammability with high electrochemical stability,” wrote Akito Sakai, Yosuke Matsumoto and their colleagues in their paper. “Although current electrolyte technologies have enhanced LMB cyclability, rational electrolyte fabrication capable of simultaneously addressing high-rate performance and safety remains a grand challenge. We report an electrolyte design concept to enable practical, safe and fast-cycling LMBs.”

To create their electrolytes, the researchers added an ionic plastic crystal called 1,1-diethylpyrrolidinium bis(fluorosulfonyl)imide (Pyr₂(2)FSI) to different conventional battery electrolytes. The resulting symmetric organic salt was found to alter the interactions between lithium ions and other charged particles, ultimately allowing them to move easily inside a battery.

“We created miniature anion–Li⁺ solvation structures by introducing symmetric organic salts into various electrolyte solvents,” wrote Sakai, Matsumoto and their colleagues. “These structures exhibit a high ionic conductivity, low desolvation barrier and interface stabilization.”

The researchers showed that their electrolytes lower the so-called desolvation barrier. As a result, they make it easier for lithium ions to reach the batteries’ electrodes, which can positively impact the battery’s charging speed and overall lifespan.

Moreover, the new electrolytes prompt the formation of a stable protective layer, known as solid-electrolyte interphase (SEI) on the lithium-metal anode. This protective layer prevents undesirable chemical reactions and the build-up of lithium, which cause batteries to degrade over time.

The team tested their electrolytes in a series of tests, introducing them in LMBs, and found that they improved the batteries’ stability and speed of cycling. In addition, the electrolytes are non-flammable, robust against overheating and far safer than many other electrolytes introduced in the past, which could facilitate their future practical use.

“Our electrolyte design enables stable, fast cycling of practical LMBs with high stability (LiNi_0.8Co_0.1Mn_0.1O₂ cell (twice-excessed Li): 400 cycles) and high power density (pouch cell: 639.5 W kg⁻¹),” wrote the researchers. “Furthermore, the Li-metal pouch cell survived nail penetration, revealing its high safety. Our electrolyte design offers a viable approach for safe, fast-cycling LMBs.”

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