Asymmetric ether solvents enhance Li-metal battery charging and stability

To fuel the future advancement of the electronics industry, engineers will need to develop batteries that can be charged quickly, have higher energy densities (i.e., can store more energy) and last longer. Among the most promising alternatives to lithium-ion (Li-ion) batteries, which power most devices on the market today, are lithium-metal batteries (LMBs).

As suggested by their name, LMBs have an anode (i.e., negative electrode) made of Li metal. Compared to Li-ion batteries, which have graphite or silicon-based anodes, LMBs can exhibit significantly higher energy densities.

Despite their potential, LMBs have been found to exhibit slow redox kinetics and poor cycling reversibility. These limitations tend to adversely impact their performance, reducing their charging speed and their efficiency over time.

Researchers at Stanford University have been trying to develop new electrolyte solvents that could improve the performance of LMBs.

In a paper, published in Nature Energy, they introduce asymmetric ether-based solvents that were found to speed up the charging of LMBs, while also boosting their stability and reliability over time.

“Our goal was to enable high-rate lithium metal batteries by designing better solvent molecules,” Rok Choi, first author of the paper, told Tech Xplore. “We drew inspiration from ethyl methyl carbonate (EMC), an asymmetric alkyl carbonate used in Li-ion batteries, and explored whether a similar asymmetric structure could enhance ether solvents for Li-metal batteries.”

Ether-based solvents have often been used as battery electrolytes. Conventional ether-based solvents are compounds containing two hydrocarbon groups linked by oxygen atoms and are symmetric.

These symmetric ether solvents have been found to slow down the rate with which lithium ions are exchanged, thus adversely impacting the speed with which a battery is charged and its stability over time.

Choi and his colleagues thus set out to explore the performance of asymmetric ether solvents, which are made up of molecules with different side groups, as electrolytes for LMBs.

“We designed solvents that minimize steric hindrance during Li⁺ desolvation,” explained Choi. “Symmetric solvents tend to block Li⁺ from the anode under an electric field, slowing charge transfer. In contrast, asymmetric solvents align in a way that facilitates faster Li⁺ reduction and desolvation.”

The researchers optimized the dipole orientation (i.e., alignment of pairs of positive and negative charges) in their solvents. They found that this improved charge transfer, thus facilitating the movement of Li ions, promoting the formation of a more stable solid-electrolyte interphase (SEI) and a uniform Li-plating layer onto an Li metal anode.

“We discovered that higher molecular asymmetry accelerates Li⁺ kinetics, leading to a more stable SEI and longer cycle life under high-rate conditions,” said Choi.

“By optimizing both the ether backbone and fluorination degree, we developed F3EME as an ideal solvent, which demonstrated over 600 cycles for anode-free pouch cells in a testing protocol designed to mimic eVTOL (electric vertical take-off and landing) applications.”

In initial experiments, the asymmetric ether solvents designed by this team of researchers were found to significantly improve the performance and stability of LMBs.

In the future, Choi and his colleagues plan to design other electrolytes with similar underlying molecular structures, while also introducing them into various Li-based batteries and further assessing their potential.

“Building on this molecular design strategy, we aim to expand our solvent portfolio for various battery systems, including Li-metal, Li-ion (with Si anodes) and Li-S batteries,” added Choi.

© 2025 Science X Network