A novel high-concentration solid polymer electrolyte for high-voltage lithium-metal batteries

Lithium metal batteries, which contain lithium metal-based anodes, are promising rechargeable batteries that could help meet the growing demands of the electronics industry. These batteries have various advantages, including high energy density and fast charging times.

Researchers have recently attempted to design new electrolytes that could further improve the performance of lithium-metal batteries. These are often organic liquid electrolytes or inorganic solid electrolytes.

Although some of these electrolytes perform better than others, the liquid and solid alternatives have significant limitations. Organic liquid electrolytes can compromise the safety of a battery, while inorganic solid electrolytes typically exhibit high interface resistance, resulting from poor contact between solid electrolytes and solid electrodes.

A potential alternative to these existing liquid and solid electrolytes is the use of polymer-based electrolytes. By taking advantage of the advantageous properties of polymers, these electrolytes could overcome the limitations of previously introduced electrolytes.

Researchers from the University of Maryland, the University of Illinois and other institutes recently introduced a new high-concentration solid polymer electrolyte for lithium-metal batteries. Their proposed electrolyte, described in an article published in Natural energycould improve the safety, stability and energy density of lithium-metal batteries, maintaining good mechanical strength, eliminating the interface in the polymer and suppressing the growth of Li dendrites.

“Polymer electrolytes can take advantage of the low contact resistance of liquid electrolytes and the high safety of solid electrolytes, but still suffer from two problems: low ionic conductivity and growth of Li dendrites at the anode and along of the interface”, Weiran Zhang, holder of a Ph.D. .D. student in the Chunsheng Wang group at UMD and first author of the paper, told Tech Xplore.

“To overcome these challenges, we took inspiration from state-of-the-art high-concentration liquid electrolytes, where a high concentration of Li salt promotes Li production.⁺ conduction and forms a protective solid electrolyte interphase (SEI) to suppress the growth of Li dendrites at the anode.

Although high Li salt concentration can facilitate the formation of inorganic-rich SEI and thus suppress Li dendrite growth, it can also compromise the mechanical strength of polymers. Zhang and his colleagues set out to design an approach to design high-concentration polymer blends that maintain good mechanical strength.

Their approach builds on knowledge gathered from previous studies to add another composition to enhance mechanical strength. However, previous studies have also shown that the interface within solid electrolytes can facilitate the growth of Li dendrites. The researchers therefore hypothesized that the polymers should be miscible with each other, which would eliminate this interface.

Based on this hypothesis, they then examined the composition of various polymers and ultimately designed a promising new polymer electrolyte. This electrolyte is based on mixtures of two miscible polymers.

“We proposed the use of two miscible polymers: a Li⁺ conductive polymer and an inert fluorine-based polymer,” Zhang explained. “The Li⁺ A conductive polymer (poly(bis(trifluoroethoxy)phosphazene)) with a high Li salt content (lithium bis(fluorosulfonyl)imide) acts as a Li-dendrite suppressor to form an SEI layer that blocks the lithium dendrite and prevents short circuits. Meanwhile, the inert polymer (Poly(vinylidene fluoride-co-hexafluoropropylene)), adds mechanical strength to the polyblends.

The researchers discovered that this miscible polyblend acquired advantageous associative properties. Specifically, they found that the proposed mixture strengthened the mechanical strength of the electrolyte, while retaining its high lithium stability and making it more resistant to lithium dendrites. The new electrolyte also exhibited exceptional high-voltage stability, which surpassed that of previously introduced polymer electrolytes.

“In summary, our novel solid polymer electrolyte combines high mechanical strength with stability, demonstrating its compatibility with lithium metal and high-voltage cathodes,” said Professor Chunsheng Wang, principal investigator in the group. “Traditional polymer electrolytes exhibited limited stability when combined with the lithium metal anode, resulting in Coulomb efficiency ranging from 90% to 98%. In our work, we improve this metric to over 99%, one of the state-of-the-art performances. reported so far. “

“Most polymer electrolytes are oxidized at low potential, so they are not electrochemically compatible with a typical 4.2 V cathode,” Zhang said. “On the other hand, our polymer electrolytes demonstrate compatibility with high energy LiNi_0.8Co_0.1Mn_0.1Oh₂ (NMC811) cathode at 4.5 V. This large improvement in stability with the Li metal anode and high voltage cathode represents a major advancement in the field of polymer electrolytes. »

This recent work by Zhang and colleagues presents a design strategy that could help overcome common limitations of polymer electrolytes. Instead of just reporting a few individual electrolytes, the team aims to introduce a design principle that could soon be used to filter other polymer mixtures and make new polymer electrolytes.

Wang and his team hope their design will facilitate the large-scale deployment and commercialization of polymer electrolytes. The proposed design strategy could also soon lead to the introduction of a new class of safe, high-energy lithium-metal batteries for small smart devices.

“So far, the concept presented in our paper has been demonstrated and implemented on a relatively small scale, such as button batteries and small pocket cells,” Wang added. “In the next step, we will aim to expand the application, including the production of electrolytes in larger quantities. We plan to integrate these electrolytes into large, high-capacity multilayer pocket cells to evaluate their commercialization potential. we will perform further tests to evaluate the flammability and safety aspects of cells using these polymer electrolytes.

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