Isotropic MOF coating reduces side reactions to boost stability of solid-state Na batteries

In recent years, energy engineers have been trying to design new reliable batteries that can store more energy and allow electronics to operate for longer periods of time before they need to be charged. Some of the most promising among these newly developed batteries are solid-state batteries, which contain solid electrolytes instead of liquid ones.

Compared to batteries with liquid electrolytes that are widely used today, solid-state batteries could exhibit higher energy densities (i.e., could store more energy) and longer lifetimes. However, many of these batteries have been found to be unstable, due to unwanted chemical reactions that occur between their high-voltage cathodes (i.e., positive electrodes) and solid electrolytes, which can speed up the degradation of the batteries’ performance over time.

These undesirable side reactions are particularly common in sodium-ion (Na⁺) solid-state batteries, which use Na⁺ ions to store and release electrical energy. This is because while Na is more abundant and cheaper than lithium, Na-ion batteries are inherently more chemically reactive than Li-ion batteries.

Researchers at the Chinese Academy of Sciences recently introduced a promising strategy to increase the durability and performance of solid-state Na-based solid-state batteries, by minimizing side reactions between their underlying cathodes and solid electrolytes. This strategy, outlined in a paper published in Nature Energy, entails the growth of a dense metal-organic framework (MOF) layer on the surface of high-voltage cathodes, which could prevent them from reacting with solid electrolytes.

“Side reactions between high-voltage cathodes and electrolytes remain a critical obstacle to the advancement of solid-state batteries—particularly for Na-ion systems—due to the higher Na⁺/Na redox potential,” wrote Yuan Liu, Huican Mao and their colleagues in their paper.

“Despite recent extensive efforts, achieving a long cycle life is still challenging at the 4.2 V cut-off (versus Na⁺/Na). We design a room-temperature isotropic epitaxial growth to achieve a relatively uniform and dense metal–organic framework epilayer on Na₃V₂O₂(PO₄)₂F surfaces.”

To assess the potential of their approach, the researchers grew a uniform MOF coating on Na₃V₂O₂(PO₄)₂F cathodes via a process known as room-temperature isotropic epitaxial growth. They then created a solid-state battery, pairing this coated electrode with a solid electrolyte based on the polymer polyethylene oxide.

“Despite using polyethylene oxide, a typical ether-based solid polymer electrolyte, the cathode with isotropic epilayer exhibits enhanced cycling performance at the 4.2 V cut-off (retaining up to 77.9% of its initial capacity after 1,500 cycles),” wrote the authors.

“Combining experimental measurements and theoretical analyses, the key factor governing isotropic epitaxial growth behavior is explicitly elucidated. Furthermore, we develop a self-designed high-sensitivity characterization method, in situ linear sweep voltammetry coupled with gas chromatography–mass spectrometry, to elucidate the failure mechanism of polyethylene oxide on Na₃V₂O₂(PO₄)₂F surfaces and to reveal the excellent electrochemical stability of the isotropic epilayer.”

In initial tests, solid-state batteries based on the team’s coated cathode material were found to perform remarkably well, exhibiting significantly fewer side reactions between the cathode and electrolyte. Notably, the strategy they employed could also be applied to other cathodes and batteries with different compositions.

Other researchers could soon draw inspiration from this study and employ similar strategies to stabilize other Na-based solid-state batteries. In the future, the isotropic epitaxial method developed by Liu, Mao and their colleagues could ultimately contribute to the large-scale deployment of durable and reliable solid-state batteries with high-energy densities.

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