Magnesium metal batteries (MMBs) have a longevity problem. As a battery material, magnesium metal (Mg) is a promising alternative to lithium for future energy systems, as it’s more abundant, safe and energy-dense. However, in most MMBs, a stubborn film of Mg deposits forms over their electrodes after just a few charging cycles. This film can deactivate (passivate) an electrode by throttling the flow of Mg2+ ions at its interface with the electrolyte, crippling battery performance.
While researchers have extensively probed at these films to design more durable MMBs, it’s been challenging to fully uncover the links between electrolyte chemistry, interfacial passivation and the structures of Mg deposits during battery operations, noted Zhi Wei Seh, a Senior Principal Scientist at the A*STAR Institute of Materials Research and Engineering (A*STAR IMRE).
“Characterisation tools such as time-of-flight secondary ion mass spectrometry (ToF-SIMS), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) capture only part of the picture,” explained Seh. “None can easily preserve and visualise the freshly deposited metal, its crystal structure and the passivation layer together at the nanoscale.”
To compound the problem, Mg surfaces are highly sensitive. Exposed to air, heat or most characterisation processes, critical interfacial states can quickly change, distorting the structural information captured.
In a recent study, Seh, A*STAR IMRE Scientist Gaoliang Yang and colleagues worked with researchers from the US and China to more accurately capture Mg metal deposition dynamics in single-salt electrolyte MMBs. The team turned to a novel tool: cryogenic transmission electron microscopy (cryo-TEM).
“Cryo-TEM allows us to directly visualise Mg deposits and interfacial structures at the nanoscale in a state close to their true electrochemical environment,” said Yang. “By rapidly freezing and maintaining samples at cryogenic temperatures, we can observe morphology, crystallography and interfacial layers at the same time, making it possible to directly link surface chemistry with deposition morphology.”
The team included collaborators from the University of Texas at Austin, Stanford University, and the University of Houston, US; as well as the Beijing Institute of Technology, ShanghaiTech University, and Central South University, China.
An unexpected discovery was the formation of irregular whisker- or seaweed-like shapes by Mg deposits: a morphology rarely reported in such MMB systems. The team found these structures were caused by a magnesium oxide (MgO)-rich passivation layer, which caused highly uneven Mg2+ ion transport.
To reveal Mg’s natural growth behaviour, the team engineered electrolytes with magnesium chloride (MgCl₂) and borohydride additives to suppress MgO passivation. Under these conditions, Mg deposits grew in closely packed hexagonal platelets, even in MMBs with different electrolyte chemistries. This suggested that irregular Mg structures seen in previous MMB studies may be due to interfacial constraints rather than the metal’s inherent properties.
“Our results show that when surface passivation is suppressed, Mg²⁺ ions can reach the electrode surface more uniformly,” said Seh. “The deposition process is no longer dominated by transport limitations or local electric fields, allowing Mg to grow according to its intrinsic crystallographic and thermodynamic preferences.”
Seh added that based on their findings, it’s not just electrolyte composition that matters for MMBs, but their interfacial chemistry: a significant shift in design philosophy.
“Future efforts should focus on engineering interphases that enable fast Mg²⁺ transport while minimising passivation,” said Yang. “This could improve reversibility, reduce overpotential and enhance cycling stability in MMBs.”
The A*STAR affiliated researchers contributing to this research are from the A*STAR Institute of Materials Research and Engineering (A*STAR IMRE).
