Magnesium batteries could one day complement lithium-ion batteries. But so far they have failed because of a stubborn weak point: the anode wears out too quickly. An international team of researchers has now filtered out the most promising solution from over 2,200 material compounds. The result: A magnesium-tin anode that lasted over 1,300 hours in laboratory tests and far exceeds the performance of previous approaches.
Magnesium is considered a promising anode material for future batteries. The inexpensive metal impresses with its high volumetric capacity of 3,833 milliamperes hours per cubic centimeter and is plentiful. In addition, under suitable conditions, it enables deposition without crystalline branches. Despite these advantages, development has so far failed due to strong passivation of the metal anode.
The cause of this wear is the high reactivity of the metal at interfaces. Components of the electrolyte spontaneously decompose on the surface and form an insulating passivation layer. This layer hinders the transport of charged ions and causes progressive polarization of the cell. Previous additive-based strategies usually only showed limited stability, while nanostructured architectures are often difficult to prepare in a scalable manner.
Magnesium battery: How a screening of 2,227 compounds found the best candidate
A research team led by Qian Wang, Hao Li and Yigang Yan used an approach for their work that is based purely on the analysis of data. Using fast, automated computer calculations, the experts examined 2,227 different compounds, each consisting of magnesium and one other element. From this they filtered out 596 candidates who are stable or almost stable.
For the subsequent practical experiments, they selected five typical elements that are well suited for mixing metals (alloying): calcium, aluminum, tin, bismuth and lanthanum. The highly toxic substances cadmium and mercury were excluded from the outset. The calculations ultimately showed that a particular magnesium-tin compound called Mg2Sn forms the most promising additional structure (secondary phase) in the material.
Stable for 1,300 hours: What makes the tin-magnesium anode so superior
In electrochemical measurements, the modified anode delivered a peak current density that exceeded the level of pure magnesium by more than 440 times. At a constant operating temperature of 50 degrees Celsius, the test cell remained stable for over 1,300 hours. The overvoltage remained constant at a low level of less than 0.05 volts.
Other mixtures tested with calcium, aluminum or lanthanum failed after around 60, 180 or 220 hours. From the results, the team derived general guidelines for building long-lasting solid-state batteries. Accordingly, the additional structure should form a continuous, stable network in order to distribute the chemical reactions evenly throughout the entire material.
In addition, the responsiveness at the contact surfaces must be activated in a controlled manner, while at the same time continuous conduction paths made of the magnesium base material are maintained. These principles link the energetic behavior at the contact surfaces, the spatial arrangement of the material structures and the speed of the chemical-electrical reactions in order to develop particularly robust battery poles made of magnesium.
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