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Mg-ion batteries: amorphization and crystallization in In-Pb anodes

Mg-ion batteries: amorphization and crystallization in In-Pb anodes

Automotive electrification and renewable energy storage are currently dominated by Li-ion battery technology, which depends on resources such as lithium, graphite, copper and certain transition metals available in limited quantities and/or unevenly distributed geographically. New battery technologies based on other alkaline or alkaline-earth ions with virtually unlimited resources may in the long term partially replace Li-ion batteries for certain applications. Magnesium-ion batteries are one of these alternative technologies, due to the high abundance of magnesium and the high volumetric and gravimetric capacities that can be achieved.

Following on from initial work on the InSb compound, a team at IRAMIS has developed a new negative electrode material for Mg-ion batteries based on the In-Pb compound. The synergistic combination of the electro-active elements In and Pb influences the reaction mechanisms and structure (amorphous/crystalline) of the products formed during the reaction with Mg. This favors high capacitance, but is subsequently detrimental to the material’s reversibility. These results illustrate the influence of electrode amorphization and crystallization processes on the electrochemical performance of batteries.

The growth of sectors such as electric vehicles and renewable energy storage is driving battery research, where efforts are now focused on developing more durable, high energy density batteries. One example is the development of magnesium metal (Mg) batteries. Magnesium is emerging as an excellent alternative to lithium, thanks in particular to its high volumetric capacity (3833 mAh/cm3), low cost and abundance in the earth’s crust. For these batteries, the use of metallic magnesium at the anode limits the choice of electrolyte to a few specific compositions, often highly corrosive and with a very narrow potential stability window. On the contrary, electrodes made of certain p-block elements react with magnesium ions in electrolytes with wider stability windows. In a recent perspective review on Mg-ion batteries, contributed by several international groups including NIMBE/LEEL, it is shown that the use of these alloys in place of Mg metal reduces the energy density of the electrochemical cell (see figure below). However, thanks to their greater compatibility with electrolytes and simpler processing, they are potentially interesting anodes for Mg-ion batteries, and synergistic effects are expected for p-block cell combinations.

Volumetric energy density versus specific energy density (relative to mass) of Mg batteries, for various cathodes (color-coded) and two types of anode composition: left, Mg metal anode; right, Mg2Snalloy-based anode [2].

After proposing an initial alloy based on InSb [3], the LEEL team explored the solid solution In-Pb [1] to determine whether, like InSb, a beneficial effect existed between the 2 elements In and Pb. Firstly, it was shown that the synthesis of the material greatly influenced its reactivity. An In-Pb powder produced by mechano-synthesis with particle sizes > 200 µm led to no reaction with Mg2+ ions (see figure below). By introducing carbon into the synthesis by grinding, the particle size is reduced by a factor of 20 to 40 and the In-Pb grains become electrochemically active.

Scanning electron microscopy (SEM) images: top, 5h ground In-Pb powder showing grains of the order of 200 µm; bottom, 5h ground In-Pb powder in the presence of carbon, showing a grain size of less than 10 µm. Reactivity with magnesium is a direct function of particle size [3].

The behavior of the In-Pb alloy was studied by electrochemical analysis and ex situ X-ray diffraction. The number of Mg2+ ions inserted into the structure (see figure below), close to 3, suggests the formation of Mg2Pb and MgIn phases. However, unlike the case of pure In or InBi electrodes, no indium-containing crystalline phase is detected after magnesiation. However, the electrical capacity obtained by inserting Mg (calculated from the electrochemical curve) exceeds that stored in the detected Mg2Pb phase. This results in the formation of an amorphous MgIn phase. This behavior echoes that of InSb, where the observed amorphization of MgIn remains dependent on reaction kinetics, while amorphization is always observed for In-Pb.

Black curve: evolution of the potential as a function of the number of Mg2+ ions inserted into the alloy during the first charge-discharge cycle, with mention of the initial InPb crystalline phase and at the end of the cycle (after demagnesiation), and at the end of charging after magnesiation (amorphous Mg2Pb) [1].

The coupling between In and Pb shows a real advantage for the first charge cycle, where a capacity of 488 mAhg-1 can be achieved, higher than the values obtained for In and Pb alone. However, a sharp reduction is observed after the first cycle, with a reversible capacity of around 300 mAh/g on subsequent cycles. A more detailed understanding of the underlying reactions is required, in particular to determine whether the presence of MgIn in amorphous and non-crystalline form has an impact on the reversibility of the material.

References

[1] “Electrochemical reactivity of In-Pb solid solution as a negative electrode for rechargeable Mg-ion batteries”,
Lucie Blondeau, Suzy Surblé, Eddy Foy, Hicham Khodja, Magali Gauthier, Journal of Energy Chemistry 55 (2020) 124-128.
[2] “Magnesium batteries: Current picture and missing pieces of the puzzle”
Robert Dominko, Jan Bitenc, Romain Berthelot, Magali Gauthier, Gioele Pagot, Vito Di Noto, Journal of Power Sources 478 (2020) 229027.
[3] Unexpected behavior of the InSb alloy in Mg-Ion batteries: unlocking the reversibility of Sb.
Lucie Blondeau, Eddy Foy, Hicham Khodja, Magali Gauthier, The Journal of Physical Chemistry C, 123 (2019) 1120.
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CEA-IRAMIS contact: Magali Gauthier(NIMBE/LEEL)

Collaboration :

Luc BARBIER