The Magnesium battery is considered a potential a high energy, sustainable successor to the lithium-ion battery, due to an almost two-fold increase in the volumetric capacity of magnesium compared to lithium (Li), decreased probability of dendritic growth, cheaper raw material costs, and high natural abundance.^[1]–[3] Current electrolytes have been found to be insufficiently stable towards the Mg electrode, leading to reduction of the electrolyte and formation of a solid electrolyte interphase (SEI), which is believed to be detrimental to performance.^[4]–[6] Our previous study ^[7] indicated a cycling mechanism at Mg surface in a Mg(TFSI)₂-based electrolyte occurring through Mg deposits and an evolution of interphase chemistry during conditioning that is critical for stable cycling in the Mg(TFSI)₂-glyme electrolyte. However, unlike Li metal batteries,^{[8], [9]} where the Li plating and nucleation mechanism has been studied in depth, this is not the case for Mg batteries.

In this study, we have combined electrochemical analysis with state-of-the-art cryo-focus ion beam scanning electron microscopy (FIB-SEM) and energy-dispersive X-ray spectroscopy (EDX), aiming to give insight into the Mg nucleation & growth mechanism. In doing so, we are able to reveal the detailed chemical and structural composition of the Mg deposits for the first time. Our studies are performed in Mg(TFSI)₂-tetraglyme electrolyte as the leading base electrolyte for the battery. By linking the structure of Mg deposits to their state of charge and cycling performance, we can conclusively demonstrate the origin of the high overpotential in the battery. In addition, we show how Mg is reversibly plated and stripped within the deposit and demonstrate how the structure and size of the Mg deposit fluctuates to accommodate this process.

Image caption:

Electron microscopy images of the cross -section of a Mg particle after discharge etched using cryo-FIB-SEM, showing. a) secondary electron images of the exposed cross-section and b) In lens secondary electron images highlighting the distinct regions of the particle: Mg-rich inner core, MgO-rich outer core and interphase.

References:

[1] G. N. Newton, L. R. Johnson, D. A. Walsh, B. J. Hwang, and H. Han, ACS Sustain. Chem. Eng., vol. 9, no. 19, pp. 6507–6509, May 2021

[2] M. Fichtner, Magnesium Batteries: Research and Applications, vol. 2020, no. 23. Royal Society of Chemistry, 2019

[3] J. W. Choi and D. Aurbach, Nat. Rev. Mater., vol. 1, no. 4, p. 16013, 2016

[4] J. Muldoon, C. B. Bucur, and T. Gregory, Chem. Rev., vol. 114, no. 23, pp. 11683–11720, Dec. 2014

[5] A. Ponrouch, J. Bitenc, R. Dominko, N. Lindahl, P. Johansson, and M. R. Palacin, Energy Storage Mater., vol. 20, no. pp. 253–262, Feb. 2019

[6] R. Attias, M. Salama, B. Hirsch, Y. Goffer, and D. Aurbach, Joule, vol. 3, no. 1, pp. 27–52, 2019

[7] C. Holc, K. Dimogiannis, E. Hopkinson, and L. R. Johnson, ACS Appl. Mater. Interfaces, vol. 13, no. 25, pp. 29708–29713, Jun. 2021

[8] Z. Yu et al. Nat Energy, vol 5, pp. 526–533 Jun. 2020

[9] B. Liu, J. G. Zhang and W. Xu, Joule, vol 2, no. 16, pp. 833-845, May 2018