Mechanistic Properties of MgH2–Based Anode As Derived from Structural Morphology Changes Versus Electrochemical Impedance in a Li-Ion Cell
Authors: El kharbachi Abdel , Hu Yang , Sørby Magnus Helgerud , Vullum Per Erik , Mæhlen Jan Petter , Fjellvåg Helmer , Hauback Bjørn Christian 
Published: February 2020

The Li-ion battery is a well advanced technology for electrical energy storage. Thorough search of active materials has resulted in a limited set of recognized materials which can satisfy the selection criteria regarding energy density, costs, safety and lifetime. However, current Li-ion batteries are based on the concept of intercalation which has an inherent limitation of maximum one lithium stored per interstitial site [1]. Conversion type anodes can offer new possibilities in this field regarding their high capacity and constitute chemistries which are in the initial stage of development along the emergence of novel materials (metal hydrides, metal oxides, complex hydrides, MnSn and many metals) and technologies (e.g. Mg- and Na-ion and redox flow batteries) [2-7].
Metal hydride-based anodes are particularly interesting owing to their high capacity and low volume expansion. It has been demonstrated that MgH2 could be a candidate for conversion-type anode for Li-ion batteries [2]. In fact, this material features high theoretical capacity 2037 mAh.g-1, compared to graphite-anode 372 mAh.g-1, and low charge-discharge polarization, being considered as an indicator of lifespan [2,8]. However, its application as anode is still a challenge owing to capacity fading after several cycles [8]. MgH2 can interact with 2Li by undergoing a conversion reaction leading to the formation of 2LiH and Mg. Commercial MgH2 (particle size 25-100 µm) has shown poor electrochemical activity and practically no discharge capacity. Even with the addition of carbon black, this material loses its charge capacity after one discharge. However, the poor electric conductivity of MgH2has to be taken in consideration. Furthermore, the electrode formulation (shape, sampling and additives) can influence the cycling performance i.e. discharge/charge capacity and overvoltage-hysteresis [8-10]. Hence, design of representative electrodes with a modulated particle size is highly desirable for the understanding of the down capacity.
A systematic investigation of the morphology – property relation of MgH2 anode for Li-ion batteries is reported in this work. In particular, the aim is to present a comprehensive study of the contribution of the structural morphology to the electrochemical cycling of the tape-casted electrodes (~26µm thick) prepared from ball-milled MgH2. Samples with different particle size and microstructure were obtained by mechanical ball-milling in various conditions in both inert (Ar) and reducing atmospheres (H2) using different milling devices and cryo-milling. The discharge/charge electrochemical curves are discussed according to the ball-milling-induced structural morphology changes revealed by PXD and TEM analysis. New electrochemical impedance spectroscopy (EIS) measurements on the morphologically reproduced hydride anodes before and after battery tests are presented as function particle size, state of charge and cycle number. Advances on the use of EIS for in-situcharacterization of conversion-type anodes are also discussed.

Acknowledgements

This work is financially supported by Research Council of Norway under the program EnergiX, Project no. 244054, LiMBAT - "Metal hydrides for Li-ion battery anodes". We acknowledge the skillful assistance from the staff of SNBL at ESRF, Grenoble, France.

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