Insight into interfacial processes and degradation mechanism in magnesium metal batteries

• Published in: Nano energy Vol. 78; p. 105338
• Main Authors: Hu, Xin-Cheng, Shen, Zhen-Zhen, Wan, Jing, Song, Yue-Xian, Liu, Bing, Yan, Hui-Juan, Wen, Rui, Wan, Li-Jun
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.12.2020
• Subjects: Degradation mechanism; In situ atomic force microscopy; In situ optical microscope; Interfacial process; Mg deposition/stripping; Degradation mechanism; In situ atomic force microscopy; Mg deposition/stripping; Interfacial process; In situ optical microscope
• ISSN: 2211-2855
• DOI: 10.1016/j.nanoen.2020.105338

Magnesium (Mg) metal batteries are attractive due to their high energy density and low cost. However, the progress of rechargeable Mg batteries is hindered by the limited options of Mg-ion electrolytes, serious electrode passivation and poor cycling performance. Direct visualization of the dynamic processes of electrochemical deposition/stripping of metallic Mg is of great significance for intensive understanding the degradation mechanism. Herein, the deposition/stripping process of Mg is directly tracked by using in situ optical microscope and in situ atomic force microscope. It is shown that the growth of deposited Mg is dynamically slow and the dissolution process is obviously irreversible in magnesium bis(trifluoromethanesulfonyl)imide (Mg(TFSI)2)-based electrolyte. Interestingly, it is found that the weak reversibility is mainly caused by the point-contact of the deposited Mg to the electrode. By introducing magnesium borohydride (Mg(BH4)2) to improve the stability of the electrolyte, the nucleation sites significantly increased and closely packed on the electrode during the deposition. Moreover, the connection between deposited Mg and electrode turns into surface-contact mode, in which ion transmission and the reaction reactivity is greatly improved, revealing that the stability of the electrolyte to the electrode could affect the nucleation process and thus change the contact mode. These findings provide direct insights into the electrolyte-dependent reactivity and performance degradation mechanisms in Mg metal batteries. The regulation mechanisms of electrolyte on anode/electrolyte interfacial reactions were obtained by in situ optical microscope and in situ atomic force microscope. The stability of the electrolyte to the electrode affects the contact mode between the deposited Mg and the anode, which in turn affects battery performance. Surface contact with large contact area provides more ion channels than point contact, which is conducive to improving cycle stability and capacity maintenance. [Display omitted] •Electrodeposition/stripping of metallic Mg in different electrolyte systems were dynamically visualized.•The reversibility of Mg electrodeposition/stripping is directly correlated to the connection mode of deposited Mg to the electrode.•The stability of electrolytes mediates the nucleation process of Mg deposition, thus influencing the surface contact mode.