In Situ Light Microscopy Observation of Sodium Dissolution/Deposition Reaction in Propylene Carbonate-Based Electrolyte

Sodium-ion batteries (SIBs) are anticipated as promising alternatives to lithium-ion batteries (LIBs), and many studies have been conducted on electrodes for SIBs using half-cells.  In general, sodium metals are used for the counter electrode of half-cells.  However, there are few reports [1] on sodium dissolution/deposition, knowledge of which an understanding of which is important for advancing the development of SIBs.  In this study, we focused on sodium metal and observed morphologies of sodium dissolution/deposition reactions in propylene carbonate-based electrolyte solution using in situlight microscopy.

Morphologies of sodium depositions were observed by using in-situ light microscopy (Lasertec Corp., ECCS B310).  The semicircular cell for in-situ light microscopy examination was a stack consisting of a sodium sheet as a counter electrode (0.2 mm thick), an electrolyte solution (1 mol/l NaPF₆/PC)-soaked polypropylene separator (19 mm in diameter), and a Cu sheet (0.01 mm thick) or sodium sheet (0.2 mm thick) as a working electrode.  Electrochemical measurements were performed by using an automatic galvanostatic discharge-charge system (Hokuto HJ1001SD8) at a constant current of 65 mA/cm²at room temperature.

The sodium deposition reaction on the sodium sheet was observed, and the morphologies were granular and needle-like (not shown here).  Figure 1 shows cross-sectional images of sodium deposition on the Cu sheet, separator, and sodium metal.    The morphologies of sodium deposition showed no difference from that on sodium sheet.  Then, needle-like sodium led to away from the Cu sheet and became “dead sodium”.  This suggests that the sodium metal dissolution/deposition efficiency is low.

 

[1] K. Matsumoto et al., J. Power Sources, 265 (2014) 36.