Analytical Study of Surface Film on the Li Metal

Introduction

Li metal is one of the most promising materials as a negative electrode because of its theoretical capacity > 3800 mAh g^-1.  However the formation of the dendritic lithium during the deposition process has to be prevented for the rechargeable batter applications. 

Though a lot of studies have been conducted to suppress the dendritic growth of lithium metal [1], it can be easily formed at a high current density or during a cycling process.  Hence it still needs more studies to prevent the dendritic growth of the lithium metal completely. 

Recently we reported that the interfacial resistance of a lithium metal electrode and an electrolyte solution mainly consists of the lithium ion transfer in SEI film [2].  This result suggests that the ionic conductive property of the SEI significantly affects to the formation process of the dendritic lithium.  In the present study, we investigated relationship between surface morphology of electrodeposited lithium and the SEI films using various electrolyte additives used in commercial Li ion batteries.

Experimental

A 1 mol dm^-3 LiPF₆EC:DEC (1:1 vol%) was employed as a standard electrolyte solution.  Three additives: 5 vol% Vinylene carbonate (VC), 5 vol% Fluoroethylene carbonate (FEC) and 1 wt% Lithium bis(oxalate)borate (LiBOB), were added to the standard solution as the SEI formation source. 

The electrodeposition of the lithium was carried out using a two-electrode Swagelok cell.  The deposited lithium electrodes were carefully rinsed with DEC and dried in argon filled glove box.  The surface morphology of the deposited lithium was observed using SEM and the SEI film was analyzed by XPS. 

Results and Discussion

Fig.1 shows SEM images of lithium electrodes deposited in the electrolyte solutions with / without VC.  The lithium electrode from VC-free electrolyte showed a smooth surface morphology, while the electrodeposited lithium form the electrolyte solution with 5 vol% VC had dendritic-shaped deposits.  It suggests that the difference of ion conductivity in the SEI films on the Li metal affects to the surface morphology of lithium as we expected. 

Fig. 2 shows XPS spectra of O 1s for the Li metal immersed in 1 mol dm^-3 LiPF₆ EC:DEC (1:1 vol%) with / without VC.  In the case of the VC-free electrolyte, a relatively broad peak corresponds to lithium alkyl carbonate overlapped with a peak of Li₂CO₃ was observed while polyether like compounds form the surface film in the case of VC containing electrolyte solution.  Furthermore Li₂CO₃and lithium alkyl carbonate was observed after 30 sec of argon etching process.  Therefore it clearly shows that the SEI composition directly affects to the surface morphology of lithium.  Further detailed   discussion concerning the the SEI composition and surface morphology would will be presented and discussed in the meeting. 

Reference

[1] K. Kanamura, S. Shiraishi, Z. Takehara, J. Electrochem. Soc., 143, 2187 (1996)

[2] H. Kuwata, M. Matsui, N. Imanishi, Abstract of The 54^thBattery Symposium in Japan, 1D18, 2013