Initial Stage of Li Electrodeposition in PC Electrolyte
Authors: Nishida Tetsuo , Fukunaka Yasuhiro , Homma Takayuki , Nohira Toshiyuki 
Published: December 2020

Li metal is an attractive material for negative electrode because it has the highest energy density (about 3600 mA h g-1). However, the Li metal dendrite is easily formed during the charging process. Its growth induces significantly poor electrochemical reversibility due to so-called dead Li and even imposes a risk of short-circuit phenomenon followed by firing or even fatal explosion accidents. Considerable efforts have been made so far in elucidating and suppressing Li dendrite formation toward practical utilization of Li metal anode(1).

As well known, many studies were published on the dendrite formation of metals such as Ag, Cu and Zn formed in the course of electrodeposition in aqueous electrolyte. Metal electrodeposition in aqueous electrolyte was studied by Volmer, Kossel, Stranski and Frank. They described that the nucleation and growth phenomena proceed through the surface diffusion of adatoms on the substrate followed by its aggregation and incorporation at the interface of growing nuclei(2). Nucleation process has been considered to be one of the most important stages for morphological variations of the deposits.

Since Li metal is generally electrodeposited in organic electrolytes because of electrochemical window restriction, the Li deposition essentially accompanies the adsorption or decomposition of the organic species on the substrate and deposited Li metal. Characteristics of so-called SEI strongly influences the charge transfer kinetics, which affects the nucleation and consequently the morphological variations. In this regard, the Li deposition process accompanying SEI formation is considered to be fundamentally different from that of metal deposition in aqueous solution. In fact, recent work revealed that Li deposition proceeded underneath SEI(3). Also, TEM observations indicated the competitive deposition on tip and root of Li dendrite(4).

In this study, we focused on the investigation of Li nucleation behavior during electrodeposition in a common propylene carbonate (PC) electrolyte by applying the double-pulsed potential technique which has been widely used for analyzing the nucleation and growth of electrochemical process. We have tried to separately control the nucleation phenomena from the growth process during the morphological variations develop to dendrite formation. Accordingly, our interest is focused to associate the surface morphological change with the Li nucleation behavior.

The double-pulsed technique used in ionic liquid electrolyte(5) was also applied to the present organic PC work. The electrolyte composed of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and PC. Water content of the electrolyte was confirmed by Kirl-Fisher titration (< 20 ppm). Electrochemical measurement was conducted with a typical three electrode cell in Ar glove box (dew point < -90 ℃), varying the pulse condition, salt concentration and temperature. A mechanically polished cross-section of Ni wire (500 μm in diameter, Nilaco Corp.) was used as the working electrode. A lithium foil (200 μm in thickness, Honjo Metal Co., Ltd.) was used as the counter electrode and the reference electrode. The working electrode was immersed in dimethyl carbonate and dried in vacuum after electrolysis, followed by SEM observation of deposits.

Fig.1 shows a typical example of chronoamperograms after applying double pulse potential. (The inset shows the time transient in the initial stage). When the first potential of -100 mV is applied, large currents flow in the beginning, which is attributed to the charging of double layer capacitance, the reduction of surface oxide layer and SEI formation. The current then decreases toward the minimum value and starts increasing again, which should correspond to the nucleation and growth. After the interruption of the first potential at 5 s, the formed Li nuclei grow moderately at the second potential of -15 mV. The gradual increase of current after 500 s is probably caused by the enlargement of effective surface area associated with nucleus growth. SEM images of the resultant deposits are shown in Fig.2. Granular deposits with the size of about 1-3 μm are relatively homogeneously distributed.

Nucleation behaviors under various experimental conditions will be discussed with the results of SEM observations and electrochemical measurements.

References

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Figure 1