Abstract
Lithium with an atomic number of 3 has a small electrochemical equivalent of 6.9 g per Faraday, and a standard electrode potential of -3.045 V, the lowest among metals. For this reason, batteries using lithium as the anode are lightweight and have a high operating voltage. Li/MnO2 batteries and Li/(CF)n batteries have been commercialized using lithium anode, but they are all primary batteries. Lithium metal anodes are prone to generate dendrite during charging. The key to their practical use as anodes for secondary batteries is whether or not dendrite formation can be suppressed. The success technique of dendrite suppression will be indispensable for not only the realization of liquid electrolyte type battery with high energy density such as Li-S batteries and Li-air batteries but also all solid-state lithium metal battery. Thus, it is urgently required to interdisciplinary accumulate our profound understanding of the lithium metal nucleation and growth phenomena.
Since Li metal is conventionally electrodeposited in organic electrolytes because of electrochemical window restriction, Li deposition essentially accompanies the adsorption or decomposition of the organic species on the substrate and deposited Li metal. In galvanostatic or constant current electrolysis, the electrolyte decomposes prior to Li deposition and so-called SEI is formed. Therefore, the nucleation and growth of Li is naturally affected by the existence of SEI layer. Characteristics of SEI strongly influences the charge and/or mass transfer kinetics, which affects the nucleation and growth process followed by the morphological variations. Recent work revealed that Li deposition proceeded underneath SEI(1). Also, TEM observations indicated SEI microstructure(2) and the competitive deposition on tip and root of Li dendrite(3).
In this study, constant current electrodeposition of Li was carried out in common organic electrolyte instead of ionic liquid(4) to focus on the effect of current density in the initial stage of the electrodeposition. The electrolyte composed of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and PC. A typical three electrode cell was used in Ar glove box (dew point < -90 ℃), varying current density, salt concentration and temperature. The working electrode was Ni wire 500 μm in diameter (Nilaco Corp.). It was coated with FEP fluoroplastic tube as a sleeve so that the portion in contact with the electrolyte was 10 mm in depth. A lithium foil (200 μm in thickness, Honjo Metal Co., Ltd.) was used as the counter electrode and the reference electrode. The cell was mounted in the temperature-controlled aluminum block. The working electrode was immersed in dimethyl carbonate to rinse the surface and dried in vacuum after electrochemical measurements, followed by XPS and UPS analysis, and SEM observation of deposits.
Fig. 1 shows the potential behavior for 0 – 10 mC cm-2 immediately after starting at 0.04 - 60 mA cm-2. In all the current densities, the potential did not immediately jump into the equilibrium potential of Li. The coulomb quantity passing before Li precipitation decreases from 8 to 4 mC cm-2 as the current density increases under the lower current densities of 0.04 - 4 mA cm-2. On the other hand, under the higher current densities of 4 - 60 mA cm-2, it is about 4 mC cm-2 which does not change much with the current density. It is deduced that the SEI formed prior to Li precipitation under constant current conditions shows differences depending on the current density and there may be a transition point around 4 mA cm-2.
The typical SEM images of the electrode surface after the electrolysis of 100 mC cm-2 at 0.2 and 8 mA cm-2 are demonstrated in Fig. 2. The appearance is quite different between lower and higher current densities. At 0.2 mA cm-2, both whiskers with several micrometers in length and granular precipitates of 300 - 400 nm in size can be seen. On the other hand, at 8 mA cm-2, no granular shapes are noticed and substantially only whiskers of around 500 nm in length are homogeneously distributed.
Li nucleation and growth behaviors will be further examined to focus the effect of not only current density but also salt concentration and temperature. Nucleation & growth behavior of Li electrodeposited in PC electrolyte may be compared with conventional metal electrodeposition researches in aqueous solution system in order to find out any similarity or dissimilarity between both systems.
References
Jana, R. E. García, Nano Energy, 41, 552–565 (2017).
Li, Y. Cui et al., Joule, 2, 2167−2177 (2018).
Li, Y. Yu et al., Science, 358, 506–510 (2017).
Nishida, K. Nishikawa, M. Rosso and Y. Fukunaka, Electrochim. Acta, 100, 333-341 (2013).
Figure 1