Galvanostatic Li Electrodeposition in LiTFSI-PC Electrolyte: Part I. Effects of Current Density in Initial Stage

Abstract

Galvanostatic electrodeposition of Li was carried out in 1M LiTFSI/PC electrolyte to investigate the effect of current density on the morphological variations during the initial stage of the electrodeposition. The solid electrolyte interphase (SEI) formation process was analyzed from the potential change combined with XPS and UPS measurements along with SEM observation of Li deposits. A significant difference in the deposited Li morphology was observed depending on the current density. The simultaneous growth of whisker-like and granular deposits was noticed at lower current density, while the experiments at higher current densities evolved rather uniform mesoscopic-sized rod development. The formation behavior of SEI prior to Li deposition also differed between lower and higher current densities, of which the transition was about 4 mA cm-2. It was deduced that the formation history of SEI affected the surface defect density heterogeneity and mass transport properties inside SEI. The event of “sprouting”, in which Li precipitates nucleated and grown in SEI are extruded from the SEI layer into the electrolyte, certainly influenced the subsequent growth mode. The diffusion coefficient of Li+ in the SEI galvanostatically formed on Ni substrate in 1M LiTFSI/PC was estimated to be in the order of 10-9 cm2 s-1.

Potentiostatic Li Electrodeposition in LiTFSI-PC Electrolyte

“…However, as we have seen in the previous section, the nucleation behavior changes with a boundary potential around −100 mV. Even at very low current densities, such as 0.04 mA cm –2 , Li electrodeposition experiences potentials lower than −100 mV in galvanostatic electrolysis, but naturally, the potential does not exceed it in potentiostatic electrolysis when the polarized potential is more positive than −100 mV. If an overpotential of −100 mV is required for nucleation and growth in the presence of SEI, then potentiostatic electrolysis at the cathodic potential more positive than −100 mV should have a different nucleation and growth mechanism from that of galvanostatic electrolysis.…”

“…This section discusses the phenomena to which each current change corresponds. For comparison, V–t curves obtained in previous galvanostatic experiments , are also shown.…”

“…First, in ordinary single-step constant potential electrolysis, a usual airtight three-electrode cell was used. The experimental method was basically the same as the galvanostatic experiment described in the previous reports , except for the potentiostatic operation. Cell assembly and disassembly were all performed in an argon glovebox (dew point < −90 °C).…”

“…SEI is formed prior to Li deposition under galvanostatic conditions, whereas SEI formation and Li deposition should occur simultaneously in potentiostatic operation. The authors have conducted galvanostatic electrodeposition in lithium bis­(trifluoromethanesulfonyl)­imide/propylene carbonate (LiTFSI/PC) electrolyte to investigate the effect of current density on the morphological variation of electrodeposited Li. , The LiTFSI/PC was selected because, in parallel with this study, the authors have been studying flame-retardant electrolytes applicable to lithium metal batteries with ternary oxide cathodes, and carbonate electrolytes are used as part of the flame-retardant electrolyte in those studies. The larger the current density, the more instantaneous the potential changes during the initial process, which are expected to approach the potentiostatic condition.…”

“…Most of the reports on Li electrodeposition are generally based on galvanostatic conditions. − This is largely due to the fact that the actual LiB is charged and discharged under constant current conditions. The author has been conducting galvanostatic Li electrodeposition at different current densities to investigate the effects of current density and associated mass transfer properties on the deposition morphology. − The morphological variation of electrodeposited Li had already started from the nucleation and growth process up to 100 mC cm –2 and was considered to be closely related to the mass transport phenomena in SEI and in the electrolyte …”