Surface Patterning of Lithium Metal: Novel Approach to Stable Li Metal Anodes
Authors: Thanapisitikul Piyathip , Gattu Bharat , Shanthi Pavithra Murugavel , Day Brian A , Datta Moni Kanchan , Kumta Prashant N 
Published: February 2020

The global demand for energy storage systems for portable consumer devices such as cell phone, laptop including electric vehicles is clearly on the rise. To date, lithium ion batteries are considered the flagship battery system due to its high volumetric/gravimetric energy and power densities. Current lithium ion batteries exhibit a power density of 100-260 Whkg-1when layered spinel and olivine cathode material such as LiCoO2, LiNiO2, LiMnO4, and LiFePO4 are coupled with a graphite anode. With the recent demand for higher energy storage capacities for use in electric vehicles research has been focused on replacing graphite with Li metal as anode material owing to its high theoretical capacity of ~3800mAh/g. However, lithium anode suffers from growth of dendritic morphology and low columbic efficiency which leads to failure of the battery.

The dendrite formation is caused by the non-uniform current density at the surface of the solid electrolyte interphase (SEI) which results in a high-surface area lithium (HSAL). A solution to prevent the HSAL formation is using uniform low current density and by increasing the active surface area, the actual current density can be kept low during plating/deplating process. Li electroplating is a well-studied phenomenon. However, the effect of electrode surface features on the resultant electrocrystallization process of Li from nucleation and growth perspective is not yet defined and well understood.

In the present work, patterning of Li electrode surface was implemented and their effect on the the nucleation and growth process is studied. Various patterned Li electrodes (plain Li, Li-300, Li-2500) were assembled in CR2025 coin cell using commercial Li foil as counter electrode and tested at different areal current densities for plating and deplating. Fig 1a-c shows the evolution of voltage of plain Li foil, patterned Li-300 and Li-2500, respectively, on repeated plating and deplating at 0.5mA/cm2. The voltage profile can be identified as two distinct regions of nucleation overpotential (EN) and growth overpotential (EG) in the electrocrystallization process of Li. Depending on the Li surface, a noticeable change in the nucleation and growth overpotential is observed. Plain Li surface shows a very high EN (0.45V) as compared to the Li-300 (0.32V) indicating the formation of higher nucleation sites which results in HSAL. Li-300 and Li-2500 on the other hand, show rapid decrease in EN on cycling with similar nucleation and growth potential (>10th cycle) indicating growth of preexisting nucleation sites instead of creating fresh nucleation sites.

For full cell testing, electrodes were made from slurry consisting of 90% Lithium Manganese Oxide (LiMn2O4) with 5% Carbon (Super P) and 5% Polyvinylidene difluoride (PVDF) cast on aluminum current collector and assembled in a CR2025 coin cell with different patterned Li electrodes. Long term cycling and rate capability tests were conducted at different current rates and their performance evaluated in terms of stability, columbic efficiency and charge/discharge capacities. Subsequently, electrical impedance spectroscopy and SEM analysis were carried out to understand the evolution of Li plating morphology and variation of electrochemical performance with different surface patterns. Results of these studies will be presented and discussed.

Figure 1