Investigating Facet Selectivity of Li Deposition on Cu Current Collector for Anode-Free Lithium Metal Batteries
Authors: Kim Yun-Jung , Noh Hyungjun , Yuk Seongmin , Lee Jinhong , Kim Hee-Tak 
Published: February 2020

Li metal is considered as one of optimal candidates for high-energy anode material because it has the highest theoretical specific capacity (3,860 mAh g-1) and the lowest redox potential (−3.04 V vs. standard hydrogen electrode). However, safety concerns and low coulombic efficiency issues, which are caused by the inhomogeneous Li deposition/dissolution and continuous corrosion by electrolytes during battery cycling, have prohibited the use of metallic Li as anode in practical Li metal batteries.

Lithium metal batteries (LMBs), which replace graphite anode with Li metal from conventional lithium ion batteries (LIBs), are considered as the most realistic approach to increase energy density of rechargeable battery. Furthermore, an anode-free LMB which features the only use of current collector in the anode compartment is the design to maximize the energy density of LMB. Because the current collector could affect the nucleation behaviors at the initial state of Li plating and the morphological development of the subsequently plated Li, it is a key component for achieving cycling stability of anode-free LMBs.

Cu current collector is the most widely adopted current collector for the anode of LIBs and LMBs due to its high conductivity, mechanical property, and electrochemical stability, but is known to have a large Li nucleation overpotential in comparison with Au or Ag. That is because the binding energies of Li atom on bulk Li is much larger than that of Li atom on bulk Cu (which are around 24~30 and 2.5~2.7 kcal mol-1, respectively). Therefore, the surficial modulation of Cu current collector is needed to improve the affinity between Li metal and Cu and induce uniform initial Li deposition on Cu current collector.

Cu collectors used for lithium ion battery, which are fabricated by electrodeposition method, have various crystalline facets on their surfaces. On the basis of the principle of nucleation and growth, the degree of crystalline misfit between adsorbent and substrate can vary depending on the surface facet of substrate. We believe that it can be a key principle for manipulating an initial Li nucleation mode on Cu substrate.

In detail, we explore a facet selective Li nucleation and growth phenomenon on Cu and demonstrate that controlling the facet structure can improve the uniformity in Li deposition and the cycling stability. Preferential Li deposition on the Cu(100) plane is demonstrated by electrochemical analysis of the Cu single crystal surfaces and by EBSD analysis of the Li-deposited Cu surfaces. DFT calculations show that a difference in the Li adsorption energy during the initial Li deposition process among the Cu facets is responsible for the facet selectivity. A majorly (100) plane-orientated Cu foil fabricated with a simple annealing method has a more uniform Li nucleation with a 6-times higher nuclei density and a two-fold enhancement in the Li cycling stability compared with a conventional Cu foil with randomly oriented surface facets. The control of the surface facet provides a new design principle for the current collector of lithium metal batteries.