Abstract
“Anode-free” batteries present a significant advantage due to their substantially higher energy density and ease of assembly in a dry air atmosphere. However, issues involving lithium dendrite growth and low cycling Coulombic efficiencies during operation remain to be solved. Solid electrolyte interphase (SEI) formation on Cu and its effect on Li plating are studied here to understand the interplay between the Cu current collector surface chemistry and plated Li morphology. A native interphase layer (N-SEI) on the Cu current collector was observed with solid-state nuclear magnetic resonance spectroscopy (ssNMR) and electrochemical impedance spectroscopy (EIS). Cyclic voltammetry (CV) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) studies showed that the nature of the N-SEI is affected by the copper interface composition. An X-ray photoelectron spectroscopy (XPS) study identified a relationship between the applied voltage and SEI composition. In addition to the typical SEI components, the SEI contains copper oxides (CuxO) and their reduction reaction products. Parasitic electrochemical reactions were observed via in situ NMR measurements of Li plating efficiency. Scanning electron microscopy (SEM) studies revealed a correlation between the morphology of the plated Li and the SEI homogeneity, current density, and rest time in the electrolyte before plating. Via ToF-SIMS, we found that the preferential plating of Li on Cu is governed by the distribution of ionically conducting rather than electronic conducting compounds. The results together suggest strategies for mitigating dendrite formation by current collector pretreatment and controlled SEI formation during the first battery charge.
Highlights
The search for higher energy density rechargeable lithium (Li) batteries has created a renaissance of interest in the Li metal anode
This procedure was used to prepare the samples for Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) analysis with the aim of exploring Li morphology and the solid electrolyte interphase (SEI) composition on Li-plated Cu
To study the composition of the SEI that forms at specific potentials with XPS, constant current was applied until the desired voltage was reached; this was followed by a constant voltage hold (Figure 1b, stages I and II)
Summary
The search for higher energy density rechargeable lithium (Li) batteries has created a renaissance of interest in the Li metal anode. Potential safety issues due to Li dendrite growth and low cycling Coulombic efficiency are delaying the practical implementation of lithium metal batteries (LMBs).[1−3] The use of an excessively thick layer of Li metal in full battery cells decreases the practical energy density[4] and increases safety concerns. A thin layer of Li metal is electroplated on the current collector during charging.[2] These types of batteries present a significant advantage due to their higher energy density and ease of assembly in a dry air atmosphere. It serves as an interphase between the metal and the electrolyte and has the properties of a composite solid electrolyte.[5,6] It has been widely demonstrated in the literature that nonhomogeneous SEI compositions and morphologies initiate uneven plating and stripping of Li metal anodes, resulting in dendritic short circuits and low Coulombic efficiencies.[4,7−9]
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