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Page 2 of 14 Kühn et al. Energy Mater 2023;3:300020 https://dx.doi.org/10.20517/energymater.2023.07

Keywords: Lithium metal battery, in coin cell atmosphere, liquid electrolyte, film-forming additive, solid electrolyte
interphase, cathode electrolyte interphase

INTRODUCTION
The application of lithium metal as the “holy grail” electrode material has been a topic in academia and
[1-3]
-1 [4,5]
industry for more than 40 years. The high theoretical specific capacity (3,860 mAh g ) and the low
standard reduction potential (-3.04 V vs. SHE) make lithium metal an ideal material for future high energy
density batteries. To date, the main challenge for commercial liquid electrolyte-based LMBs to overcome is
inhomogeneous stripping/plating coinciding with the formation of high surface area lithium (HSAL), also
[6]
known as dendritic morphology . There have been a variety of approaches to stabilize the lithium metal
[7-9]
anode, the solid electrolyte interphase (SEI) , and improve lithium stripping/plating behavior. These
include stabilizing functional electrolyte additives [2,10-12] , lithium metal pre-treatment and artificial SEIs [13-17] ,
lithium metal host materials [18-20] , and the application of external pressure [21,22] . Multiple publications have
also suggested guidelines and benchmarks for battery performance characterization to enhance research
comparability [23-27] . Another external factor that can influence the performance of battery electrodes is the
storage atmosphere. In particular, the degradation of cathode materials in an ambient atmosphere has been
the subject of numerous publications [28,29] . However, the reactivity of lithium metal with its surrounding
atmosphere, resulting in different compositions of the native passivation layer covering the lithium surface,
has only recently come into focus [7,30,31] . Furthermore, the influence of different storage atmospheres on
lithium metal electrode performance has - to the best of our knowledge - only been reported by
Momma et al. . Ambient atmosphere storage conditions can also have an impact via the electrolyte on
[32]
[34]
lithium ion batteries and LMBs . This field of research is especially relevant for Li||air batteries [35,36] , as
[33]
this cell setup allows for an unlimited amount of oxygen to enter the battery during its lifetime. On the
other hand, in the case of common research coin cell battery setups, the in coin cell atmosphere (ICCA,
Figure 1) is limited to the volume of gas trapped inside the cell during crimping. The isolated impact of the
ICCA has not been analyzed to date, despite it having an impact on the comparability of all obtained
scientific results gathered using a coin cell setup. Furthermore, results generated within the confines of a
glovebox (GB) ICCA might not be straightforwardly transferrable to industrial applications, since large-
scale battery production is mostly conducted in clean and/or dry room (DR) atmospheres.

In an attempt to close this knowledge gap, we report on the significant impact of this remaining trapped
ICCA on the chemical and electrochemical characteristics of lithium metal electrodes as well as the
performance and lifetime of transition metals (NMC811) using LMBs by comparing a DR and a super-clean
water-oxygen-nitrogen-free inert gas argon GB atmosphere. After precautions were taken to ensure safe
storage conditions, thus ensuring equal starting conditions for each experiment, Li||Li and NMC811||Li cells
wereassembledin aGB andaDR.Stripping/Plating experiments,operando electrochemical impedance
spectroscopy (EIS), post-mortem scanning electron microscopy (SEM), and X-ray photoelectron
spectroscopy (XPS) analysis were conducted for Li||Li symmetric cells. The impact of the ICCA on LMB full
cell performance was evaluated by galvanostatic cycling in NMC811||Li cells and subsequent post-mortem
attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy analysis of the CEI. In
addition, based on the recorded ATR-FTIR analysis, a new decomposition mechanism of FEC on NMC811
was proposed and supported by different quantum chemistry calculations. Furthermore, the systematic
analysis of two atmospheres with two additive-containing electrolytes and one baseline organic carbonate-
based liquid electrolyte revealed a differently pronounced ICCA influence depending on the electrolyte
formulation. The following electrolytes were chosen based on an equal molar approach: a baseline
electrolyte [BE; EC:EMC 3:7 (w/w), 1.2 M LiPF ] and two additive electrolytes containing the popular
6
[7]
additives FEC (AE-FEC; BE +6.09 wt.% FEC) and VC (AE-VC; BE +5 wt.% VC) .