*1.1. Motivation*

Lithium metal has always been one of the most attractive candidates for anode materials in lithium batteries. This is due to its potential to extend the energy density of conventional lithiumion batteries. State-of-the-art Li-ion cells, depending on the cell chemistry, can deliver a specific energy density of 130 Wh·kg−<sup>1</sup> to 250 Wh·kg−<sup>1</sup> [1]. This is already behind the U.S. Department of Energy's (DOE) target for advanced batteries for electric vehicles [2]. Lithium has a theoretical specific capacity of 3860 mAh·g−<sup>1</sup> and a higher redox potential of *φ*Li||H2 = −3.04 V versus standard hydrogen electrodes in comparison to electrodes based on graphite. This means using lithium (Li) instead of conventional intercalating anode materials like graphite (LiC6), which has a theoretical specific capacity of 372 mAh·g−<sup>1</sup> that can increase the specific energy and volumetric energy density of cells significantly. In one study, researchers reported a 35% increase in specific energy and 50% increase in volumetric energy density when the graphite electrode is replaced with a Li metal electrode [3]. In addition to the aforementioned change in electrode material, they considered a solid electrolyte for the Li-metal cell and a liquid

**Citation:** Momeni Boroujeni, S.; Fill, A.; Ridder, A.; Birke, K.P. Influence of Temperature and Electrolyte Composition on the Performance of Lithium Metal Anodes. *Batteries* **2021**, *7*, 67. https://doi.org/10.3390/ batteries7040067

Academic Editor: Seokheun Choi

Received: 30 July 2021 Accepted: 29 September 2021 Published: 14 October 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

electrolyte for the conventional Li-ion cell in their estimates [3]. However, commercializing Li-metal batteries has been paused due to various challenges in both production and performance of lithium cells [4]. The issues with the production are that the Li surface is highly reactive and sensitive to humidity, oxygen and nitrogen, which are all present in the air atmosphere [5,6]. The challenges of the performance are given by the unstable and different Li growth morphology, low Coulombic efficiency, and considerable volume change during cycling [1]. Lithium metal cannot be utilized with known carbonate electrolytes because its electrochemical potential causes the electrolyte to continuously decompose until a passivating solid electrolyte interface (SEI) is built up [7]. One approach to utilizing the Li-metal is creating a stable and uniform SEI layer on the lithium surface that can withstand significant volumetric changes of Li during cycling. This is of critical concern to ensure safe and efficient lithium metal cells. Local variations in the SEI layer's composition might cause uneven Li deposition, resulting in changes in Li-ion conductivity across the electrode or SEI rupture, which can facilitate the creation of Li dendrites. Another approach is to use solid membranes, such as solid polymer electrolytes (SPE), which are less reactive to the Li [8] and their soft nature could withstand the extensive volume change of the Li anode [9]. In this study we focus on compatible liquid electrolytes in lithium cells. More experiments investigating the influence of different electrolyte compositions and the cycling conditions on effective SEI layer formation are much needed.
