*Article* **Influence of Temperature and Electrolyte Composition on the Performance of Lithium Metal Anodes**

**Sanaz Momeni Boroujeni \*, Alexander Fill, Alexander Ridder and Kai Peter Birke**

Institute of Photovoltaics, Electrical Energy Storage Systems, University of Stuttgart, 70569 Stuttgart, Germany; alexander.fill@ipv.uni-stuttgart.de (A.F.); alexander.ridder@ipv.uni-stuttgart.de (A.R.);

peter.birke@ipv.uni-stuttgart.de (K.P.B.)

**\*** Correspondence: sanaz.momeni@ipv.uni-stuttgart.de

**Abstract:** Lithium metal anodes have again attracted widespread attention due to the continuously growing demand of cells with higher energy density. However, the lithium deposition mechanism and the affecting process of influencing factors, such as temperature, cycling current density, and electrolyte composition are not fully understood and require further investigation. In this article, the behavior of lithium metal anode at different temperatures (25, 40, and 60 ◦C), lithium salts, electrolyte concentrations (1 and 2 M), and the applied cell current (equivalent to 0.5 C, 1 C, and 2 C). is investigated. Two different salts were evaluated: lithium bis(fluorosulfonyl)imide (LiFSI) and lithium bis(trifluoromethanesul-fonyl)imide (LiTFSI). The cells at a medium temperature (40 ◦C) show the highest Coulombic efficiency (CE). However, shorter cycle life is observed compared to the experiments at room temperature (25 ◦C). Regardless of electrolyte type and C-rate, the higher temperature of 60 ◦C provides the worst Coulombic efficiency and cycle life among those at the examined temperatures. A higher C-rate has a positive effect on the stability over the cycle life of the lithium cells. The best performance in terms of long cycle life and relatively good Coulombic efficiency is achieved by fast charging the cell with high concentration LiFSI in 1,2-dimethoxyethane (DME) electrolyte at a temperature of 25 ◦C. The cell has an average Coulombic efficiency of 0.987 over 223 cycles. In addition to galvanostatic experiments, Electrochemical Impedance Spectroscopy (EIS) measurements were performed to study the evolution of the interface under different conditions during cycling.

**Keywords:** lithium battery; temperature dependency; ether based electrolyte, *insitu* deposited lithium-metal electrode; Coulombic efficiency; lithium deposition morphology
