*1.2. Relevant Literature*

The high reactivity of metallic Li, which causes difficulty during the production process, can also be a source of performance challenges. Potential corrosive reactions at the surface of Li metal often lead to an increase in interfacial resistance, a reduction in Coulombic efficiency (CE) and a poor lifetime. Additionally, the large volume expansion of the electrode during repeated Li deposition/dissolution will seriously deteriorate the interfacial stability and in general increase the gap between theoretical and practical energy density of the Li cells. Continuous interface reactions, together with the surface enlargement due to new depositions, consume the fresh Li more and more during the cycle life of an Li metal anode. This means that an excess of lithium and electrolytes are strongly needed to increase the cycle life and improve the stability of Li metal cells [10]. There are studies implying that 20% is the optimum excess of lithium; a greater excess of lithium will increase the possibility of side reactions and consequently shorten the cycle life of the cells [11]. This required excess of Li and electrolytes is another limitation to increasing practical energy density in Li-metal cells.

The function and properties of Li cells are strongly dependent on the growth morphologies. However, predicting the kinetic structures is difficult as they are influenced by different parameters. There are studies [12–15] investigating the parameters that have an influence on the shape, morphology and growth of metal particles. Tao Yang et al. [12] proposed three different growth modes: reaction limited, diffusion limited and the socalled reaction–diffusion balance mode. The reaction limited mode is dominated by a slow reaction rate. The reaction in the case of Li cells, which is the focus of this paper, is the nucleation and transformation of an Li ion to deposited Li metal:

$$Li^{+} + \text{e}^{-} \to Li.\tag{1}$$

The slow reaction rate combined with a sufficient mass transport diffusion in the electrolyte from the cathode to the Li metal anode leads to a rich concentration of reactants (Li ions) around the nucleus which further leads to a classic crystallization process. The diffusion limited mode is dominant if the reaction rate is much faster compared to the mass transport diffusion, which causes a lack of reactants around the nucleus. In both mentioned conditions—reaction limited and diffusion limited—the particles' growth is slow. However, in the reaction–diffusion balance mode, the morphology is dendritic and the growth speed is fast. In this region, there is a concentration gradient around the nucleus. Yang and his group [12] examined the morphology evolution on silver, gold and copper, coming to the

same conclusion that the relation between diffusion and reaction rate is the key factor in predicting the shape of particle growth. With this as the background, the importance of operation temperature and applied current, as the key kinetics definers for the performance of lithium metal anodes, should be emphasized.

There are studies showing that a local temperature rise can have healing effects on Li dendrites at different current densities [13,14]. By increasing the temperature, the diffusion of Li ions in the bulk of the electrolyte will be faster [15]; therefore, based on Tao Yang's model, in comparison to the cell performed at *T*Cell = 25 ◦C, elevated temperature should move the cells either in the direction of reaction limited or to a balanced region. Yehu et al. [13] investigated the temperature influence on the stability and efficiency of the Li metal anode. They found out that in their studied system, temperature rise has a positive influence on the efficiency and life time of the cells. They also studied the impact of different temperatures and electrolyte compositions on the morphology of Li depositions. They found out that bigger Li spheres form at the initial stage of deposition at elevated temperatures, leading to a lower specific area of plated Li and consequently to reducing the probability of dendritic formation [13].

At the same time, there are studies showing that the temperature rise leads to more unstable lithium deposition [16–18]. The structural uniformity and mechanical strength of the Solid Electrolyte Interface (SEI) play important roles in defining the type of deposition as they directly influence the dynamic of Li plating and stripping [19]. As the SEI layer consists of reduced and decomposed electrolyte components, different electrolytes induce totally different SEI layers. One influencing component in the electrolyte is the used Li salt. The salt lithium hexafluorophosphate LiPF6 exhibits poor thermostability [20]; Lithium perchlorate LiClO4 can strongly oxidize the Li metal [21] and causes low safety. An alternative salt given by lithium bis(fluorosulfonyl)imide (LiFSI) is reported to form a robust SEI protecting layer [22]. In this work, the influence of LiFSI and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and their concentrations in electrolytes on the cycle life of Li metal cells is studied.
