# Feasible Energy Density Pushes of Li-Metal vs. Li-Ion Cells
## 2 Understanding the Need for a Li-Metal Anode  
The performance of the LIBs is determined by cathode, anode and electrolyte. However, as the anode materials provide much more specific capacity than the cathode materials, the overall cell capacity is limited by the cathode active material. For example NMC 811 (LiNi\({}_{0.8}\)Mn\({}_{0.1}\)Co\({}_{0.1}\)O\({}_{2}\)) offers 220 mAh/g of capacity where graphite and Li-metal offer 372 mAh/g and 3860 mAh/g of capacity, respectively [16]. An overview of the characteristics of different cathode and anode active materials are given in Table 1.  
In order to meet the requirements of next generation battery systems for electromobility, the battery should provide an energy density of more than 300 Wh/kg at cell level to achieve a range of more than 600 km per charge. If we compare today's high specific cathode material NMC 811 with graphite and Li-metal anode, we can see that according to capacity ratios (C\({}_{\text{cathode}}\)/C\({}_{\text{anode}}\)), each gram of NMC 811 corresponds to either 0.6 g of graphite or 0.06 g of Li-metal (see Supplementary Materials). Assuming an extra 15% of weight is coming from other cell materials (separator, current collector and electrolyte) [16], theoretical capacity (C) and energy density (ED) of the cells can be calculated by the following equations:  
\[\text{C}_{\text{cell}}=\frac{\text{C}_{\text{cathode}}}{\text{Weight}_{\text {cathode}}+\text{Weight}_{\text{anode}}}/(1.15) \tag{2}\]  
\[\text{ED}=\text{C}_{\text{cell}}\times\text{V}_{\text{cell}} \tag{3}\]  
As it is sketched in the Figure 1, using a high-capacity anode with lower redox potential results in high ED cells. The challenging point here is not only to achieve the highest ED but to keep this high ED in the cell for many cycles. For example, another high capacity silicon (Si) anode (4200 mAh/g) is hard to consider as a \"next-generation\" EV battery component, due to its volumetric expansion (up to 400%) during cycling [20]. In order to increase its cycle life, many approaches, such as using Si nano-wires and/or mixing Si with different ratios of graphite, were proposed [20; 21; 22; 23]. As one of the biggest hurdles in the development of LIBs is the long charging time, other important criteria for material selection are the electrochemical and physical performances of the material at high C rates (>1C). The goal for future EVs is to reduce charging time to less than 15 min [24]. Nevertheless, charging the battery at higher rates accelerates battery degradation and capacity fading [24; 25; 26]. This capacity fading mainly occurs because of two different phenomena: (i) deposition of metallic lithium onto the graphite anode (Li-plating) [24; 25; 26] and (ii) failure/fracture of electrode particles due to diffusion-induced stresses [27; 28; 29]. For the first, as the deposition takes place in dendritic form, it leads to a safety risk through short circuits. Additionally, the volume expansion due to Li-plating deforms the stable solid electrolyte interphase (SEI) which protects liquid electrolyte decomposition [30]. For the second, [PERSON] et al., showed that the charging &