### Benefits of Zinc-Air Batteries  
As already explained in the previous section, lithium-ion accumulators are currently used in numerous battery applications. Especially when considering the further storage demand due to electromobility and the expansion of renewable energies, a shortage of the necessary cell materials is expected. In particular, cobalt, which is used for the production of the positive electrode, is only available on a small scale on earth and has a high toxicity. The problem is further intensified by the fact that the majority of cobalt deposits are located in countries with poor working conditions, making it difficult to switch to other suppliers. Zinc is a relatively frequent element on Earth, representing 76 ppm of the Earth's crust, making it the 24 th most frequent element [11]. It is thus more common than lithium (60 ppm), cobalt (40 ppm), or lead (18 ppm) [12; 13; 11]. Consequently, the price of zinc per ton is much lower than that of lithium [14]. In addition, the deposits are distributed all over the world, so that better working conditions can be guaranteed. Large deposits exist in the United States, Canada, Australia, and China, but there are also zinc deposits in Europe that are being actively mined [11]. Another major advantage of zinc is that it is completely recyclable. For example, it is estimated that in North America more than 33% of the utilized zinc is made from recycled materials. Globally, the recycling rate is about 40% of the produced zinc, with the recycling loops in Europe being largely closed for zinc recycling. Further advantages of metal-air batteries, and thus of zinc-air batteries, are particularly high theoretical specific energies and theoretical energy densities [15; 16]. The theoretical specific energy is given by Equation (1). \[e=\frac{V_{\text{cell}}\cdot M_{\text{reaction}}}{\text{charge carrier transfer per mole}}, \tag{1}\]  
Here, \(M_{\text{reaction}}\) is the mass of the reactants, for a substance amount of one mole. For illustration, Figure 3 shows the theoretical specific energies of different metal-air batteries in comparison to current battery technologies. It can be seen that the theoretical specific energy of metal-air batteries is several times higher than that of other battery technologies. For metal-air batteries, one differentiates whether the oxygen content is included or excluded in the calculation. This seems confusing at first, but is based on practical operation, as oxygen is usually used from ambient air and is not inside the cells. The lithium-air cells stand out with the highest specific energy. However, this is a primary cell technology. This means that lithium-air batteries can only be discharged once, as superoxide ions form during the discharge process and react with the electrolyte, preventing a recharge [17]. Aluminum-air batteries are also primary cells and cannot be recharged. In contrast, operation as a secondary cell is theoretically possible with magnesium-air batteries. Metallic magnesium even has the advantage that dendrite formation does not occur during the charging process. However, there are no commercial magnesium-air secondary cells yet, as research is currently being carried out on electrolytes and electrodes that work with magnesium ions [18]. Compared to the lithium-air primary cell, the theoretical specific energy of zinc-air cells is much smaller. In addition to the positive aspects of gaining raw materials, they also offer the advantage that secondary cell operation is possible. In relation to current secondary cell technologies, the theoretical specific energy is still much higher. However, it  
Figure 3: Theoretical specific energy of different battery technologies in [W h kg\({}^{-1}\)] [16; 19]. should be noted that the zinc-air battery as a secondary cell has not yet been optimized to the extent that lithium-ion technology has. Therefore, the actual specific energies of current cells are approximately at the level of current lithium-ion cells [20]. Another advantage of the zinc-air technology is the low price.