**5. In energy storage**

Magnesium is under research in the energy sector as a possible replacement or improvement on lithium-ion batteries in certain applications. In the early 1990s, the potential use of magnesium batteries was recognized based on V2O5, TiS2, or Ti2S4 cathode materials and magnesium metal anodes [20]. Magnesium has a (theoretical) energy density per unit mass under half that of lithium (18.8 MJ/kg vs 42.3 MJ/kg) in comparison to that of lithium as an anode material. Still, a volumetric energy density is around 50% higher (32.731 GJ/m3 vs 22.569 GJ/m3 ), making its use advantageous [21]. Magnesium anodes do not exhibit dendrite formation compared to metallic lithium, except in certain nonaqueous solutions and at current densities below ca. 1 mA/ cm2 . Such dendrite-free Mg deposition allows magnesium metal to be used without an intercalation compound at the anode, thus raising the theoretical maximum relative volumetric energy density to around five times that of a lithium graphite electrode [22]. Additionally, modeling and cell analysis have indicated that magnesium-based batteries may have a cost advantage over lithium due to the abundant magnesium on earth and the scarcity of lithium deposits [20, 23].

Another viable emerging energy storage technology under research is the Magnesium-Sulfur rechargeable battery (**Figure 6**) that uses magnesium ion as its charge carrier, magnesium metal as the anode, and sulfur paste as a cathode. To increase the electronic conductivity of the cathode, sulfur is usually mixed with carbon to form a cathode composite. Currently, efforts on rechargeable magnesium battery research are underway at Apple, Toyota, and Pellion Technologies [6] and in several universities.

Successful research outcome would be of great interest since, in theory, the Mg/S chemistry can provide 1722 Wh/kg energy density with a voltage of ~1.7 V, addressing most future energy challenges.

### **Figure 6.**

*Schematic diagram of Mg-S batteries showing the working principle and formation of magnesium-polysulfides (Mg-PS), which passivate the anode surface [20].*
