Borate-Free Electrolytes for Calcium Metal Batteries

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Since Li-ion batteries are close to their theoretical energy densities, metal batteries gain increasing attention. Although the main focus is on lithium metal batteries, they are still considered “unsafe” due to dendrite formation and therefore the risk of thermal runaway. [1] Alkaline earth metals, such as calcium and magnesium, are significantly less prone to dendrite formation. Additionally, calcium is the 5th most abundant element in the earth crust and hence cheaper and easier to access than lithium metal. Thus, it is a promising alternative anode material. Calcium metal has a low standard reduction potential ( 2.87 V vs. SHE) and a high theoretical volumetric capacity of 2072 mAh cm-3. However, so far reversible electrodeposition/-dissolution is barely possible due to the formation of a strongly insulating passivation layer. Furthermore, due to the size of the cation and its divalent nature, the mobility and the kinetics are slower compared to lithium. Therefore, new electrolytes which prevent passivation and at the same time enable sufficient conductivity must be investigated. [2,3]
There are various possibilities to design electrolytes, such as the use of additives in liquid electrolytes or switching to ionic liquid-based or polymer systems. Several studies on borate-based electrolytes can be found in literature, but herein, we present several borate-free options for electrolytes compatible with calcium metal anodes. Another crucial aspect is the preparation of the calcium metal anodes, to ensure that the electrode is not passivated already before cell assembly. Herein, apart from the electrolytes, our focus will be on pretreatment of calcium metal foil.

Acknowledgements: The authors would like to acknowledge financial support from the European Union through the Horizon 2020 framework program for research and innovation within the project “VIDICAT” (829145).

References:
[1] Bieker, G.; Winter, M.; Bieker, P., Physical Chemistry Chemical Physics 2015, 17 (14), 8670-8679.
[2] Ponrouch, A.; Bitenc, J.; Dominko, R.; Lindahl, N.; Johansson, P.; Palacin, M. R., Multivalent rechargeable batteries. Energy Storage Materials 2019, 20, 253-262.
[3] Li, Z.; Fuhr, O.; Fichtner, M.; Zhao-Karger, Z., Towards stable and efficient electrolytes for room-temperature rechargeable calcium batteries. Energy & Environmental Science 2019, 12 (12), 3496-3501.

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