Molten Salt Electrolytes for Calcium Batteries

Licentiate thesis, 2023

Batteries are in higher demand than ever before as well as increased requirements in terms of energy and power density. The desire for higher energy density has meant that electrolytes are pushed above their electrochemical stability limits and hence they decompose, but typically this is under control and a new beneficial phase is created at the electrode surface; the solid electrolyte interphase (SEI).

Here calcium batteries (CaBs) are targeted. The large abundance of Ca in the Earth’s crust and its relatively low electrochemical potential make CaBs a very attractive alternative. However, Ca metal anodes commonly experience issues with ion-isolating phases, associated with organic electrolytes and in particular their solvents, which turn the SEIs from favorable to detrimental. Herein, we create completely solvent-free electrolytes in the form of molten salt electrolytes (MSEs), i.e. binary and multi-component systems of inorganic cations and anions, whereby this issue potentially can be avoided.

For all studies herein, studying thermal properties has been crucial, both to better understand the compositional entropy effect of MSEs, which is shown to reduce the melting temperature from >100 °C to 55-75 °C, and as input to a predictive model for the solubility of possible SEI components, including those for CaBs. Future work based on these findings includes refined MSEs and detailed studies of electrochemical properties of MSEs for/in CaBs, as well as CaB full-cell tests.