Electrolytes and interfaces in calcium and magnesium rechargeable batteries

Abstract

The development of next-generation batteries (post Li) will have a great impact in the upcoming years of transition from fossil fuels to renewable sources of energy. Among the many next-generation battery concepts, the ones based on divalent metals as negative electrodes stand out (particularly Ca and Mg), given their combination of low standard redox potential and high natural abundance. Development of Ca and Mg metal batteries have been historically hampered, as only a few organic electrolytes allow reversible electroplating/stripping of the metal, which is required for a continuous and stable operation of the metallic negative electrode. In this thesis, some efforts on the optimization of organic electrolytes for Ca metal plating are presented, with parallels to Mg application. The electrolyte formulation, which is based on a mixture of salt, solvent, and additives, is studied in this work from three complementary points of view: preparation of calcium salts, physicochemical properties of the obtained electrolytes, and evaluation of their electrochemical performance, which includes also the possible reductive decomposition of the electrolyte and the formation of a passivation layer on the electrode. Although they are separated in different chapters, these are not isolated parameters, and their interactions are discussed across this document. Particularly, the cation solvation appears as a physical parameter of critical importance from the three points of view: being influenced by salt and solvent choice, it is responsible for the macroscopic properties (as ionic conductivity and ionicity), and affects, in turn, the dynamics of electroplating, and the nature of the passivation layer formed. Therefore, some insides in the cation solvation-shell manipulation are presented as a route to optimize the operation of a Ca metal as the negative electrode. Additionally, a study on the passivation layers formed on metallic calcium electrodes is presented, as a function of the electrolyte formulation. The borate-based passivation layer, which is produced in-situ by anion or additive decomposition, was shown to promote calcium plating in contrast to a carbonate-based passivation layer, which appeared to be fully blocking for Ca2+ cations. Further studies will determine a more precise composition of the borate-based layer and will pave the way for artificially generated passivation layers to be used in future calcium batteries.

Date of Award	5 Apr 2022
Original language	English
Supervisor	Alexandre Ponrouch (Director) & Patrik Johansson (Director)