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Calcium and magnesium metal anode based batteries

Periodic Reporting for period 2 - CAMBAT (Calcium and magnesium metal anode based batteries)

Reporting period: 2018-07-01 to 2019-12-31

Li-ion battery is ubiquitous and has emerged as the major contender to power electric vehicles, yet Li-ion is slowly but surely reaching its limits and controversial debates on lithium supply cannot be ignored. New sustainable battery chemistries must be developed and the most appealing alternatives are to use Ca or Mg metal anodes which would bring a breakthrough in terms of energy density relying on much more abundant elements. Since Mg and Ca do not appear to be plagued by dendrite formation like Li, metal anodes could thus safely be used. While standard electrolytes forming stable passivation layers at the electrode/electrolyte interfaces enabled the success of the Li-ion technology, the migration of divalent cations through a passivation layer was thought to be impossible. Thus, all research efforts to date have been devoted to the formulation of electrolytes that do not form such layer. This approach comes with complex electrolyte, highly corrosive and with narrow electrochemical stability window leading to incompatibility with high voltage cathodes thus penalizing energy density. The main goal of CAMBAT is to acquire fundamental knowledge on electrolytes and interfaces enabling divalent cation (Ca2+ and Mg2+) transport and use them for the development of energy storage devices based on Ca or Mg metal anodes. Such battery technologies having the prospect for higher energy density and lower cost than the state of the art Li-ion.
First efforts in CAMBAT were dedicated to better understand the physico-chemical properties of Ca and Mg based electrolytes and comparison with Li based electrolytes were made in order to highlights the specificities of divalent cations. Ca and Mg salt solubility and ion pair formation were found to be a significant issue, calling for the development of new highly dissociative salts. Aiming at understanding the degree of ion pairs formation further, we conducted Walden analysis and Raman spectroscopy for a large variety of salt, solvent and concentrations. We concluded that the solvent donicity and salt concentration play a major role, as well as the nature of the cation, with ion pair formation following the trend: Mg>Ca>Li. Considering the prohibitive binding energies of contact ion pairs which is expected to play a major role in interfacial processes, this work provides clear electrolyte design strategies to engineer the solvation structure and possibly improve power performances of divalent systems.

In parallel, a detailed investigation on the passivation layers being formed at the surface of Ca metal electrode was performed in order to identify the passivation layer component allowing for Ca2+ migration. Borates were identified as potential passivation layer component allowing for Ca plating. A simple pre-passivation procedure was then tested, allowing us to transfer a passivated electrode (the passivation layer containing borates) in an electrolyte that was optimized in terms of ion pairing but which does not usually allow for Ca plating. After this pre-passivation step, not only Ca plating could be observed, but an enhanced electrochemical response – ca. 4 times higher current densities was observed. This strongly suggests that the nature of the passivation layer is key to enable Ca plating and that the composition of the electrolyte plays a major role in the overall plating kinetics. Both the cation mobility in the electrolyte and the interfacial phenomena such as de-solvation are thus interconnected properties of utmost importance for practical Ca batteries.
CAMBAT is based on an unconventional approach in divalent cation based system and focusses on using electrolyte that lead to the formation of passivation layer at the interface with the metal anode. Such passivation layered are generally assumed to be fully cation blocking and thus prevent Ca or Mg plating. The knowledge acquired on the nature and properties of the passivation layer will be crucial for the development of Ca and Mg metal anode based batteries. Another important parameter to take into consideration is the poor mobility of the divalent cation in the electrolyte due the high degree of ion pair formation and the larger size of the cation primary solvation shell. By improving our understanding of divalent cation mobility in a passivation layer and in the electrolyte, CAMBAT will allows to significantly improve power performances of divalent cation based batteries.