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 highlight the specificities of divalent cations. Ca and Mg salt solubility and ion pair formation (poor dissociation of the salt in solution) were found to be significant issues, calling for the development of new highly dissociative salts. Within CAMBAT, several new Ca and Mg salts were thus prepared and documented in scientific articles. Besides, a promising strategy for improving salt dissociation was identified, namely the use of solvent with high donicity (tendency to be included in the cation’s primary solvation shell in solution). Considering the prohibitive binding energies of formed contact ion pairs (CIPs), their presence was found to play a major role in interfacial processes, penalizing the kinetics of metal (Ca or Mg) plating. This work provides clear electrolyte design strategies to engineer the cation solvation structure and improve power performances of divalent cation based batteries.
The composition and morphology of the passivation layers being formed at the surface of Ca metal electrode were also systematically investigated in order to identify the passivation layer component allowing for Ca2+ migration. In this respect, borate-containing cross-linked polymers were identified as components potentially allowing for Ca plating. Via prior pre-passivation procedure, a passivated electrode (the passivation layer containing borates) was produced and transferred in fresh cell containing 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 density – was observed, suggesting that the nature of the passivation layer is key to enable Ca plating and the composition of the electrolyte plays a major role in the overall plating kinetics. Both the cation mobility in the electrolyte and interfacial phenomena such as de-solvation are thus interconnected properties of utmost importance for practical Ca batteries. The nature of such passivation layer and its formation mechanism were further investigated by DFT calculations.
On the other hand, potential cathode materials were also explored. TiS2 layered material, when tested in ionic liquid (IL) based electrolytes, was found to preferentially intercalate the IL cation [Pyr14]+ leading to drastic amorphization and poor cyclability. By contrast, very promising cycling (> 100 cycles) and power performances (80% of capacity retention during fast charge/discharge, < 30 min) was recorded for organic cathode materials in both Ca and Mg cells and represent the most promising cathode candidates so far for rechargeable divalent cation based batteries. The reaction mechanisms for such organic cathodes were investigated by means of operando infrared spectroscopy using synchrotron radiation.
