Periodic Reporting for period 4 - CAMBAT (Calcium and magnesium metal anode based batteries)
Berichtszeitraum: 2021-07-01 bis 2022-06-30
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. More sustainable battery chemistries must be developed quickly, 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. While standard electrolytes forming stable passivation layers (formed by accumulation of electrolyte decomposition products) 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 in divalent batteries to date have been devoted to the formulation of electrolytes that do not form such layer. This approach comes with complex electrolytes, highly corrosive and with narrow electrochemical stability window leading to incompatibility with high voltage cathodes, thus penalizing energy density. The main goal of CAMBAT was 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 have the prospect for higher energy density and lower cost than the state of the art Li-ion while being much more sustainable.
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.
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.
CAMBAT is based on an unconventional approach in divalent cation based systems and focusses on using electrolytes leading to the formation of passivation layers at the interface with the metal anode. Such passivation layers are generally assumed to be fully cation blocking and preventing Ca or Mg plating. The knowledge acquired within the project on the nature of Ca2+ conducting passivation layer components (cross-linked polymer containing borates) will be crucial for the development of Ca and Mg metal anode based batteries. In addition, these results constitute a paradigm change in the research on battery electrolytes with divalent cations. The fact that an ideal passivation layer (conducting divalent cations) can be formed during a pre-passivation step of the metal anode opens the door to completely new electrolyte formulations, specifically designed for improved metal plating kinetics and stability against high potential cathode materials, thus enabling improved power performances and high energy density rechargeable batteries. While the divalent research community has been struggling with poor reversible capacity of inorganic cathode materials with Ca2+ and Mg2+, CAMBAT clearly points at organic type cathodes as presenting capacity, power and cyclability performances far above any other cathode material developed so far. Being also far more sustainable and low cost, such organic cathode materials are expected to even further enhance the competitiveness of Ca and Mg based rechargeable batteries.