Interfaces and Interphases in Ca and Mg Batteries

The development of high energy density battery technologies based on divalent metals as the negative electrode is very appealing. Ca and Mg are especially interesting choices due to their combination of low standard reduction potential and natural abundance. One particular problem stalling the technological development of these batteries is the low efficiency of plating/stripping at the negative electrode, which relates to several factors that have not yet been looked at systematically; the nature/concentration of the electrolyte, which determines the mass transport of electro-active species (cation complexes) toward the electrode; the possible presence of passivation layers, which may hinder ionic transport and hence limit electrodeposition; and the mechanisms behind the charge transfer leading to nucleation/growth of the metal. Different electrolytes are investigated for Mg and Ca, with the presence/absence of chlorides in the formulation playing a crucial role in the cation desolvation. From a R&D point-of-view, proper characterization alongside modeling is crucial to understand the phenomena determining the mechanisms of the plating/stripping processes. The state-of-the-art is here presented together with a short perspective on the influence of the cation solvation also on the positive electrode and finally an attempt to define guidelines for future research in the field

Operando Synchrotron X-ray Diffraction in Calcium Batteries : Insights into the Redox Activity of 1D CaCoMO (M = Co and Mn)

1D CaCoMO (M = Co z = 0, M = Mn z = 1, and M = Fe z = 0.4) were prepared and tested electrochemically. While the iron-containing phase was not found to be active, the iron- and manganese-containing phases were found to be potentially interesting as positive electrode materials for calcium metal-based high-energy battery technologies and were investigated by operando synchrotron X-ray diffraction. Results indicate that electrochemically driven calcium deintercalation from the crystal structure (ca. 0.7 mol per formula unit) takes place upon oxidation in both cases. The oxidized phases have incommensurate modulated crystal structures with the space group R 3 m (00γ)0s and a = 9.127(1) Å, c = 2.4226(3) Å and c = 4.1857(3) Å, and γ = 0.579 (M = Co) and a = 9.217(1) Å, c = 4.9076(4) Å and c = 4.3387(4) Å, and γ = 1.139 (M = Mn), which exhibit differences due to the presence of manganese and Mn/Co ordering. The degree of calcium re-intercalation within the structure was found to be extremely limited, if any. Complementary experiments carried out in lithium cells did not show any reversibility either, thus pointing at intrinsic structural/migration constraints in the oxidized phase rather than slow kinetics of high desolvation energies associated with divalent ion charge carriers

On the Reliability of Half-Cell Tests for Monovalent (Li+, Na+) and Divalent (Mg2+, Ca2+) Cation Based Batteries

A comprehensive study is reported entailing a comparison of Li, Na, K, Mg, and Ca based electrolytes and an investigation of the reliability of electrochemical tests using half-cells. Ionic conductivity, viscosity, and Raman spectroscopy results point to the cationsolvent interaction to follow the polarizing power of the cations, i.e. Mg2+ > Ca2+ > Li+ > Na+ > K+ and to divalent cation based electrolytes having stronger tendency to form ion pairs - lowering the cation accessibility and mobility. Both increased temperature and the use of anions with delocalized negative charge, such as TFSI, are effective in mitigating this issue. Another factor impeding the divalent cations mobility is the larger solvation shells, as compared to those of monovalent cations, that in conjunction with stronger solvent - cation interactions contribute to slower charge transfer and ultimately a large impedance of Mg and Ca electrodes. An important consequence is the non-reliability of the pseudo-reference electrodes as these present both significant potential shifts as well as unstable behaviors. Finally, experimental protocols in order to achieve consistent results when using half-cell set-ups are proposed

Frustrated tunnelling ionization during strong-field fragmentation of D3+

We reveal surprisingly high kinetic energy release in the intense-field fragmentation of D[subscript 3][superscript +] to D[superscript +] + D[superscript +] + D with 10[superscript 16]Wcm[superscript −2], 790 nm, 40 fs (and 7 fs) laser pulses. This feature strongly mimics the behaviour of the D[superscript +] + D[superscript +] + D[superscript +] channel. From the experimental evidence, we conclude that the origin of the feature is due to frustrated tunnelling ionization, the first observation of this mechanism in a polyatomic system. Furthermore, we unravel evidence of frustrated tunnelling ionization in dissociation, both two-body breakup to D + D[subscript 2][superscript +] and D[superscript +] + D[subscript 2], and three-body breakup to D[superscript +] + D + D

Elucidating isotopic effects in intense ultrafast laser-driven D2H+ fragmentation

The triatomic hydrogen molecular ion is instrumental as a benchmark toward understanding the strong-field dynamics of polyatomic molecules. Using a crossed-beams coincidence three-dimensional momentum imaging method, we demonstrate clear isotopic effects in the fragmentation of D[subscript 2]H[superscript +] induced by 7 fs (40 fs), 790 nm laser pulses at an intensity of 10[superscript 16] W/cm² (5×10[superscript 15] W/cm²). Our experiment uniquely separates all fragmentation channels and provides kinematically complete information for the nuclear fragments. For example, we show that for dissociative ionization of D[subscript 2]H[superscript +] there is a large difference in branching ratios of the two-body channels, namely, H[superscript +]+D[superscript +][subscript 2] dominates D[superscript +]+HD[superscript +], whereas there is minimal difference in branching ratios between the dissociation channels H[superscript +]+D[subscript 2] and D[superscript +]+HD