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. Mg 2+ > Ca 2+ > 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 Although the lithium-ion battery is currently being considered as the most promising technology for electric vehicle propulsion, the development of alternative and complementary battery chemistries and technologies is of great importance, especially aiming at large-scale applications, i.e. the grid for which the cost in $/kWh and sustainability are crucial indicators. Indeed, the implementation of lithium based technology at large scale faces a significant challenge, since the controversial debates on lithium availability and cost cannot be overlooked. Amongst several chemistries possible the most appealing alternatives involve the use of sodium (Na), magnesium (Mg) or calcium (Ca) for mainly two reasons. The prime is the abundance of the raw materials, i.e. Na, Mg, and Ca being the 6 th , 8 th , and 5 th most abundant elements in the Earth's crust, vs. 25 th for Li, making them 20 to 50 times cheaper than Li, e.g. $5000/ton, $135-165/ton, $265/ton, and $100/ton for Li 2 CO 3 , Na 2 CO 3 , MgO 2 , and CaCO 3 , 1 respectively. Performance wise, the low cost alternatives of Na, Mg, and Ca technologies would also benefit from high standard reduction potentials, ca. −2.71, −2.37, and −2.87 V vs. SHE for Na, Mg, and Ca, respectively, as compared to −3.04 V for Li, and large theoretical electrochemical capacities, both gravimetric and volumetric, for the metal electrodes (Fig. 1).Sodium metal anodes are already used in the liquid state (m.p. ∼97• C) in the Na/S technology 2 and room-temperature Na-ion technology is currently intensively investigated with hundreds of papers appearing per year, with progress being summarized in several review papers amongst which 3-5 are the most recent. For Mg and Ca metal anodes, the situation is radically different. For the Mg battery technology, proof-of-concept was achieved as late as in 2000, 6 although i...

A comparative study of the electrochemical intercalation of Ca2+ and Mg2+ in layered TiS2 using alkylcarbonate-based electrolytes is reported, and for the first time, reversible electrochemical Ca2+ insertion is proved in this compound using both X-ray diffraction and differential absorption X-ray tomography at the Ca L2 edge. Different new phases are formed upon M2+ insertion that are structurally characterized, their amount and composition being dependent on M2+ and the experimental conditions. The first phase formed upon reduction is found to be the result of an ion-solvated intercalation mechanism, with solvent molecule(s) being cointercalated with the M2+ cation. Upon further reduction, new non-cointercalated calcium-containing phases seem to form at the expense of unreacted TiS2. The calculated activation energy barriers for Ca2+ migration in TiS2 (0.75 eV) are lower than those previously reported for Mg (1.14 eV) at the dilute limit and within the CdI2 structural type. DFT results indicate that the expansion of the interlayer space lowers the energy barrier and favors a different pathway for Ca2+ migration.

HIGHLIGH TS • Controversial reports exist in the lit erature regarding Ca/Mg intercalation in V 2 0 s-• Crystal structure elucidation of inter calated phases bas remained elusive to date. • Electrochemical and ex"'5itu XRD ex periments evidenoe formation of pro tonated phases. • Degradation of V 2 0s bas also been observed un der some conditions. • Oxidation of AV20s (A= Ca, Mg) prepared by solid state reaction was not feasible.

The electrochemical oxidation of a transition metal oxide through calcium extraction is achieved for the first time. The 1D framework of Ca3Co2O6 is maintained upon oxidation and the new phase formed exhibits a modulated structure. The process occurs at high potential and is partially reversible, which opens prospects for a calcium battery proof-of-concept.

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.

Density Functional Theory (DFT) calculations are used to investigate basic electrochemical characteristics of Si-based anodes in Calcium Ion Batteries (CIBs). The calculated average voltage of Ca alloying with fcc-Si to form the intermetallic Ca x Si phases (0.5 < x ≤2) is of 0.4V, with a volume variation of 306%. Decalciation of the lower Ca content phase, CaSi 2, is predicted at an average voltage between 0.57 V (formation of Si-fcc, 65% volume variation) and 1.2 V (formation of metastable deinserted-Si phase, 29% volume variation). Experiments carried out in conventional alkyl carbonate electrolytes do evidence that electrochemical "decalciation" of CaSi 2 is possible at moderate temperatures. The decalciation process from CaSi 2 is confirmed by different characterization techniques.

Layered MgMoN2 was prepared by solid state reaction at high temperature between Mo and Mg3N2 in N2 which represents a simple synthetic pathway compared to the previously reported method that used NaN3 as nitrogen source. The crystal structure of MgMoN2 was studied by synchrotron X-ray and neutron powder diffraction. The feasibility of oxidizing this compound and concomitantly extracting magnesium from the structure was assessed by both chemical and electrochemical approaches, using different protocols. The X-ray diffraction patterns of oxidized samples do not exhibit any relevant difference with respect to that of the as prepared MgMoN2 and no differences in the cell parameters are deduced from Rietveld refinements. No hints pointing at the presence of any amorphous phase are observed either. These results are rationalized through DFT calculated energy barriers for Mg 2+ ion migration in MgMoN2.

A comparative study of the reduction of TiS2 in diverse electrolyte formulations involving Ca(BF4)2 and Ca(TFSI)2 salts was carried out at different temperatures (from 25 °C to 100 °C). While for the former salt intercalation of calcium is only observed at high temperatures, calcium intercalated phases are also observed for the latter even at room temperature. The nature of the electrolyte does also have an impact on the relative amounts of the phases formed. Since Ca(TFSI)2 based electrolytes do not enable calcium plating, cycling was attempted using activated carbon as counterelectrode, and the reversibility of the process was ascertained. Even if corrosion of stainless steel current collectors and side reactions do still prevent proper cyclability, the results achieved should contribute to the establishment of reliable and viable cell set-up and methodology for the unambiguous study of the intercalation process in multivalent battery systems.