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Current societal challenges in terms of energy storage have prompted an intensification in the research aiming at unravelling new high energy density battery technologies. These would have the potential of having disruptive effects in the world transition towards a less carbon-dependent energy economy through transport, both by electrification and renewable energy integration. Aside from controversial debates on lithium supply, the development of new sustainable battery chemistries based on abundant elements is appealing, especially for large-scale stationary applications. Interesting alternatives are to use sodium, magnesium or calcium instead of lithium. While for the Na-ion case, fast progresses are expected as a result of chemical similarities with lithium and the cumulated Li-ion battery know-how over the years, for Ca and Mg the situation is radically different. On the one hand, the possibility to use Ca or Mg metal anodes would bring a breakthrough in terms of energy density; on the other, development of suitable electrolytes and cathodes with efficient multivalent ion migration are bottlenecks to overcome. ; Funding from the European Union's Horizon 2020 research and innovation programme is acknowledged, both from the European Research Council (A. Ponrouch ERC-2016-STG, CAMBAT grant agreement no. 715087) and FETOPEN-1-2016-2017 (CARBAT, grant agreement no. 766617). Acknowledgements. Authors acknowledge ALISTORE-ERI members and NAIADES (H2020, 646433) partners for helpful discussions and are grateful to Fanny Barde (Toyota Motor Europe) and M. Elena Arroyo-de Dompablo for fruitful collaboration. Authors are grateful to Ministry of Science, Innovation and Universities for support through MAT2017-86616-R and 'Severo Ochoa' Programme for Centres of Excellence in R&D (SEV-2015-0496). ; Peer reviewed

This paper examines Vicenta Arnal Yarza's (1902-1960) life and scientific contributions to electrochemistry in the late 1920s and the 1930s. In Spain, this was a time of ambitious programs to modernize and internationalize scientific research, which included the training and career promotion of some women. Vicenta Arnal's pathway shifted from the early years of her international scientific education with top scientists in the field to a domestic career devoted to improving the teaching and popularizing of chemistry under the constraints of the Franco dictatorship, which brought about a dramatic regression in gender equity. ; Authors acknowledge Alejandro Santos for being able to retrieve Vicenta Arnal's scientific publications despite our limited information input, and are grateful to Josefina Perez-Arantegui (Universidad de Zaragoza) for invaluable comments and to the Alfaro-García family for the permission to publish the image in Fig. 1. MRP thanks Spain's Agencia Estatal de Investigación Severo Ochoa Program for Centers of Excellence in R&D (CEX2019–000917-S). ; With funding from the Spanish government through the 'Severo Ochoa Centre of Excellence' accreditation (CEX2019-000917-S). ; Peer reviewed

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. ; J.D.F.-S. and D.S.T. contributed equally to this work. Funding from the European Union's Horizon 2020 research and innovation programme ERC-2016-STG (CAMBAT Grant Agreement No. 715087) and H2020 FETOPEN-1-2016-2017 (CARBAT, Grant Agreement No. 766617) is gratefully acknowledged. ICMAB-CSIC members thank the Spanish Agencia Estatal de Investigación Severo Ochoa Programme for Centres of Excellence in R&D (CEX2019-000917-S). J.D.F.-S. is grateful to 2020-MSCA-COFUND-2016 (DOC-FAM, Grant Agreement No. 754397). J.D.F.-S. contributed to this work in the frame of the Doctoral Degree Program in Materials Science by the Universitat Autònoma de Barcelona. ; With funding from the Spanish government through the 'Severo Ochoa Centre of Excellence' accreditation (CEX2019-000917-S). ; Peer reviewed

The currently emerging sodium-ion battery technology is in need of an optimized standard organic solvent electrolyte based on solid and directly comparable data. With this aim we have made a systematic study of "simple" electrolyte systems consisting of two sodium salts (NaTFSI and NaPF6) dissolved in three different alkyl carbonate solvents (EC, PC, DMC) within a wide range of salt concentrations and investigated: (i) their more macroscopic physico-chemical properties such as ionic conductivity, viscosity, thermal stability, and (ii) the molecular level properties such as ion-pairing and solvation. From this all electrolytes were found to have useful thermal operational windows and electrochemical stability windows, allowing for large scale energy storage technologies focused on load levelling or (to a less extent) electric vehicles, and ionic conductivities on par with analogous lithium-ion battery electrolytes, giving promise to also be power performant. Furthermore, at the molecular level the NaPF6-based electrolytes are more dissociated than the NaTFSI-based ones because of the higher ionic association strength of TFSI compared to PF6− while two different conformers of DMC participate in the Na+ first solvation shells – a Na+ affected conformational equilibrium and induced polarity of DMC. The non-negligible presence of DMC in the Na+ first solvation shells increases as a function of salt concentration. Overall, these results should both have a general impact on the design of more performant Na-conducting electrolytes and provide useful insight on the very details of the importance of DMC conformers in any cation solvation studies. ; We acknowledge funding received from the European Union's Horizon 2020 research and innovation programme under grant agreement no. 646433 (NAIADES), the ALISTORE-ERI members for many fruitful discussions, and also the funding received from the Swedish Research Council the Swedish Energy Agency (#37671-1), the Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning, the Severo Ochoa Programme for Centres of Excellence in R&D (SEV-2015-0496). DM especially acknowledges the European Union for a H2020 MSCA-IF grant no. 74343. PJ is grateful for the continuous financial support from several of Chalmers Areas of Advance: Materials Science, Energy, and Transport. We also acknowledge the computational resources provided by the Swedish National Infrastructure for Computing (SNIC) at Chalmers Centre for Computational Science and Engineering (C3SE). Author Information ; Peer reviewed

Sustainability combined with high energy density prospects makes Fe-based oxides attractive as cathodes for calcium rechargeable batteries. This work presents a DFT evaluation of the CaFe2+nO4+n (0 < n < 3) family, for which both the average intercalation voltage and the theoretical specific capacity decrease with the increasing n value. The term n = 1/4, Ca4Fe9O17, meets the most appealing characteristics: a calculated average voltage of 4.16 V, a theoretical specific capacity of 230 mA h g−1 and the lowest energy barrier for Ca migration so far predicted for an existing oxide (0.72 eV). To overcome the previously reported synthesis difficulties, we employed a novel synthesis procedure in sealed quartz tubes followed by quenching in water. The XRD and SAED patterns of the prepared Ca4Fe9O17 powder reveal a certain degree of stacking defects along the c axis. Attempts to deinsert Ca ions from Ca4Fe9O17 by chemical means (NO2BF4 in ACN) and in electrochemical Ca cells were unsuccessful, although some hints of oxidation are observed in Li cells with the LP30 electrolyte. The suitability of Ca4Fe9O17 as a Ca cathode is pending further studies utilizing Ca-electrolytes with high anodic stability. ; This research was funded by the European Union's Horizon 2020 research and innovation programme H2020 FETOPEN-1- 2016-2017 (CARBAT, grant agreement no. 766617). The authors are grateful for access to the computational facilities from Universidad de Oviedo (MALTA-Consolider cluster), to ALBA synchrotron for the provision of beam time and to Dr F. Fauth for his help during data collection and also acknowledge Dr. J. Oró-Solé for assistance with electron diffraction and Dr. Clemens Ritter for helpful discussions. ICMAB authors are also grateful to the Spanish Ministry for Economy, Industry and Competitiveness Severo Ochoa Programme for Centres of Excellence in R&D (SEV-2015-0496). ; Peer reviewed

This paper is part of the JES Focus Issue on Challenges in Novel Electrolytes, Organic Materials, and Innovative Chemistries for Batteries in Honor of Michel Armand. ; 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. ; Funding from the European Union's Horizon 2020 research and innovation programme H2020 FETOPEN-1-2016-2017 (CARBAT, grant agreement No. 766617) and ERC-2016-STG, (CAMBAT grant agreement No 715087) is gratefully acknowledged. Authors are grateful to the Spanish Ministry for Economy, Industry and Competitiveness Severo Ochoa Programme for Centres of Excellence in R&D (SEV-2015-0496). ; Peer reviewed

Battery research initiatives and giga-scale production generate an abundance of diverse data spanning myriad fields of science and engineering. Modern battery development is driven by the confluence of traditional domains of natural science with emerging fields like artificial intelligence and the vast engineering and logistical knowledge needed to sustain the global reach of battery Gigafactories. Despite the unprecedented volume of dedicated research targeting affordable, high-performance, and sustainable battery designs, these endeavours are held back by the lack of common battery data and vocabulary standards, as well as, machine readable tools to support interoperability. An ontology is a data model that represents domain knowledge as a map of concepts and the relations between them. A battery ontology offers an effective means to unify battery-related activities across different fields, accelerate the flow of knowledge in both human- and machine-readable formats, and support the integration of artificial intelligence in battery development. Furthermore, a logically consistent and expansive ontology is essential to support battery digitalization and standardization efforts, such as, the battery passport. This review summarizes the current state of ontology development, the needs for an ontology in the battery field, and current activities to meet this need. ; The authors acknowledge BATTERY 2030+ funded by the European Union's Horizon 2020 research and innovation programme under Grant Agreement No. 957213 and the project BIG-MAP with funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 957189. ; With funding from the Spanish government through the 'Severo Ochoa Centre of Excellence' accreditation (CEX2019-000917-S). ; Peer reviewed

Ca2MnO3.5 and CaV2O4 were found to be potentially interesting as positive electrode materials for calcium metal-based high-energy density batteries with DFT-predicted average voltages of 3.7 V and 2.5 V and energy barriers for Ca migration of 1.1 eV and 0.6 eV, respectively. Both compounds were prepared by solid state reaction under reducing atmospheres. Optimum conditions to achieve Ca2MnO3.5 comprised the reduction of Ca2MnO4 under NH3 gas at 420⁰C with a flow rate of 1200 ml/min while CaV2O4 was achieved by reduction of CaV2O6 at 700 ⁰C under H2 flow. Electrochemical oxidation of Ca2MnO3.5 in lithium or calcium cells resulted in the formation of an orthorhombic phase with cell parameter (a= 5.2891(1), b= 10.551(2), c= 12.1422(1)). Operando synchrotron radiation diffraction experiments indicate that the charge compensation mechanism is not related to Ca2+ extraction but to intercalation of F− (originated from electrolyte salt decomposition) into the anion vacancy position, as confirmed by EELS and EDS. This process was found to be irreversible. In the case of CaV2O4, oxidation induces the electrochemical extraction of calcium with the formation of an orthorhombic phase (space group Pbnm) with cell parameters a = 10.72008(9) Å, b = 9.20213(2) Å and c = 2.89418(3) Å. The process was also investigated via operando synchrotron radiation diffraction, with the oxidized phase being found to reintercalate Ca2+ ions upon reduction, with the formation of a solid solution. Preliminary cycling tests reveals a decrease in the polarization after the first cycle and call for further investigation of this system. ; This research was funded by the European Union's Horizon 2020 research and innovation programme H2020 FETOPEN-1- 2016-2017 (CARBAT, grant agreement no. 766617). The authors are grateful for access to the computational facilities from Universidad de Oviedo (MALTA-Consolider cluster) and to ALBA synchrotron for beamtime (proposal 2021024935). ICMAB-CSIC members thank the Spanish Agencia Estatal de Investigación ...