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Divalent cation based batteries are being considered as potential high energy density storage devices. The optimization of electrolytes for these technologies is, however, still largely lacking. Recent demonstration of the feasibility of Ca and Mg plating and stripping in the presence of a passivation layer or an artificial interphase has paved the way for more diverse electrolyte formulations. Here we exhaustively evaluate several Ca based electrolytes with different salts, solvents and concentrations, via measuring physico-chemical properties and using vibrational spectroscopy. Some comparisons with Mg and Li based electrolytes are made in order to highlight the unique properties of the Ca2+ cation. The Ca-salt solubility is found to be a major issue, calling for development of new highly dissociative salts. Nonetheless, reasonable salt solubility and dissociation was achieved using TFSI, BF4 and triflate anion based electrolytes and high permittivity solvents such as EC, PC, gBL and DMF. The local Ca2+ coordination is concentration dependent and rather complex, possibly involving bidentate coordination and participation of the nitrogen atom of DMF. The ionicity and the degree of ion-pair formation were both investigated and found to be strongly dependent on the nature of the cation, solvent donicity and salt concentration. The large ion-ion interaction energies of the contact ion-pairs, confirmed by DFT calculations, are expected to play a major role in the interfacial processes, and thus we here provide electrolyte design strategies to engineer the cation solvation and possibly improve the power performance of divalent battery systems., Funding from the European Union’s Horizon 2020 research and innovation program H2020 is acknowledged: European Research Council (ERC-2016-STG, CAMBAT grant agreement No. 715087), FETOPEN-1-2016-2017 (CARBAT, grant agreement No. 766617), and H2020-MSCA-COFUND-2016 (DOC-FAM, grant agreement No. 754397). A.P. is grateful to the Spanish Ministry for Economy, Industry and Competitiveness Severo Ochoa Program for Centers of Excellence in R&D (SEV-2015-0496). P.J. acknowledges financial support from the Swedish Energy Agency (contract #37671-1, Next Generation Batteries) and Chalmers Battery Initiative, part of the profile Materials for Energy Applications jointly managed by the Areas of Advance Materials Science and Energy at the Chalmers University of Technology, We acknowledge support by the CSIC Open Access Publication Initiative through its Unit of Information Resources for Research (URICI).

While less mature than Li-ion battery, technologies based on Na, K, Mg and Ca are attracting more and more attention from the battery community. New material (cathode, anode or electrolyte) testing for these post Li systems commonly involves the use of an electrochemical setup called half-cell in which metal counter and reference electrodes are used. Here we firstly describe the different issues that become critical when moving away from Li with respect to the cell hardware (cell design, current collector, separator, insulator) and the nature of counter and reference electrodes. Workarounds are given and a versatile setup is proposed to run reliable electrochemical tests for post Li battery materials in general, in a broad range of electrolyte compositions., Funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (Grant Agreement No. 715087) and doctoral cofund program H2020-MSCA-COFUND-2016 (DOC-FAM, Grant Agreement No. 754397) is acknowledged. Financial support from the Spanish Ministry for Economy, Industry and Competitiveness through the Severo Ochoa Programme for Centres of Excellence in R&D (SEV-2015-0496) is also acknowledged. R.D. acknowledges support of the Beatriu de Pinós postdoctoral programme of the Government of Catalonia’s Secretariat for Universities and Research of the Ministry of Economy and Knowledge (2017 BP 00187), We acknowledge support by the CSIC Open Access Publication Initiative through its Unit of Information Resources for Research (URICI).

The development of practical multivalent-ion batteries critically depends on the identification of suitable positive electrode materials. To gain a better understanding of the intercalation chemistry of multivalent ions, model frameworks can be used to study the distinct specificities of possible multivalent ions, thus expanding our knowledge on the emerging “beyond Li battery” technology. Here, we compare the intercalation chemistry of Mg2+, Zn2+, and Ca2+ ions in a disordered layered-type structure featuring water interlayers and cationic vacancies as possible host sites. The thermodynamics of cation-inserted reactions performed on the model structure indicated that these reactions are thermodynamically favorable with Zn2+ being the least stable ion. Galvanostatic measurements confirmed that the structure is inactive toward Zn2+ intercalation, while Mg2+ can be reversibly inserted (0.37 Mg2+ per formula unit) with minor changes in the atomic arrangement, as demonstrated by pair distribution function analysis. Moreover, we demonstrate that nonsolvated Mg2+ was intercalated in the structure. Finally, the intercalation of Ca2+ performed at 100 °C with Ca(BF4)2 in propylene carbonate induced the collapse of the layered structure releasing water molecules that contribute to the degradation of the electrolyte, as revealed by the presence of CaF2 at the electrode level. The decomposition of the structure led to the formation of an electrochemically active phase featuring a strong long-range disorder, yet a short-range order close to that found in perovskite structures, particularly with corner-shared TiO6 octahedra. We, hence, hypothesize that defective CaTiO3-based perovskite could be explored as viable cathode materials for rechargeable Ca-based batteries., The research leading to these results received funding from the French National Research Agency under Idex@Sorbonne University for the Future Investments Program (no. ANR-11-IDEX-0004-02). Funding from the European Union’s Horizon 2020 research is acknowledged: European Research Council (ERC-2016-STG, CAMBAT grant agreement no. 715087). The work done at the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under contract no. DE-AC02-06CH11357. A.P. is grateful to the Spanish Ministry for Economy, Industry and Competitiveness Severo Ochoa Programme for Centres of Excellence in R&D (SEV-2015-0496). We thank IMPC FR2482 for SEM-FEG and TEM instrumentation funded by CNRS, Sorbonne Université and C’Nano projets of Region Ile-de-France.

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

Tapia-Ruiz, Nuria et al., Increasing concerns regarding the sustainability of lithium sources, due to their limited availability and consequent expected price increase, have raised awareness of the importance of developing alternative energy-storage candidates that can sustain the ever-growing energy demand. Furthermore, limitations on the availability of the transition metals used in the manufacturing of cathode materials, together with questionable mining practices, are driving development towards more sustainable elements. Given the uniformly high abundance and cost-effectiveness of sodium, as well as its very suitable redox potential (close to that of lithium), sodium-ion battery technology offers tremendous potential to be a counterpart to lithium-ion batteries (LIBs) in different application scenarios, such as stationary energy storage and low-cost vehicles. This potential is reflected by the major investments that are being made by industry in a wide variety of markets and in diverse material combinations. Despite the associated advantages of being a drop-in replacement for LIBs, there are remarkable differences in the physicochemical properties between sodium and lithium that give rise to different behaviours, for example, different coordination preferences in compounds, desolvation energies, or solubility of the solid–electrolyte interphase inorganic salt components. This demands a more detailed study of the underlying physical and chemical processes occurring in sodium-ion batteries and allows great scope for groundbreaking advances in the field, from lab-scale to scale-up. This roadmap provides an extensive review by experts in academia and industry of the current state of the art in 2021 and the different research directions and strategies currently underway to improve the performance of sodium-ion batteries. The aim is to provide an opinion with respect to the current challenges and opportunities, from the fundamental properties to the practical applications of this technology., The authors gratefully acknowledge RS2E and Alistore-ERI for funding their research into Na-ion batteries. The funding received from the European Union’s Horizon 2020 research and innovation programme under Grant Agreement No. 646433 (NAIADES), the Swedish Research Council, the Swedish Energy Agency (#37671-1), and the Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning (FORMAS), are all gratefully acknowledged. The many fruitful discussions within ALISTORE-ERI, and especially with M Rosa Palacín, have been most valuable. P J is also grateful for the continuous support from several of the Chalmers Areas of Advance: Materials Science and Energy. Funding from the European Union’s innovation program H2020 is acknowledged: H2020-MSCA-COFUND-2016 (DOC-FAM, Grant Agreement No. 754397). A Ponrouch is grateful to the Spanish Ministry for Economy, Industry and Competitiveness Severo Ochoa Programme for Centres of Excellence in R&D (SEV-2015-0496).

Hard carbon materials were prepared from different precursors (phenolic resin and commercially available cellulose and lignin) under different pyrolysis and processing conditions using industrially adapted syntheses protocols. The study of their microstructural features enabled to assess that the nature of the precursor and the temperature of pyrolysis are the major factors determining the carbon yield and the surface area, the latter one having a major effect on the electrochemical capacity. Finally, the presence of surface groups and physisorbed water can also play a role both on the maximum reversible capacity achievable (by influencing the interaction of sodium ions with the hard carbon surface) and the irreversible capacity. Phenolic resin combining high carbon yield (~50%), tap density (0.7 gcm-3) and reversible capacity (249 mAh/g) was found to be the precursor producing the most suitable hard carbon for practical use in Na-ion batteries. Cellulose can be a good candidate as well, the lower carbon yield being counterbalanced by its lower price and higher capacity (280 mAh/g).

As for other multivalent systems, the interface between the calcium (Ca) metal anode and the electrolyte is of paramount importance for reversible plating/stripping. Here, we combined experimental and theoretical approaches to unveil the potential solid electrolyte interphase (SEI) components enabling facile Ca plating. Borates compounds, in the form of cross-linked polymers are suggested as divalent conducting component. A pre-passivation protocol with such SEI is demonstrated and allows to broaden the possibility for electrolyte formulation. We also demonstrated a 10-fold increase in Ca plating kinetics by tuning the cation solvation structure in the electrolyte limiting the degree of contact ion pair., Funding from the European Union's Horizon 2020 research and innovation program H2020 is acknowledged: European Research Council (ERC-2016-STG, CAMBAT grant agreement no. 715087) and H2020-MSCA-COFUND-2016 (DOC-FAM, grant agreement no. 754397). A. Ponrouch is grateful to the Spanish Ministry for Economy, Industry and Competitiveness Severo Ochoa Programme for Centres of Excellence in R&D (SEV-2015-0496) and to Prof. D. Lemordant (Tours University, France) for stimulating discussions. P. Canepa acknowledges funding from the National Research Foundation under his NRFF NRFF12-2020-0012 and the ANR-NRF NRF2019-NRF-ANR073 Na-MASTER. P. Canepa acknowledges the National Supercomputing Centre, Singapore (https://www.nscc.sg). This work has been done in the frame of the Doctoral Degree Program in Materials Science by the Universitat Autònoma de Barcelona. The FTIR experiments were performed at MIRAS beamline at ALBA Synchrotron with the collaboration of ALBA staff.

A study of the Solid Electrolyte Interphase (SEI) on hard-carbon (HC) electrodes in sodium half-cells is presented. Electrochemical performances over > 100 cycles were compared with two different salts (NaPF6, NaTFSI) and two different electrolyte additives (FEC, DMCF) in a mixture of EC and DMC solvents. The best electrochemical performances were observed with NaPF6 salt in conjunction with 3% FEC. The DMCF additive had a detrimental effect in all electrolyte combinations. The chemical characterization of the SEI was carried out by X-ray Photoelectron Spectroscopy (XPS) and showed that the best electrochemical behavior was related to an SEI composition based on sodium ethylene dicarbonate and NaF, whereas poorer electrochemical performances were associated to either low NaF or high Na2CO3 content. The results reported herein provide an insight on the SEI chemistry on hard carbon electrodes in sodium cells after long-term cycling, as a complement to previous studies dealing with the first cycles., Authors acknowledge the European Commission for funding through H2020 NAIADES project (LCE10–2014, grant nb. 646433). EI, MRP and AP acknowledge Ministerio de Economía y Competititivad (Spain) for Severo Ochoa Programme for Centres of Excellence in R&D (SEV-2015-0496). This work has been done in the frame of the Doctoral Degree Program in Chemistry by the Universitat Autònoma de Barcelona. JF, CC and RD acknowledge Agence Nationale de la Recherche (ANR) for its “Investment for the Future” Program through I-SITE E2S-UPPA funding.

In this work, aprotic and protic ionic liquid (IL)-based electrolytes designed for calcium-based energy storage systems are investigated. We have shown that these electrolytes display good transport properties and electrochemical stabilities comparable with those of IL-based electrolytes proposed for lithium and sodium-based systems. The use of these electrolytes in electrochemical double layer capacitors (EDLCs) leads to the realization of devices displaying good capacitances paired with a high reversibility and stability. Their use in combination with TiS2 cathode appears more problematic as the cation of the ILs is inserting in the layered structure of this material during the charge process. In this latter case a careful design of the cation appears necessary to guarantee selective insertion of Ca2+ and reversible charge-discharge process., AB and TS wish to thank the Friedrich Schiller University of Jena for the financial support. Funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (ERC-2016-STG, CAMBAT grant agreement No 715087) is acknowledged. RD acknowledges support of the Beatriu de Pinós postdoctoral programme of the Government of Catalonia's Secretariat for Universities and Research of the Ministry of Economy and Knowledge (2017 BP 00187). A. Ponrouch is grateful to the Spanish Ministry for Economy, Industry and Competitiveness Severo Ochoa Programme for Centres of Excellence in R&D (SEV-2015-0496). We thank Dr. R. Verrelli for providing us with TiS2 electrodes and for helpful discussion.

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...

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 cell potential of magnesium metal anode batteries is currently limited by the lack of stable and non-corrosive electrolytes. In a recent issue of Nature Chemistry, Son et al. demonstrated an artificial interphase on Mg anode that extends the electrolyte stability range while enabling magnesium transport. This approach paves the way for practical assessment of high-voltage cathodes and the assembly of high-energy-density Mg cells. Extension of the concept to other multivalent systems is also exciting.

Batteries based on Ca hold the promise to leapfrog ahead regarding increases in energy densities and are especially attractive as Ca is the 5th most abundant element in the Earth's crust. The viability of Ca metal anodes has recently been shown by approaches that either use wide potential window electrolytes at moderately elevated temperatures or THF-based electrolytes at room temperature. This paper provides realistic estimates of the practical energy densities for Ca-based rechargeable batteries at the cell level, calculated using open source models for several concepts. The results from the Ca metal anode batteries indicate that doubled or even tripled energy density as compared to the state-of-the-art Li-ion batteries is viable if a practical proof-of-concept can be achieved.

We present herein a systematic study of solvent co-intercalation during electrochemical reduction of titanium disulfide in lithium cells using state of the art in situ cells and synchrotron X-ray diffraction. To understand the role of the electrolyte components, four salts (LiBF4, LiBOB, LiPF6 and LiTFSI) and three solvents (ethylene carbonate, propylene carbonate and dimethyl carbonate) were investigated. Various types of in situ cells were assembled and X-ray diffraction patterns were collected in operando upon cycling. Co-intercalated phase formation was found to be triggered by the presence of propylene carbonate and to be electrochemically driven. This co-intercalated phase is formed in the early stages of reduction, with cell parameters a = 3.514 Å, c = 17.931 Å, corresponding approximately to a tripling of the pristine TiS2 cell along the c-axis. This phase does not seem to evolve upon further oxidation and hence induces an overall loss of capacity. Whereas the nature of the anion does not appear to influence the co-intercalated phase formation, the content of propylene carbonate in the electrolyte is clearly correlated to both its amount and the extent of capacity loss., The authors acknowledge funding from Ministry of Science and Innovation through grant MAT2017–86616-R, from the "Severo Ochoa" Programme for Centres of Excellence in R&D (CEX2019-000917-S). This project has received funding from the European Union's Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement N° 754397. The ALBA synchrotron is acknowledged for provision of beamtime within the in-house project program (proposals 2019013225 and 2020014004).

Hybrid electrolytes aimed at application in sodium-ion batteries (SIB) consisting of an organic solvent mixture (EC:PC) and different ionic liquids (ILs); EMImTFSI, BMImTFSI, and Pyr13TFSI, and with the NaTFSI salt providing the Naþ charge carriers have here been extensively studied. The physico-chemical and electrochemical characterisation includes ionic conductivity, viscosity, density, cation coordination and solvation, various safety measures, and electrochemical stability window (ESW). Hybrid electrolytes with 10e50% of IL content were found to have ionic conductivities on par with comparable organic solvent based electrolytes, but with highly enhanced safety properties. A systematic Raman spectroscopy study of the cation coordination and solvation before and after electrolyte safety tests by ignition suggest that IL cations and TFSI remain stable when ignited while organic solvents are consumed. Finally, the solid electrolyte interphase (SEI) formed when using hybrid electrolytes has both better mechanical and electrochemical stability than the SEI derived from pure IL based electrolytes. For a half-cell with a hard carbon (HC) electrode and a hybrid electrolyte with a composition of 0.8 m NaTFSI in EC0.45:PC0.45:- Pyr13TFSI0.10 encouraging results were obtained for IL based electrolytes e ca. 182 mAhg1 at C/10 over 40 cycles., We acknowledge the ALISTORE-ERI members for many fruitful discussions and the Swedish Research Council (VR), the Swedish Energy Agency (STEM), the Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning (FORMAS) and Ministerio de Ciencia e Innovacion (Spain, grant MAT2014-53500- R) for research funding. DM is grateful for a travel grant to ICMAB from Chalmers Area of Advance Energy. PJ acknowledges the continuous funding from several of Chalmers Areas of Advance: Energy, Materials Science, and Transport.

Density functional theory (DFT) calculations are used to investigate the 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 CaxSi phases (0.5 b x ≤ 2) is of 0.4 V, with a volume variation of 306%. Decalciation of the lower Ca content phase, CaSi2, 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 show evidence that electrochemical “decalciation” of CaSi2 is possible at moderate temperatures. The decalciation of CaSi2 is confirmed by different characterization techniques., We are grateful to the Toyota Battery Research division at Higashi Fuji (M6) for financial support. Diffraction experiments were performed at MSPD beamline at ALBA Synchrotron and the collaboration of François Fauth (ALBA staff) is sincerely acknowledged. ICMAB authors acknowledge the Spanish Ministry of Economy and Competitiveness (“Severo Ochoa” Programme for Centres of Excellence in R&D (SEV- 2015-0496)).

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).

International audience; Vanadium pentoxide has been investigated for multivalent ion battery technologies but the structural characterization of inserted phases is poor, and conflicting reports exist in the literature. This study presents a critical overview of controversial aspects related to Mg and Ca insertion in α-V2O5 under diverse conditions by combined electrochemical and ex-situ XRD experiments. Galvanostatic tests are carried out in dry and wet alkyl carbonate-based electrolytes at RT and 100 °C. The formation of protonated phases with negligible divalent ion content if any is evidenced by Rietveld refinements of the XRD data, unambiguously dismissing the presence of AV2O5 (A: Mg, Ca) as electrochemical reduction products. Furthermore, thermal instability of V2O5 at 100 °C in alkyl carbonate solvents is demonstrated by XRD and TEM analysis and the formation of an orthorhombic phase with increased a parameter, most likely due to degradation favored by both water and temperature, is observed for both Mg and Ca. In order to assess the feasibility of the reverse reaction, fully intercalated AV2O5 (A = Ca, Mg) phases were also prepared by solid state reaction and oxidation attempted both electrochemically and chemically without evidence of any significant amount of Mg2+ or Ca2+ extraction, further corroborating the sluggish diffusion kinetics of divalent cations in α-V2O5.

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.

Arrays of Pt@RuO2 core–shell nanotubes were prepared by electrodeposition of RuO2 on Pt nanotubes and evaluated as electrochemical microsupercapacitor electrodes. Specific capacitance of ca. 1585 and 1280 F g − 1 RuO 2 is obtained at sweep rates of 2 and 500 mV s−1, respectively, demonstrating high utilization factor of RuO2 due to facilitated transport of protons and electrons. The specific geometric capacitance for the Pt@RuO2 core shell nanotubes electrode is 320 mF cm−2 at 2 mV s−1 and 256 mF cm−2 at 500 mV s−1.

The development of a rechargeable battery technology using light electropositive metal anodes would result in a breakthrough in energy density1. For multivalent charge carriers (Mn+), the number of ions that must react to achieve a certain electrochemical capacity is diminished by two (n = 2) or three (n = 3) when compared with Li+ (ref. 2). Whereas proof of concept has been achieved for magnesium3, 4, 5, the electrodeposition of calcium has so far been thought to be impossible6 and research has been restricted to non-rechargeable systems7, 8, 9, 10. Here we demonstrate the feasibility of calcium plating at moderate temperatures using conventional organic electrolytes, such as those used for the Li-ion technology. The reversibility of the process on cycling has been ascertained and thus the results presented here constitute the first step towards the development of a new rechargeable battery technology using calcium anodes., Authors are grateful to F. Fauth for his assistance during data collection at the ALBA Synchrotron. The authors thank the Toyota Battery Research division at Higashi Fuji (M6) for their financial support.

International audience; A comprehensive study is reported entailing optimization of sodium ion electrolyte formulation and compatibility studies with positive and negative electrode materials. EC:PC:DMC and EC:PC:DME were found to exhibit optimum ionic conductivities and lower viscosities. Yet, hard carbon negative electrode materials tested in such electrolytes exhibit significant differences in performance, rooted in the different resistivity of the SEI, which results in too large polarization and concomitant loss of capacity at low potentials when DME is used as a co-solvent. EC0.45:PC0.45:DMC0.1 was found to be the optimum composition resulting in good rate capability and high capacity upon sustained cycling for hard carbon electrodes. Its compatibility with positive Na3V2(PO4)2F3 (NVPF) electrodes was also confirmed, which led to the assembly of full Na-ion cells displaying an operation voltage of 3.65 V, very low polarisation and excellent capacity retention upon cycling with ca. 97 mA h g−1 of NVPF after more than 120 cycles together with satisfactory coulombic efficiency (>98.5%) and very good power performance. Such values lead to energy densities comparable to those of the current state-of-the-art lithium-ion technology.

International audience; The possibility of interfacial storage at low potential for electrode materials reacting through conversion reactions was evaluated. The amount of charge that could be stored through the proposed interfacial mechanism was estimated for a range of different materials and found to be much lower than those observed experimentally. Moreover, the slope of the potential decay and the influence of the current in the extent of stored capacity for experiments carried out in composite electrodes containing Co3O4 are not consistent with a capacitive-like mechanism. In summary, no evidence for capacitive storage could be found, our results being in agreement with the process taking place at low potential being solely related to electrolyte decomposition.

Arrays of Ru, Pt, and PtRu one dimensional 1D nanowires NWs and nanotubes NTs were prepared by electrodeposition through the porous structure of an anodic aluminum oxide AAO membrane. In each case, micrometer-long NW and NT were formed with an outer diameter of ca. 200 nm, close to the interior diameter of the porous AAO membrane. Arrays of NW and NT can be formed by varying the metallic salt concentration, the applied potential, and the conductivity of the electrolyte. The Ru and Pt deposition rates were measured in the various deposition conditions, using an electrochemical quartz crystal microbalance. The mechanisms responsible for the formation of Ru and Pt NW and NT are discussed based on the observed deposition rates and models found in the literature. Finally, it is shown that arrays of PtRu alloy NT and NW can be readily prepared and their compositions can be varied over the whole compositional range by changing the metallic salt concentration of the electrodeposition bath.

Sodium-ion batteries (SIBs) are currently being considered for large-scale energy storage. Optimization of SIB electrolytes is, however, still largely lacking. Here we exhaustively evaluate NaPF6 in diglyme as an electrolyte of choice, via both physicochemical properties and extensive electrochemical tests including half as well as full cells. Fundamentally, the ionic conductivity is found to be quite comparable to carbonate based electrolytes and to obey the fractional Walden rule with viscosity. We find Na metal to work well as a reference electrode and the electrochemical stability, evaluated potentiostatically for various electrodes and corroborated by DFT calculations, to be satisfactory in the entire voltage range 0–4.4 V. Galvanostatic cycling at C/10 of half and full cells using Na3V2(PO4)3 (NVP) or Na3V2(PO4)2F3 (NVPF) as cathodes and hard carbon (HC) as anodes indicates rapid capacity fading in cells with HC anodes, possibly originating in a lack of a stable SEI or by trapping of sodium. Aiming to understand this capacity fade further, we conducted a GC/MS analysis to determine electrolyte reduction products and to propose reduction pathways, concluding that oligomer and/or alkoxide formation is possible. Overall, the promising results should warrant further investigations of diglyme based electrolytes for modern SIB development, albeit avoiding HC anodes., We acknowledge the European Commission for the financial support for “Na-ion Battery Demonstration for Electric Storage” (NAIADES) under research grant agreement 646433 and especially L. Simonin and Y. Chatillon (CEA, Grenoble, France) for the NVPF material and for sharing high purity NaPF6. We also acknowledge funding from the Swedish Energy Agency from “Batterifonden” under ontract 37671-1 “Next generation batteries” and the ALISTORE-ERI community at large, but especially the SIB brainstorms for many fruitful discussions. Furthermore, we wish to acknowledge the Chalmers Master Card foundation for the travel and accommodation grants for K.W. All calculations were carried out at the Wrocław Centre for Networking and Supercomputing (www.wcss.pl), grant 346.

Summary This article reviews the progress in the development of a possible battery technology based on calcium, which is an abundant element and has an interesting standard reduction potential. The main bottleneck has been to find electrolytes enabling reversible plating and stripping of calcium, which has been overcome recently. Ongoing efforts focus in optimizing them to enable effective operation of calcium anodes at room temperature and within a large redox potential operation window, despite the formation of a passivation layer at the electrolyte/anode interface. In parallel, researchers are looking for suitable cathode materials enabling reasonably fast insertion and de-insertion of Ca2+ ions, which need to be probed using diverse complementary characterization techniques to avoid biased interpretation of results derived from possible side reactions.

Rechargeable battery technologies based on the use of metal anodes coupled to multivalent charge carrier ions (such as Mg2+, Ca2+ or Al3+) have the potential to deliver breakthroughs in energy density radically leap-frogging the current state-of-the-art Li-ion battery technology. However, both the use of metal anodes and the migration of multivalent ions, within the electrolyte and the electrodes, are technological bottlenecks which make these technologies, all at different degrees of maturity, not yet ready for practical applications. Moreover, the know-how gained during the many years of development of the Li-ion battery is not always transferable. This perspective paper reviews the current status of these multivalent battery technologies, describing issues and discussing possible routes to overcome them. Finally, a brief section about future perspectives is given.

Na-ion batteries have regained attention because they offer sustainability advantages over the Li-ion technology, hence their interest for massive electrochemical storage. Although the Na-ion electrochemistry is analogous to that of the Li-ion concept, there are a few notable differences such as the stability of carbonated-based electrolytes toward the Li or Na metal anodes. Herein we report on the positive effect of FEC as electrolyte additive on the efficiency of Na-half cells which unfortunately comes with a side effect involving the sudden onset of polarization during discharge at unpredictable capacity values. We show that these anomalies, associated to an inefficient plating-stripping at the sodium metal anode, do not appear in full Na-ion cells. Therefore, FEC-based electrolytes are not the panacea and alternative additives must be sought to enable building of reliable half cells with metal anodes., Authors acknowledge EC for funding through H2020 NAIADES project (LCE10-2014, Contract number 646433), L. Simonin and Y. Chatillon from CEA for sending the Na3V2(PO4)2F3 materials, and members of both the RS2E Na-ion task force and ALISTORE-ERI for fruitful discussions.

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).

In recent years, several organic electrolytes have been reported allowing calcium metal electrodeposition. In all cases, some degree of passivation on the surface of the calcium electrode was reported. Following a recent report on borate-based passivation layer playing a crucial role for Ca plating in electrolyte solutions containing Ca(BF4)2, here we report the use of a boron-containing additive. The presence of this additive allows Ca plating and stripping using a Ca(TFSI)2 containing electrolyte, which otherwise lead to the formation of a fully Ca2+ blocking passivation layer., Funding from the European Union’s Horizon 2020 research and innovation program H2020 is acknowledged: European Research Council (ERC-2016-STG, CAMBAT grant agreement no. 715087) and H2020-MSCA-COFUND-2016 (DOC-FAM, grant agreement no. 754397). A. Ponrouch is grateful to the Spanish Ministry for Economy, Industry and Competitiveness Severo Ochoa Programme for Centres of Excellence in R&D (CEX2019-000917-S). This work has been done in the frame of the Doctoral Degree Program in Materials Science by the Universitat Autònoma de Barcelona. Alistore-European Research Institute is gratefully acknowledged for financial support through the postdoc grant to C. Bodin.

Calcium-metal-anode based batteries have recently gained much attention owing to their higher theoretical capacity when compared to the commercial lithium-ion cells. However, electrodeposition of metallic calcium is challenging and currently there is only a few reported organic electrolytes allowing reversible plating/stripping, including Ca(BF4)2 in carbonate solvents. Many of the commercial salts are sold as hydrates, which is the case of Ca(BF4)2, however the drying of a divalent-metal cation organic electrolyte is not trivial. Herein, several procedures for drying BF4−-based electrolytes are explored and discussed. It is shown that the tetrafluoroborate anion can easily get hydrolyzed during some drying protocols producing impurities, and thus, it is necessary to prepare the salt in anhydrous conditions to ensure low water and contaminant contents. Two different synthetic routes are presented as alternatives to the commercial hydrated salt.