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Among the different next-generation battery technologies, the ones based on divalent cations have recently gained particular attention due to their higher raw material availability and potentially higher energy density when compared with Li-ion. Such is the case of calcium, being the 5th most abundant element in the earth crust and having a standard redox potential only 170 mV above metallic lithium. However, the development of a secondary calcium-metal battery had been hampered by the lack of organic electrolytes allowing for reversible plating and stripping, which was only recently reported by our group and others employing different electrolyte formulations. The different electrolyte components – salt(s), solvent(s) and additive(s) – are expected to play a significant role in the operation of a calcium-metal battery. Here we will show how the solvation structure of the cation evolves in relation to the solvent employed and the concentration of the salt. Combining conductivity measurements with Raman spectroscopy data, a high tendency to form contact ion pairs is evidenced, which significantly affects the mobility of the ions in the liquid media. The use of different anions and their bonding strength with Ca2+ in solution will be discussed. Combined, the anion and solvent characteristics dictate the solvation of the cation, which in turn can affect the Ca plating and stripping kinetics. Additionally, the presence of electrolyte contaminants will be explored. The crucial yet very challenging drying of electrolyte containing Ca(BF4)2 in carbonate solvents will be presented. The difficulties to obtain anhydrous and contaminant free solutions from commercially available salts will be discussed and two alternative routes for anhydrous synthesis of the target salt will be presented.

New battery technologies have received increased attention in recent years, especially those based on the use of divalent cations such as calcium and magnesium. Their high abundance (calcium and magnesium being, respectively, the 5th and 8th most abundant element in the earth crust) and the possible safe use of metal anodes could result in more sustainable devices with lower cost and higher energy density systems when compared to Li-ion. Feasibility of plating/stripping of calcium metal has been demonstrated recently1 in very few organic electrolyte formulations. In all cases, some degree of electrolyte reduction and formation of passivation layer were reported. In particular, using Ca(BF4)2 in a mixture of carbonate solvents, a fully conformal passivation layer is formed at the surface of the metal anode.2 Yet this layer allows for Ca plating and stripping, thus exhibiting similar behavior as a solid electrolyte interphase (SEI). However, the nature and composition of such layer remain mostly unknown. In this communication, an in-depth analysis of the SEI will be presented. The influence of the electrolyte formulation (salt, solvent and additive) on the formation of the SEI and how it affects Ca plating and stripping kinetics were investigated. Among other technics, XPS, EELS and ToF-SIMS measurements allowed to detail the composition of the SEI. The homogeneity, the morphology and the microstructure were studied by means of TEM and FTIR microspectroscopy. Ponrouch, A. et al. Towards a calcium-based rechargeable battery. Nat. Mater. 15, 169 (2015). Forero-Saboya, J. et al. Understanding the nature of the passivation layer enabling reversible calcium plating. Energy Environ. Sci. 13, 3423–3431 (2020).

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).

Various metals have been used as battery anodes in electrochemical cells ever since the birth of the batteries with Volta’s pile and in the first commercialized primary (Zn/MnO2, Leclanché 1866) and secondary (Pb/acid, Planté 1859) batteries. Li-MoS2 cells, employing Li metal anodes, with specific energies two to three times higher than both Ni/Cd and Pb/acid cells, were withdrawn from the market due to safety issues related to dendrites growth. Instead, electrodeposition of Mg and Ca appears to be less prone to dendrite formation.[1,2] Pioneering work by Aurbach et al. in the early 1990’s showed a surface-film controlled electrochemical behavior of Ca and Mg metal anodes in electrolytes with conventional organic solvents.[3,4] The lack of metal plating was attributed to the poor divalent cation migration through the passivation layer. Nevertheless, recent demonstration of Ca and Mg plating and stripping in the presence of a passivation layer or an artificial interphase [2,5,6] has paved the way for assessment of new electrolyte formulations with high resilience towards oxidation. However, several challenges remain to be tackled for the development of Ca and Mg based batteries.[7,8] Among these, the need for reliable electrochemical test protocols, mass transport limitations and high desolvation energies (due to strong cation-solvent and cation–anion interactions) are implied.[9, 10] Here, the reliability of electrochemical set-ups involving multivalent chemistries is discussed, and a systematic investigation on the impact of the electrolyte formulation on the cation solvation structure and transport is presented. References 1. M. Matsui, J. Power Sources, 196 (2011) 7048. 2. A. Ponrouch, C. Frontera, F. Bardé, M.R. Palacín, Nat. Mater., 15 (2016) 169. 3. D. Aurbach, R. Skaletsky, Y. Gofer, J. Electrochem. Soc.138 (1991) 3536. 4. Z. Lu, A. Schechter, M. Moshkovich, D. Aurbach, J. Electroanal. Chem. 466 (1999) 203. 5. D. Wang, X. Gao, Y. Chen, L. Jin, C. Kuss, P. G. Bruce, Nat. Mater. 17 (2018) 16. 6. S.-B. Son, T. Gao, S. P. Harvey, K. X. Steirer, A. Stokes, A. Norman, C. Wang, A. Cresce, K. Xu, C. Ban, Nat. Chem. 10 (2018) 532. 7. A. Ponrouch, M.R. Palacín, Current Opinion in Electrochemistry 9 (2018) 1. 8. A. Ponrouch, J. Bitenc, R. Dominko, N. Lindahl, P. Johansson, M.R. Palacin, Energy Storage Materials 20 (2019) 253. 9. D. S. Tchitchekova, D. Monti, P. Johansson, F. Bardé, A. Randon-Vitanova, M. R. Palacı́n, A. Ponrouch, J. Electrochem. Soc., 164 (2017) A1384. 10. J. D. Forero-Saboya, E. Marchante, R. B. Araujo, D. Monti, P. Johansson, A. Ponrouch, J. Phys. Chem. C (2019).

Divalent metal batteries have gained much attention recently given the high gravimetric capacities of calcium and magnesium metal electrodes (1340 and 2000 mAh/g respectively), and thus, holding promise as high energy density next-generation battery technologies. Although calcium metal possesses lower reduction potential (-2.76 V vs SHE) than magnesium (-2.38 V vs SHE), the development of calcium metal batteries has been hampered by the lack of electrolytes allowing for reversible electrodeposition. After the first demonstration of reversible calcium plating/stripping in and organic electrolyte based on Ca(BF4)2 in carbonate solvents [1], the attention devoted to calcium-based batteries have substantially increased. Recently, other electrolyte systems allowing for reversible plating/stripping were reported [2–4]. One of the crucial parameters to consider in the devolvement of divalent metal batteries is the presence of undesired impurities, particularly water, as it will affect greatly the surface chemistry of the calcium metal anode and prevent electrodeposition. As normally alkaline-earth metal salts are more hygroscopic that their alkaline counterparts, obtaining ultra-dry electrolytes is a key challenge which needs to be addressed. In the case of commercial Ca(BF4)2, most likely synthesised in aqueous solution, the water content is usually as high as 30wt%. In the present study we evaluate different drying methods and report on their effect on the stability of the BF4 - anion. The performance of the differently dried electrolytes with regard to the calcium electrodeposition is evaluated and discussed. An anhydrous synthetic route of Ca(BF4)2 will also be presented and compared to the commercially available salt. References 1. A. Ponrouch, C. Frontera, F. Bardé, and M. R. Palacín, Nat. Mater., 15, 169–172 (2016). 2. D. Wang et al., Nat. Mater., 17, 16–20 (2017). 3. Z. Li, O. Fuhr, M. Fichtner, and Z. Zhao-Karger, Energy Environ. Sci. (2019). 4. A. Shyamsunder, L. E. Blanc, A. Assoud, and L. F. Nazar, ACS Energy Lett., 4, 2271–2276 (2019).

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).

Carbon coating on battery electrode active material powders is currently achieved through chemical procedures involving dispersing the powder in a liquid medium with a carbon precursor followed by thermolysis at high temperatures (ca. 700°C). This procedure has the drawback of not being applicable to materials which may decompose or reduce under such conditions. We present herein an alternative procedure based on physical deposition of carbon, carried out at room temperature under dry conditions, hence avoiding the limitations mentioned above and being generally applicable to any electrode active material. Homogeneous conformal coating was achieved and results are herein presented regarding improved electronic conductivity and limited side reactions recorded upon oxidation and reduction for carbon coated powder active materials in Li cells.

Raman spectroscopy is a powerful tool to study molecular level interactions in e.g. battery electrolytes. As cations and solvents interact, this affects the ion-pair formation and the nature of the solvation shells, which both are key factors affecting solution equilibria and ion transport. While Raman spectroscopy has been extensively used to study lithium-ion battery (LIB) electrolytes, studies on alternative battery technology electrolytes are scarce. Most concern sodium-ion batteries (SIB) and the interpretations are eased by the chemical similarity of Li and Na — both being monovalent. For divalent battery technologies such as Ca and Mg the differences in cation solvation can be expected to be larger, due to the fundamental differences between mono- and divalent ions, and prevents the full application of LIB electrolyte know-how and hence calls for further studies. The recently unveiled calcium battery technology [1] is today limited in scope of electrolytes, basically two compositions dominate [1, 2]. The lack of variety and understanding considerably slows down the Ca battery development, why more in depth analysis of the Ca2+ solvation is called for, to highlight issues hindering fast cation transport within the electrolytes. Here monovalent electrolytes composed of NaPF6 or NaTFSI dissolved in mixtures of ethylene carbonate (EC) and dimethyl carbonate (DMC), i.e. EC:DMC (1:1) were investigated for concentrations ranging from 0.3–1.0 M. They were selected based on the similarities with common LIB electrolytes either commercialized or extensively studied at the laboratory scale [3]. Divalent electrolytes composed of Ca(TFSI)2 and Ca(BF4)2 dissolved in a mixture of ethylene carbonate (EC) and propylene carbonate (PC), i.e. EC:PC (1:1) were chosen for their compatibility with available Ca electrodes [4, 5]. Both Raman spectroscopy and density functional theory calculations were employed to understand quantitatively how the speciation and solvation differ between these monovalent and divalent electrolytes, and ultimately how it could affect ion transport within both electrolytes and electrodes. In addition to the ion conductivities also the viscosities were measured to enable us to apply the fractional Walden rule [6] and qualitatively estimate the ion-pairing as function of both temperature and salt concentration. We observe that the solvation shell configurations change with the salt concentration for both multivalent and divalent formulations; the sizes of the divalent cationic complexes are nearly twice as large as the monovalent but the solvent coordination order is similar; EC tends to predominantly coordinate the cations, followed by PC or DMC. Finally, the Walden plots indicate that formation of contact ion-pairs (CIP) and/or aggregates (AGG) may be more likely for the BF4 based electrolytes, independent of the nature of the cation. References [1] A. Ponrouch, C. Frontera, F. Bardé, M.R. Palacín, Nature Materials, 15 (2016) 169-172. [2] D. Wang, X. Gao, Y. Chen, L. Jin, C. Kuss, P.G. Bruce, Nature Materials, 17 (2017) 16. [3] K. Xu, Chem Rev, 114 (2014) 11503-11618. [4] D.S. Tchitchekova, C. Frontera, A. Ponrouch, C. Krich, F. Bardé, M.R. Palacín, Dalton Transactions, 47 (2018) 11298-11302. [5] D.S. Tchitchekova, A. Ponrouch, R. Verrelli, T. Broux, C. Frontera, A. Sorrentino, F. Bardé, N. Biskup, M.E. Arroyo-de Dompablo, M.R. Palacín, Chemistry of Materials, 30 (2018) 847-856. [6] P. Walden, Z. Phys. Chem, 55 (1906) 207-249.

The study of intercalation compounds [1] led to the identification of redox reactions in host transition metal compounds involving reversible (usually topotactic) insertion and deinsertion of Mn+ ions. Studies with Li+ turned out to be a cornerstone in the development of the Li-ion battery technology [2] that has synergistically boosted portable electronics, while intercalation of multivalent cations (n>1) remained an academic curiosity. Yet, the current energy needs for batteries require significant improvement in energy density, beyond the capabilities of the traditional Li-ion technology, and multivalent cation chemistries represent a thrilling alternative in terms of the amount of energy that can be delivered. Ca metal based batteries have been almost unexplored as reversible plating and stripping of Ca with conventional organic electrolytes at moderate temperature was reported only recently [3,4]. The alkylcarbonate based electrolytes used in this work offer a wide electrochemical stability window, thus inciting the exploration of possible high voltage cathode materials with reliable electrochemical set-ups [5]. Development of potential multivalent host structures is challenging and the choice of cathode materials extremely limited, as divalent cation insertion/extraction is associated with sluggish solid state diffusion kinetics and high activation energies for the charge transfer kinetics. Layered intercalation compounds, with expandable interlayer space like TiS2, V2O5, MoO3, etc., are typically the most studied, but fundamental understanding of the reaction mechanisms in these phases is limited [6,7]. In this work we have explored the possibility of using Ca3Co2O6 cobaltite as Ca-based battery cathode [8]. The pristine compound has a 1D structure of CoO6 octahedra and prisms alternating along chains. By means of ex-situ synchrotron X-ray powder diffraction we have realized that Ca can be extracted by electrochemical methods driving to an incommensurate structure of the type Can+2Con+1O3n+3 (or, equivalently Ca1+xCoO3), characterized by the alternation of n octahedra and one prism along the Co-chain. The reversibility of the redox process seems to be rather limited in the present experimental conditions, indicating some kinetic or thermodynamic issues. Structurally related phases have been also explored as strategy for identifying alternative Ca2+ hosts. References: M.S. Whittingham, Prog. Solid St. Chem. (1978), 12, 41-99. G.E. Blomgren, J. Electrochem. Soc. (2017), 164, A5019-A5025. A. Ponrouch, C. Frontera, F. Bardé and M. R. Palacín, Nat. Mater. (2016), 15, 169-172. A. Ponrouch and M. R. Palacín, Current Opinion in Electrochemistry (2018), 9, 1-7. D. S. Tchitchekova, D. Monti, P. Johansson, F. Bardé, A. Randon-Vitanova, M. R. Palacín and A. Ponrouch, J. Electrochem. Soc. (2017), 164, A1384-A1392. D. S. Tchitchekova, A. Ponrouch, R. Verrelli, T. Broux, C. Frontera, A. Sorrentino, F. Bardé, M. E. Arroyo-de Dompablo and M. R. Palacín, Chem. Mater. (2018), 30, 847. R. Verrelli, A. P. Black, C. Pattanathummasid, D. S. Tchitchekova, A. Ponrouch, J. Oró-Solé, C. Frontera, F. Bardé, P. Rozier and M. R. Palacín, J. Power Sources (2018), 407, 162-172. D. S. Tchitchekova, C. Frontera, A. Ponrouch, C. Krich, F. Bardé and M. R. Palacín, Dalton Trans. (2018), 47, 11298-11302.

Due to serious concerns about future availability and price increase of lithium minerals, several alternatives have been investigated to substitute lithium-ion batteries in high demand applications. Divalent-cation chemistries, such as calcium and magnesium, promise a high energy capacity relying on much more abundant elements (5th and 8th, respectively, in the earth crust) than lithium (25th). Until recently, the development of such battery concepts was focused on electrolytes that do not form passivation films (solid electrolyte interphases - SEI) on the metal surface [1]. These electrolytes, however, suffer from a low stability towards oxidation and thus, limit the output voltage of the assembled cells. Previous works from our group demonstrated calcium plating and stripping in SEI-forming electrolytes at moderate temperatures [2]. Specifically, redox activity of calcium metal was observed in a Ca(BF4)2 electrolyte dissolved in a mixture of cyclic carbonates, while no activity was observed when TFSI was used as anion. The current study focuses on understanding the chemical composition differences between the SEI layers formed in the two systems analysed. A systematic characterization of the SEI formed on the Ca metal anode in various electrolyte formulations will be presented and the most suitable SEI compounds in terms of divalent cation mobility will be discussed. References [1] J. Muldoon, C. B. Bucur and T. Gregory, Chem. Rev., 2014, 114, 11683–11720. [2] A. Ponrouch, C. Frontera, F. Bardé and M. R. Palacín, Nat. Mater., 2016, 15, 169–172.

Batteries based on naturally abundant, light metal anodes (such as Ca and Mg) and multivalent ion host cathodes can potentially achieve very high energy densities at relatively low cost and environmental impact, thus representing a compelling alternative to currently available Li-ion systems. While reversible Ca metal plating and stripping in conventional alkyl carbonate based electrolytes has been accomplished [1,2], unraveling cathode materials with fast and reversible ion mobility at high operating voltages remains a major open challenge, mainly hampered by the slow diffusion kinetics in the solid state of multivalent ions. Thus, besides the exploration of new materials, revisiting traditional layered intercalation hosts appears as a very useful tool to gain further insight into the fundamentals of divalent ion intercalation. In this context, a thorough study of the electrochemical intercalation of Ca2+ in layered TiS2, in alkyl carbonate based electrolytes, is herein presented [3]. Fundamental insights on the insertion process are acquired through X-ray diffraction. Ca2+ insertion is unambiguously proved by using both X-ray diffraction and differential absorption X-ray tomography at the Ca L2 edge and the reversibility of the process is demonstrated at moderate temperature. Different phases can be formed upon reduction of pristine TiS2, whose amount and composition dependon the experimental conditions employed. A comparative study with Mg2+ containing electrolytes and other conventional intercalation hosts, such as V2O5, was also carried out. Careful examination of results highlights the potential relevance of side reactions in these system and the need to use several complementary characterization techniques to unambiguously assess divalent ion intercalation. [1]A. Ponrouch, C. Frontera, F. Bardé and M. R. Palacín, Nat. Mater. (2016), 15, 169. [2] A. Ponrouch, M. R. Palacín, Curr. Opin. Electrochem. (2018), 1. [3] Tchitchekova D.S., Ponrouch A., Verrelli R., Broux T., Frontera C., Sorrentino A., Biskup N., Arroyo-de Dompablo M.E., Bardé F., Palacín M.R., Chem. Mat. (2018), 30, 847.

Various metals have been used as battery anodes in electrochemical cells ever since the birth of the batteries with Volta’s pile and in the first commercialized primary (Zn/MnO2, Leclanché 1866) and secondary (Pb/acid, Planté 1859) batteries. Li-MoS2 cells, employing Li metal anodes, with specific energies two to three times higher than both Ni/Cd and Pb/acid cells, were withdrawn from the market due to safety issues related to dendrites growth. Instead, electrodeposition of Mg and Ca appears to be less prone to dendrite formation.[1,2] Pioneering work by Aurbach et al. in the early 1990’s showed a surface-film controlled electrochemical behavior of Ca and Mg metal anodes in electrolytes with conventional organic solvents.[3,4] The lack of metal plating was attributed to the poor divalent cation migration through the passivation layer. Nevertheless, recent demonstration of Ca and Mg plating and stripping in the presence of a passivation layer or an artificial interphase [2,5,6] has paved the way for assessment of new electrolyte formulations with high resilience towards oxidation. However, several challenges remain to be tackled for the development of Ca based batteries.[7] Among these, the need for reliable electrochemical test protocols, mass transport limitations and high desolvation energies (due to strong cation-solvent and cation–anion interactions) are implied.[8] Here, the reliability of electrochemical set-ups involving multivalent chemistries is discussed, and a systematic investigation on the impact of the electrolyte formulation on the cation solvation structure and transport is presented. Finally, a systematic characterization of the SEI formed on the Ca metal anode in various electrolyte formulations using complementary techniques allowed for the identification of the most suitable SEI compounds in terms of divalent cation mobility. Reference s : [1] M. Matsui, J. Power Sources, 196 (2011) 7048. [2]. A. Ponrouch, C. Frontera, F. Bardé, M.R. Palacín, Nat. Mater., 15 (2016) 169. [3] D. Aurbach, R. Skaletsky, Y. Gofer, J. Electrochem. Soc.138 (1991) 3536. [4] Z. Lu, A. Schechter, M. Moshkovich, D. Aurbach, J. Electroanal. Chem. 466 (1999) 203. [5] D. Wang, X. Gao, Y. Chen, L. Jin, C. Kuss, P. G. Bruce, Nat. Mater. 17 (2018) 16. [6] S.-B. Son, T. Gao, S. P. Harvey, K. X. Steirer, A. Stokes, A. Norman, C. Wang, A. Cresce, K. Xu, C. Ban, Nat. Chem. 10 (2018) 532. [7] A. Ponrouch, M.R. Palacín, Current Opinion in Electrochemistry 2018,doi.org/10.1016/j.coelec.2018.02.001. [8] D. S. Tchitchekova, D. Monti, P. Johansson, F. Bardé, A. Randon-Vitanova, M. R. Palacı́n, A. Ponrouch, J. Electrochem. Soc., 164 (2017) A1384. Figure 1: Scheme of a Ca metal anode-based battery with the main problems/ requirements outlined.[7] Figure 1

While the current state-of-the-art in rechargeable batteries is the Li-ion technology, research efforts are intensified towards the development of alternative technologies to satisfy the ever-increasing demand for enhanced energy density. The development of rechargeable batteries based on the intercalation reaction of multivalent cations (Ca2+, Mg2+, Al3+) is a current challenge for electrochemical energy storage. Designing such batteries demands the identification of competitive electrode materials and electrolytes for each particular active ion. In the last years, we have combined computational and experimental techniques in the search of potential cathode materials that could reversibly insert Mg/Ca ions [1-4]. First principles calculations are used to predict three fundamental electrochemical properties of materials: average intercalation voltage, specific capacity and ionic mobility. Benchmarking the computational results with experimental evidences is a necessary step to accelerate reliable materials design based on DFT calculations. Therefore, our experimental investigations comprise the synthesis, characterization and electrochemical testing of materials. In this work, we confront computational and experimental results for selected oxides, nitrides, sulphides and silicate materials. We will show that the combined computational/experimental approaches lead to proof of concept and rationalization for the reversible electrochemical intercalation and deintercalation of calcium in TiS2 [4]. Acknowledgments: Authors are grateful for financial support from Ministerio de Ciencia e Innovación (grant MAT2014-53500-R), the European Union H2020-FETOPEN funded project CARBAT-766617 and Toyota Motor Europe. References: [1]. M.E. Arroyo-de Dompablo, C. Krich, J. Nava-Avendaño, M.R. Palacin, F. Barde. Phys. Chem. Chem. Phys. 18 (2016) 19966. [2]. M.E. Arroyo-de Dompablo, C. Krich, J. Nava-Avendaño, N. Biskup, M.R. Palacin, F. Barde. Chem. Mater. 28 (2016) 6886. [3] R. Verrelli, M.E. Arroyo-de Dompablo, D. Tchitchekova, A.P. Black, C. Frontera, A. Fuertes, M.R. Palacin. Phys. Chem. Chem. Phys., 19 (2017) 26435 [4]. D. Tchitchekova, A. Ponrouch, R. Verrelli, T. Broux, C. Frontera, A. Sorrentino, F. Barde, N. Biskup, M.E. Arroyo-de Dompablo, M.R. Palacin. Chem. Mater. 30 (2018) 847

Arrays of well aligned Ru nanowires and nanotubes were prepared by electrodeposition through porous anodic aluminum oxide membranes. It is shown that Ru nanowires are formed at low cathodic deposition overpotential, while nanotubes are prepared at higher cathodic overpotential. The inner diameter of the nanotube can be controlled by the deposition potential, and it varies from 0 to 70% of the outer diameter as the deposition potential is changed from -0.30 to -0.45 V vs SCE.

Lithium metal anodes are widely used as counter (or also reference) electrodes in the so termed half cell tests, used mostly at laboratory scale within the battery research community. These are intended to assess the electrochemical performance of a given compound as either positive or negative electrode material for Lithium ion batteries in a simple way, avoiding the assembly of full cells in which electrode balancing can severely affect performance. The reliability of extrapolating such half cell testing results to potential performance in full cells is linked to the stability of the Solid Electrolyte Interphase (SEI) typically formed on the surface of lithium metal anodes as a result of electrolyte degradation reactions. Reliability and representativity of analogous half test cells in Sodium ion battery research has been generally taken for granted. In this context, the aim of our work was to perform a comparative study of the composition, morphology and stability of the SEI formed on lithium and sodium with state-of-the art electrolytes. The impact of the use of Li or Na counterelectrodes in half cell configuration when testing hard carbon electrodes was also investigated. Our results clearly point at the existence of significant differences which cast some doubts on the representativity of half-cell tests and call to exercise care in the extrapolation of their results.1,2 Figure 1. Charge/discharge curves of symmetric Li/Li (red curve) and Na/Na (blue curve) cells cycled at 0.05 mA/cm2 (25˚C) and using 1 M LiPF6 or NaPF6 in EC0.5DMC0.5. Poorer cyclability was observed for Na cells when compared with Li cells. References: 1. D. Iermakova, R. Dugas, M.R. Palacín, A. Ponrouch, J. Electrochem. Soc., 162 (2015) A7060. 2. R. Dugas, A. Ponrouch, G. Gachot, R. David, M.R. Palacín, J.M. Tarascon, J. Electrochem. Soc., 163 (2016) A1. Figure 1

Various metals have been used as battery anodes in electrochemical cells ever since the birth of batteries with Volta’s pile and also in the first commercialized primary (Zn/MnO2, Leclanché 1866) and secondary (Pb/acid, Planté 1859) batteries. The idea and prospects of building a technology based on lithium are much more recent, as it required moving away from aqueous electrolytes. However, the first Li-MoS2 cells with specific energy two or three times higher than the current Ni/Cd or Pb/acid cells were withdrawn from the market after safety difficulties were experienced with overheating on recharge related to dendrite growth. As an alternative, secondary Li-ion batteries avoiding the use of lithium metal anodes were commercialized by Sony in 1991. In contrast with Li metal anode, electrodeposition of Mg and Ca does not seem to be plagued with dendrite formation.1,2 In addition to the low cost of the raw materials ($5000/ton, $100/ton and $265/ton for Li2CO3, CaCO3 and MgO2, respectively), such alternative technologies would benefit from high standard reduction potentials (ca. -2.87 V and -2.37 V vs NHE for Ca and Mg, respectively, as compared to -3 V for Li) and high theoretical electrochemical capacities (both gravimetric and volumetric) for the metal electrode, while relying on the 5th (Ca) and 8th (Mg) most abundant elements on the Earth’s crust, respectively, whereas Li is the 25th. These metals are thus interesting candidates as anodes in rechargeable batteries. Pioneering research work by Aurbach et al.3,4 allowed to conclude that the electrochemical behavior of Ca and Mg metal anodes in conventional organic electrolytes is surface-film controlled, as is the case for Li, but Ca and Mg plating is virtually impossible, which was attributed to the lack of divalent cation transport through the passivation layer (solid electrolyte interphase or SEI). Therefore, research has been focusing on electrolyte formulation in which no SEI is formed. Recently Ca plating and stripping through a stable SEI layer has been demonstrated, setting out the basis for the development of new electrolytes for divalent cation based batteries with high resilience upon oxidation.2 The effect of several factors influencing the Ca deposition/stripping will be presented. Also, the reliability of the so called half-cell configuration in which Mg or Ca pseudo reference and counterelectrodes are used will be discussed and a systematic evaluation of the non-polarizability and stability in the electrolytic environment will be presented for these metal electrodes. Fig ure 1. Theoretical gravimetric and volumetric capacities for different anodes: metals and Li-ion. References: [1] M. Matsui, J. Power Sources 196 (2011) 7048. [2] A. Ponrouch, C. Frontera, F. Bardé, M.R. Palacín, Nat. Mater. 15 (2016) 169. [3]. D. Aurbach, R. Skaletsky, Y. Gofer, J. Electrochem. Soc. 138 (1991) 3536. [4]. Z. Lu, A. Schechter, M. Moshkovich, D. Aurbach, J. Electroanal. Chem. 466 (1999) 203. Figure 1

The development of a rechargeable battery technology using light electropositive metal anodes would bring in a breakthrough in energy density, especially if it involves multivalent charge carriers. While effective electrolytes have been developed for magnesium which has enabled to achieve proof-of-concept for magnesium batteries, the electrodeposition of calcium was thought to be impossible and research restricted to non rechargeable systems. Calcium is an especially attractive alternative as it is the fifth most abundant element on earth crust and its standard reduction potential is only 170 mV above that of lithium, enabling significantly larger cell potential than that achievable with magnesium or aluminium. The talk will revisit these aspects discussing the feasibility of reversible calcium plating/stripping using conventional alkylcarbonate electrolytes which impacts the prospects of developing a new calcium based rechargeable battery technology.

Our society is in urgent need of more efficient, affordable and environmentally friendly energy storage technologies. While lithium-ion batteries (LIB) is undeniably the main track of development and outstanding at present, complementary technologies are needed for managing applications such as renewable energy integration. One particularly appealing alternative is to explore the feasibility of a technology analogous to LIB – sodium ion batteries (SIB) – using the basic LIB know-how and with sodium being more abundant and less expensive. As in any battery technology, the electrolyte choice is as important as that of the electrodes to ensure successful operation. Considering both safety and “environmental impact” ionic liquids (ILs) have emerged as an interesting option for LIB. ILs exhibit both non-volatility and non-flammability, but unfortunately their high viscosities and comparatively low conductivities are expected to considerably limit the rate performance. Mixed electrolytes, i.e. with ILs as additives to an organic electrolyte, are a compromise suggested to improve the safety – while still providing an appreciable conductivity. Here, the possibility of expanding this strategy also to the SIB technology has been explored. We here present a systematic study of several electrolytes consisting of a sodium salt (NaTFSI) dissolved in mixtures of ILs and organic solvents, using electrochemical techniques (cyclic voltammetry, chronopotentiometry) coupled to various safety tests (e.g. self-extinguishing time (SET) and flash point (FP)). Imidazolium (BMImTFSI/EMImTFSI) and pyrrolidinium (PYR13TFSI) ILs were used as additives to optimized ethylene carbonate: propylene carbonate (EC:PC) electrolytes, while Hard Carbon (HC) and Na3V2(PO4)3 (NVP) were used as anode and cathode, respectively. Electrochemical tests were performed on half cells to evaluate the cycling abilities and the electrochemical stability of each electrolyte/electrode combination. HC tested with the BMImTFSI-EC:PC electrolytes exhibit specific capacities of ca 250 mAhg-1 and 140 mAhg-1 at C/5 and 1C rates with capacity retentions of 95% and 98%, respectively. For PYR13TFSI-EC:PC ~150 mAhg-1 at C/10 rate a Coulombic efficiency of ca 98% was achieved. The NVP electrode, exhibits capacities of ~80 mAhg-1 with the BMImTFSI-EC:PC electrolytes at C/10 rate, and of 80, 75 and 46 mAhg-1 at C/10, C/5 and C, respectively with the PYR13TFSI-EC:PC electrolyte. A successful full SIB cell was assembled based on the best choices of the above materials as a proof of concept. As a major argument for the use of ILs is an increased safety, it was encouraging that the safety tests, carried out in air for many different compositions, unambiguously proved the addition of ILs to decrease the SET, as well as the flame intensity, and simultaneously raise the FP.

Recently, there has been a pressing demand for massive energy storage so as to enable the development of electric vehicles and facilitate the use of renewable energies. Li-ion batteries, which have already conquered the portable electronic market, are penetrating the EV’s market and stand as a serious contender for grid-related applications. Therefore, their performances must be improved cost-wise while preserving their energy density and safety attributes. This calls either to design new and high energy density materials based on abundant elements which can be synthesized viaeco-compatible processes or the exploration of alternative technologies. Numerous alternative technologies such as Li-air, Li/S or Na-ion are presently considered. The last technology, will be the focus of the present report. There are several reasons for that: i) sodium resources are in principle unlimited, evenly distributed worldwide and their cost is extremely low; ii) Na does not alloy with Al enabling the use of cheap Al current collectors; last but not least iii) Na has similar intercalation chemistry to that of Li. Moreover, sodium has already been successfully implemented in today’s commercialized high temperature Na/S1 cells for MW size electrochemical energy storage systems and for Na/NiCl2 ZEBRA-type systems2for electric vehicles. Based on both our present understanding of this technology and recent research advances, done worldwide and in our group, at the electrode/electrolyte level we have reached confidence, from safety data regulation sheets, that making Na-ion batteries could present 20-30% cost reduction per kWh as compared to Li-ion technology. To secure such an optimism we launched a French project, involving several partners, aiming to benchmark the Na-ion secondary batteries in terms of sustainability, cost, safety and performances building 18650 and pouch cells. We initially focused on the Sb//1M NaPF6//Na3V2(PO4)2F3 Na-ion technology based on the expertise being developed in the groups forming the consortium on Na3V2(PO4)2F3,3 Electrolyte4 and Sb alloys5. Fundamental studies on the materials processing and electrodes optimization will be first presented. Then, their stability with respect to various electrolytes formulations based on various carbonates solvents and additives (FEC and/or VC) will be discussed in terms of SEI layer formation in both half and complete cells. At last, the performances of a prototype battery (18650 type cell, and pouch cell) together with a preliminary cost estimate will be discussed. Other electrochemical results such as power rate performances and battery hazard analyses will be presented as well. References: [1] J Broadhead - US Patent 4,054,728, 1977. [2] B.L. Ellis, L.F. Nazar, Current Opinion in Solid State and Materials Science, 16 (2012) 168- 177. [3] A. Ponrouch, R. Dedryvère, D. Monti, A.E. Demet, J.M. Ateba Mba, L. Croguennec, C. Masquelier, P. Johansson and M.R. Palacin, Energy & Environmental Science 2013, 6(8), 2361−2369. [4] A. Ponrouch, E. Marchante, M. Courty, J.-M. Tarascon, M.R. Palacin, Energy & Environmental Science, 5 (2012) 8572-8583. [5] A. Darwiche, C. Marino, M.T. Sougrati, B. Fraisse, L. Stievano, L. Monconduit, Journal of the American Chemical Society, 134 (2012) 20805-20811.

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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.