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A simplified method is presented to extract the transport parameters of a liquid electrolyte solution from an experiment in a four-electrode cell. By observing that both polarisation and relaxation tend exponentially to a finite value with the same time constant, it becomes possible to precisely evaluate the limiting voltage attained upon polarisation though no steady state can be reached in practice, and use this input to evaluate the cation transference number as a function of the thermodynamic factor. This procedure simplifies the data analysis by avoiding complete fitting of the polarisation with a complicated numerical model. Such approach can favour the deployment of the four-electrode method to evaluate electrolyte transport parameters, which are crucial for the development of better performing batteries., Funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (Grant Agreement No. 715087). We thank the Spanish Agencia Estatal de Investigacion Severo Ochoa Programme for Centres of Excellence in R&D (CEX2019-000917-S). R.D. acknowledges support of the Beatriu de Pinos postdoctoral programme of the Government of Catalonia's Secretariat for Universities and Research of the Ministry of Economy and Knowledge (2017 BP 00187). We thank Charles Delacourt and Mohammad Farkhondeh for useful discussion., With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).

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

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).

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.

This special collection contains a selection of research articles corresponding to contributions presented at Symposium C of the European Materials Science Meeting Edition 2019 that was held in Nice, France, from the 26th to the 31st of May 2019.

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.

Electrochemical performance of hard carbon (HC) prepared from sugar pyrolysis was investigated against sodium anodes at temperatures ranging from −15 °C up to 75 °C. Carbon coating was performed on HC decreasing its surface area and allowing significant improvement in terms of the first cycle coulombic efficiency and thus, irreversible capacity loss. Electrochemical performances of HC at various temperatures pointed at the IR drop of the cell being determinant for achieving the highest reversible capacity (450 mAh/g at C/10, 75 °C) and rate capability (340 mAh/g at 2C, 75 °C) ever reported. Stable cycling was also achieved at −15 °C with reversible capacity of ca. 265 mAh/g. Keywords: Na-ion batteries, Negative electrode, Hard carbon, Temperature, Electrolyte, Coating

Na2Ti3O7 is an interesting negative electrode material for sodium ion batteries given its electrochemical capacity and low operation potential. Unfortunately its prospects of practical application are hindered by an unacceptable capacity fading upon cycling. In this work we combine experiments and DFT calculations to investigate the origin of such a phenomenon. Different electrode technologies and different electrolytes have been targeted for electrochemical testing while the stability of Na2Ti3O7 and the fully reduced Na4Ti3O7 has been studied. The calculated elastic constants and vibrational modes support the mechanical and dynamical stability of the Na4Ti3O7 structure. In situ XRD measurements corroborate the reversibility of the insertion reaction as no structural damage is detected after 50 cycles. An intriguing reactivity of Na2Ti3O7 with the electrolyte upon storage is observed, which coupled to electrochemical measurements points to this being the main factor behind capacity fading.

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.

We report on the hydrothermal synthesis of LiMn 1 − x Fe x PO 4 where x = 0.1, 0.2, 0.5, 0.8 and LiMn 0.9 Mg 0.1 PO 4 at 260 °C, using water–EMI_TFSI mixture as reacting medium. Electrochemical tests carried out in galvanostatic mode at 100 oC show that LiMn 0.5 Fe 0.5 PO 4 presents the best performance in terms of capacity (ca.120 mAh/g), therefore this capacity is equally parted between 3.5 and 4.1 V plateaus. This contrasts with LiMn 0.9 Mg 0.1 PO 4 which is more attractive energy density wise, as it exhibits a single reversible plateau having a capacity of 120 mAh/g at higher voltage (ca. 4.1 V vs. Li + /Li).

The performance of MnV2O6 (MnV) and its [{Mn(Bpy)}(VO3)2]≈(H2O)1.16 (MnBpy) and [{Mn(Bpy)0.5}(VO3)2]≈(H2O)0.62 (MnBpy0.5) hybrid derivative compounds was investigated against sodium and lithium counter electrodes. For MnV2O6 stable capacities of 850 mAh/g were achieved in lithium cells, the best value reported so far. The whole capacity is ascribed to a conversion reaction in which the amorphization of the compounds takes place. No significant differences in the capacities for the inorganic compound and the hybrid ones were observed. Interestingly, the potential hysteresis decreases in the hybrid compounds. The difference between Li and Na cell capacity most probably comes from the difference of standard potential of the two redox couples Li+/Li and Na+/Na of about ca. 0.3 V leading to an incomplete conversion reaction and thus lowers capacity in the case of Na cells. The Raman and IR ex-situ experiments after cycling indicate that the bipyridine organic ligands are completely decomposed during the electrochemical testing. The IR studies in MnV inorganic and MnBpy and MnBpy0.5 hybrid electrodes after the electrochemical cycling, suggest that the SEI formation and bipyridine degradation give rise to different aliphatic compounds.

Storing electrochemical energy by means of fully reducing transition metal compounds to their metallic state is attractive due to the large amounts of charge that are produced. However, while more reversible than originally envisioned, electrochemical conversion reactions are accompanied by large inefficiencies. These inefficiencies are associated with hysteresis between the potentials of reduction and re-oxidation, as well as the loss of capacity when reversing back to the initial state. This work presents a series of results collected from different kinds of electroanalytical experiments. The use of data from conventional powder electrodes as well as thin films offered the opportunity to compare measurements of common occurrence in the literature with more sophisticated experiments carried out at higher temperature, which provided a complete picture of the kinetic nature of the processes involved. This picture is supportive of the thermodynamic nature of potential hysteresis, and indicates that coulombic inefficiencies stem from the low reversibility of the conversion reaction. Taken together, the results cast a clear light on the significant challenges that lie ahead if this kind of reactivity is to become technologically relevant in devices such as Li-ion batteries.

Electrochemical performance of hard carbon prepared from sugar pyrolysis was investigated against sodium anodes. Specific surface area and graphitization degree are determinant for achieving the highest reversible capacity ever reported (more than 300 mAh/g at C/10 after 120 cycles) with excellent rate capability. Such results were obtained using additive free EC:PC based electrolyte which appears to induce the formation of a more conducting solid electrolyte interphase (SEI) than that produced in the presence of 2% fluoroethylene carbonate (FEC). Keywords: Sodium ion batteries, Negative electrode, Hard carbon, Electrolyte, FEC additive

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.

A systematic formulation study has been performed on Co 3 O 4 based composite electrodes. The influence of diverse parameters has been studied such as: calendaring pressure applied, composition of the composite electrode, nature of the carbon additives (carbon super P, carbon nanofibers and carbon nanotubes) and the size of the active material particles. The influence of electrochemical parameters (lower cut-off voltage, initial C rate, and rate changes or open circuit potential steps during cycling) was also ascertained. These results are discussed in the perspective of the specific issues associated with materials reacting through conversion reactions (first cycle coulombic inefficiency, voltage hysteresis and capacity fade). Finally, basing on electrochemical impedance spectroscopy measurements and systematic record of the IR drop term of the cells upon cycling, a mechanism to account for the generally observed capacity fade on these materials is proposed.

Highly {100} oriented Pt deposits were prepared by electrodeposition from a 10 mM HCl, 100 mM KCl and Na 2 PtCl 6 .xH 2 O electrolyte. The deposits were prepared in the form of thin fi lms and array of nanowires. A qualitative assessment of the proportion of {100} oriented Pt surfaces was obtained through X-ray diffraction measurements and cyclic voltammetry in 0.5 M H 2 SO 4 . The effect of the deposition potential, E dep , temperature of the electrolyte, T dep , platinum salt concentration [Na 2 PtCl 6 .xH 2 O], and nature of the substrate were investigated. It was shown that the proportion of {100} oriented Pt surfaces reaches a maximum for E dep = −0.35 V vs SCE. Moreover, this proportion increases steadily as T dep and [Na 2 PtCl 6 .xH 2 O] are decreased from 75 to 25 ° C and from 2.5 to 0.25 mM, respectively. Scanning electron microscopy and high-resolution transmission electron microscopy micrographs indicate that the more oriented samples are made of pine tree-like structures that are effectively single crystals, and that the growth facets appear to be close to the {001} plane. This observation also clearly indicates that the plane exposed during the CV experiment is also {001}. As suggested by these micrographs, the fi lms and nanowires are highly porous and roughness factors as large as 1000 were obtained on highly {100} oriented Pt nanowires. The predominance of {100} facets is attributed to their energetically favoured growth in the presence of hydrogen, and is shown to be signifi cantly enhanced when the mass transport of Pt 4 + is limited. Due to the predominance of {100} facets, the normalized electrocatalytic activity ( μ A cm − 2 Pt ) for the electro-oxidation of hydrazine and ammonia is higher than non-oriented polycrystalline Pt by a factor of 4 and 2.7, respectively.

PtxCo100−x (60 ≤ x ≤ 100) 1D nanostructures (nanowires) on titanium are prepared by direct electrodeposition throughout a porous anodic aluminum oxide membrane and analyzed for methanol electrooxidation. For comparison, thin films of the same composition are also prepared. The morphology and composition are characterized by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). A model based on the simultaneous deposition of Pt and Co underpotential deposition (UPD) on Pt was devised to understand how the composition varies with the electrodeposition potential. The deposition rates were determined by means of an Electrochemical Quartz Crystal Microbalance (EQCM). PtxCo100−x alloy formation was confirmed by X-ray diffraction (XRD). Electrocatalytic properties for methanol oxidation are investigated by cyclic voltammetry (CV) in 1.0 M CH3OH + 0.5 M H2SO4. The results show that PtCo nanowires yield to an enhancement (by a factor of 5) of the exposed surface area (m2 g−1). In the presence of CH3OH, If/Ib ratio of Pt rich nanowires is systematically higher than for thin films with the same composition. If/Ib ratio of Pt90Co10 nanowires is 1.59, larger than that of pure Pt (If/Ib = 0.76–0.87) and commercially available PtCo deposited on carbon support (If/Ib = 1.32).

The influence of the slurry mixing procedure on the homogeneity and electrochemical performance of composite electrodes for Li-ion batteries was studied. MCMB graphite and Co3O4 were used as probe active materials, since they exhibit different reaction mechanisms with lithium. Three different mixing procedures were used, (i) magnetic stirring, (ii) magnetic stirring with additional sonication steps and (iii) ball milling. SEM studies allow to determine that the mixing procedure has a significant effect in the microstructure of the active material. Indeed, ball milling was found to destroy the pristine MCMB microstructure and to induce agglomeration in nanosized Co3O4. While magnetic stirring is much less aggressive to that respect, sonication steps were found to enhance the homogeneity of the mixture without inducing major modifications in the microstructure of the materials and hence to have a beneficial impact in the final electrochemical performance.

Two types of platinum deposits were obtained by potentiostatic deposition onto Ti substrates, namely platinum black (Pt B) and platinum nanowires (Pt NW) with the latter being achieved through a porous anodic aluminum oxide (AAO) membrane at the solution/substrate interface. Surface characterization of these deposits was performed using scanning electron microscopy (SEM) and cyclic voltammetry (CV) in sulphuric acid solution. Surface properties for Pt NW revealed a predominant presence of (1 0 0) crystallographic planes, not present in Pt B deposit grown in the same conditions. Also, Pt NW exhibits an increased resistance to electrochemically active surface area (EASA) loss upon potential cycling in acidic solution, as compared to Pt B. Keywords: Platinum, Surface order, Nanowires, (1 0 0) Orientation

PtRu 1D nanostructures on titanium are prepared and analysed as electrocatalysts for methanol electrooxidation. The morphology and composition of the 1D nanostructure are characterized by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). The electrocatalytic properties of such catalysts for methanol oxidation are investigated by cyclic voltammetry (CV) and chronoamperometry (CA) in 1.0 M CH3OH + 0.5 M H2SO4 aqueous solution. The results show that Pt46Ru54 nanotubes yields to a five-fold improvement of the mass specific activity and to a three-fold improvement of the long-term poisoning rate as compared to PtRu black of similar composition. Keywords: Methanol, PtRu, Nanotube, Electrocatalysts, Electrooxidation

Platinum was electrochemically deposited in the forms of nanowires and nanotubes and compared to Pt black and Pt wire electrodes. The resistance to CO poisoning was measured in CO saturated solution by recording the current decrease in the H adsorption/desorption region through continuous sweeping of the electrode potential between 0.00 and 0.45 V vs RHE. For Pt deposits prepared with the same deposition charge, it is shown that the maximum coverage of CO, θCO,max, decreases as one goes from Pt black, to nanowires and to nanotubes. In the case of the longest nanotube prepared, θCO,max is as low as 0.44. Keywords: CO poisoning, Nanowire, Nanotube, Platinum, Cyclic voltammetry

A first review of hard carbon materials as negative electrodes for sodium ion batteries is presented, covering not only the electrochemical performance but also the synthetic methods and microstructures. The relation between the reversible and irreversible capacities achieved and microstructural features is described and illustrated with specific experiments while discussing also the effect of the electrolyte. A summary of the current knowledge is given while emphasizing the possibility of further performance improvements by thoroughly mastering structure-property relationships and also discussing the main current bottleneck to maximize energy density in real applications: the first cycle irreversible capacity. Finally, a short conclusion and perspectives session is provided highlighting necessary developments in the field to turn the present optimistic research prospects into tangible practical products.

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.

[EN] A facile and potentially scalable synthesis route to obtain SnO2–carbon composites was developed. SnO2 nanoparticles were deposited on the surface of two types of graphitic carbon: (a) commercial porous graphite (HG) and (b) graphitic carbon nanostructures. The synthesis procedure consists of two simple steps: (i) room temperature formation/deposition of SnO2 nanocrystals and (ii) thermal treatment at 350 °C to generate SnO2 nanoparticles (size ~3.5 nm) over the carbon surface. The electrochemical performance of the graphitic carbons and the SnO2–carbon composites as anode materials in Li-ion rechargeable batteries was investigated. In all cases, tape casting electrode fabrication allowed almost full active material utilization. Good cyclabilities were achieved, with HG and HG–SnO2 showing capacities of 356 and 545 mAh g−1, respectively after 50 cycles., We acknowledge the financial support received from the Spanish Ministry of Science and Innovation (MAT2011-24757 and MAT2008-00407). M. S. thanks the Spanish Ministry of Science and Innovation for the award of a Postdoctoral Mobility contract.

In this study, platinum was electroplated onto bare Ti substrates (Pt black) and through a porous AAO membrane (Pt nanowires). The morphology of the deposits was observed by scanning electron microscopy. Preferential orientation along the (100) direction into the bulk material was evidenced through XRD analysis, as well as at the nanowire surface by using electrochemical characterization. These highly oriented Pt nanowires exhibited an increased activity for the electrocatalytic oxidation of hydrazine oxidation, as compared to Pt black.

Electrolytes are essential for the proper functioning of any battery technology and the emerging Na-ion technology is no exception. Hence, a major focus on battery research is to identify the most appropriate formulation so as to minimize interface reactions and enhance both cell performances and safety aspects. In order to identify suitable electrolyte formulations for Na-ion chemistry we benchmarked various electrolytes containing diverse solvent mixtures (cyclic, acyclic carbonates, glymes) and Na-based salts having either F-based or perchlorate anions and measured viscosity, ionic conductivity, and thermal and electrochemical stability. The binary EC:PC solvent mixture has emerged as the best solvent formulation and has been used to test the performance of Na/hard carbon cells with both NaClO4 and NaPF6 as dissolved salts. Hard carbon electrodes having reversible capacities of 200 mA h g−1 with decent rate capability and excellent capacity retention (>180 cycles) were demonstrated. Moreover, DSC heating curves demonstrated that fully sodiated hard carbon cycled in NaPF6–EC:PC exhibits the highest exothermic onset temperature and nearly the lowest enthalpy of reaction, thus making this electrolyte most attractive for the development of Na-ion batteries.

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.

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.

Calcium-metal batteries have received growing attention recently after several studies reporting successful metal plating and stripping with organic electrolytes. Given the low redox potential of metallic calcium, its surface is commonly covered by a passivation layer grown by the accumulation of electrolyte decomposition products. The presence of borate species in this layer has been shown to be a key parameter allowing for Ca2+ migration and favoring Ca electrodeposition. Here, boron-based additives are evaluated in order to tune the SEI composition, morphology, and properties. The decomposition of a BF3-based additive is studied at different potentiostatic steps and the resulting SEI layer was thoroughly characterized. SEI growth mechanism is proposed based on both experimental data and DFT calculations pointing at the formation of boron-crosslinked polymeric matrices. Several boron-based adducts are explored as SEI-forming additives for calcium-metal batteries paving the way to very rich chemistry leading to Ca2+ conducting SEI., Funding from the European Union's Horizon 2020 research and innovation program H2020 are acknowledged: European Research Council (ERC-2016-STG, CAMBAT, grant agreement no. 715087 and ERC-2020-STG, HiPeR−F, grant agreement no. 950625) 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 Programme for Centres of Excellence in R&D (CEX2019-000917-S). D.F., C.C. and R.D. thank the French National Research Agency (STORE-EX Labex Project ANR-10-LABX-76-01) for financial support. K.R. and M.L. gratefully acknowledge the research funding by the Slovenian Research Agency (P1-0045, N1-0189). Alistore-European Research Institute is gratefully acknowledged for financial support through the postdoc grant to C.B. The SR-FTIR experiments were performed at MIRAS beamline at ALBA Synchrotron with the collaboration of ALBA staff. All DFT calculations were carried out at the Wroclaw Centre for Networking and Supercomputing within grant no. 346., With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).

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.