Towards a calcium-based rechargeable battery

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.Peer reviewe

Achievements, Challenges, and Prospects of Calcium Batteries

This Review flows from past attempts to develop a (rechargeable) battery technology based on Ca via crucial breakthroughs to arrive at a comprehensive discussion of the current challenges at hand. The realization of a rechargeable Ca battery technology primarily requires identification and development of suitable electrodes and electrolytes, which is why we here cover the progress starting from the fundamental electrode/electrolyte requirements, concepts, materials, and compositions employed and finally a critical analysis of the state-of-the-art, allowing us to conclude with the particular roadblocks still existing. As for crucial breakthroughs, reversible plating and stripping of calcium at the metal-anode interface was achieved only recently and for very specific electrolyte formulations. Therefore, while much of the current research aims at finding suitable cathodes to achieve proof-of-concept for a full Ca battery, the spectrum of electrolytes researched is also expanded. Compatibility of cell components is essential, and to ensure this, proper characterization is needed, which requires design of a multitude of reliable experimental setups and sometimes methodology development beyond that of other next generation battery technologies. Finally, we conclude with recommendations for future strategies to make best use of the current advances in materials science combined with computational design, electrochemistry, and battery engineering, all to propel the Ca battery technology to reality and ultimately reach its full potential for energy storage

Towards standard electrolytes for sodium-ion batteries: physical properties, ion solvation and ion-pairing in alkyl carbonate solvents.

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

Towards standard electrolytes for sodium-ion batteries: physical properties, ion solvation and ion-pairing in alkyl carbonate solvents

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

Interfaces and Interphases in Ca and Mg Batteries

The development of high energy density battery technologies based on divalent metals as the negative electrode is very appealing. Ca and Mg are especially interesting choices due to their combination of low standard reduction potential and natural abundance. One particular problem stalling the technological development of these batteries is the low efficiency of plating/stripping at the negative electrode, which relates to several factors that have not yet been looked at systematically; the nature/concentration of the electrolyte, which determines the mass transport of electro-active species (cation complexes) toward the electrode; the possible presence of passivation layers, which may hinder ionic transport and hence limit electrodeposition; and the mechanisms behind the charge transfer leading to nucleation/growth of the metal. Different electrolytes are investigated for Mg and Ca, with the presence/absence of chlorides in the formulation playing a crucial role in the cation desolvation. From a R&D point-of-view, proper characterization alongside modeling is crucial to understand the phenomena determining the mechanisms of the plating/stripping processes. The state-of-the-art is here presented together with a short perspective on the influence of the cation solvation also on the positive electrode and finally an attempt to define guidelines for future research in the field

Multivalent Batteries—Prospects for High Energy Density: Ca Batteries

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

Amorphous Iron Oxyhydroxide Nanosheets: Synthesis, Li Storage, and Conversion Reaction Kinetics

We present a facile approach to synthesize amorphous iron oxyhydroxide nanosheet from the surfactant-assisted oxidation of iron sulfide nanosheet. The amorphous iron oxyhydroxide nanosheet is porous and has a high surface area of 223 m(2) g(-1). The lithium storage properties of the amorphous iron oxyhydroxide are characterized: it is a conversion-reaction electrode material, and it demonstrates superior rate capabilities (e.g., discharge capacities as high as 642 mAh g(-1) are delivered at a current density of 2 C). The impedance spectroscopy analysis identifies a RC series subcircuit originated by the conversion-reaction process. Investigation of the conversion-reaction kinetics through the RC subcircuit time constant reproduces the hysteresis in the discharge/charge voltage profile. Hysteresis is then connected to underlying thermodynamics of the conversion reaction rather than to a kinetic limitation.We thank financial support from Generalitat Valenciana (ISIC/2012/008 Institute of Nanotechnologies for Clean Energies) and FP7 European project ORION (Large CP-IP 229036-2). We thank the financial support by the Singapore National Research Foundation (NRF) through its Campus for Research Excellence and Technological Enterprise (CREATE) program

Unlocking the potential of weberite-type metal fluorides in electrochemical energy storage

Sodium-ion batteries (NIBs) are a front-runner among the alternative battery technologies suggested for substituting the state-of-the-art lithium-ion batteries (LIBs). The specific energy of Na-ion batteries is significantly lower than that of LIBs, which is mainly due to the lower operating potentials and higher molecular weight of sodium insertion cathode materials. To compete with the high energy density of LIBs, high voltage cathode materials are required for NIBs. Here we report a theoretical investigation on weberite-type sodium metal fluorides (SMFs), a new class of high voltage and high energy density materials which are so far unexplored as cathode materials for NIBs. The weberite structure type is highly favorable for sodium-containing transition metal fluorides, with a large variety of transition metal combinations (M, M’) adopting the corresponding Na2MM’F7 structure. A series of known and hypothetical compounds with weberite-type structure were computationally investigated to evaluate their potential as cathode materials for NIBs. Weberite-type SMFs show two-dimensional pathways for Na+ diffusion with surprisingly low activation barriers. The high energy density combined with low diffusion barriers for Na+ makes this type of compounds promising candidates for cathode materials in NIBs

Synthesis, Magnetic Anisotropy and Optical Properties of Preferred Oriented Zinc Ferrite Nanowire Arrays

Preferred oriented ZnFe2O4 nanowire arrays with an average diameter of 16 nm were fabricated by post-annealing of ZnFe2 nanowires within anodic aluminum oxide templates in atmosphere. Selected area electron diffraction and X-ray diffraction exhibit that the nanowires are in cubic spinel-type structure with a [110] preferred crystallite orientation. Magnetic measurement indicates that the as-prepared ZnFe2O4 nanowire arrays reveal uniaxial magnetic anisotropy, and the easy magnetization direction is parallel to the axis of nanowire. The optical properties show the ZnFe2O4 nanowire arrays give out 370–520 nm blue-violet light, and their UV absorption edge is around 700 nm. The estimated values of direct and indirect band gaps for the nanowires are 2.23 and 1.73 eV, respectively