Analysis of Minerals as Electrode Materials for Ca-based Rechargeable Batteries

Mg3Si3(MoO6)2 as a High-Performance Cathode Active Material for Magnesium-Ion Batteries

The natural abundance of magnesium together with its high volumetric energy capacity and less-dendritic anodes makes Mg-ion batteries an appealing alternative to the widely used Li-ion batteries. However, Mg cathode materials under current investigation suffer from various shortcomings such as low operation voltage and high energy barrier for ion migration, resulting in poor battery performance. Here, we propose a garnet-type intercalation cathode active material, Mg₃Si₃(MoO₆)₂, for high-performance Mg-ion batteries. Through first-principles density functional theory calculations, it is demonstrated that Mg₃Si₃(MoO₆)₂ possesses a high average discharge voltage (2.35 V vs Mg/Mg²⁺), a low ion migration barrier (∼0.2 eV), and a minimal volume change (∼4%) concurrently, which comprises excellent intercalation cathode chemistry. The small energy barrier for ion migration is shown to arise from the favorable change in the Mg coordination along the migration route within the garnet host. These findings present an additional direction to develop competent Mg-ion batteries for future energy storage applications.

Gel Polymer Electrolytes Based on Cross-Linked Poly(ethylene glycol) Diacrylate for Calcium-Ion Conduction

Calcium batteries are promising alternatives to lithium batteries owing to their high energy density, comparable reduction potential, and mineral abundance. However, to meet practical demands in high-performance applications, suitable electrolytes must be developed. Here, we report the synthesis and characterization of polymer gel electrolytes for calcium-ion conduction prepared by the photo-cross-linking of poly(ethylene glycol) diacrylate (PEGDA) in the presence of solutions of calcium salts in a mixture of ethylene carbonate (EC) and propylene carbonate (PC) solvents. The results show room-temperature conductivity between 10^-5 and 10^-4 S/cm, electrochemical stability windows of ∼3.8 V, full dissociation of the salt, and minimal coordination with the PEGDA backbone. Cycling in symmetric Ca metal cells proceeds but with increasing overpotentials, which can be attributed to interfacial impedance between the electrolyte and calcium surface, which inhibits charge transfer. Calcium may still be plated and stripped yielding high-purity deposits and no indication of significant electrolyte breakdown, indicating that high overpotentials are associated with an electrically insulating, yet ion-permeable solid electrolyte interface (SEI). This work provides a contribution to the study and understanding of polymer gel materials toward their improvement and application as electrolytes for calcium batteries.

A first-principles comparative study of lithium, sodium, potassium and calcium storage in two-dimensional Mg2C

Two-dimensional (2D) materials have recently emerged as potential candidates for high-capacity lithium-ion batteries anode materials because of their compelling physicochemical and structural properties. In the present study, we use first-principles calculations to investigate the performance of 2D Mg₂C as anode materials for Li, Na, K and Ca-ions batteries. The calculated average open-circuit voltage are 0.37, 0.50, 0.03 and 0.06 eV vs Li, Na, K, Ca. No significant structural deformations are observed on the 2D Mg₂C upon the adsorption of Li, Na, K or Ca and the metallic characteristic of the 2D Mg₂C is retained. The metallic behaviour of both pristine and adsorbed Mg₂C ensures the desirable electric conductivity, implying the advantages of 2D Mg₂C for batteries. The Na and K atoms show an extremely high diffusivity on the 2D Mg₂C with a low energy barrier of 0.08 and 0.04 eV respectively, which is about an order of magnitude smaller than that of Li atom. For the Na and K atoms, the theoretical storage capacity can reach up to 1770 mAh g^-1, nearly two times that of the Li atom of 885 mAh g^-1. Our study suggests that the 2D Mg₂C is a promising anode material which offers a fast ion diffusion and high storage capacity.

Recent Advances and Perspectives on Calcium-Ion Storage: Key Materials and Devices

The urgent demand for cost-effective energy storage devices for large-scale applications has led to the development of several beyond-lithium energy storage systems (EESs). Among them, calcium-ion batteries (CIBs) are attractive due to abundant calcium resources, excellent volumetric and gravimetric capacities of Ca metal anode, and potential high energy density coming from the multivalent feature of Ca-ion. Therefore, the exploration of CIBs electrode materials and the construction of CIBs devices are gaining increasing research interest. Relevant publications cover a wide range of materials by both theoretical and experimental investigations, whereas the performances of rocking-chair CIBs have been unsatisfactory. Meanwhile, multi-ion strategies using more than one ion as the charge carrier have been demonstrated to be feasible and promising options in realizing room temperature CIBs. The summary and reflection of previous studies would provide useful information for future exploration and optimization. In this circumstance, this paper overviews the reported CIBs electrode materials, including both anode and cathode, and presents the latest progress of multi-ion strategies in CIBs. Fundamental challenges, potential solutions, and opportunities are accordingly proposed, mimicking other more mature EESs. This review may promote the development of electrode materials and accelerate the construction of low-cost and high-performance CIBs.

Defects and Calcium Diffusion in Wollastonite

Wollastonite (CaSiO3) is an important mineral that is widely used in ceramics and polymer industries. Defect energetics, diffusion of Ca ions and a solution of dopants are studied using atomistic-scale simulation based on the classical pair potentials. The energetically favourable defect process is calculated to be the Ca-Si anti-site defect cluster in which both Ca and Si swap their atomic positions simultaneously. It is calculated that the Ca ion migrates in the ab plane with an activation energy of 1.59 eV, inferring its slow diffusion. Favourable isovalent dopants on the Ca and Si sites are Sr2+ and Ge4+, respectively. Subvalent doping by Al on the Si site is a favourable process to incorporate additional Ca in the form of interstitials in CaSiO3. This engineering strategy would increase the capacity of this material.

Geo-inspired crystallization engineering: multifunctional materials design and fabrication at nanoscale and beyond

Crystallization engineering aims to design and develop solutions for the optimum conversion of natural resources for use by humans, by using crystallization. Crystallization is a cross-scale process, from atoms, ions and molecules in microscale to bulk crystals in macroscale. Fabricating nanomaterials with desired performances is an open issue with multiscale challenges during crystallization. For innovation in crystallization engineering, geology may provide various sources of inspiration such as structures, compositions and formation conditions, if mineral materials can be regarded as novel artificial materials. This review shows us some geo-inspirations that enable people to create and engineer novel materials with satisfactory performance.

Defects and Dopants in CaFeSi2O6: Classical and DFT Simulations

Calcium (Ca)-bearing minerals are of interest for the design of electrode materials required for rechargeable Ca-ion batteries. Here we use classical simulations to examine defect, dopant and transport properties of CaFeSi2O6. The formation of Ca-iron (Fe) anti-site defects is found to be the lowest energy process (0.42 eV/defect). The Oxygen and Calcium Frenkel energies are 2.87 eV/defect and 4.96 eV/defect respectively suggesting that these defects are not significant especially the Ca Frenkel. Reaction energy for the loss of CaO via CaO Schottky is 2.97 eV/defect suggesting that this process requires moderate temperature. Calculated activation energy of Ca-ion migration in this material is high (>4 eV), inferring very slow ionic conductivity. However, we suggest a strategy to introduce additional Ca2+ ions in the lattice by doping trivalent dopants on the Si site in order to enhance the capacity and ion diffusion and it is calculated that Al3+ is the favourable dopant for this process. Formation of Ca vacancies required for the CaO Schottky can be facilitated by doping of gallium (Ga) on the Fe site. The electronic structures of favourable dopants were calculated using density functional theory (DFT).

Appraisal of calcium ferrites as cathodes for calcium rechargeable batteries: DFT, synthesis, characterization and electrochemistry of Ca4Fe9O17

Sustainability combined with high energy density prospects makes Fe-based oxides attractive as cathodes for calcium rechargeable batteries. This work presents a DFT evaluation of the CaFe2+nO4+n (0 < n < 3) family, for which both the average intercalation voltage and the theoretical specific capacity decrease with the increasing n value. The term n = 1/4, Ca4Fe9O17, meets the most appealing characteristics: a calculated average voltage of 4.16 V, a theoretical specific capacity of 230 mA h g-1 and the lowest energy barrier for Ca migration so far predicted for an existing oxide (0.72 eV). To overcome the previously reported synthesis difficulties, we employed a novel synthesis procedure in sealed quartz tubes followed by quenching in water. The XRD and SAED patterns of the prepared Ca4Fe9O17 powder reveal a certain degree of stacking defects along the c axis. Attempts to deinsert Ca ions from Ca4Fe9O17 by chemical means (NO2BF4 in ACN) and in electrochemical Ca cells were unsuccessful, although some hints of oxidation are observed in Li cells with the LP30 electrolyte. The suitability of Ca4Fe9O17 as a Ca cathode is pending further studies utilizing Ca-electrolytes with high anodic stability.

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.