Cathode Materials and Chemistries for Magnesium Batteries: Challenges and Opportunities

Rechargeable magnesium batteries hold promise for providing high energy density, material sustainability, and safety features, attracting increasing research interest as post-lithium batteries. With the progressive development of Mg electrolytes with enhanced (electro-)chemical stability, tremendous efforts have been devoted to the exploration of high-energy cathode materials. In this review, recent findings related to Mg cathode chemistry are summarized, focusing on the strategies that promote Mg2+ diffusion by targeting its interaction with the cathode hosts. The critical role of the cathode–electrolyte interfaces is elaborated, which remains largely unexplored in Mg systems. The approaches to optimization of cathode–electrolyte combinations to unlock the kinetic limitations of Mg2+ diffusion, enabling fast electrochemical processes of the cathodes, are highlighted. Furthermore, representative conversion chemistries and coordination chemistries that bypass bulk Mg2+ diffusion are discussed, with particular attention given to their key challenges and prospects. Finally, the hybrid systems that combine the fast kinetics of the monovalent cathode chemistries and high-capacity Mg anodes are revisited, calling for further practical evaluation of this promising strategy. All in all, the aim is to provide fundamental insights into the cathode chemistry, which promotes the material development and interfacial regulations toward practical high-performance Mg batteries

Towards stable and efficient electrolytes for room-temperature rechargeable calcium batteries

Rechargeable calcium (Ca) batteries have the prospect of highenergy and low-cost. However, the development of Ca batteries is hindered due to the lack of efficient electrolytes. Herein, we report novel calcium tetrakis(hexafluoroisopropyloxy)borate Ca[B(hfip)₄]₂ based electrolytes exhibiting reversible Ca deposition at room temperature, a high oxidative stability up to 4.5 V and high ionic conductivity >8 mS cm¯¹. This finding opens a new approach towards room-temperature rechargeable calcium batteries

Calcium-tin alloys as anodes for rechargeable non-aqueous calcium-ion batteries at room temperature

Rechargeable calcium batteries possess attractive features for sustainable energy-storage solutions owing to their high theoretical energy densities, safety aspects and abundant natural resources. However, divalent Ca-ions and reactive Ca metal strongly interact with cathode materials and non-aqueous electrolyte solutions, leading to high charge-transfer barriers at the electrode-electrolyte interface and consequently low electrochemical performance. Here, we demonstrate the feasibility and elucidate the electrochemical properties of calcium-tin (Ca–Sn) alloy anodes for Ca-ion chemistries. Crystallographic and microstructural characterizations reveal that Sn formed from electrochemically dealloying the Ca–Sn alloy possesses unique properties, and that this in-situ formed Sn undergoes subsequent reversible calciation/decalciation as CaSn(3). As demonstration of the suitability of Ca–Sn alloys as anodes for Ca-ion batteries, we assemble coin cells with an organic cathode (1,4-polyanthraquinone) in an electrolyte of 0.25 M calcium tetrakis(hexafluoroisopropyloxy)borate in dimethoxyethane. These electrochemical cells are charged/discharged for 5000 cycles at 260 mA g(−1), retaining a capacity of 78 mAh g(−1) with respect to the organic cathode. The discovery of new class of Ca–Sn alloy anodes opens a promising avenue towards viable high-performance Ca-ion batteries

Growth and Optical Properties of the Whole System of Li(Mn1−x_{1-x},Nix_{x})PO4_{4} (0 ≤ x ≤ 0.5) Single Crystals

A series of single crystals of Li(Mn$_{1-x}$,Ni$_{x}$)PO$_{4}$ (x = 0.00, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.08, 0.10, 0.15, 0.20, and 0.50) have been grown to large sizes up to 5 mm in diameter and 120 mm in length using the floating zone method for the first time. The comprehensive characterizations of the as-grown crystals were performed before further physical property measurements. The composition of the grown crystals was determined by energy-dispersive X-ray spectroscopy. The crystal structures were characterized by the X-ray powder diffraction method with a GSAS fitting for structural refinement, which reveals a high phase purity of the as-obtained crystals. The polarized microscopic images and Laue patterns prove the excellent quality of the single crystals. Oriented cuboids with sizes of 2.7 × 3.8 × 2.1 mm$3{1-x}$ along the a, b, and c crystalline directions were cut and polished for further anisotropic magnetic and transparent measurements. We also first proposed a new potential application in the non-linear optical (NLO) and laser generation application for LiMPO$_{4}$ (M = transition metal) materials. The optical and laser properties, such as the absorption spectra and the second harmonic generation (SHG), have been investigated and have furthermore confirmed the good quality of the as-grown single crystals

Establishing a Stable Anode–Electrolyte Interface in Mg Batteries by Electrolyte Additive

Simple magnesium salts with high electrochemical and chemical stability and adequate ionic conductivity represent a new-generation electrolyte for magnesium (Mg) batteries. Similar to other Mg electrolytes, the simple-salt electrolyte also suffers from high charge-transfer resistance on the Mg surface due to the adsorbed species in the solution. In the current study, we built a model Mg cell system with the Mg[B(hfip)4]2/DME electrolyte and Chevrel phase Mo6S8 cathode, to demonstrate the effect of such anode–electrolyte interfacial properties on the full-cell performance. It was found that the cell required additional activation cycles to achieve its maximal capacity. The activation process is mainly attributed to the conditioning of the anode–electrolyte interface, which could be boosted by introducing an additive amount of Mg(BH4)2 to the Mg[B(hfip)4]2/DME electrolyte. Electrochemical and spectroscopic analyses revealed that the Mg(BH4)2 additive helps to remove the native oxide layer and promotes the formation of a solid electrolyte interphase layer on Mg. As a result, the full cell with the additive-containing electrolyte delivered a stable capacity from the second cycle onward. Further battery tests showed a reversible cycling for 600 cycles and an excellent rate capability, indicating good compatibility of the Mg(BH4)2 additive. The current study not only provides fundamental insights into the interfacial phenomena in Mg batteries but also highlights the facile tunability of the simple-salt Mg electrolytes

Fast kinetics of multivalent intercalation chemistry enabled by solvated magnesium-ions into self-established metallic layered materials

Rechargeable magnesium batteries are one of the most promising candidates for next-generation battery technologies. Despite recent significant progress in the development of efficient electrolytes, an on-going challenge for realization of rechargeable magnesium batteries remains to overcome the sluggish kinetics caused by the strong interaction between double charged magnesium-ions and the intercalation host. Herein, we report that a magnesium battery chemistry with fast intercalation kinetics in the layered molybdenum disulfide structures can be enabled by using solvated magnesium-ions ([Mg(DME)x]2+). Our study demonstrates that the high charge density of magnesium-ion may be mitigated through dimethoxyethane solvation, which avoids the sluggish desolvation process at the cathode-electrolyte interfaces and reduces the trapping force of the cathode lattice to the cations, facilitating magnesium-ion diffusion. The concept of using solvation effect could be a general and effective route to tackle the sluggish intercalation kinetics of magnesium-ions, which can potentially be extended to other host structures

The Electronic Structural and Defect-Induced Absorption Properties of a Ca2_{2}B10_{10}O14_{14}F6_{6} Crystal

Comprehensive ab initio electronic structure calculations were performed for a newly developed deep-ultraviolet (DUV) non-linear optical (NLO) crystal Ca$_{2}$B$_{10}$O$_{14}$F$_{6}$ (CBOF) using the first principle method. Fifteen point defects including interstitial, vacancy, antisite, Frenkel, and Schottky of Ca, O, F, and B atoms in CBOF were thoroughly investigated as well as their effects on the optical absorption properties. Their formation energies and the equilibrium concentrations were also calculated by ab initio total energy calculations. The growth morphology was quantitatively analyzed using the Hartman–Perdok approach. The formation energy of interstitial F (Fi) and antisite defect O$_{F}$ were calculated to be approximately 0.33 eV and 0.83 eV, suggesting that they might be the dominant defects in the CBOF material. The absorption centers might be induced by the O and F vacancies (V$_{F}$, V$_{O}$), interstitial B and O (O$_{i}$, B$_{i}$), and the antisite defect O substitute of F (O$_{F}$), which might be responsible for lowering the damage threshold of CBOF. The ionic conductivity might be increased by the Ca vacancy (V$_{ca}$), and, therefore, the laser-induced damage threshold decreases