Periodic density functional theory calculations have been performed to study the migration of various charge carriers in spinel-type MgSc 2 Se 4. This compound exhibits low barriers for Mg ion diffusion, making it a potential candidate for solid electrolytes in Mg-ion batteries. In order to elucidate the decisive factors for the ion mobility in spinel-type phases, the diffusion barriers of other mono-and multivalent ions (Li + , Na + , K + , Cs + , Zn 2+ , Ca 2+ , and Al 3+) in the MgSc 2 Se 4 framework have been determined as well. This allows for disentangling structural and chemical factors, showing that the ion mobility is not solely governed by size and charge of the diffusing ions. Finally, our results suggest that charge redistribution and rehybridization caused by the migration of the multivalent ions increase the resulting migration barriers.

 10 À4 % of Li. The ionic radii of Mg 2þ , 0.86 Å, and Li þ , 0.90 Å, are rather similar, [1] but Mg has the advantage of being a bivalent ion, which leads to a higher volumetric capacity of Mg metal anodes compared to Li, 3833 mAh cm À3 versus 2062 mAh cm À3 , and also to a low reduction potential of À2.37 V versus the standard hydrogen electrode (SHE) compared to À3.05 V of Li. [9,10] Furthermore, Mg-ion batteries (MIBs) exhibit a low tendency for dendrite formation [11][12][13][14][15] and a high melting point.A high multivalent ionic conductivity of 1-10 mS cm À1 has been achieved in MIBs at high temperatures. [16,17] However, a major problem for MIBs lies in the sluggish kinetics during intercalation at room temperature. [2,18] It should be noted that the design of chemically stable electrodes with high ionic conductivity is highly desirable, [2,[19][20][21][22][23] as a low ionic mobility can severely limit the performance of batteries.To address the slow migration of Mg ions in cathode materials at low temperatures, Chevrel phases and layered and spinel TiS 2 structures have been studied in detail. [24] A Mg-ion migration barrier of about 550 meV was found in cubic Ti 2 S 4 using galvanostatic intermittent titration technique measurements. Note that typically maximum migration barriers of %525 meV for micron-sized particles and %650 meV for nanosized particles are assumed to be compatible with an adequate battery operation. [25] Studies on the sulfide and selenide spinel frameworks indicate low-energy barriers for Mgion diffusion comparable to those of LIBs. [26] In contrast, oxide spinel cathode materials exhibit high migration barriers for Mg ions, which are caused by the relatively strong Coulombic attraction between the guest Mg 2þ and host oxygen lattice, [23] which leads to a lower ion mobility. The smaller electronegativity of sulfur and selenium lattices enlarges the lattice constant of these materials and thus also their ion mobility as typically diffusion barriers become smaller for larger lattice constants. Nevertheless, the increase of the ion mobility through the lowering of diffusion barriers is also accompanied by lower Mg insertion energies into the spinel structures, which lowers the voltage [27,28] and thus causes a reduction of the energy densities of chalcogenide materials.Recently, MgSc 2 Se 4 has been found to be a super ionic conductor exhibiting a high Mg-ion conductivity of 0.1 mS cm À1 at room temperature. [26] This high ion mobility not only makes MgSc 2 Se 4 a promising cathode material for MIBs, but also suggests that it could be used as a solid electrolyte. However, solid

Rechargeable sodium-ion batteries are viable candidates as nextgeneration energy storage devices. Nonetheless, the development of high-potential and stable cathode materials is still one among the open tasks. Here, we propose a combined experimental/theoretical approach to shed light on the effect of magnesium doping on the layered P2-Na 0.67 Mn 0.75 Ni 0.25 O 2 cathode material. The P2-Na 0.67 Mn 0.75 Ni 0.25 O 2 baseline material and doped P2-Na 0.67 Mn 0.75 Ni 0.20 Mg 0.05 O 2 , synthesized via coprecipitation route followed by thermal treatment, have been physically and chemically characterized via X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM), as well as electrochemically via galvanostatic cycling and galvanostatic intermittent titration technique (GITT). The Mg-doped material showed stabilization of the high potential plateau and improved cycle life. The analysis of the phase transition with synchrotron operando XRD (SXRD) shows multiple possible intermediate phases ("Z-phase") rather than a pure OP4-like structure. Based on our experimental data and periodic density functional theory (DFT) calculations, the stability of the O2, P2, and OP4 phases for the pristine and Mg-doped systems was investigated to elucidate the origin of the "Z"phase formation in the Mg-doped material.

Enabling high Mg ion mobility, spinel-type materials are promising candidates for cathode or solid electrolyte applications. To elucidate the factors governing the observed high mobility of multivalent ions, periodic DFT calculations of various charge carriers (A = Li, Na, K, Mg, Ca, Zn and Al) in the ASc 2 S 4 and ASc 2 Se 4 spinel compounds were performed, resulting in the identification of a Brønsted-Evans-Polanyi-type scaling relation for the migration barriers of the various charge carriers. Combining this scaling relation with the derivation of a descriptor, solely based on easily accessible observables, constitutes a conceptual framework to investigate ion mobility in d 0 -metal-based spinel chalcogenides with significantly reduced computational effort. This approach was exemplarily verified for various d 0 -metal-based spinel chalcogenide compounds AB 2 X 4 (B = Sc, Y, Ga, In, Er and Tm; X = O, S and Se) and led to the identification of d 0 -metal-based CaB 2 O 4 spinels as promising compounds possibly enabling high Ca ion mobility.

In the area of sustainable energy storage, batteries based on multivalent ions such as magnesium have been attracting considerable attention due to their potential for high energy densities. Furthermore, they are typically also more abundant than, e.g., lithium. However, as a challenge their low ion mobility in electrode materials remains.This study addresses the ionic conductivity in spinel host materials which represent a promising class of cathode and solid-electrolyte materials in Mg-ion batteries. Based on periodic density functional theory calculations, we identify the critical parameters which determine the mobility and insertion of ions. We will in particular highlight the critical role that trigonal distortions of the spinel structure play for the ion mobility. In detail, we will show that it is the competition between coordination and bond length that governs the Mg site preference in ternary spinel compounds upon trigonal distortions. This can only be understood by also taking covalent interactions into account. Furthermore, our calculations suggest that anionic redox plays a much more important role in sulfide and selenide spinels than in oxide spinels. Based on our theoretical study, we rationalize the impact of the metal distribution in the host material and the ion concentration on the diffusion process. Furthermore, cathode-related challenges for practical devices will be addressed. Our findings shed light on the fundamentional mechanisms underlying ionic conductivity in solid hosts and thus may contribute to improve ion transport in battery electrodes.

Mg batteries with oxide cathodes have the potential to significantly surpass existing Li-ion technologies in terms of sustainability, abundance, and energy density. However, Mg intercalation at the cathode is often severely hampered by the sluggish kinetics of Mg2+ migration within oxides. Here we report a combined theoretical and experimental study addressing routes to identify cathode materials with an improved Mg-ion mobility. Using periodic density functional theory calculations, Mg2+ migration in oxide spinels has been studied, revealing key features that influence the activation energy for Mg2+ migration. Furthermore, the electronic and geometrical properties of the oxide spinels as well as their stability have been analyzed for a series of different transition metals in the spinels. We find that electronegative transition metals enable a high Mg-ion mobility in the oxide spinel frameworks and thus a favorable cathode functionality. Based on the theoretical findings, some promising candidates have been identified, prepared and structurally characterized. Our combined theoretical and experimental findings open up an avenue toward the utilization of functional cathode materials with improved Mg2+ transport properties for Mg-metal batteries.

In the area of sustainable energy storage, batteries based on multivalent ions such as magnesium have been attracting considerable attention due to their potential for high energy densities. Furthermore, they are typically also more abundant than, e.g., lithium. However, as a challenge their low ion mobility in electrode materials remains. This study addresses the ionic conductivity in spinel host materials which represent a promising class of cathode and solid-electrolyte materials in Mg-ion batteries. Based on periodic density functional theory calculations, we identify the critical parameters which determine the mobility and insertion of ions. We will in particular highlight the critical role that trigonal distortions of the spinel structure play for the ion mobility. In detail, we will show that it is the competition between coordination and bond length that governs the Mg site preference in ternary spinel compounds upon trigonal distortions. This can only be understood by also taking covalent interactions into account. Furthermore, our calculations suggest that anionic redox plays a much more important role in sulfide and selenide spinels than in oxide spinels. Based on our theoretical study, we rationalize the impact of the metal distribution in the host material and the ion concentration on the diffusion process. Furthermore, cathode-related challenges for practical devices will be addressed. Our findings shed light on the fundamentional mechanisms underlying ionic conductivity in solid hosts and thus may contribute to improve ion transport in battery electrodes.

Ion mobility in electrolytes and electrodes is an important performance parameter in electrochemical devices, particularly in batteries. In this review, we concentrate on the charge carrier mobility in crystalline battery materials where the diffusion basically corresponds to hopping processes between lattice sites. However, in spite of the seem- ing simplicity of the migration process in crystalline materials, the factors governing mobility in these materials are still debated. There are well-accepted factors contributing to the ion mobility such as the size and the charge of the ions, but they are not sufficient to yield a complete picture of ion mobility. In this review, we will critically discuss possible factors influencing ion mobility in crystalline battery materials. To gain insights in these factors, we discuss chemical trends in batteries, both as far as the charge carriers as well as the host materials are concerned. Furthermore, we will also address fundamental questions, for example about the nature of the migrating charge carriers.