Ab initio study of Li, Mg and Al insertion into rutile VO2: fast diffusion and enhanced voltages for multivalent batteries

Order-Disorder Transition in Rutile VO2(M) Electrodes during Li Intercalation and Extraction

Transition metal oxides are widely employed as electrode materials in Li-ion batteries. During battery operation, Li ions are intercalated and extracted from the framework of the electrode structure, causing structural transitions. In some materials, the process can drive order-disorder transitions; however, insights into such processes are generally lacking, although they are essential for our understanding of battery aging and in the design of new sustainable battery chemistries. Herein, we investigate the intercalation-induced order-disorder transition in rutile VO2(M) electrodes by means of galvanostatic charge/discharge cycling, operando powder X-ray diffraction, and total X-ray scattering with pair distribution function analysis. The study reveals that the rutile structure transforms irreversibly into a highly disordered layered Li x VO2 structure, which is capable of reversibly intercalating Li ions. Our findings point out general trends for the intercalation-driven transitions in rutile oxides.

Phase-engineered cathode for super-stable potassium storage

The crystal phase structure of cathode material plays an important role in the cell performance. During cycling, the cathode material experiences immense stress due to phase transformation, resulting in capacity degradation. Here, we show phase-engineered VO2 as an improved potassium-ion battery cathode; specifically, the amorphous VO2 exhibits superior K storage ability, while the crystalline M phase VO2 cannot even store K+ ions stably. In contrast to other crystal phases, amorphous VO2 exhibits alleviated volume variation and improved electrochemical performance, leading to a maximum capacity of 111 mAh g-1 delivered at 20 mA g-1 and over 8 months of operation with good coulombic efficiency at 100 mA g-1. The capacity retention reaches 80% after 8500 cycles at 500 mA g-1. This work illustrates the effectiveness and superiority of phase engineering and provides meaningful insights into material optimization for rechargeable batteries.

Fast kinetics of monoclinic VO2(B) bulk upon magnesiation via DFT+U calculations

Although magnesium rechargeable batteries (MRBs) have gained considerable attention, research relating to MRBs is still in its infancy. One issue is that magnesium ions are difficult to reversibly (de)intercalate in most electrode materials. Among various available cathodes, VO₂(B) is a promising layered cathode material for use in MRBs. Totally different from monolayer VO₂, the magnesiation mechanism in monoclinic bulk VO₂(B) has not been clearly clarified to this day. For the first time, we systematically investigated the influence of magnetism and van der Waals (vdW) forces on the electronic structure and diffusion kinetics of magnesium in bulk VO₂(B) using a series of DFT+U calculations. The Mg diffusivity can reach a high value of 1.62 × 10^-7 cm² s^-1 at 300 K, which is comparable to Li⁺. These results demonstrate that VO₂(B) is a potential host material with high mobility and fast kinetics.

A first principles study of corundum V2O3 material as a promising anode for Li/Mg/Al-ion batteries

In this work the electrochemical properties of corundum V2O3 are calculated using the first principle calculations. Our results highly recommend V2O3 as promising anode for both MIBs and AIBs.

First-principles prediction of layered MoO2and MoOSe as promising cathode materials for magnesium ion batteries

Magnesium ion battery is one of the promising next-generation energy storage systems. Nevertheless, lack of appropriate cathode materials to ensure massive storage and efficient migration of Mg cations is a big obstacle for development of Mg-ion batteries. Herein, by means of first principles calculations, the geometric structure, electronic structure, Mg intercalation behavior and Mg diffusion behavior of the layered MoO₂and two MoOSe (MoOSe(I) and MoOSe(V)) were systematically investigated. Layered MoO₂shows semiconductor properties, while MoOSe displays metallic characteristics which ensure higher conductivity. The Mg cations tend to intercalate into octahedral sites for both MoO₂and MoOSe. The maximum Mg-storage phases of the layered MoO₂, MoOSe(I) and MoOSe(V) correspond to Mg_0.666MoO₂, Mg_0.666MoOSe(I) and Mg_0.666MoOSe(V), with theoretical specific capacities of 279, 191 and 191 mAh g^-1, respectively. The calculated discharge plateaus of MoO₂and two MoOSe are all about 1 V, which ensure that the layered MoO₂and MoOSe electrodes can act as cathodes for Mg-ion batteries in the early stage. Moreover, comparing with other cathodes, the diffusion barrier of Mg cations and volume expansion during Mg intercalation process are competitive. The results suggest that layered MoO₂and MoOSe are the promising cathode materials for Mg-ion batteries.

Density-Based Descriptors of Redox Reactions Involving Transition Metal Compounds as a Reality-Anchored Framework: A Perspective

Description of redox reactions is critically important for understanding and rational design of materials for electrochemical technologies, including metal-ion batteries, catalytic surfaces, or redox-flow cells. Most of these technologies utilize redox-active transition metal compounds due to their rich chemistry and their beneficial physical and chemical properties for these types of applications. A century since its introduction, the concept of formal oxidation states (FOS) is still widely used for rationalization of the mechanisms of redox reactions, but there exists a well-documented discrepancy between FOS and the electron density-derived charge states of transition metal ions in their bulk and molecular compounds. We summarize our findings and those of others which suggest that density-driven descriptors are, in certain cases, better suited to characterize the mechanism of redox reactions, especially when anion redox is involved, which is the blind spot of the FOS ansatz.

First-Principle Insights Into Molecular Design for High-Voltage Organic Electrode Materials for Mg Based Batteries

Low cost, scalability, potentially high energy density, and sustainability make organic magnesium (ion) battery (OMB) technologies a promising alternative to other rechargeable metal-ion battery solutions such as secondary lithium ion batteries (LIB). However, most reported OMB cathode materials have limited performance due to, in particular, low voltages often smaller than 2 V vs. Mg²⁺/Mg and/or low specific capacities compared to other competing battery technologies, e.g., LIB or sodium ion batteries. While the structural diversity of organic compounds and the large amount of possible chemical modifications potentially allow designing high voltage/capacity OMB electrode materials, the large search space requires efficient exploration of potential molecular-based electrode materials by rational design strategies on an atomistic scale. By means of density functional theory (DFT) calculations, we provide insights into possible strategies to increase the voltage by changes in electronic states via functionalization, by strain, and by coordination environment of Mg cations. A systematic analysis of these effects is performed on explanatory systems derived from selected prototypical building blocks: five- and six-membered rings with redox-active groups. We demonstrate that voltage increase by direct bandstructure modulation is limited, that strain on the molecular scale can in principle be used to modulate the voltage curve and that the coordination/chemical environment can play an important role to increase the voltage in OMB. We propose molecular structures that could provide voltages for Mg insertion in excess of 3 V.

Sulfur- and Selenium-Containing Compounds Potentially Exhibiting Al Ion Conductivity

We have created a set of crystalline model structures exhibiting straight lines of Al³⁺ connected to chalcogenides (O^2- , S^2- , and Se^2- ) connected to metal cations of varying valence (Sr²⁺ , Y³⁺ , Zr⁴⁺ , Nb⁵⁺ , and Mo⁶⁺ ). They were relaxed with density functional theory computations and analysed by Bader partitioning. As Al³⁺ ions are supposed to strongly interact with their atomic environment, we studied the electron density topology induced by higher-valent cations in the extended chemical neighbourhood of Al. In fact, we found a general decrease of ionic charges and an increasing displacement of the chalcogenides towards higher-valent ions for the heavier chalcogens. Therefore, we comprehensively screened S- and Se-containing compounds for candidates theoretically exhibiting low migration barriers for Al³⁺ ions by using Voronoi-Dirichlet partitioning and bond valence site energy calculations. The basis for this search is the Inorganic Crystal Structure Database. Indeed, we could extract six promising candidates with low Al³⁺ migration barriers. which are even lower than the barriers for any other element inside of these materials. This will encourage efforts towards preparing suitable Al³⁺ conductors.

Machine Learning the Voltage of Electrode Materials in Metal-Ion Batteries

Machine-learning (ML) techniques have rapidly found applications in many domains of materials chemistry and physics where large data sets are available. Aiming to accelerate the discovery of materials for battery applications, in this work, we develop a tool ( http://se.cmich.edu/batteries ) based on ML models to predict voltages of electrode materials for metal-ion batteries. To this end, we use deep neural network, support vector machine, and kernel ridge regression as ML algorithms in combination with data taken from the Materials Project database, as well as feature vectors from properties of chemical compounds and elemental properties of their constituents. We show that our ML models have predictive capabilities for different reference test sets and, as an example, we utilize them to generate a voltage profile diagram and compare it to density functional theory calculations. In addition, using our models, we propose nearly 5000 candidate electrode materials for Na- and K-ion batteries. We also make available a web-accessible tool that, within a minute, can be used to estimate the voltage of any bulk electrode material for a number of metal ions. These results show that ML is a promising alternative for computationally demanding calculations as a first screening tool of novel materials for battery applications.

First-principles study of VPO4O as a cathode material for rechargeable Mg batteries

The electrochemical properties of VPO₄O as a cathode for Mg batteries were studied by performing first principles calculations.

Non-catalytic hydrogenation of VO2 in acid solution

Hydrogenation is an effective way to tune the property of metal oxides. It can conventionally be performed by doping hydrogen into solid materials with noble-metal catalysis, high-temperature/pressure annealing treatment, or high-energy proton implantation in vacuum condition. Acid solution naturally provides a rich proton source, but it should cause corrosion rather than hydrogenation to metal oxides. Here we report a facile approach to hydrogenate monoclinic vanadium dioxide (VO₂) in acid solution at ambient condition by placing a small piece of low workfunction metal (Al, Cu, Ag, Zn, or Fe) on VO₂ surface. It is found that the attachment of a tiny metal particle (~1.0 mm) can lead to the complete hydrogenation of an entire wafer-size VO₂ (>2 inch). Moreover, with the right choice of the metal a two-step insulator–metal–insulator phase modulation can even be achieved. An electron–proton co-doping mechanism has been proposed and verified by the first-principles calculations.

Hydrogenation is an effective way to tune the property of metal oxides. Here, the authors report a simple approach to hydrogenate VO₂ in acid solution under ambient conditions by placing a small piece of low workfunction metal on VO₂ surface.