Investigating the role of structural water on the electrochemical properties of α-V2O5 through density functional theory

The α polymorph of V₂O₅ is one of the few known cathodes capable of reversibly intercalating multivalent ions such as Mg, Ca, Zn and Al, but suffers from sluggish diffusion kinetics. The role of H₂O within the electrolyte and between the layers of the structure in the form of a xerogel/aerogel structure, though, has been shown to lower diffusion barriers and lead to other improved electrochemical properties. This density functional theory study systematically investigates how and why the presence of structural H₂O within α-V₂O₅ changes the resulting structure, voltage, and diffusion kinetics for the intercalation of Li, Na, Mg, Ca, Zn, and Al. We found that the coordination of H₂O molecules with the ion leads to an improvement in voltage and energy density for all ions. This voltage increase was attributed to the extra host sites for electrons present with H₂O, thus leading to a stronger ionization of the ion and a higher voltage. We also found that the increase in interlayer distance and a potential "charge shielding" effect drastically changes the electrostatic environment and the resulting diffusion kinetics. For Mg and Ca, this resulted in a decrease in diffusion barrier from 1.3 eV and 2.0 eV to 0.89 eV and 0.4 eV, respectively. We hope that our study motivates similar research regarding the role of water in both V₂O₅ xerogels/aerogels and other layered transition metal oxides.

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.

A cobalt-based metal-organic framework and its derived material as sulfur hosts for aluminum-sulfur batteries with the chemical anchoring effect

With the urgent need to explore high-performance electrochemical energy storage systems, rechargeable Al-ion batteries (AIBs) have attracted attention from researchers and engineers due to their traits, such as abundance and safety. Among all the issues waiting to be solved, the development of a reliable positive electrode material with high specific capacity is an absolute priority for the commercialization of AIBs. Sulfur has a natural advantage when used as the active material, and its theoretical specific capacity is as high as 1675 mA h g-1. MOFs and MOF-derived materials have been proved to be promising hosts for Li-S batteries. Herein, we report a novel Al-S battery system employing MOF (ZIF-67) and MOF-derived materials as sulfur host materials. After being chemically combined with sulfur, the composite still maintains its unique well-defined polyhedron morphology. The voltage hysteresis phenomenon is effectively alleviated with the aid of the host matrix. DFT calculations confirm that ZIF-67 and carbonized ZIF-67-700 polyhedrons can act as an anchor point towards sulfur (S8) and polysulfides (Al2S3, Al2S6, Al2S12, and Al2S18), preventing the detrimental dissolution and shuttle effect. These findings can enlighten future researchers regarding Al-S batteries and broaden the application of MOFs in the field of electrochemical energy storage systems.

Insights into Synergistic Effect of Acid on Morphological Control of Vanadium Oxide: Toward High Lithium Storage

Morphological control is a fundamental challenge of nanomaterial development. Commonly, hierarchical nanostructures cannot be induced by a single driving force, but obtained through balancing multiple driving forces. Here, a feasible strategy is reported based on the synergistic effect of proton and acid anion, leading to the morphological variation of vanadium oxide from nanowire, bundle, to hierarchical nanoflower (HNF). Protons can only induce the formation of nanowire through reducing the pH value ≤ 2. However, acid anions with strong coordination ability, e.g., phosphate radicals, can also participate in morphological regulation at high concentration. Through coordinating with exposed vanadium ions, the enrichment of phosphate radicals at ledge and kink changes the growth directions, giving rise to the advanced structures of bundle and HNF. The lithium ion batteries using HNF as a cathode achieve a 30% improved initial discharge specific capacity of 436.23 mAh g-1 at a current density of 0.1 A g-1, reaching the theoretical maximum value of vanadium oxide based on insertion/desertion of three lithium ions, in addition to strong cyclic stability at 1 A g-1.

Computational Screening of Layered Materials for Multivalent Ion Batteries

Batteries based on multivalent ion (such as Al³⁺, Ca²⁺, and Mg²⁺) intercalation materials have attracted extensive research interest due to their impressive capacity improvement and cost reduction compared with Li-ion batteries. However, the materials for state-of-the-art multivalent ion batteries still suffer from drawbacks such as sluggish ion mobility, poor rate performance, and low cyclic stability, bringing challenges for the design and investigation of new materials. Layered cathode materials are widely applied in current commercial batteries due to their outstanding ionic conductivity and structural stability, which may also hold the key for the cathodes of multivalent batteries. Therefore, combining database screening and density functional theory computations, we evaluated the layered compounds in Materials Project database by theoretical capacity, thermodynamic stability, experimental availability, voltage, volume variation, electronic conductivity, and ionic migration barrier and achieved over 20 kinds of layered cathode materials for multivalent batteries. Through Mg ion substitution for Ca sites, we further achieved several kinds of cathode materials for Mg-ion batteries with ideal stability, voltage, and ion diffusion barriers. We hope the methodology and screened materials could promote the development of multivalent ion batteries.

In Operando Synchrotron Diffraction and in Operando X-ray Absorption Spectroscopy Investigations of Orthorhombic V2O5 Nanowires as Cathode Materials for Mg-Ion Batteries

Orthorhombic V₂O₅ nanowires were successfully synthesized via a hydrothermal method. A cell-configuration system was built utilizing V₂O₅ as the cathode and 1 M Mg(ClO₄)₂ electrolyte within acetonitrile, together with Mg _xMo₆S₈ ( x ≈ 2) as the anode to investigate the structural evolution and oxidation state and local structural changes of V₂O₅. The V₂O₅ nanowires deliver an initial discharge/charge capacity of 103 mAh g^-1/110 mAh g^-1 and the highest discharge capacity of 130 mAh g^-1 in the sixth cycle at C/20 rate in the cell-configuration system. In operando synchrotron diffraction and in operando X-ray absorption spectroscopy together with ex situ Raman and X-ray photoelectron spectroscopy reveal the reversibility of magnesium insertion/extraction and provide information on the crystal structure evolution and changes of the oxidation states during cycling.

Tailoring the electrochemical activity of magnesium chromium oxide towards Mg batteries through control of size and crystal structure

Chromium oxides with the spinel structure have been predicted to be promising high voltage cathode materials in magnesium batteries. Perennial challenges involving the mobility of Mg2+ and reaction kinetics can be circumvented by nano-sizing the materials in order to reduce diffusion distances, and by using elevated temperatures to overcome activation energy barriers. Herein, ordered 7 nm crystals of spinel-type MgCr2O4 were synthesized by a conventional batch hydrothermal method. In comparison, the relatively underexplored Continuous Hydrothermal Flow Synthesis (CHFS) method was used to make highly defective sub-5 nm MgCr2O4 crystals. When these materials were made into electrodes, they were shown to possess markedly different electrochemical behavior in a Mg2+ ionic liquid electrolyte, at moderate temperature (110 °C). The anodic activity of the ordered nanocrystals was attributed to surface reactions, most likely involving the electrolyte. In contrast, evidence was gathered regarding the reversible bulk deintercalation of Mg2+ from the nanocrystals made by CHFS. This work highlights the impact on electrochemical behavior of a precise control of size and crystal structure of MgCr2O4. It advances the understanding and design of new cathode materials for Mg-based batteries.

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.

Layered H0.68Ti1.83O4/reduced graphene oxide nanosheets as a novel cathode for rechargeable magnesium batteries

Rechargeable magnesium batteries (RMBs) are promising devices for energy storage owing to their high volumetric energy density and high safety, but they still suffer from the lack of suitable cathodes.

The potential application of black and blue phosphorene as cathode materials in rechargeable aluminum batteries: a first-principles study

Developing a suitable cathode material for rechargeable aluminum-ion batteries (AIBs) is currently recognized as a key challenge in pushing AIBs from lab-level to industrial application.

The effect of protons on the Mg2+ migration in an α-V2O5 cathode for magnesium batteries: a first-principles investigation

Via first-principles calculations, we showed that the Mg-ion diffusion energy barrier in α-V₂O₅ can be substantially decreased through hydrogenation.

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

Vanadium oxides are among the most promising materials that can be used as electrodes in rechargeable metal-ion batteries. In this work, we systematically investigate thermodynamic, electronic, and kinetic properties associated with the insertion of Li, Mg and Al atoms into rutile VO₂. Using first-principles calculations, we systematically study the structural evolution and voltage curves of Li_xVO₂, Mg_xVO₂ and Al_xVO₂ (0 < x < 1) compounds. The calculated lithium intercalation voltage starts at 3.50 V for single-atom insertion and decreases to 2.23 V for full lithiation, to the LiVO₂ compound, which agrees well with the experimental results. The Mg insertion features a plateau about 1.6 V up to Mg_0.5VO₂ and then another plateau-like region at around 0.5 V up to Mg₁VO₂. The predicted voltage curve for Al insertion starts at 1.98 V, followed by two plateaus at 1.48 V and 1.17 V. The diffusion barrier of Li, Mg and Al in the tunnel structure of VO₂ is 0.06, 0.33 and 0.50 eV, respectively. The demonstrated excellent Li, Mg and Al mobility, high structural stability and high specific capacity suggest promising potential of rutile VO₂ electrodes especially for multivalent batteries.

Investigation of the water-stimulated Mg2+ insertion mechanism in an electrodeposited MnO2 cathode using X-ray photoelectron spectroscopy

In water-containing organic electrolyte, the charge storage of amorphous MnO₂ combines Mg(OH)₂ formation at the cathode surface and Mg insertion.

Hybrid DFT investigation of the energetics of Mg ion diffusion in α-MoO3

Mg ion diffusion barriers in α-MoO₃ are simulated using the GGA, GGA+U, and hybrid DFT approaches.

Ab initio study of adsorption and diffusion of lithium on transition metal dichalcogenide monolayers

Using first principles calculations, we studied the stability and electronic properties of transition metal dichalcogenide monolayers of the type MX2 (M = Ti, Zr, Hf, V, Nb, Ta, Mo, Cr, W; X= S, Se, Te). The adsorption and diffusion of lithium on the stable MX2 phase was also investigated for potential application as an anode for lithium ion batteries. Some of these compounds were found to be stable in the 2H phase and some are in the 1T or 1T' phase, but only a few of them were stable in both 2H/1T or 2H/1T' phases. The results show that lithium is energetically favourable for adsorption on MX2 monolayers, which can be semiconductors with a narrow bandgap and metallic materials. Lithium cannot be adsorbed onto 2H-WS2 and 2H-WSe2, which have large bandgaps of 1.66 and 1.96 eV, respectively. The diffusion energy barrier is in the range between 0.17 and 0.64 eV for lithium on MX2 monolayers, while for most of the materials it was found to be around 0.25 eV. Therefore, this work illustrated that most of the MX2 monolayers explored in this work can be used as promising anode materials for lithium ion batteries.

Structural water engaged disordered vanadium oxide nanosheets for high capacity aqueous potassium-ion storage

Aqueous electrochemical energy storage devices using potassium-ions as charge carriers are attractive due to their superior safety, lower cost and excellent transport properties compared to other alkali ions. However, the accommodation of potassium-ions with satisfactory capacity and cyclability is difficult because the large ionic radius of potassium-ions causes structural distortion and instabilities even in layered electrodes. Here we report that water induces structural rearrangements of the vanadium-oxygen octahedra and enhances stability of the highly disordered potassium-intercalated vanadium oxide nanosheets. The vanadium oxide nanosheets engaged by structural water achieves high capacity (183 mAh g⁻¹ in half-cells at a scan rate of 5 mV s⁻¹, corresponding to 0.89 charge per vanadium) and excellent cyclability (62.5 mAh g⁻¹ in full cells after 5,000 cycles at 10 C). The promotional effects of structural water on the disordered vanadium oxide nanosheets will contribute to the exploration of disordered structures from earth-abundant elements for electrochemical energy storage.

The authors report that the interplay between structural water and highly disordered vanadium oxide can stabilize the layered metal oxides and enhanced their performance for aqueous potassium-ion storage based on neutron scattering measurements and electrochemical characterizations.

Comparison of Li, Na, Mg and Al-ion insertion in vanadium pentoxides and vanadium dioxides

We investigate and compare main vanadium oxide phases for Li, Na, Mg and Al-ion batteries.

Mapping the Challenges of Magnesium Battery

Rechargeable Mg battery has been considered a major candidate as a beyond lithium ion battery technology, which is apparent through the tremendous works done in the field over the past decades. The challenges for realization of Mg battery are complicated, multidisciplinary, and the tremendous work done to overcome these challenges is very hard to organize in a regular review paper. Additionally, we claim that organization of the huge amount of information accumulated by the great scientific progress achieved by various groups in the field will shed the light on the unexplored research domains and give clear perspectives and guidelines for next breakthrough to take place. In this Perspective, we provide a convenient map of Mg battery research in a form of radar chart of Mg electrolytes, which evaluates the electrolyte under the important components of Mg batteries. The presented radar charts visualize the accumulated knowledge on Mg battery and allow for navigation of not only the current research state but also future perspective of Mg battery at a glance.

Defect-Mediated Lithium Adsorption and Diffusion on Monolayer Molybdenum Disulfide

Monolayer Molybdenum Disulfide (MoS₂) is a promising anode material for lithium ion batteries because of its high capacities. In this work, first principle calculations based on spin density functional theory were performed to investigate adsorption and diffusion of lithium on monolayer MoS₂ with defects, such as single- and few-atom vacancies, antisite, and grain boundary. The values of adsorption energies on the monolayer MoS₂ with the defects were increased compared to those on the pristine MoS₂. The presence of defects causes that the Li is strongly bound to the monolayer MoS₂ with adsorption energies in the range between 2.81 and 3.80 eV. The donation of Li 2s electron to the defects causes an enhancement of adsorption of Li on the monolayer MoS₂. At the same time, the presence of defects does not apparently affect the diffusion of Li, and the energy barriers are in the range of 0.25–0.42 eV. The presence of the defects can enhance the energy storage capacity, suggesting that the monolayer MoS₂ with defects is a suitable anode material for the Li-ion batteries.

Tavorite-FeSO4F as a potential cathode material for Mg ion batteries: a first principles calculation

The electrochemical and Mg ion diffusion properties of tavorite-Mg0.5FeSO4F were studied by using first principles calculations. A discharge voltage of about 2.52 V versus Mg/Mg(2+) corresponding to the redox couples of Fe(3+)/Fe(2+) was predicted for tavorite-Mg0.5FeSO4F, and the experimental diffusion coefficient for the Mg-vacancy in Mg0.5-xFeSO4F is expected to be of the same order of magnitude as that of the Li-vacancy in Li1-xFeSO4F.

Evaluating pristine and modified SnS2 as a lithium-ion battery anode: a first-principles study

Li intercalation and diffusion in pristine and modified SnS2 interlayer are studied by a first-principles approach. The results predict that the octahedral interstitial site is energetically favored for Li intercalation. The minimum energy path of Li diffusion in SnS2 interlayer is investigated by climbing image nudged elastic band method. It is found that Li atom diffuses from one energetically favored octahedral interstitial site to the neighbor one via tetrahedral interstitial site. The expansion of interlayer spacing is beneficial for decreasing the diffusion barrier. Ce dopant negatively impacts the Li diffusivity although it can optimize the interlayer spacing. Geometric structures of LixSnS2 (0 < x ≤ 3) are investigated to understand the lithiation-induced volume expansion and atomic structure change. The lithiation process can be divided into two stages. When Li content (x in LixSnS2) is less than 1, the volume expansion is not dramatic and only S atoms capture electrons from Li atoms. When Li content is larger than 1, Sn(4+) cations are significantly reduced, S-Sn-S trilayer gradually decomposes, and LixS2 (1 ≤ x ≤ 3) layer forms between two Sn monolayers. The mechanism of volume expansion is elucidated in this study.

Transformers: the changing phases of low-dimensional vanadium oxide bronzes

In this feature article, we explore the electronic and structural phase transformations of ternary vanadium oxides with the composition M_xV₂O₅where M is an intercalated cation.

First-principles evaluation of multi-valent cation insertion into orthorhombic V2O5

A detailed exploration and systematic evaluation of V₂O₅ as a cathode material for multi-valent batteries using first-principles.

Recent progress in theoretical and computational investigations of Li-ion battery materials and electrolytes

Advancements and progress in computational and theoretical investigations of Li-ion battery materials and electrolytes are reviewed and assessed.

The solvation structure of Mg ions in dichloro complex solutions from first-principles molecular dynamics and simulated X-ray absorption spectra

The knowledge of Mg solvation structure in the electrolyte is requisite to understand the transport behavior of Mg ions and their dissolution/deposition mechanism at electrolyte/electrode interfaces. In the first established rechargeable Mg-ion battery system [D. Aurbach et al. Nature 2000, 407, 724], the electrolyte is of the dichloro complex (DCC) solution family, Mg(AlCl2BuEt)2/THF, resulting from the reaction of Bu2Mg and EtAlCl2 with a molar ratio of 1:2. There is disagreement in the literature regarding the exact solvation structure of Mg ions in such solutions, i.e., whether Mg(2+) is tetra- or hexacoordinated by a combination of Cl(-) and THF. In this work, theoretical insight into the solvation complexes present is provided based on first-principles molecular dynamics simulations (FPMD). Both Mg monomer and dimer structures are considered in both neutral and positively charged states. We found that, at room temperature, the Mg(2+) ion tends to be tetracoordinated in the THF solution phase instead of hexacoordinated, which is the predominant solid-phase coordination. Simulating the X-ray absorption spectra (XAS) at the Mg K-edge by sampling our FPMD trajectories, our predicted solvation structure can be readily compared with experimental measurements. It is found that when changing from tetra- to hexacoordination, the onset of X-ray absorption should exhibit at least a 1 eV blue shift. We propose that this energy shift can be used to monitor changes in the Mg solvation sphere as it migrates through the electrolyte to electrolyte/electrode interfaces and to elucidate the mechanism of Mg dissolution/deposition.

Comparison of tetragonal and cubic tin as anode for Mg ion batteries

Using first-principles calculation based on density functional theory, diffusion of Mg atom into α- and β-Sn was investigated. The diffusion barriers are 0.395 and 0.435 eV for an isolated Mg atom in the α- and β-Sn, respectively. However, the diffusion barriers of the Mg atom decrease in the α-Sn, whereas they increase in the β-Sn, when an additional Mg atom was inserted near the original diffusing Mg atom, which is mainly due to strong binding of Mg-Mg atoms in the β-Sn. Therefore, it is better to use the α-Sn, rather than the β-Sn, as an anode material for Mg ion batteries.

MgVPO4F as a one-dimensional Mg-ion conductor for Mg ion battery positive electrode: a first principles calculation

MgVPO₄F is proposed as a cathode material for rechargeable Mg ion batteries for the first time.

Theoretical study on the initial stage of a magnesium battery based on a V2O5 cathode

The barrierless α to δ phase transformation is proposed as a possible explanation for the full reversibility of a Mg battery with a V₂O₅ cathode.