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

Unconventional Charge Transport in MgCr2O4 and Implications for Battery Intercalation Hosts

Ion transport in solid-state cathode materials prescribes a fundamental limit to the rates batteries can operate; therefore, an accurate understanding of ion transport is a critical missing piece to enable new battery technologies, such as magnesium batteries. Based on our conventional understanding of lithium-ion materials, MgCr₂O₄ is a promising magnesium-ion cathode material given its high capacity, high voltage against an Mg anode, and acceptable computed diffusion barriers. Electrochemical examinations of MgCr₂O₄, however, reveal significant energetic limitations. Motivated by these disparate observations; herein, we examine long-range ion transport by electrically polarizing dense pellets of MgCr₂O₄. Our conventional understanding of ion transport in battery cathode materials, e.g., Nernst-Einstein conduction, cannot explain the measured response since it neglects frictional interactions between mobile species and their nonideal free energies. We propose an extended theory that incorporates these interactions and reduces to the Nernst-Einstein conduction under dilute conditions. This theory describes the measured response, and we report the first study of long-range ion transport behavior in MgCr₂O₄. We conclusively show that the Mg chemical diffusivity is comparable to lithium-ion electrode materials, whereas the total conductivity is rate-limiting. Given these differences, energy storage in MgCr₂O₄ is limited by particle-scale voltage drops, unlike lithium-ion particles that are limited by concentration gradients. Future materials design efforts should consider the interspecies interactions described in this extended theory, particularly with respect to multivalent-ion systems and their resultant effects on continuum transport properties.

Facile Preparation of CuCo2 S4 /Cu7.2 S4 Nanocomposites as High-Performance Cathode Materials for Rechargeable Magnesium Batteries*

Rechargeable magnesium batteries (RMBs) have been considered a promising energy-storage device due to their high energy density and high safety, but they still suffer from a lack of high-rate performance and cycle performance of the cathode. Nanosized CuCo₂ S₄ /Cu_7.2 S₄ composites have been synthesized for the first time by a facile solvothermal method. Herein, the magnesium ion storage behavior when applied in the cathode for RMBs is discussed. Electrochemical results demonstrated that the CuCo₂ S₄ /Cu_7.2 S₄ composites exhibit a high initial discharge capacity of 256 mAh g^-1 at 10 mA g^-1 and 123 mAh g^-1 at 300 mA g^-1 at room temperature and an outstanding long-term cyclic stability over 300 cycles at 300 mA g^-1 . Furthermore, the electrochemical storage mechanism demonstrated that the storage process of magnesium ion in the CuCo₂ S₄ /Cu_7.2 S₄ cathode is mainly driven by strong pseudocapacitive effects.

Control of crystal size tailors the electrochemical performance of α-V2O5 as a Mg2+ intercalation host

α-V₂O₅ has been extensively explored as a Mg²⁺ intercalation host with potential as a battery cathode, offering high theoretical capacities and potentials vs. Mg²⁺/Mg. However, large voltage hysteresis is observed with Mg insertion and extraction, introducing significant and unacceptable round-trip energy losses with cycling. Conventional interpretations suggest that bulk ion transport of Mg²⁺ within the cathode particles is the major source of this hysteresis. Herein, we demonstrate that nanosizing α-V₂O₅ gives a measurable reduction to voltage hysteresis on the first cycle that substantially raises energy efficiency, indicating that mechanical formatting of the α-V₂O₅ particles contributes to hysteresis. However, no measurable improvement in hysteresis is found in the nanosized α-V₂O₅ in latter cycles despite the much shorter diffusion lengths, suggesting that other factors aside from Mg transport, such as Mg transfer between the electrolyte and electrode, contribute to this hysteresis. This observation is in sharp contrast to the conventional interpretation of Mg electrochemistry. Therefore, this study uncovers critical fundamental underpinning limiting factors in Mg battery electrochemistry, and constitutes a pivotal step towards a high-voltage, high-capacity electrode material suitable for Mg batteries with high energy density.

Database of ab initio L-edge X-ray absorption near edge structure

The L-edge X-ray Absorption Near Edge Structure (XANES) is widely used in the characterization of transition metal compounds. Here, we report the development of a database of computed L-edge XANES using the multiple scattering theory-based FEFF9 code. The initial release of the database contains more than 140,000 L-edge spectra for more than 22,000 structures generated using a high-throughput computational workflow. The data is disseminated through the Materials Project and addresses a critical need for L-edge XANES spectra among the research community.

Enhanced charge storage of nanometric ζ-V2O5 in Mg electrolytes

V2O5 is of interest as a Mg intercalation electrode material for Mg batteries, both in its thermodynamically stable layered polymorph (α-V2O5) and in its metastable tunnel structure (ζ-V2O5). However, such oxide cathodes typically display poor Mg insertion/removal kinetics, with large voltage hysteresis. Herein, we report the synthesis and evaluation of nanosized (ca. 100 nm) ζ-V2O5 in Mg-ion cells, which displays significantly enhanced electrochemical kinetics compared to microsized ζ-V2O5. This effect results in a significant boost in stable discharge capacity (130 mA h g-1) compared to bulk ζ-V2O5 (70 mA h g-1), with reduced voltage hysteresis (1.0 V compared to 1.4 V). This study reveals significant advancements in the use of ζ-V2O5 for Mg-based energy storage and yields a better understanding of the kinetic limiting factors for reversible magnesiation reactions into such phases.

Probing Mg Intercalation in the Tetragonal Tungsten Bronze Framework V4Nb18O55

While commercial Li-ion batteries offer the highest energy densities of current rechargeable battery technologies, their energy storage limit has almost been achieved. Therefore, there is considerable interest in Mg batteries, which could offer increased energy densities in comparison to Li-ion batteries if a high-voltage electrode material, such as a transition-metal oxide, can be developed. However, there are currently very few oxide materials which have demonstrated reversible and efficient Mg²⁺ insertion and extraction at high voltages; this is thought to be due to poor Mg²⁺ diffusion kinetics within the oxide structural framework. Herein, the authors provide conclusive evidence of electrochemical insertion of Mg²⁺ into the tetragonal tungsten bronze V₄Nb₁₈O₅₅, with a maximum reversible electrochemical capacity of 75 mA h g^-1, which corresponds to a magnesiated composition of Mg₄V₄Nb₁₈O₅₅. Experimental electrochemical magnesiation/demagnesiation revealed a large voltage hysteresis with charge/discharge (1.12 V vs Mg/Mg²⁺); when magnesiation is limited to a composition of Mg₂V₄Nb₁₈O₅₅, this hysteresis can be reduced to only 0.5 V. Hybrid-exchange density functional theory (DFT) calculations suggest that a limited number of Mg sites are accessible via low-energy diffusion pathways, but that larger kinetic barriers need to be overcome to access the entire structure. The reversible Mg²⁺ intercalation involved concurrent V and Nb redox activity and changes in crystal structure, as confirmed by an array of complementary methods, including powder X-ray diffraction, X-ray absorption spectroscopy, and energy-dispersive X-ray spectroscopy. Consequently, it can be concluded that the tetragonal tungsten bronzes show promise as intercalation electrode materials for Mg batteries.

Energy storage emerging: A perspective from the Joint Center for Energy Storage Research

Energy storage is an integral part of modern society. A contemporary example is the lithium (Li)-ion battery, which enabled the launch of the personal electronics revolution in 1991 and the first commercial electric vehicles in 2010. Most recently, Li-ion batteries have expanded into the electricity grid to firm variable renewable generation, increasing the efficiency and effectiveness of transmission and distribution. Important applications continue to emerge including decarbonization of heavy-duty vehicles, rail, maritime shipping, and aviation and the growth of renewable electricity and storage on the grid. This perspective compares energy storage needs and priorities in 2010 with those now and those emerging over the next few decades. The diversity of demands for energy storage requires a diversity of purpose-built batteries designed to meet disparate applications. Advances in the frontier of battery research to achieve transformative performance spanning energy and power density, capacity, charge/discharge times, cost, lifetime, and safety are highlighted, along with strategic research refinements made by the Joint Center for Energy Storage Research (JCESR) and the broader community to accommodate the changing storage needs and priorities. Innovative experimental tools with higher spatial and temporal resolution, in situ and operando characterization, first-principles simulation, high throughput computation, machine learning, and artificial intelligence work collectively to reveal the origins of the electrochemical phenomena that enable new means of energy storage. This knowledge allows a constructionist approach to materials, chemistries, and architectures, where each atom or molecule plays a prescribed role in realizing batteries with unique performance profiles suitable for emergent demands.

Atomic-scale investigation of enhanced lithium, sodium and magnesium storage performance from defects in MoS2/graphene heterostructures

MoS₂ is of great interest as an anode material of batteries due to its high theoretical reversible capacity; in particular, a defective MoS₂/graphene heterostructure exhibits excellent cycling stability. However, very little is known about the diffusion and ion storage mechanism at the atomistic level. To provide insights into the issue, we have developed and used first principles calculations and an atom intercalation/deintercalation algorithm to model the adsorption, diffusion, insertion and removal of Li, Na and Mg in pristine and defective MoS₂/graphene systems. First, the adsorption of Li, Na and Mg is generally more stable in the defective MoS₂/graphene structure. Mg and Li prefer to diffuse in the structure with disulfide defects, while Na prefers to diffuse in the molybdenum defective structure. Next, we found that the atomic configurations of both pristine and defective MoS₂/graphene are not restored to their original states after the insertion and removal of Li, Na and Mg, which is related to the irreversible capacity loss of the system. Furthermore, by excluding the amount of lithium atoms related to the unrestored sulfur atoms, an algorithm was proposed to calculate the reversible capacity and it was verified by experimental results. We have also demonstrated that the introduction of defects leads to significant increase in the theoretical capacities of the Na and Mg systems, however, decreasing the capacity retention rate of Mg.

Rapid synthesis of MgCo2O4 and Mg2/3Ni4/3O2 nanocrystals in supercritical fluid for Mg-ion batteries

High-quality spinel MgCo₂O₄ and rocksalt Mg_2/3Ni_4/3O₂ nanocrystal cathodes allow fast extraction/insertion of magnesium ions for high energy-density batteries.