Mg-ion Diffusion Research Articles

Silicene/BN heterostructure as high-performance anode material for Mg-ion batteries

Finding a suitable anode material is a key part of the development of Mg-ion batteries (MIBs) to commercialize. In this work, an outstanding Silicene/BN heterostructure was discovered as a candidate anode material for MIBs by using first-principles calculations. The mixed sp2/sp3 orbital hybridization of silicon atoms makes Silicene monolayer easy to adsorb Mg atoms. The introduction of BN monolayer can effectively buffer the structural deformation and improve the electrochemical performances. Compared to pristine Silicene, the Silicene/BN heterostructure exhibited higher structural stability and Mg adsorption capacity. The diffusion of Mg ions in the heterostructure interlayer was easier with a low barrier of 0.160 eV than that on the Silicene monolayer. A comparable low open circuit voltage (0.112 V) was also observed. These results indicated the potential of Silicene/BN heterostructure as a high-performance anode material for MIBs.

Alkali-Ion-Assisted Activation of ε-VOPO4 as a Cathode Material for Mg-Ion Batteries.

Rechargeable multivalent-ion batteries are attractive alternatives to Li-ion batteries to mitigate their issues with metal resources and metal anodes. However, many challenges remain before they can be practically used due to the low solid-state mobility of multivalent ions. In this study, a promising material identified by high-throughput computational screening is investigated, ε-VOPO4, as a Mg cathode. The experimental and computational evaluation of ε-VOPO4 suggests that it may provide an energy density of >200Whkg-1 based on the average voltage of a complete cycle, significantly more than that of well-known Chevrel compounds. Furthermore, this study finds that Mg-ion diffusion can be enhanced by co-intercalation of Li or Na, pointing at interesting correlation dynamics of slow and fast ions.

A DFT-based design of B/N/P-co-doped oxo-triarylmethyl as a robust anode material for magnesium-ion batteries

Magnesium-ion batteries (MIBs) are emerging as a promising alternative to lithium-ion batteries (LIBs) due to their superior safety features and cost-effectiveness. In this work, heteroatoms-co-doped oxo-triarylmethyl (B/N/P@oxTAM) as a favorable anode material for MIBs was investigated using density functional theory calculations. The B/N/P@oxTAM is a highly porous structure and has a better affinity for Mg-ions to attach to vacancy site. Partial density of states, open-circuit voltage, theoretical specific capacity, and diffusion energy barrier were calculated and discussed. A significant decrease in the HOMO-LUMO gap with no structural deformation occurred, suggesting the high cycling performance of B/N/P@oxTAM for MIBs. Moreover, the designed anode material demonstrated full loading with six Mg-ions at different active sites, indicating a high theoretical specific capacity of 513.75 mAh g−1 and a low open-circuit voltage of 0.07 V. The presence of a heterocyclic ring (borabenzene) with a diffusion energy barrier of 0.039 eV increased the diffusion of Mg-ions. Therefore, B/N/P@oxTAM can be used as a viable anode material for MIBs with extended life cycle and quick charge-discharge rates due to its low open-circuit voltage and diffusion energy barrier, as well as its high theoretical specific capacity value.

Uncovering the remarkable electrochemical performance of B2N2 monolayer as a promising candidate for Mg-ion batteries

Nowadays, owing to their potential applications in next-generation self-powered electronic devices, such as energy-harvesting smart garments from body movements and roll-up displays, electrochemical energy storages are enjoying considerable interest. Nonetheless, the development of such technologies has been hindered due to the rarity of high-efficacy electrodes with specific electrochemical performance. Amongst prospective electrodes, researchers have investigated 2D flexible and lightweight materials which have unique physiochemical attributes such as high conductance, high surface metal diffusivity, a hydrophilic surface and mechanical strength. Within this piece of research, a 2D orthorhombic di-boron di-nitride monolayer (o-B2N2ML), which is a boron nitride allotrope, was investigated. Moreover, several determining electronic chemical factors were investigated, such as theoretical capacity, equilibrium voltage and binding strength. Interestingly, the Mg-ion battery had a specific capacity of up to 1125 mAh.g−1. Also, the diffusion of Mg-ions was accelerated due to the presence of a o-B2N2 ring with a diffusion barrier (DB) of 0.26 eV. The low OCV and the low DB of the o-B2N2ML demonstrate that it can be used for practical purposes with long service life and fast rates of charge and discharge. The results also indicated the possible use of the o-B2N2ML as an anode material with high efficiency in MIBs.

Potential use of silicon carbide monolayer as an anode in rechargeable Mg-ion batteries

In this study, we evaluated the possible application of a two-dimensional silicon carbide monolayer, which is a similar to material graphene, as an anode material in rechargeable Mg-ion batteries based on density functional theory computations. In particular, we investigated the related electronic structures, structural geometry changes, Mg-ion diffusion characteristics, and the corresponding electrochemical characteristics during charging. The adsorption energy of Mg is higher on the six-ring member than other sites. The maximum theoretical capacity of SiC is as high as 683.99 mAh/g, which is accompanied by a negligible change in the Si–C spacing band. We also considered the changes in the open circuit voltage caused by the adatom concentrations due to the intercalation of Mg in Si. The open-circuit voltage range determined for the SiC monolayer after the adsorption of Mg adtoms also indicates that the SiC monolayer can be considered a promising anode material. Considerable charge transport of approximately one electron occurs in SiC nano-sheets from the Mg atom-, thereby making them electrically conductive, which is a requirement for an acceptable anode material. Furthermore, the SiC monolayer can be regarded as the best candidate for use as an anode material in magnesium-ion batteries because of its properties.

Reversible Mg metal anode in conventional electrolyte enabled by durable heterogeneous SEI with low surface diffusion barrier

The popularization of Mg metal batteries (MMBs) is plagued by Mg anode usually suffering from severe passivation and exhibiting an extremely high overpotential in conventional electrolytes. Here, a solvent-assisted additive displacement strategy is proposed to design a durable and heterogeneous solid electrolyte interphase (SEI) with low surface diffusion barrier, composed of MgCl2-rich top layer and organosilicon-dominated bottom layer. This SEI can tolerate long-term anode cycling and therefore permanently protect Mg anode against passivation in conventional electrolyte. Different from traditional SEI with porosity accumulation, this hybrid SEI is condensable with the attenuation of monolithic MgCl2 into nanodomains, which are infused and embedded in the Si-O and Si-C reinforced organic matrix, without the compromise of Mg-ion diffusion. This anti-passivation strategy enables a highly reversible Mg metal anode (over 600 cycles) in Mg(TFSI)2 based electrolyte with the enhanced interfacial kinetics, reduced overpotential (∼350 mV), large critical current density (5.6 mA cm−2) and excellent compatibility with conversion cathode without extra electrolyte additive. This work opens the avenue for the construction of highly tolerant and conductive SEI to displace fragile and passivated SEI to inspire the application of “simple” salt electrolyte in MMBs.

Exploring the Full Potential of Functional Si2BN Nanoribbons As Highly Reversible Anode Materials for Mg-Ion Battery

Efficient energy storage devices like rechargeable batteries have a vital role in the modern society to cater for an ever-increasing demand of energy. In this context, magnesium-ion batteries (MIBs) have emerged as high-capacity energy storage systems. However, the progress in this area is hindered due to the lack of suitable anode materials for efficient Mg2+ ion storage and diffusion. In this study, using state-of-the-art density functional theory (DFT) simulations, we have systematically investigated novel one-dimensional Si2BN nanoribbons as anode materials for MIBs applications. Our calculations confirm the structural stability and metallic character of pristine (Si2BN) and hydrogen functionalized (Si2BN-H) nanoribbons upon Mg adsorptions. We find Mg adsorption energies in the ranges of -1.2 to -1.8 (-1.8 to -2.0) eV for 25% (20%) coverages in Si2BN (Si2BN-H), respectively, which are strong enough to mitigate the Mg aggregation. Maximum specific capacities of 661.865 (550.421) mAh g-1 and open-circuit voltages of 0.7-1.1 (0.6-0.8) V are found for Si2BN (Si2BN-H), respectively. Diffusion barrier calculations based on nudge elastic band (NEB) methods reveal a relatively low barrier of 0.14 eV, which guarantees a robust diffusion of Mg ions and faster charge/discharge capability of Si2BN nanoribbons. These intriguing features confirm the potential of functional Si2BN nanoribbons as promising anode materials for MIBs.

Synthesis of Mg-Rich Mg-M-O(M = Fe, Ni, Mn) Cathode Materials Using Layered Double Hydroxide Andpositive Electrode Characteristics and Average / Local Structure

1. Introduction The Mg rechargeable battery (MRB) can be regarded as a next-generation secondary battery that replaces the Li-ion battery, since MRB is expected to have a high volume energy density due to divalent charge carrier, abundant resources, and high safety. However, the diffusion of Mg ions in solids is slower than that of Li ions, and thus there have been no reports of excellent positive electrode materials and they have not been put to practical use. As a new electrode material, we have focused on the disordered rocksalt type positive electrode materials, and tried to synthesize the material with various Mg contents by a reverse co-precipitation method 1). However, the obtained materials had lower Mg contents than expected, and it is hard to prepare a Mg-excess material. Therefore, in this study, as a new synthesis process, we used layered double hydroxide (LDH) as a precursor, and pyrolyzed them to synthesize Mg-excess new positive electrode material Mg-MO (M = Fe, Ni, Mn). For the positive electrode material, the electrode characteristics and the crystal structure were investigated in this study. 2. Experimental The precursor (layered double hydroxide) was synthesized by a coprecipitation method, and the sample was synthesized by pyrolyzing the precursor 2). As the raw materials, MgCl2 · 6H2O, MCl3 · 6H2O (M = Fe, Ni), and MnCl2 · 4H2O were dissolved in ion-exchanged water so that Mg: M became 2: 1, and then the pyrolysis was performed at 500 ° C. The obtained sample was subjected to powder X-ray diffraction measurement to identify the phase and calculate the lattice constant, and the metal composition was analyzed by ICP-AES. A three-electrode cell (manufactured by Toyo System Co., Ltd.) was utilized for a charge / discharge test at an operating temperature of 90 ° C, and the positive electrode characteristics were evaluated. In addition, synchrotron X-ray diffraction measurement (BL19B2, SPring-8) was performed, and the average structure was clarified by Rietveld analysis (RIETAN-FP). Synchrotron X-ray total scattering measurement (BL04B2, SPring-8) were also performed, and the local structure was investigated by reverse Monte Carlo analysis (RMC Profile). 3. Results As a result of X-ray diffraction measurement, the peaks could be assigned to the spinel structure (Fd-3m) in the Fe system and the rock salt structure (Fm-3m) in the Ni system. Composition analysis by ICP-AES revealed that all the samples prepared by this new process had Mg-rich composition. As a result of the charge-discharge test, a discharge capacity of Mg1.33Ni0.66O2 was only about 4.5 mAh / g although the capacity increased to about 30 mAh / g by partial substitution for Ni. On the other hand, in the Fe-based sample, the capacity was about 60 mAh / g for Mg2FeO4-δ, and excellent cycle characteristics could be confirmed. Moreover, the discharge capacity of Mg2Fe0.66Ni0.33O2 was about 170mAh / g at the 2nd discharge. Reverse Monte Carlo analysis was performed to clarify the cause of the small discharge capacity of the Ni-based samples. As a result, it was found that Ni tends to aggregate in Mg1.33Ni0.66O2 and the domain can be considered not to participate in charge / discharge. On the other hand, similar phenomenon was not observed in Mg1.33Ni0.5Mn0.16O2. From this, it can be concluded that the amount of Mg and/or synthetic process must be reexamined to improve the electrode property.This work was supported by JST ALCA-SPRING Grant Number JPMJAC1301, Japan.

Electrolyte-Dependent SEI Formation and Its Consequences on Mg Electrode Cycling

Recently, there has been significant research into magnesium batteries due to their potential as a beyond Li-ion technology. This is due to a number of beneficial properties of the Mg negative electrode, including high volumetric capacity, twice that of metallic Li (Figure 1) and lack of dendritic growth on cycling.1,2 However, current electrolytes are insufficiently stable to the Mg electrode, leading to reduction of the electrolyte and formation of a solid electrolyte interphase (SEI), which is believed to be detrimental to performance.3-7 Furthermore, a majority of reports have used noble metal electrodes to assess the suitability of electrolytes with Mg, which is not representative of the conditions in a practical Mg battery.8,9 Here, we probe the cycling behavior of bulk Mg electrodes in two electrolytes, one Grignard-based (BuMgCl in THF) and the other glyme ether-based (Mg(TFSI)2 in TEGDME). Cyclic voltammetry identified different plating and stripping behaviour in each electrolyte, with the Grignard demonstrating plating/stripping typical for a bulk electrode, while Mg stripping in the glyme ether resulted in a finite peak in the cyclic voltammetry. This unexpected result suggests a protective SEI forms on the Mg surface in the glyme ether, which critically still allows Mg-ion diffusion. The nature of the SEI in both electrodes was characterised using FTIR spectroscopy and SEM imaging, confirming that the nature of the SEI is dependent on the electrolyte and that it affects the way in which Mg electrodes cycle. Our results suggest that it is possible to protect Mg electrodes with a stable SEI and allow the use of electrolytes with increased oxidative stability. Reference s : [1] M. Matsui, J. Power Sources 196 (2011) 7048–7055.[2] J. Song, E. Sahadeo, M. Noked, S. B. Lee, J. Phys. Chem. Lett. 7 (2016) 1736–1749.[3] M.-S. Park, J.-G. Kim, Y.-J. Kim, N.-S. Choi, J.-S. Kim, Isr. J. Chem. 55 (2015) 570–585.[4] S. Y. Ha, Y. W. Lee, S. W .Woo, B. Koo, J. S. Kim, J. Cho, K. T. Lee, N. S. Choi, ACS Appl Mater Interfaces 6 (2014) 4063–4073.[5] I. Shterenberg, M. Salama, H. D. Yoo, Y. Gofer, J.-B. Park, Y.-K. Sun, D. Aurbach, J. Electrochem. Soc. 162 (2015) A7118-A7128.[6] O. Tutusaus, R. Mohtadi, N. Singh, T. S. Arthur, F. Mizuno, ACS Energy Lett. 2 (2017) 224-229.[7] M, S. Ding, T. Diemant, R. J. Behm, S. Passerini, G. A. Giffin, J. Electrochem. Soc. 165, (2018) A1983-A1990[8] K. Ta, K. A. See, A. A. Gewirth, J. Phys. Chem. C 122 (2018) 13790–13796.[9] J. G. Connell, B. Genorio, P. Papa, D. Strmcnik, V. R. Stamenkovic, N. M. Markovic, Chem. Mater. 28 (2016) 8268–8277. Figure 1

Reductive solvothermal synthesis of MgMn2O4 spinel nanoparticles for Mg-ion battery cathodes

Rechargeable Mg-ion batteries have gained significant attention as promising alternatives to Li-ion batteries. Owing to its high theoretical energy density and relatively high Mg-ion diffusivity, spinel oxide MgMn2O4 is a viable candidate as a cathode material; however, its poor rate capability limits its applicability. Decreasing the particle size can effectively address this problem by enhancing Mg-ion diffusion. In this paper, we demonstrate the conventional solvothermal synthesis of MgMn2O4 spinel nanoparticles. Solvothermal process is one of the most fundamental methods for nanoparticle synthesis because of its simple and flexible synthetic conditions. In the alcohol solvothermal conditions, spinel type MgMn2O4 nanoparticles of approximately 10–15 nm are successfully synthesized using amorphous MnO2 as a precursor. We note that controlling Mg2+ solvation and oxidation/reduction conditions in the reaction solution is crucial for the effective intercalation of Mg2+ into the MnO6 octahedral framework. Although the obtained MgMn2O4 nanoparticles aggregate to form submicron secondary particles, the aggregation can be suppressed by compositing them with the carbon nanotubes dispersed in the reaction solution. The composite exhibits a discharge capacity of 60 mAh g−1 with maintaining 80% of capacity retention after the 10th cycle.

Pseudocapacitive Ti-Doped Niobium Pentoxide Nanoflake Structure Design for a Fast Kinetics Anode toward a High-Performance Mg-Ion-Based Dual-Ion Battery.

Magnesium-ion batteries (MIBs) have received increasing attention for next-generation energy storage recently because of the natural abundance, high capacity, and dendrite-free deposition of Mg. However, their applications are hindered by irreversible Mg anode plating in conventional electrolytes and the lack of cathode materials, demonstrating high working voltage, satisfactory Mg2+ diffusivity, and long cycling life. In this work, we first developed a novel magnesium-ion based dual-ion battery (Mg-DIB) by utilizing expanded graphite as the cathode and Ti-doped niobium pentoxide nanoflakes (Ti-Nb2O5 NFs) as the anode. The Ti-Nb2O5 NFs showed hierarchical structures of microspheres with diameters of 4-5 μm assembled by nanoflakes. For the first time, the Mg-ion storage mechanism in Ti-Nb2O5 NFs was investigated. Benefiting from the hierarchical structure design and pseudocapacitive intercalation behavior of Mg ions, the Ti-Nb2O5 NF anode exhibited fast Mg-ion diffusion. Consequently, the Mg-DIB exhibited a high discharge capacity of 93 mA h g-1 at 1 C (1 C corresponding to 100 mA g-1), along with good long-term cycling performance with a capacity retention of 79% at 3 C after 500 cycles. The Mg-DIB also demonstrated a capacity retention of 77% at 5C, indicating its good rate performance. Moreover, the Mg-DIB exhibited a high discharge medium voltage of ∼1.83 V, thus enabling a high energy density of 174 W h kg-1 at 183 W kg-1 and 122 W h kg-1 at a high power density of 845 W kg-1, among the best of the reported magnesium-ion full batteries. Our work provides a new strategy to improve the performance of MIBs and other rechargeable batteries.

Atomic-scale studies of garnet-type Mg3Fe2Si3O12: Defect chemistry, diffusion and dopant properties

Materials with low cost, environmentally benign, high structural stability and high Mg content are of considerable interest for the construction of rechargeable Mg-ion batteries. In the present study, atomistic simulations are used to provide insights into defect and diffusion properties of garnet type Mg 3 Fe 2 Si 3 O 12 . Calculations reveal that the Mg–Fe anti-site defect cluster (0.44 eV/defect) is the lowest energy intrinsic defect process. Three dimensional Mg-ion migration pathway with the activation energy of 2.19 eV suggests that Mg-ion diffusion in this material is slow. Favourable isovalent dopants are found to be Mn 2+ , Ga 3+ and Ge 4+ on the Mg, Fe and Si sites respectively. While the formation of Mg interstitials required for the capacity is facilitated by Al doping on the Si site, Mg vacancies needed for the vacancy assisted Mg-ion diffusion are enhanced by Ge doping on the Fe site. The electronic structures of favourable dopants are calculated and discussed using density functional theory. • Defect processes and Mg transport in Mg 3 Fe 2 Si 3 O 12 are studied theoretically. • The energetically favourable intrinsic defect type is the Mg–Fe anti-site. • Activation energy of migration of Mg ions via the vacancy mechanism is 2.19 eV. • Subvalent doping by Al on the Si site can eenhance the capacity.

How does the active site in the MoSe2 surface affect its electrochemical performance as anode material for metal-ion batteries?

In this work, we implement density functional theory (DFT) simulations to study how active sites in the MoSe2 surface affect its electrochemical performance as anode material for metal-ion batteries (metal ions including Li, Na, K, Mg, and Al ions). Se vacancy MoSe2 (0 0 1) and MoSe2 (1 1 0) are considered to provide active sites for metal ions. The adsorption energies, diffusion energy barriers, electronic conductivity and open circuit voltage profile (OCV) are evaluated. According to our calculations, MoSe2 (0 0 1) with Se vacancy occupied by Mg ion shows similar diffusion energy barriers and comparable OCV to the pristine MoSe2 (0 0 1). Moreover, Mg ion shows much lower diffusion barriers than the other ions. Besides that, MoSe2 (1 1 0) shows much higher adsorption energies than the (0 0 1) section, indicating its high capacity for metal-ion batteries. However, MoSe2 (1 1 0) also exhibits much higher diffusion energy barriers, which would inhibit its rate capability. Comparatively, MoSe2 (0 0 1) with Se vacancies can improve the conductivity of MoSe2, and the conductivity increases with the number of Se vacancies increases. Consequently, MoSe2 (0 0 1) with single Se vacancy has comprehensive electrochemical performance for Mg-ion battery anode, including low Mg-ion diffusion energy, comparable OCV, and high electronic conductance.

Computer modeling investigation of MgV2O4 for Mg-ion batteries

MgV2O4 is a vanadium spinel considered for rechargeable magnesium ion batteries. Its defect chemistry, solution of dopants, and the diffusion of Mg ions are investigated using advanced atomistic modeling techniques. The energetically most favorable defect is Mg–V anti-site cluster (0.53 eV/defect) assuming that a small percentage of Mg2+ and V3+ ions would exchange their positions, particularly at higher temperatures. Reaction energies for the loss of MgO via MgO Schottky and the formation of Mg vacancies via Mg Frenkel are calculated to be 5.13 eV/defect and 5.23 eV/defect, respectively, suggesting that the concentrations of these two defects will not be significant. The most favorable diffusion mechanism of Mg ions is a three-dimensional pathway, where the activation energy of migration is 0.52 eV. The formation of Mg interstitials and O vacancies can be facilitated by doping with Co2+ at the V site in MgV2O4. The electronic structures of the favorable dopants calculated using the density functional theory are discussed.

Mg6MnO8 as a Magnesium-Ion Battery Material: Defects, Dopants and Mg-Ion Transport

Rechargeable magnesium ion batteries have recently received considerable attention as an alternative to Li- or Na-ion batteries. Understanding defects and ion transport is a key step in designing high performance electrode materials for Mg-ion batteries. Here we present a classical potential-based atomistic simulation study of defects, dopants and Mg-ion transport in Mg6MnO8. The formation of the Mg–Mn anti-site defect cluster is calculated to be the lowest energy process (1.73 eV/defect). The Mg Frenkel is calculated to be the second most favourable intrinsic defect and its formation energy is 2.84 eV/defect. A three-dimensional long-range Mg-ion migration path with overall activation energy of 0.82 eV is observed, suggesting that the diffusion of Mg-ions in this material is moderate. Substitutional doping of Ga on the Mn site can increase the capacity of this material in the form of Mg interstitials. The most energetically favourable isovalent dopant for Mg is found to be Fe. Interestingly, Si and Ge exhibit exoergic solution enthalpy for doping on the Mn site, requiring experimental verification.

Magnesium Storage Performance and Mechanism of 2D-Ultrathin Nanosheet-Assembled Spinel MgIn2 S4 Cathode for High-Temperature Mg Batteries.

Magnesium batteries have the potential to be a next generation battery with large capability and high safety, owing to the high abundance, great volumetric energy density, and reversible dendrite-free capability of Mg anodes. However, the lack of a stable high-voltage electrolyte, and the sluggish Mg-ion diffusion in lattices and through interfaces limit the practical uses of Mg batteries. Herein, a spinel MgIn2 S4 microflower-like material assembled by 2D-ultrathin (≈5.0 nm) nanosheets is reported and first used as a cathode material for high-temperature Mg batteries with an ionic liquid electrolyte. The nonflammable ionic liquid electrolyte ensure the safety under high temperatures. As prepared MgIn2 S4 exhibits wide-temperature-range adaptability (50-150 °C), ultrahigh capacity (≈500 mAh g-1 under 1.2 V vs Mg/Mg2+ ), fast Mg2+ diffusibility (≈2.0 × 10-8 cm2 s-1 ), and excellent cyclability (without capacity decay after 450 cycles). These excellent electrochemical properties are due to the fast kinetics of magnesium by the 2D nanosheets spinel structure and safe high-temperature operation environment. From ex situ X-ray diffraction and transmission electron microscopy measurements, a conversion reaction of the Mg2+ storage mechanism is found. The excellent performance and superior security make it promising in high-temperature batteries for practical applications.

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

The scarce inventory of cathode materials with reasonable diffusion of Mg ions is the main obstacle in the development of rechargeable magnesium batteries. In this regard, vanadium pentoxide (V2O5) has been reported to be a candidate cathode material for Mg batteries. In this study, via first-principles calculations, we showed that the Mg-ion diffusion energy barrier in α-V2O5 could be substantially decreased through hydrogenation. It is found that the Mg-ion migration energy barrier in HxV2O5 is gradually decreased with an increase in H concentration. When the H concentration x reaches 2, the migration barrier is decreased to 0.56 eV from that in α-V2O5 without hydrogenation (1.28 eV). This indicates that the Mg diffusion kinetics can be substantially improved through hydrogenation, and the resultant energy barrier makes Mg diffusion acceptable even at room temperature. The mechanism of the H-enhanced Mg-diffusion has also been studied, and it has been found that H atoms not only can expand the Mg-diffusion pathway, but also have a screening effect on the interactions between Mg ions and the α-V2O5 lattice.

Water-Activated VOPO4 for Magnesium Ion Batteries.

Rechargeable Mg batteries, using high capacity and dendrite-free Mg metal anodes, are promising energy storage devices for large scale smart grid due to low cost and high safety. However, the performance of Mg batteries is still plagued by the slow reaction kinetics of their cathode materials. Recent discoveries demonstrate that water in cathode can significantly enhance the Mg-ion diffusion in cathode by an unknown mechanism. Here, we propose the water-activated layered-structure VOPO4 as a novel cathode material and examine the impact of water in electrode or organic electrolyte on the thermodynamics and kinetics of Mg-ion intercalation/deintercalation in cathodes. Electrochemical measurements verify that water in both VOPO4 lattice and organic electrolyte can largely activate VOPO4 cathode. Thermodynamic analysis demonstrates that the water in the electrolyte will equilibrate with the structural water in VOPO4 lattice, and the water activity in the electrolyte alerts the mechanism and kinetics for electrochemical Mg-ion intercalation in VOPO4. Theoretical calculations and experimental results demonstrate that water reduces both the solid-state diffusion barrier in the VOPO4 electrode and the desolvation penalty at the interface. To achieve fast reaction kinetics, the water activity in the electrolyte should be larger than 10-2. The proposed activation mechanism provides guidance for screening and designing novel chemistry for high performance multivalent-ion batteries.

Concomitant Intercalation Behavior of Li-Mg Dual Cations into Mo6S8

There is a growing demand for high energy-density rechargeable batteries to construct high performance electric vehicles and renewable energy storage systems. As a candidate of post Li-ion batteries (LIBs), Mg rechargeable batteries (MRBs) have a great potential to attain considerable energy density higher than current commercial LIBs. This is mainly because Mg shows a non-dendritic growth behavior during charging. This feature allows Mg metal to be used as an anode material, which is in a sharp contrast to Li metal anode showing the dangerous dendritic growth. However, during developing intercalation-type cathode materials for MRBs, we inevitably face great difficulties as follows. The divalent Mg cations show an inherent large coulombic interaction with host materials; there are only a few materials can allow the intercalation of Mg ions. Although the operating potential is relatively low (about 1.1 V vs. Mg), the Chevrel compound Mo6S8 is the unique material that Mg ions can be intercalated reversibly at room temperature with an excellent cyclability [1]. Even in such excellent MRB cathode materials, Mg ions show a sluggish diffusion behavior and large overpotential inevitably occurs at a certain level of current value. The sluggish diffusion behavior of Mg cations is seemingly insurmountable unless operating temperature is elevated (~100 ℃). However, while we have been investigating the feasibility of rocking-chair type Li-Mg dual-salt batteries [2], we have found an interesting phenomenon that intercalation of Mg ions into Mo6S8 can be accelerated by concomitant intercalation with Li ions in a Li-Mg dual-salt electrolyte, LiTFSA-Mg(TFSA)2/triglyme(G3), even at room temperature. After fully discharging the Mo6S8 cathode in electrolyte with the same concentration (0.5 mol L-1) of Li and Mg salts, almost the same amount of Li and Mg ions are intercalated, amounting to the full capacity of Mo6S8 (~120 mAh g-1). In addition, Li and Mg ions show a cooperative intercalation behavior in Li-Mg dual-salt electrolyte. During discharge, Li ions are firstly intercalated and occupy the inner sites of Mo6S8 cathode, then subsequently intercalated Mg ions exchange the sites with the pre-intercalated Li ions. Further importantly, the intercalation potential of Mg in the dual-salt electrolyte is found to be higher than that in the single Mg-salt electrolyte. Thus, with the aid of Li ions, the diffusion of Mg ions seems to be accelerated, which would reduce the overpotential significantly. Similar experiments were also carried out in a Cl-contained Li-Mg dual-salt electrolyte, LiHMDS-Mg(HMDS)2-AlCl3/G3. In contrast, the cooperative intercalation behavior is not observed; Li ions were preferentially intercalated into Mo6S8 in the whole discharge process. This result is consistent with the previous studies [3,4] on Daniell-type Li-Mg dual-salt batteries, in which Mo6S8 was used as cathodes and Cl-contained all-phenyl complex (APC) electrolyte was employed as the base of Li-Mg dual-salt electrolytes. The reason why the same phenomenon cannot be observed in these Cl-contained electrolytes would be attributed to the different coordination state of Mg ions in these electrolytes. Thus, the cooperative intercalation behavior of Li and Mg ions indicates that the inherent sluggish diffusion of Mg ions in solid can be improved by concomitant intercalation with rapid-diffusion Li ions. Furthermore, the coordination state of Mg ions in electrolytes is, of course, suggested to be an important factor for the intercalation of Mg ions.

Enhanced Magnesium-Ion Charge Transport and Storage within Layered Vanadium Pentoxide-Poly(ethylene oxide) Nanocomposites

Cathode materials that have high reversible magnesium-ion capacities and reasonable operating voltages are needed for rechargeable magnesium batteries. Layered materials are a particularly promising class of materials for electrochemical Mg-ion charge storage, however additional research is needed to increase Mg-ion capacities, improve capacity retension upon cycling, and understand the nature of interlayer interactions. Our work has investigated hydrated vanadium pentoxide (V2O5) xerogels that incorporate poly(ethylene oxide) (PEO) between the layers as an approach to improve Mg-ion charge storage and control interlayer structure and dynamics. V2O5 nanosheets were grown in the presence of PEO to allow inimate interaction with the V2O5 layers and allow control of the PEO content in V2O5-PEO nanocomposites. X-ray diffraction and high resolution transmission electron microscopy demonstrate that the interlayer spacing between V2O5 layers was increased by incorporating PEO within the layers. The Mg-ion diffusion coefficient was shown to be strongly dependent on not only interlayer spacing, but also interlayer composition. With specific polymer compositions, Mg-ion diffusion within V2O5-PEO nanocomposites was significantly increased compared with V2O5 xerogels. Raman spectroscopy supports that the polymer interacts with the V2O5 lattice resulting in interlayer water that is less strongly bound to the V2O5 lattice. The V2O5-PEO nanocomposite with specific PEO compositions exhibited significantly improved Mg-ion specific capacity compared with V2O5 xerogels and showed improved capacity retension upon cycling. Our work supports that in addition to interlayer spacing, controlling interlayer interactions between V2O5 layers, PEO, H2O, and Mg-ions allows significant improvements in Mg-ion charge transport and storage. The design of layered materials with controlled interlayer compositions provides a novel route to significantly improve charge storage and transport in layered materials.