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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.

Oxides with the CaFe2O4-type structure have been predicted as being suitable hosts for reactions of intercalation of light cations such as Li and Mg because of their favorable cationic diffusion. A...

Racemates have recently received attention as nonlinear optical and piezoelectric materials. Here, a machine-learning-assisted composition space approach was applied to synthesize the missing M = Ti, Zr members of the Δ,Λ-[Cu(bpy)2(H2O)]2[MF6]2·3H2O (M = Ti, Zr, Hf; bpy = 2,2'-bipyridine) family (space group: Pna21). In each (CuO, MO2)/bpy/HF(aq) (M = Ti, Zr, Hf) system, the polar noncentrosymmetric racemate (M-NCS) forms in competition with a centrosymmetric one-dimensional chain compound (M-CS) based on alternating Cu(bpy)(H2O)22+ and MF62- basic building units (space groups: Ti-CS (Pnma), Zr-CS (P1), Hf-CS (P2/n)). Machine learning models were trained on reaction parameters to gain unbiased insight into the underlying statistical trends in each composition space. A human-interpretable decision tree shows that phase selection is driven primarily by the bpy:CuO molar ratio for reactions containing Zr or Hf, and predicts that formation of the Ti-NCS compound requires that the amount of HF present be decreased to raise the pH, which we verified experimentally. Predictive leave-one-metal-out (LOO) models further confirm that behavior in the Ti system is distinct from that of the Zr and Hf systems. The chemical origin of this distinction was probed via fluorine K-edge X-ray absorption spectroscopy. Pre-edge features in the F1s X-ray absorption spectra reveal the strong ligand-to-metal π bonding between Ti(3d - t2g) and F(2p) states that distinguishes the TiF62- anion from the ZrF62- and HfF62- anions.

A series of hybrid electrolytes composed of diglyme and ionic liquids (ILs) have been investigated for Na-O

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.

While α-V2O5 has traditionally been considered as a promising oxide to reversibly intercalate high levels of Mg2+ at high potential, recent reports indicate that previously observed electrochemical...

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.

Oxides undergoing reversible electrochemical cycling of Mg2+ ions would enable novel battery concepts beyond Li+, capable of storing large amounts of energy. However, materials showing this chemical reactivity are scarce. Suitable candidates require small particles to shorten transport lengths, together with chemically complex structures that promote cation mobility, such as spinel. These goals pose a challenge for materials chemists. Here, nanocrystals of spinel-type Mg0.5Mn2.5O4 were prepared using colloidal synthesis, and their electrochemical activity is presented. Cycling in an aqueous Mg2+ electrolyte led to a reversible transformation between a reduced spinel and an oxidized layered framework. This reaction involves large amounts of capacity because of the full oxidation to Mn4+, through the extraction of both Mg2+ and, in the first cycle, Mn2+ ions. Re-formation of the spinel upon reduction resulted in enrichment with Mg2+, indicating that its insertion is more favorable than that of Mn2+. Incorpo...

In this report, the feasibility of reversible Ca2+ or Zn2+ intercalation into a crystalline cubic spinel Mn2O4 cathode has been investigated using electrochemical methods in an aqueous electrolyte....

Rechargeable Mg batteries are promising candidates for high energy density storage in theory, when a Mg metal anode is combined with an oxide cathode material. Despite the widely observed sluggish Mg2+ diffusion in most oxide lattices, recent first-principles calculations predicted low diffusion barriers in the calcium ferrite (CF)-type post-spinel structures. In the present work, we experimentally examine the prospect of CF-type NaV1.25Ti0.75O4 as a Mg cathode. The Na+ ions, which lie in the ion migration pathway, need to be removed or exchanged with Mg2+ to allow Mg2+ de/intercalation. Partial desodiation was achieved through chemical and electrochemical methods, as proven by X-ray diffraction and X-ray absorption spectroscopy, but deep desodiation was accompanied by partial amorphization of the material. Mg2+ ion exchange at moderate temperature (80 °C) resulted in the formation of Na0.19Mg0.41V1.25Ti0.75O4; however, phase transformation was observed when higher temperatures were applied to attempt com...

The energy density of rechargeable batteries utilizing metals as anodes surpasses that of Li ion batteries, which employ carbon instead. Among possible metals, magnesium represents a potential alternative to the conventional choice, lithium, in terms of storage density, safety, stability, and cost. However, a major obstacle for metal-based batteries is the identification of electrolytes that show reversible deposition/dissolution of the metal anode and support reversible intercalation of ions into a cathode. Traditional Grignard-based Mg electrolytes are excellent with respect to the reversible deposition of Mg, but their limited anodic stability and compatibility with oxide cathodes hinder their applicability in Mg batteries with higher voltage. Non-Grignard electrolytes, which consist of ethereal solutions of magnesium(II) bis(trifluoromethanesulfonyl)imide (Mg(TFSI)2), remain fairly stable near the potential of Mg deposition. The slight reactivity of these electrolytes toward Mg metal can be remedied b...

A comprehensive study on the effects of experimental parameters on the composition and size of manganese oxide nanocrystals was completed using colloidal chemistry. The reactions studied involved the thermolysis of Mn2+ acetate and Mn3+ acetylacetonate in oleylamine. Temperature was found to be the dominant factor affecting the composition and size of the products. Reactions completed below 200 °C favored the formation of nanocrystals smaller than 20 nm, with the presence of even impurity amounts of oxidizing agents leading to the formation of Mn3O4. Nanocrystals of MnO could only be synthesized below 200 °C if Mn2+ acetate was used, and the reaction was carefully controlled to have no O2 and H2O contamination. In turn, particle growth was rapid above this temperature. In this case, regardless of the oxidizing agents used or oxidation state of the Mn precursor, nanocrystals of MnO formed after annealing for at least 1 h at temperatures higher than 200 °C. This finding suggests the role of oleylamine as solvent, surfactant, and reducing agent at sufficiently high annealing temperatures. These results increase the understanding of redox stability of manganese during the colloidal synthesis of semiconductor metal oxide nanocrystals.

The energy density of rechargeable batteries utilizing metals as anodes surpasses that of Li ion batteries, which employ carbon instead. Among possible metals, magnesium represents a potential alternative to the conventional choice, lithium, in terms of storage density, safety, stability, and cost. However, a major obstacle for metal-based batteries is the identification of electrolytes that show reversible deposition/dissolution of the metal anode and support reversible intercalation of ions into a cathode. Traditional Grignard-based Mg electrolytes are excellent with respect to the reversible deposition of Mg, but their limited anodic stability and compatibility with oxide cathodes hinder their applicability in Mg batteries with higher voltage. Non-Grignard electrolytes, which consist of ethereal solutions of magnesium(II) bis(trifluoromethanesulfonyl)imide (Mg(TFSI)

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.

A new family of fluorine-free solid-polymer electrolytes, for use in sodium-ion battery applications, is presented. Three novel sodium salts withdiffuse negative charges: sodium pentacyanopropenide (NaPCPI), sodium 2,3,4,5-tetracyanopirolate (NaTCP) and sodium 2,4,5-tricyanoimidazolate (NaTIM) were designed andtested in a poly(ethylene oxide) (PEO) matrix as polymer electrolytes for anall-solid sodium-ion battery. Due to unique, non-covalent structural configurations of anions, improved ionic conductivities were observed. As an example, “liquid-like” high conductivities (>1 mS cm−1) were obtained above 70 °C for solid-polymer electrolyte with a PEO to NaTCP molar ratio of 16:1. All presented salts showed high thermal stability and suitable windows of electrochemical stability between 3 and 5 V. These new anions open a new class of compounds with non-covalent structure for electrolytes system applications.

Rechargeable Mg batteries are considered as promising candidates for energy storage. The Mg metal anode with high volumetric capacity (3833 mAh mL-1) and low redox potential (-2.37 V vs. S.H.E.) provides high energy density for the system, while a low price, good safety and long term utilization is maintained by the inexpensiveness, air stability and limited-dendritic growth of Mg. However, the sluggish solid diffusion of the divalent Mg ion presents limited candidates for cathode materials. One strategy to facilitate Mg2+ mobility is to utilize polarizable anions in the lattice so as to weaken the ionic interaction, as noted with the first Mg cathode (the Chevrel phase Mo6S8) as well as with more recent spinel and layered phases of titanium sulfide.1-3 Although the sulfide materials exhibit reasonable Mg2+ mobility, they limit the voltage and have relatively high mass, thus lowering the energy density of the cathode. Switching to oxides is the solution towards the search for high energy density cathodes; however, a structure that allows facile Mg2+de/intercalation is obviously necessary. In this presentation, we will discuss the prospect of oxide materials for Mg2+ de/intercalation. One such cathode candidate is the “calcium ferrite” (CF) –type structured oxide in which a low Mg2+ migration barrier has been predicted by first principles calculations.4 A sodium vanadium-titanium oxide compound with the CF structure was obtained by solid state synthesis. Na+ ions reside in the spacious tunnels of the CF structure in this material, and need to be either removed or replaced by Mg2+ in order to allow Mg2+ de/intercalation. Desodiation (Fig. 1a) was carried out with both electrochemical and chemical methods, resulting in ~ 55% and 85% Na+ extraction, respectively. The chemically desodiated material exhibits ~ 85 mAh g-1 discharge capacity in a Mg full cell (Fig. 1b), showing a promising Mg2+ intercalation capability in the structure, although only half of this is reversible in these preliminary studies. XAS studies suggest that vanadium is the redox centre. We also examined Mg2+ ion exchange for the CF material, finding that ~ half of the Na+ ions were replaced with Mg2+ (Fig. 1a). Interestingly, we observe a phase transformation reaction accompanying major desodiation or ion exchange. This demonstrates a trend towards the rearrangement of the polyhedral framework in the structure when the large ion tunnel is emptied or occupied by the smaller Mg2+ ion, the origins of which will be discussed in this talk. Nevertheless, the promising electrochemistry of the desodiated material upon Mg2+ de/intercalation, as well as the ability to partially exchange Na+ with Mg2+, suggests moderately good Mg2+ diffusion in the CF structure, and sheds light on the possibility of iso-structural compounds as cathode materials for Mg batteries that are stable or metastable upon Mg2+de/intercalation. Other promising cathode materials for Mg batteries, not strictly limited to oxides, will also be the topic of the presentation. References: 1. D. Aurbach, Z. Lu, A. Schechter, Y. Gofer, H. Gizbar, R. Turgeman, Y. Cohen, M. Moshkovich and E. Levi, Nature 407, 724 (2000). 2. X. Sun, P. Bonnick, V. Duffort, M. Liu, Z. Rong, K. A. Persson, G. Ceder and L. F. Nazar, Energy Environ. Sci. 9, 2273 (2016). 3. X. Sun, P. Bonnick and L. F. Nazar, ACS Energy Lett. 1, 297 (2016). 4. C. Ling and F. Mizuno, Chem. Mater. 25, 3062 (2013). Figure 1

Li-ion batteries are the top choice in electrochemical energy storage for portable devices and mobile vehicles. However, the limited storage capacities do not quite yet meet the emerging energy demands in transportation and grid markets. Multivalent batteries based on divalent cations have attracted wide attention with high energy density/capacity in different framework of oxide cathodes. For instance, spinel structure M2O4, has been theoretically predicted as one of the most suitable oxide cathode for the reversible intercalation with Mg2+ [1]. Further, reversible, yet sluggish Mg intercalation into Mn2O4 host, prepared from de-lithiated LiMn2O4 electrode, was reported in aqueous electrolyte environments [2]. In order to enhance kinetics, reduction of particles to the smallest possible sizes is desired. Further, spinel-type Mn­2O4 can only be prepared by delithiation of LiMn2O4. Therefore, the direct synthesis of the parent spinel oxide, MgMn2O4, in the form nanocrystals would be desirable to investigate the fundamental limits of Mg intercalation within the spinel oxide framework, preferably in non-aqueous electrolytes. Here, we report a series of Mg-based spinel nanocrystals with different phase (MgxMn3-xO4 and MgxCr3-xO4), size (5nm~50nm), and shape (cube and sheet) via a low temperature hydrothermal process, followed by calcination (Figure 1). All samples were characterized with X-ray diffraction and spectroscopy, as well as electron microscopy in order to define the products of electrochemical reactions in non-aqueous cells. Our goal was to explore spinel oxide nanocrystals with optimal size, phase, and shape and establish size-structure-property relationship for the experimental realization of Mg reversible intercalation in spinel oxide hosts. 1. Liu, M., et al., Energy Environ. Sci. 2015, 8, 964. 2. Kim, C. et al., Adv. Mater. 2015, 27, 3377. Figure 1 (a) Representative STEM image of 50nm MgMn­2O4 nanocubes (b) Representative STEM image of 2~5nm thickness MgMn­2O4 nanosheets Figure 1

Efficiently using anodes based on the deposition of light metals would be a breakthrough that would produce leaps in energy density of rechargeable batteries. Magnesium is one of such metal electrodes that are not susceptible to dendrite formation. This unique property stems from (1) the thermodynamic stability in Grignard-based electrolytes and (2) the inherent preference to form three-dimensional deposits. However, the use of Grignard-based electrolytes results in severe chemical reactions between the nucleophilic electrolytes and electrophilic cathodes such as oxide materials. Accordingly, a lot of effort has been paid to find non-nucleophilic electrolyte solutions that deposit and dissolve Mg metal reversibly. Among non-nucleophilic anions, bis(trifluoromethylsulfonyl)imide (CF3SO2)2N–, TFSI) anion has shown exceptional stability on Mg metal anode. And the ethereal solution of the magnesium salt (MgTFSI2) has shown quite reversible Mg metal deposition and dissolution with coulombic efficiency of ~90% and reasonable voltage stability window of about 3 V. The most important difference from the Grignard-based electrolytes is that the TFSI-based electrolyte solution is non-nucleophilic and possibly compatible with oxide cathodes, which can increase the voltage and energy density of Mg rechargeable batteries. Herein, we report the mechanisms of degradation in the Mg metal anode and electrolyte after hundreds of cycles at high current density of 2 mA/cm2. Although the Mg metal anode operated without a significant problem at current densities lower than 0.1 mA/cm2, degradation was evident after long-term cycling at high current density. Overall, the Mg metal became mechanically brittle, with microscopy showing extremely porous structures (Fig. 1a). In addition, large hemispherical deposits up to several hundred micrometers were scattered or aggregated at the Mg surface (Fig. 1b). In turn, the color of the electrolyte turned to dark brown upon cycling, indicating a change in the chemical structure of the solvent. The macroscopic consequence was the short circuiting of the cells after several cycles at a high current density. These results suggest that the lemma of dendrite-free metal deposition is possible when the system meets several conditions such as absolute chemical stability of the metal in the electrolyte, adequate level of current density and charge density, and other engineering factors such as organic or inorganic additives that complexes with the metal ion and lead to more uniform metal deposition. Those understandings will serve as step stones to get to non-nucleophilic electrolytes, in which metal anodes operate without dendrite formation. Figure 1

Li-ion batteries dominate the mobile device market due their high energy density; and their application is being extended to the automotive and grid storage industries. However, charge storage limitations limit the applicability of classic Li-ion technologies into these new markets, thus triggering interest in new energy storage concepts. Multivalent ion intercalation is an attractive alternative because, while conceptually similar to Li-ion, they store more charge per intercalated species and observe less dramatic structural rearrangement in the host. Recently, reversible electrochemical Mg intercalation into spinel-type Mn2O4 has been reported, where Mg2+ ions occupy vacant tetrahedral sites [1]. First principles calculations by Lui et al. argue that other multivalent ions (i.e. Ca2+, Zn2+, Al3+, Y3+) may be intercalated into spinel host framework to yield high voltage cathode materials for multivalent batteries [2]. In this work, cubic phase Mn2O4 was prepared electrochemically by delithiating commercially available spinel-type LiMn2O4 in a non-aqueous environment. Following, Mn2O4electrodes were reduced in concentrated aqueous solutions of Ca-, Mg- and Zn nitrate to -0.4 V/SCE. As-prepared electrodes were characterized using powder x-ray diffraction. As shown in the figure, the cubic structure is preserved upon delithiation and significant structural changes occurred upon reduction; indicating the formation of a tetragonal spinel (likely tetrahedral site occupation) in addition to the parent cubic phase. X-ray absorption spectroscopy and atomic resolution microscopic methods will also be used to help understand the structural changes as these divalent cations are intercalated into a spinel host framework. References: Kim, C., et al., Adv. Mater. 2015, 27, 3377 Miao, L., et al., Energy Environ. Sci. 2015, 8, 964 Figure 1

Sodium has emerged as a major contender to lithium for rechargeable battery applications. Owing to its strengths are its high abundance, low production costs and suitable redox potential (Na/Na+, Eo = -2.71 V, not much less than Li/Li+, E˚=-3.05 V vs. standard hydrogen electrode). However, much like the lithium-ion battery industry, researchers are aware of how critical the electrolyte plays into cell performance. The novel imidazolate sodium salts: sodium 4,5-dicyano-2 (trifluoromethyl)imidazolate (NaTDI), sodium 4,5-dicyano-2-(pentafluoroethyl)imidazolate (NaPDI) and sodium pentacyanopropenide (NaPCPI) are presented [1]. Novel salts present interesting properties for electrolytes with applications in sodium and sodium-ion batteries. These salts may be synthesized easily and have low cost of production on the industrial scale, but also possess very interesting ionic conductivity in solution containing EC/DMC (Graph 1.). Moreover, the salts studied exhibit electrochemical stability against anode (Graph 2.) and cathode. Electrode materials have been produced by two techniques: tape casting (cathode and anode) and Microwave Plasma Chemical Vapor Deposition (anode) [2]. Presented anodes are based on germanium and antimony. References: [1] L. Niedzicki, E. Karpierz, A. Bitner, M. Kasprzyk, G.Z. Zukowska, M. Marcinek, W. Wieczorek, Optimization of the lithium-ion cell electrolyte composition through the use of the LiTDI saltElectrochim. Acta 117C (2014) 224-229 [2] M. Marcinek, L. Hardwick, G. Żukowska, R. Kostecki, Microwave Plasma Chemical Vapor Deposition of Graphitic Carbon Thin Films, Carbon 48 (2010) 1552-1557 Figure 1