Reversible Magnesium Metal Cycling in Additive-Free Simple Salt Electrolytes Enabled by Spontaneous Chemical Activation.

Rechargeable magnesium (Mg) batteries can offer higher volumetric energy densities and be safer than their conventional counterparts, lithium-ion batteries. However, their practical implementation is impeded due to the passivation of the Mg metal anode or the severe corrosion of the cell parts in conventional electrolyte systems. Here, we present a chemical activation strategy to facilitate the Mg deposition/stripping process in additive-free simple salt electrolytes. By exploiting the simple immersion-triggered spontaneous chemical reaction between reactive organic halides and Mg metal, the activated Mg anode exhibited an overpotential below 0.2 V and a Coulombic efficiency as high as 99.5% in a Mg(TFSI)2 electrolyte. Comprehensive analyses reveal simultaneous evolution of morphology and interphasial chemistry during the activation process, through which stable Mg cycling over 990 cycles was attained. Our activation strategy enabled the efficient cycling of Mg full-cell candidates using commercially available electrolytes, thereby offering possibilities of building practical Mg batteries.

Reversible Mg-Metal Batteries Enabled by a Ga-Rich Protective Layer through One-Step Interface Engineering.

Practical applications of Mg-metal batteries (MMBs) have been plagued by a critical bottleneck─the formation of a native oxide layer on the Mg-metal interface─which inevitably limits the use of conventional nontoxic electrolytes. The major aim of this work was to propose a simple and effective way to reversibly operate MMBs in combination with Mg(TFSI)2-diglyme electrolyte by forming a Ga-rich protective layer on the Mg metal (GPL@Mg). Mg metal was carefully reacted with a GaCl3 solution to trigger a galvanic replacement reaction between Ga3+ and Mg, resulting in the layering of a stable and ion-conducting Ga-rich protective film while preventing the formation of a native insulating layer. Various characterization tools were applied to analyze GPL@Mg, and it was demonstrated to contain inorganic-rich compounds (MgCO3, Mg(OH)2, MgCl2, Ga2O3, GaCl3, and MgO) roughly in a double-layered structure. The artificial GPL on Mg was effective in greatly reducing the high polarization for Mg plating and stripping in diglyme-based electrolyte, and the stable cycling was maintained for over 200 h. The one-step process suggested in this work offers insights into exploring a cost-effective approach to cover the Mg-metal surface with an ion-conducting artificial layer, which will help to practically advance MMBs.

Al-compatible boron-based electrolytes for rechargeable magnesium batteries.

Galvanic couple-assisted dissolution of Mg metal in an ethereal solution containing tris(2H-hexafluoroisopropyl)borate was utilized to prepare an efficient Al-compatible electrolyte for rechargeable Mg batteries. The electrolyte exhibited conditioning-free Mg deposition, high oxidative stability (3.5 V vs. Mg/Mg2+), and a superb electrochemical performance with various cathodes on an Al current collector.

Magnesiophilic Graphitic Carbon Nanosubstrate for Highly Efficient and Fast-Rechargeable Mg Metal Batteries.

The high volumetric energy density of rechargeable Mg batteries (RMBs) gives them a competitive advantage over current Li ion batteries, which originates from the high volumetric capacity (∼3833 mA h cm-3) of bivalent Mg metal anodes (MMAs). On the other hand, despite their importance, there are few reports on research strategies to improve the electrochemical performance of MMAs. This paper reports that catalytic carbon nanosubstrates rather than metal-based substrates, such as Mo, Cu, and stainless steel, are essential in MMAs to improve the electrochemical performance of RMBs. In particular, three-dimensional macroporous graphitic carbon nanosubstrates (GC-NSs) with high electrical conductivities can accommodate Mg metal with significantly higher rate capabilities and Coulombic efficiencies than metal substrates, resulting in a more stable and longer-term cycling performance over 1000 cycles. In addition, while metal-based substrates suffered from undesirable Mg peeling-off, homogeneous Mg metal deposition is well-guided in GC-NSs owing to the better affinity of the Mg2+ ion. These results are supported by density functional theory calculations and ex-situ characterization.

Exceptionally Reversible Li-/Na-Ion Storage and Ultrastable Solid-Electrolyte Interphase in Layered GeP5 Anode.

In this study, we synthesize two layered and amorphous structures of germanium phosphide (GeP5) and compare their electrochemical performances to better understand the role of layered, crystalline structures and their ability to control large volume expansions. We compare the results obtained with those of previous, conventional viewpoints addressing the effectiveness of amorphous phases in traditional anodes (Si, Ge, and Sn) to hinder electrode pulverization. By means of both comprehensive experimental characterizations and density functional theory calculations, we demonstrate that layered, crystalline GeP5 in a hybrid structure with multiwalled carbon nanotubes exhibits exceptionally good transport of electrons and electrolyte ions and tolerance to extensive volume changes and provides abundant reaction sites relative to an amorphous structure, resulting in a superior solid-electrolyte interphase layer and unprecedented initial Coulombic efficiencies in both Li-ion and Na-ion batteries. Moreover, the hybrid delivers excellent rate-capability (symmetric and asymmetric) performance and remarkable reversible discharge capacities, even at high current rates, realizing ultradurable cycles in both applications. The findings of this investigation are expected to offer insights into the design and application of layered materials in various devices.

Nanoscale Zirconium-Abundant Surface Layers on Lithium- and Manganese-Rich Layered Oxides for High-Rate Lithium-Ion Batteries.

Battery performance, such as the rate capability and cycle stability of lithium transition metal oxides, is strongly correlated with the surface properties of active particles. For lithium-rich layered oxides, transition metal segregation in the initial state and migration upon cycling leads to a significant structural rearrangement, which eventually degrades the electrode performance. Here, we show that a fine-tuning of surface chemistry on the particular crystal facet can facilitate ionic diffusion and thus improve the rate capability dramatically, delivering a specific capacity of ∼110 mAh g-1 at 30C. This high rate performance is realized by creating a nanoscale zirconium-abundant rock-salt-like surface phase epitaxially grown on the layered bulk. This surface layer is spontaneously formed on the Li+-diffusive crystallographic facets during the synthesis and is also durable upon electrochemical cycling. As a result, Li-ions can move rapidly through this nanoscale surface layer over hundreds of cycles. This study provides a promising new strategy for designing and preparing a high-performance lithium-rich layered oxide cathode material.

Surface Modification of the LiFePO4 Cathode for the Aqueous Rechargeable Lithium Ion Battery.

The LiFePO4 surface is coated with AlF3 via a simple chemical precipitation for aqueous rechargeable lithium ion batteries (ARLBs). During electrochemical cycling, the unfavorable side reactions between LiFePO4 and the aqueous electrolyte (1 M Li2SO4 in water) leave a highly resistant passivation film, which causes a deterioration in the electrochemical performance. The coated LiFePO4 by 1 wt % AlF3 has a high discharge capacity of 132 mAh g-1 and a highly improved cycle life, which shows 93% capacity retention even after 100 cycles, whereas the pristine LiFePO4 has a specific capacity of 123 mAh g-1 and a poor capacity retention of 82%. The surface analysis results, which include X-ray photoelectron spectroscopy and transmission electron microscopy results, show that the AlF3 coating material is highly effective for reducing the detrimental surface passivation by relieving the electrochemical side reactions of the fragile aqueous electrolyte. The AlF3 coating material has good compatibility with the LiFePO4 cathode material, which mitigates the surface diffusion obstacles, reduces the charge-transfer resistances and improves the electrochemical performance and surface stability of the LiFePO4 material in aqueous electrolyte solutions.

Critical Role of pH Evolution of Electrolyte in the Reaction Mechanism for Rechargeable Zinc Batteries.

The reaction mechanism of α-MnO2 having 2×2 tunnel structure with zinc ions in a zinc rechargeable battery, employing an aqueous zinc sulfate electrolyte, was investigated by in situ monitoring structural changes and water chemistry alterations during the reaction. Contrary to the conventional belief that zinc ions intercalate into the tunnels of α-MnO2 , we reveal that they actually precipitate in the form of layered zinc hydroxide sulfate (Zn4 (OH)6 (SO4 )⋅5 H2 O) on the α-MnO2 surface. This precipitation occurs because unstable trivalent manganese disproportionates and is dissolved in the electrolyte during the discharge process, resulting in a gradual increase in the pH value of the electrolyte. This causes zinc hydroxide sulfate to crystallize from the electrolyte on the electrode surface. During the charge process, the pH value of the electrolyte decreases due to recombination of manganese on the cathode, leading to dissolution of zinc hydroxide sulfate back into the electrolyte. An analogous phenomenon is also observed in todorokite, a manganese dioxide polymorph with 3×3 tunnel structure that is an indication for the critical role of pH changes of the electrolyte in the reaction mechanism of this battery system.

Investigation of the Na Intercalation Mechanism into Nanosized V2O5/C Composite Cathode Material for Na-Ion Batteries.

There is a significant interest to develop high-performance and cost-effective electrode materials for next-generation sodium ion batteries. Herein, we report a facile synthesis method for nanosized V2O5/C composite cathodes and their electrochemical performance as well as energy storage mechanism. The composite exhibits a discharge capacity of 255 mAh g(-1) at a current density of 0.05 C, which surpasses that of previously reported layered oxide materials. Furthermore, the electrode shows good rate capability; discharge capacity of 160 mAh g(-1) at a current density of 1 C. The reaction mechanism of V2O5 upon sodium insertion/extraction is investigated using ex situ X-ray diffraction (XRD) and synchrotron based near edge X-ray absorption fine structure (NEXAFS) spectroscopy. Ex situ XRD result of the fully discharged state reveals the appearance of NaV2O5 as a major phase with minor Na2V2O5 phase. Upon insertion of sodium into the array of parallel ladders of V2O5, it was confirmed that lattice parameter of c is increased by 9.09%, corresponding to the increase in the unit-cell volume of 9.2%. NEXAFS results suggest that the charge compensation during de/sodiation process accompanied by the reversible changes in the oxidation state of vanadium (V(4+) ↔ V(5+)).

Elucidating the intercalation mechanism of zinc ions into α-MnO2 for rechargeable zinc batteries.

The intercalation mechanism of zinc ions into 2 × 2 tunnels of an α-MnO2 cathode for rechargeable zinc batteries was revealed. It involves a series of single and two-phase reaction steps and produces buserite, a layered compound with an interlayer spacing of 11 Å as a discharge product.

Electrochemically-induced reversible transition from the tunneled to layered polymorphs of manganese dioxide.

Zn-ion batteries are emerging energy storage systems eligible for large-scale applications, such as electric vehicles. These batteries consist of totally environmentally-benign electrode materials and potentially manufactured very economically. Although Zn/α-MnO2 systems produce high energy densities of 225 Wh kg(-1), larger than those of conventional Mg-ion batteries, they show significant capacity fading during long-term cycling and suffer from poor performance at high current rates. To solve these problems, the concrete reaction mechanism between α-MnO2 and zinc ions that occur on the cathode must be elucidated. Here, we report the intercalation mechanism of zinc ions into α-MnO2 during discharge, which involves a reversible phase transition of MnO2 from tunneled to layered polymorphs by electrochemical reactions. This transition is initiated by the dissolution of manganese from α-MnO2 during discharge process to form layered Zn-birnessite. The original tunneled structure is recovered by the incorporation of manganese ions back into the layers of Zn-birnessite during charge process.

Sulfur/graphitic hollow carbon sphere nano-composite as a cathode material for high-power lithium-sulfur battery.

The intrinsic low conductivity of sulfur which leads to a low performance at a high current rate is one of the most limiting factors for the commercialization of lithium-sulfur battery. Here, we present an easy and convenient method to synthesize a mono-dispersed hollow carbon sphere with a thin graphitic wall which can be utilized as a support with a good electrical conductivity for the preparation of sulfur/carbon nano-composite cathode. The hollow carbon sphere was prepared from the pyrolysis of the homogenous mixture of the mono-dispersed spherical silica and Fe-phthalocyanine powder in elevated temperature. The composite cathode was manufactured by infiltrating sulfur melt into the inner side of the graphitic wall. The electrochemical cycling shows a capacity of 425 mAh g-1 at 3 C current rate which is more than five times larger than that for the sulfur/carbon black nano-composite prepared by simple ball milling.

Polysulfide dissolution control: the common ion effect.

A Li-sulfur cell with a high discharge capacity of over 1300 mAh g(-1) at a C/10 rate, and a controlled overcharge amount less than 1%, was manufactured by synthesizing a carbon-sulfur nano-composite via a wet milling process, and suppressing polysulfide dissolution using an electrolyte with a highly concentrated lithium salt.

Synthesis of a metallic mesoporous pyrochlore as a catalyst for lithium­O2 batteries.

The lithium­O2 'semi-fuel' cell based on the reversible reaction of Li and O2 to form Li2O2 can theoretically provide energy densities that exceed those of Li-ion cells by up to a factor of five. A key limitation that differentiates it from other lithium batteries is that it requires effective catalysts (or 'promoters') to enable oxygen reduction and evolution. Here, we report the synthesis of a novel metallic mesoporous oxide using surfactant templating that shows promising catalytic activity and results in a cathode with a high reversible capacity of 10,000 mAh g(−1) (∼1,000 mAh g(−1) with respect to the total electrode weight including the peroxide product). This oxide also has a lower charge potential for oxygen evolution from Li2O2 than pure carbon. The properties are explained by the high fraction of surface defect active sites in the metallic oxide, and its unique morphology and variable oxygen stoichiometry. This strategy for creating porous metallic oxides may pave the way to new cathode architectures for the Li­O2 cell.

Screening for superoxide reactivity in Li-O2 batteries: effect on Li2O2/LiOH crystallization.

Unraveling the fundamentals of Li-O(2) battery chemistry is crucial to develop practical cells with energy densities that could approach their high theoretical values. We report here a straightforward chemical approach that probes the outcome of the superoxide O(2)(-), thought to initiate the electrochemical processes in the cell. We show that this serves as a good measure of electrolyte and binder stability. Superoxide readily dehydrofluorinates polyvinylidene to give byproducts that react with catalysts to produce LiOH. The Li(2)O(2) product morphology is a function of these factors and can affect Li-O(2) cell performance. This methodology is widely applicable as a probe of other potential cell components.