Understanding the Cathode-Electrolyte Interfacial Chemistry in Rechargeable Magnesium Batteries.

Rechargeable magnesium batteries (RMBs) have garnered significant attention due to their potential to provide high energy density, utilize earth-abundant raw materials, and employ metal anode safely. Currently, the lack of applicable cathode materials has become one of the bottleneck issues for fully exploiting the technological advantages of RMBs. Recent studies on Mg cathodes reveal divergent storage performance depending on the electrolyte formulation, posing interfacial issues as a previously overlooked challenge. This minireview begins with an introduction of representative cathode-electrolyte interfacial phenomena in RMBs, elaborating on the unique solvation behavior of Mg2+, which lays the foundation for interfacial chemistries. It is followed by presenting recently developed strategies targeting the promotion of Mg2+ desolvation in the electrolyte and alternative cointercalation approaches to circumvent the desolvation step. In addition, efforts to enhance the cathode-electrolyte compatibility via electrolyte development and interfacial engineering are highlighted. Based on the abovementioned discussions, this minireview finally puts forward perspectives and challenges on the establishment of a stable interface and fast interfacial chemistry for RMBs.

A Perspective on Uniform Plating Behavior of Mg Metal Anode: Diffusion Limited Theory versus Nucleation Theory.

Utilizing metal anode is the most attractive way to meet the urgent demand for rechargeable batteries with high energy density. Unfortunately, the formation of dendrites, which is caused by uneven plating behavior, always threaten the safety of the batteries. To explore the origin of different plating behavior and predict the plating morphology of anode under a variety of operating conditions, multifarious models have been developed. However, abuse of models has led to conflictive views. In this perspective, to clarify the controversial reports on magnesium (Mg) metal plating behavior, the previously proposed models are elaborated that govern the plating process. Through linking various models and clarifying their boundary conditions, a scheme is drawn to illustrate the strategy for achieving the most dense and uniform plating morphology, which also explains the seemingly contradictory of diffusion limited theory and nucleation theory on uniform plating. This perspective will undoubtedly enhance the understanding on the metal anode plating process and provide meaningful guidance for the development of metal anode batteries.

Cation Co-Intercalation with Anions: The Origin of Low Capacities of Graphite Cathodes in Multivalent Electrolytes.

Dual-ion batteries involving anion intercalation into graphite cathodes represent promising battery technologies for low-cost and high-power energy storage. However, the fundamental origins regarding much lower capacities of graphite cathodes in earth abundant and inexpensive multivalent electrolytes than in Li-ion electrolytes remain elusive. Herein, we reveal that the limited anion-storage capacity of a graphite cathode in multivalent electrolytes is rooted in the abnormal multivalent-cation co-intercalation with anions in the form of large-sized anionic complexes. This cation co-intercalation behavior persists throughout the stage evolution of graphite intercalation compounds and leads to a significant decrease of sites practically viable for capacity contribution inside graphite galleries. Further systematic studies illustrate that the phenomenon of cation co-intercalation into graphite is closely related to the high energy penalty of interfacial anion desolvation due to the strong cation-anion association prevalent in multivalent electrolytes. Leveraging this understanding, we verify that promoting ionic dissociation in multivalent electrolytes by employing high-permittivity and oxidation-tolerant co-solvents is effective in suppressing multivalent-cation co-intercalation and thus achieving increased capacity of graphite cathodes. For instance, introducing adiponitrile as a co-solvent to a Mg2+-based carbonate electrolyte leads to 83% less Mg2+ co-intercalation and a ∼29.5% increase in delivered capacity of the graphite cathode.

Cathode Electrolyte Interphase (CEI) Endows Mo6 S8 with Fast Interfacial Magnesium-Ion Transfer Kinetics.

Magnesium (Mg) metal secondary batteries have attracted much attention for their high safety and high energy density characteristics. However, the significant issues of the cathode/electrolyte interphase (CEI) in Mg batteries are still being ignored. In this work, a significant CEI layer on the typical Mo6 S8 cathode surface has been unprecedentedly constructed through the oxidation of the chloride-free magnesium tetrakis(hexafluoroisopropyloxy)borate (Mg[B(hfip)4 ]2 ) salt under a proper charge cut-off voltage condition. The CEI has been identified to contain Bx Oy effective species originating from the oxidation of [B(hfip)4 ]- anion. It is confirmed that the Bx Oy species is beneficial to the desolvation of solvated Mg2+ , speeding up the interfacial Mg2+ transfer kinetics, thereby improving the Mg2+ -storage capability of Mo6 S8 host. The firstly reported CEI in Mg batteries will give deeper insights into the interface issues in multivalent electrochemical systems.

Epitaxial Electrocrystallization of Magnesium via Synergy of Magnesiophilic Interface, Lattice Matching, and Electrostatic Confinement.

Rechargeable magnesium batteries are particularly advantageous for renewable energy storage systems. However, the inhomogeneous Mg electrodeposits greatly shorten their cycle life under practical conditions. Herein, the epitaxial electrocrystallization of Mg on a three-dimensional magnesiophilic host is implemented via the synergy of a magnesiophilic interface, lattice matching, and electrostatic confinement effects. The vertically aligned nickel hydroxide nanosheet arrays grown on carbon cloth (abbreviated as "Ni(OH)2@CC") have been delicately designed, which satisfy the essential prerequisite of a low lattice geometrical misfit with Mg (about 2.8%) to realize epitaxial electrocrystallization. Simultaneously, the ionic crystal nature of Ni(OH)2 displays a periodic and hillock-like electrostatic potential field over its exposed facets, which can precisely capture and confine the reduced Mg0 species onto the local electron-enriched sites at the atomic level. The Ni(OH)2@CC substrate undergoes sequential Mg-ion intercalation, underpotential deposition, and electrocrystallization processes, during which the uniform, lamellar Mg electrodeposits with a locked crystallographic orientation are formed. Under practical conditions (10 mA cm-2 and 10 mAh cm-2), the Ni(OH)2@CC substrate exhibits stable Mg stripping/plating cycle performances over 600 h, 2 orders of magnitude longer than those of the pristine copper foil and carbon cloth substrates.

Uneven Stripping Behavior, an Unheeded Killer of Mg Anodes.

Uniform magnesium (Mg) plating/stripping under high areal capacity utilization is critical for the practical application of Mg-metal anodes in rechargeable Mg batteries. However, the failure of the Mg-metal anode when cycling under a practical areal capacity (>4 mA h cm-2 ), is of rising concern. The mechanism behind these failures remains controversial. In this work, it is illustrated that the initial plating Mg can be undoubtedly uniform in a wide range of current densities (≤5 mA cm-2 ) and under a practical areal capacity (6 mA h cm-2 ). However, an unusual self-accelerating pit growth is observed in the Mg stripping side under moderate current densities (0.1-1 mA cm-2 ), which severely deteriorates the anode integrity and subsequent Mg plating uniformity, causing failure of the Mg-metal anode or short circuit of the battery. The stripping morphology depends on the applied current density, as non-uniformity versus the current density displays a volcano plot during the stripping process. Through in situ spectroscopy, it is illustrated that this current-dependent behavior is determined by the evolution of chlorine-containing complex ions near the interface. This research reminds that the plating/stripping process of the Mg-metal anode must be considered comprehensively for practical Mg-metal batteries.

Charge-Compensation in a Displacement Mg2+ Storage Cathode through Polyselenide-Mediated Anion Redox.

Chalcogenides have been viewed as important conversion-type Mg2+ -storage cathodes to fulfill the high volumetric energy density promise of magnesium (Mg) batteries. However, the low initial Columbic efficiency and the rapid capacity degradation remain challenges for the chalcogenide cathodes, as the clear Mg2+ -storage mechanism has yet to be clarified. Herein, we illustrate that the charge storage mechanism of the Cu2-x Se cathode is a reversible displacement reaction along with a polyselenide (PSe) mediated solution process of anion-compensation. The unique anion redox improves charge storage, while the dissolution of PSe also leads to performance degradation. To address this issue, we introduce Mo6 S8 into the Cu2-x Se cathode to immobilize PSe, which significantly improves performance, especially the reversible capacity (from 140 mAh g-1 to 220 mAh g-1 ). This work provides inspiration for the modification of the Mg2+ -storage cathode, which is a milestone for high-performance Mg batteries.

Current Design Strategies for Rechargeable Magnesium-Based Batteries.

As a next-generation electrochemical energy storage technology, rechargeable magnesium (Mg)-based batteries have attracted wide attention because they possess a high volumetric energy density, low safety concern, and abundant sources in the earth's crust. While a few reviews have summarized and discussed the advances in both cathode and anode materials, a comprehensive and profound review focusing on the material design strategies that are both representative of and peculiar to the performance improvement of rechargeable Mg-based batteries is rare. In this mini-review, all nine of the material design strategies and approaches to improve Mg-ion storage properties of cathode materials have been comprehensively examined from both internal and external aspects. Material design concepts are especially highlighted, focusing on designing "soft" anion-based materials, intercalating solvated or complex ions, expanding the interlayer of layered cathode materials, doping heteroatoms into crystal lattice, size tailoring, designing metastable-phase materials, and developing organic materials. To achieve a better anode, strategies based on the artificial interlayer design, efficient electrolyte screening, and alternative anodes exploration are also accumulated and analyzed. The strategy advances toward Mg-S and Mg-Se batteries are summarized. The advantages and disadvantages of all-collected material design strategies and approaches are critically discussed from practical application perspectives. This mini-review is expected to provide a clear research clue on how to rationally improve the reliability and feasibility of rechargeable Mg-based batteries and give some insights for the future research of Mg-based batteries as well as other multivalent-ion battery chemistries.

Facilitated magnesium atom adsorption and surface diffusion kinetics via artificial bismuth-based interphases.

Robust bismuth-based interphases, comprised of bismuth and bismuth oxides, were developed using galvanic replacement reactions. Facilitated Mg atom adsorption and distinct interfacial Mg atom migration were demonstrated, greatly lowering the electrochemical energy penalty (23 mV for the nucleation process and 69 mV for the growth process at 1.0 mA cm-2).

Polymer Electrolytes - New Opportunities for the Development of Multivalent Ion Batteries.

Batteries, as highly concerned energy conversion system, have a great development prospect in various fields, especially in the field of energy powered vehicles. Multivalent ion batteries are getting more attention due to their low cost, high abundance in earth crust, high capacity and safety compared with Lithium batteries. Despite above advantages, several problems still need to be solved before multivalent ion batteries achieve large-scale application, such as interfacial parasitic reaction, anode passivation, and dendrites. The replacement of liquid electrolytes with gel polymer electrolytes (GPEs) which pose high safety, high mechanical strength and simplified battery system, is an effective strategy to inhibit dendrite growth and improve electrochemical performance. This review mainly discusses the advantages and challenges of multivalent ion batteries including zinc, magnesium, calcium and aluminum batteries. Meanwhile, the major targets of this review are introducing the recent developments and making a summary of the future trends of GPEs in the multivalent ion batteries.

Uniform Magnesium Electrodeposition via Synergistic Coupling of Current Homogenization, Geometric Confinement, and Chemisorption Effect.

Unevenly distributed magnesium (Mg) electrodeposits have emerged as a major obstacle for Mg-metal batteries. A comprehensive design matrix is reported for 3D magnesiophilic hosts, which regulate the uniform Mg electrodeposition through a synergistic coupling of homogenizing current distribution, geometric confinement, and chemisorptive interaction. Vertically aligned nitrogen- and oxygen-doped carbon nanofiber arrays on carbon cloth (denoted as "VNCA@C") are developed as a proof of concept. The evenly arranged short nanoarray architecture helps to homogenize the surface current density and the microchannels built in this 3D host allow the preferential nucleation of Mg due to their geometrical confinement effect. Besides, the nitrogen-/oxygen-doped carbon species exhibit strong chemisorptive interaction toward Mg atoms, providing preferential nucleation sites as demonstrated by first-principle calculation results. Electrochemical analysis reveals a peculiar yet highly reversible microchannel-filling growth behavior of Mg metals, which empowers the delicately designed VNCA@C host with the ability to deliver a reduced nucleation overpotential of 429 mV at 10.0 mA cm-2 and an elongated Mg plating/stripping cycle life (110 cycles) under high current density of 10.0 mA cm-2 . The proposed design matrix can be extended to other metal anodes (such as lithium and zinc) for high-energy-density batteries.

A Novel Regulation Strategy of Solid Electrolyte Interphase Based on Anion-Solvent Coordination for Magnesium Metal Anode.

Magnesium (Mg) metal anode is a highly desirable candidate among various high energy density metal anodes, possessing higher volumetric capacity and better safety characteristic compared to lithium metal. However, most Mg salts in conventional Mg electrolytes easily react with Mg metal to form blocking layers, leading to inferior reversibility of Mg plating/stripping. Here, a stable Mg2+ -conducting solid electrolyte interphase (SEI) is successfully constructed on Mg metal anode by regulating the molecular-orbital-energy-level toward an aluminum(III)-centered anion Mg salt through anion-solvent coordination. Of which, the LUMO energy level of perfluorinated pinacolatoaluminate (Al(O2 C2 (CF3 )4 )2 - , abbreviated as FPA) anion has been adjusted by coordinating with solvent molecule (tetrahydrofuran) for facilitating the formation of advantageous SEI. The existence of SEI formed by FPA anion greatly improves the reversibility and long-term stability of Mg plating/stripping process. More importantly, based on this aluminum(III)-centered Mg electrolyte, the Mo6 S8 /Mg batteries can achieve a fantastic cycle performance of 9000 cycles, proving the beneficial effect of SEI on the cycling stability of Mg battery system. These findings open up a promising avenue to construct stable and compatible SEI on Mg metal anode, and lay significant foundations for the successful development of rechargeable Mg metal batteries.

A Stable Solid Electrolyte Interphase for Magnesium Metal Anode Evolved from a Bulky Anion Lithium Salt.

Rechargeable magnesium (Mg) metal batteries are a promising candidate for "post-Li-ion batteries" due to their high capacity, high abundance, and most importantly, highly reversible and dendrite-free Mg metal anode. However, the formation of passivating surface film rather than Mg2+ -conducting solid electrolyte interphase (SEI) on Mg anode surface has always restricted the development of rechargeable Mg batteries. A stable SEI is constructed on the surface of Mg metal anode by the partial decomposition of a pristine Li electrolyte in the electrochemical process. This Li electrolyte is easily prepared by dissolving lithium tetrakis(hexafluoroisopropyloxy)borate (Li[B(hfip)4 ]) in dimethoxyethane. It is noteworthy that Mg2+ can be directly introduced into this Li electrolyte during the initial electrochemical cycles for in situ forming a hybrid Mg2+ /Li+ electrolyte, and then the cycled electrolyte can conduct Mg-ion smoothly. The existence of this as-formed SEI blocks the further parasitic reaction of Mg metal anode with electrolyte and enables this electrolyte enduring long-term electrochemical cycles stably. This approach of constructing superior SEI on Mg anode surface and exploiting novel Mg electrolyte provides a new avenue for practical application of high-performance rechargeable Mg batteries.

A Novel Bifunctional Self-Stabilized Strategy Enabling 4.6 V LiCoO2 with Excellent Long-Term Cyclability and High-Rate Capability.

Although the theoretical specific capacity of LiCoO2 is as high as 274 mAh g-1, the superior electrochemical performances of LiCoO2 can be barely achieved due to the issues of severe structure destruction and LiCoO2/electrolyte interface side reactions when the upper cutoff voltage exceeds 4.5 V. Here, a bifunctional self-stabilized strategy involving Al+Ti bulk codoping and gradient surface Mg doping is first proposed to synchronously enhance the high-voltage (4.6 V) performances of LiCoO2. The comodified LiCoO2 (CMLCO) shows an initial discharge capacity of 224.9 mAh g-1 and 78% capacity retention after 200 cycles between 3.0 and 4.6 V. Excitingly, the CMLCO also exhibits a specific capacity of up to 142 mAh g-1 even at 10 C. Moreover, the long-term cyclability of CMLCO/mesocarbon microbeads full cells is also enhanced significantly even at high temperature of 60 °C. The synergistic effects of this bifunctional self-stabilized strategy on structural reversibility and interfacial stability are demonstrated by investigating the phase transitions and interface characteristics of cycled LiCoO2. This work will be a milestone breakthrough in the development of high-voltage LiCoO2. It will also present an instructive contribution for resolving the big structural and interfacial challenges in other high-energy-density rechargeable batteries.

Fast magnesiation kinetics in α-Ag2S nanostructures enabled by an in situ generated silver matrix.

Here we first present a novel cathode material of α-Ag2S for rechargeable Mg batteries. Due to the unique cation displacement reaction mechanism and the in situ generated uniformly distributed silver matrix, α-Ag2S nanostructures deliver remarkably improved magnesiation kinetics compared to conventional transition metal sulfides.

A Crosslinked Polytetrahydrofuran-Borate-Based Polymer Electrolyte Enabling Wide-Working-Temperature-Range Rechargeable Magnesium Batteries.

A polymer-based magnesium (Mg) electrolyte is vital for boosting the development of high-safety and flexible Mg batteries by virtue of its enormous advantages, such as significantly improved safety, potentially high energy density, ease of fabrication, and structural flexibility. Herein, a novel polytetrahydrofuran-borate-based gel polymer electrolyte coupling with glass fiber is synthesized via an in situ crosslinking reaction of magnesium borohydride [Mg(BH4 )2 ] and hydroxyl-terminated polytetrahydrofuran. This gel polymer electrolyte exhibits reversible Mg plating/stripping performance, high Mg-ion conductivity, and remarkable Mg-ion transfer number. The Mo6 S8 /Mg batteries assembled with this gel polymer electrolyte not only work well at wide temperature range (-20 to 60 °C) but also display unprecedented improvements in safety issues without suffering from internal short-circuit failure even after a cutting test. This in situ crosslinking approach toward exploiting the Mg-polymer electrolyte provides a promising strategy for achieving large-scale application of Mg-metal batteries.

Multifunctional Sandwich-Structured Electrolyte for High-Performance Lithium-Sulfur Batteries.

Due to its high theoretical energy density (2600 Wh kg-1), low cost, and environmental benignity, the lithium-sulfur (Li-S) battery is attracting strong interest among the various electrochemical energy storage systems. However, its practical application is seriously hampered by the so-called shuttle effect of the highly soluble polysulfides. Herein, a novel design of multifunctional sandwich-structured polymer electrolyte (polymer/cellulose nonwoven/nanocarbon) for high-performance Li-S batteries is demonstrated. It is verified that Li-S battery with this sandwich-structured polymer electrolyte delivers excellent cycling stability (only 0.039% capacity decay cycle-1 on average exceeding 1500 cycles at 0.5 C) and rate capability (with a reversible capacity of 594 mA h g-1 at 4 C). These electrochemical performances are attributed to the synergistic effect of each layer in this unique sandwich-structured polymer electrolyte including steady lithium stripping/plating, strong polysulfide absorption ability, and increased redox reaction sites. More importantly, even with high sulfur loading of 4.9 mg cm-2, Li-S battery with this sandwich-structured polymer electrolyte can deliver high initial areal capacity of 5.1 mA h cm-2. This demonstrated strategy here may open up a new era of designing hierarchical structured polymer electrolytes for high-performance Li-S batteries.