Tailoring Coordination in Conventional Ether-Based Electrolytes for Reversible Magnesium-Metal Anodes.

Rechargeable magnesium (Mg) batteries based on conventional electrolytes are seriously plagued by the formation of the ion-blocking passivation layer on the Mg metal anode. By tracking the Mg2+ solvation sheath, this work links the passivation components to the Mg2+ -solvents (1,2-dimethoxyethane, DME) coordination and the consequent thermodynamically unstable DME molecules. On this basis, we propose a methodology to tailor solvation coordination by introducing the additive solvent with extreme electron richness. Oxygen atoms in phosphorus-oxygen groups compete with that in carbon-oxygen groups of DME for the coordination with Mg2+ , thus softening the solvation sheath deformation. Meanwhile, the organophosphorus molecules in the rearranged solvation sheath decompose on the Mg surface, increasing the Mg2+ transport and electrical resistance by three and one orders of magnitude, respectively. Consequently, the symmetric cells exhibit superior cycling performance of over 600 cycles with low polarization.

Defect-Free Metal-Organic Framework Membrane for Precise Ion/Solvent Separation toward Highly Stable Magnesium Metal Anode.

Metallic magnesium batteries are promising candidates beyond lithium-ion batteries; however, a passive interfacial layer because of the electro-reduction of solvents on Mg surfaces usually leads to ultrahigh overpotential for the reversible Mg chemistry. Inspired by the excellent separation effect of permselective metal-organic framework (MOF) at angstrom scale, a large-area and defect-free MOF membrane directly on Mg surfaces is here constructed. In this process, the electrochemical deprotonation of ligand can be facilitated to afford the self-correcting of intercrystalline voids until a seamless membrane formed, which can eliminate nonselective intercrystalline diffusion of electrolyte and realize selective Mg2+ transport but precisely separate the solvent molecules from the MOF channels. Compared with the continuous solvent reduction on bare Mg anode, the as-constructed MOF membrane is demonstrated to significantly stabilize the Mg electrode via suppressing the permeation of solvents, hence contributing to a low-overpotential plating/stripping in conventional electrolytes. The concept is demonstrated that membrane separation can serve as solid-electrolyte interphase, which would be widely applicable to other energy-storage systems.

Ionic Liquid-Mediated Mass Transport Channels for Ultrahigh Rate Lithium-Ion Batteries.

Excellent mass transport capability is indispensable to building a high-power lithium-ion battery (LIB) system. Nanomaterials with enhanced electrochemical properties have been used for next-generation high-performance LIBs. However, due to the high surface free energy, nanomaterials tend to form agglomeration. The resulting insufficient mass transport channels limit the high rate performance of nanomaterials. Here, we increase the electrolyte accessibility of nanomaterials through a facile ionic liquid (IL) mediation method. The fluidity and affinity enable the IL to infiltrate into the interstitials of nanomaterial aggregations under capillary force, enhancing the electrochemically active contact area and ensuring rapid mass transport. As a proof of concept, IL-mediated LiFePO4 electrodes delivered extraordinary rate performance (112 and 95 mAh g-1 at 200 and 300 C, respectively). In terms of simplicity, the IL mediation can be used as a general strategy to achieve an ultrahigh rate for LIBs.

MXene Frameworks Promote the Growth and Stability of LiF-Rich Solid-Electrolyte Interphases on Silicon Nanoparticle Bundles.

Silicon-based materials are the desirable anodes for next-generation lithium-ion batteries; however, the large volume change of Si during the charging/discharging process causes electrode fracture and an unstable solid-electrolyte interphase (SEI) layer, which severely impair their stability and Coulombic efficiency. Herein, a bundle of silicon nanoparticles is encapsulated in robust micrometer-sized MXene frameworks, in which the MXene nanosheets are precrumpled by capillary compression force to effectively buffer the stress induced by the volume change, and the abundant covalent bonds (Ti-O-Ti) between adjacent nanosheets formed through a facile thermal self-cross-linking reaction further guarantee the robustness of the MXene architecture. Both factors stabilize the electrode structure. Moreover, the abundant fluorine terminations on MXene nanosheets contribute to an in situ formation of a highly compact, durable, and mechanically robust LiF-rich SEI layer outside the frameworks upon cycling, which not only shuts down the parasitic reaction between Si and an organic electrolyte but also enhances the structural stability of MXene frameworks. Benefiting from these merits, the as-prepared anodes deliver a high specific capacity of 1797 mA h g-1 at 0.2 A g-1 and a high capacity retention of 86.7% after 500 cycles at 2 A g-1 with an average Coulombic efficiency of 99.6%. Significantly, this work paves the way for other high-capacity electrode materials with a strong volume effect.

Three-Dimensional Magnesiophilic Scaffolds for Reduced Passivation toward High-Rate Mg Metal Anodes in a Noncorrosive Electrolyte.

Magnesium ion batteries are a promising alternative of the lithium counterpart; however, the poorly ion-conductive passivation layer on Mg metal makes plating/stripping difficult. In addition to the generally recognized chemical passivation, the interphase is dynamically degraded by electrochemical side reactions. Especially under high current densities, the interphase thickens, exacerbating the electrode degradation. Herein, we adopt 3D Mg3Bi2 scaffolds for Mg metal, of which the high surface area reduces the effective current density to avoid continuous electrolyte decomposition and the good Mg affinity homogenizes nucleation. The greatly alleviated passivation layer could serve as a stable solid/electrolyte interface instead. The symmetric cell delivers a low overpotential of 0.21 and 0.50 V at a current density of 0.1 and 4 mA cm-2, respectively, and a superior cycling performance over 300 cycles at 0.5 mA cm-2 in a noncorrosive conventional electrolyte. This work proves that the control of dynamic passivation can enable high-power density Mg metal anodes.

Electrostatic Shielding Guides Lateral Deposition for Stable Interphase toward Reversible Magnesium Metal Anodes.

Compared with lithium, magnesium shows a low propensity toward dendritic deposition due to its low surface self-diffusion barriers. However, due to the intrinsic surface roughness of the metal and the nonuniformity of the formed solid-electrolyte interphase, uneven deposition of Mg still happens, which brings about high local current density and continuous proliferation of the interphase, greatly exacerbating the passivation. Unfortunately, little attention has been paid to the deposition uniformity and the interfacial stability of Mg metal anodes, which result in a potential penalty. Herein, we modify the electrolyte with cathodically stable cations to guide smooth deposition via an electrostatic shielding strategy. The cations adsorbed on the initial protuberances effectively homogenize the charge flux by repulsing the incoming Mg2+ away from the tips. Importantly, we prove the lateral growth can benefit the interphase stability and electrochemical reversibility.

Decreasing Ion-Diffusion Barrier Enables Superior Na-Ion Storage by Synergizing Hierarchical Architecture and Lattice Distortion.

Compared with nanosized materials, the long-pathway isolation of the interior part from the electrolyte for bulk electrode materials may result in high ionic diffusion barrier, leading to the poor rate behavior. Either the modification of lattice or the construction of a porous structure is generally effective to decrease the ion-diffusion barrier; however, achieving these multiscaled modulations simultaneously via a facile approach is still a challenge. Herein, we manipulate a bifunctional dopant to prepare micron-sized Na3V2(PO4)3 with extraordinary synergy of hierarchical architecture and lattice distortion. The cations Zn2+ not only substitute partial V3+ to enhance the solid-phase ion diffusivity but also stabilize the lattice structure due to the pillar effect. Additionally, the anions CH3COO- also participate in the reaction to modulate the porous architecture. The analysis results of galvanostatic intermittent titration technique, cyclic voltammetry, and electrochemical impedance spectroscopy demonstrate that the rational design of morphology and structure compounding lowers the ion-diffusion barrier and strengthens the Na+ migration kinetics. When evaluated as the cathode electrode, the optimal composite exhibits improved Na+ ion transport kinetics and superior rate behaviors of 72.2 and 58.7 mAh g-1 at 100 and 200C, respectively.

Rational Design of the Robust Janus Shell on Silicon Anodes for High-Performance Lithium-Ion Batteries.

The high-capacity silicon anode is regarded as a promising electrode material for next-generation lithium-ion batteries. Unfortunately, its practical application is still severely hindered by electrode fracture and unstable solid electrolyte interphase during cycling. Herein, we design a structure of encapsulating silicon in a robust "janus shell", in which an internal graphene shell with sufficient void space is used to absorb the mechanical stress induced by volume expansion, and the conformal carbon outer shell is introduced to strongly bond the loosely stacked graphene shell and simultaneously seal the nanopores on the surface. With the ultrastable janus carbon shell, the excellent structural integrity of the electrode and stable solid electrolyte interphase layer could be effectively preserved, resulting in an impressive cycling behavior. Indeed, the as-synthesized anodes demonstrate superior cycle stability and excellent rate performance, delivering a high reversible capacity of 1416 mA h g-1 at a current density of 0.2 A g-1 and 852 mA h g-1 at a high current density of 5 A g-1. Remarkably, the superior capacity retention of 88.5% could be achieved even after 400 cycles at a high current density of 2 A g-1. More importantly, this work opens up a novel avenue to address high-capacity anodes with a large volume change.