Rechargeable sodium batteries are a promising technology for low‐cost energy storage. However, the undesirable drawbacks originating from the use of glass fiber membrane separators have long been overlooked. A versatile grafting–filtering strategy was developed to controllably tune commercial polyolefin separators for sodium batteries. The as‐developed Janus separators contain a single–ion‐conducting polymer‐grafted side and a functional low‐dimensional material coated side. When employed in room‐temperature sodium–sulfur batteries, the poly(1‐[3‐(methacryloyloxy)propylsulfonyl]‐1‐(trifluoromethanesulfonyl)imide sodium)‐grafted side effectively enhances the electrolyte wettability, and inhibits polysulfide diffusion and sodium dendrite growth. Moreover, a titanium‐deficient nitrogen‐containing MXene‐coated side electrocatalytically improved the polysulfide conversion kinetics. The as‐developed batteries demonstrate high capacity and extended cycling life with lean electrolyte loading.

Rechargeable multivalent metal (e.g., Ca, Mg or, Al) batteries are ideal candidates for large–scale electrochemical energy storage due to their intrinsic low cost. However, their practical application is hampered by the low electrochemical reversibility, dendrite growth at the metal anodes, sluggish multivalent–ion kinetics in metal oxide cathodes and, poor electrode compatibility with non–aqueous organic–based electrolytes. To circumvent these issues, here we report various aqueous multivalent–ion batteries comprising of concentrated aqueous gel electrolytes, sulfur–containing anodes and, high-voltage metal oxide cathodes as alternative systems to the non–aqueous multivalent metal batteries. This rationally designed aqueous battery chemistry enables satisfactory specific energy, favorable reversibility and improved safety. As a demonstration model, we report a room–temperature calcium-ion/sulfur| |metal oxide full cell with a specific energy of 110 Wh kg–1 and remarkable cycling stability. Molecular dynamics modeling and experimental investigations reveal that the side reactions could be significantly restrained through the suppressed water activity and formation of a protective inorganic solid electrolyte interphase. The unique redox chemistry of the multivalent–ion system is also demonstrated for aqueous magnesium–ion/sulfur||metal oxide and aluminum–ion/sulfur||metal oxide full cells.

Potassium-ion batteries based on conversion/alloying reactions have high potential applications in newgeneration large-scale energy storage. However, their applications are hindered by inherent large volume variations and sluggish kinetics of the conversion/alloying-type electrode materials during the repeated insertion and extraction of bulky K + ions. Although some efforts have been focused on this issue, the reported potassium-ion batteries still suffer from poor cycling lifespans. Here, a superior stable antimony selenide (Sb 2 Se 3 ) anode is reported for high-performance potassium-ion batteries through a combined strategy of conductive encapsulation and two-dimensional confinement. The Sb 2 Se 3 nanorods are uniformly coated with a conductive N-doped carbon layer and then confined between graphene nanosheets. The synergistic effects between conductive coating and confinement effectively buffer the large volumetric variation of the conversion/alloying anodes, which can maintain structural stability for superior cyclability. The as-prepared anodes exhibit a high reversible specific capacity of ~590 mA h g −1 and an outstanding cycling stability over 350 cycles. In-situ and ex-situ characterizations reveal a high structural integration of the large-volume-change Sb 2 Se 3 anodes during a reversible K storage mechanism of two-step conversion and multi-step alloying processes. This work can open up a new possibility for the design of stable conversion/alloying-based anodes for highperformance potassium-ion batteries.

However, the development of metal-Se batteries has been blocked by the severe shuttle effect of polyselenides, particularly in ether-based electrolyte, where the polyselenides diffuse across the separator and react with the metal anode. [12,13] Tremendous efforts have been devoted to the design of advanced cathode host materials to mitigate the shuttle effect. [14,15] These designed cathode strategies include: employing porous carbon materials as the hosts; [7,[16][17][18][19] coating the cathode materials with graphene, metal oxide, or conductive polymers; [20][21][22][23][24] and heteroatoms doping. [25][26][27] However, these cathode engineering would inevitably reduce the mass ratio of selenium by introducing additional components, leading to decreased energy density. [28] Recently, functional separators have been prepared to inhibit the shuttle effect in Se-or S-based batteries. [17,[29][30][31][32][33][34] For example, MXenes (M n+1 X n T x , where M = transition metal, X = carbon/nitrogen, n = 1, 2, and 3, and T x represents surface functional groups) modified separators have been proved to be effective in suppressing the shuttle effect due to their anisotropic shape and large 2D dimensions that increases the diffusion pathway of polysulfide/polyselenide species. [35][36][37] In addition, MXene shows strong adsorption to polysulfides due to the Lewis acid-base interaction between polysulfides and Ti sites, [38,39] which may also work for polyselenides. Nazar and co-workers have shown that the surface environment on MXene enhances such Lewis acid-base chemisorption via thiosulfate formation. [38] However, this enhanced adsorption undergoes a two-step process with sluggish kinetics. Although the shuttle effect can be mitigated, the battery showed slow redox reaction, especially under high current densities. In addition, the MXene nanosheets also suffer from restacking and poor electrolyte affinities due to the van der Waals (vdW) forces and rich hydrophilic surface groups (OH and F), respectively. [40,41] It is well known that the hydrophilic groups on MXene have low compatibility to organic electrolyte. [42] The inferior separator-electrolyte contact will lead to a poor solid-electrolyte interface (SEI) and slow electrolyte infiltration process. [43] Herein, we developed a novel self-assembled cetrimonium bromide (CTAB)/carbon nanotube (CNT)/Ti 3 C 2 T x MXene Selenium (Se), due to its high electronic conductivity and high energy density, has recently attracted considerable interest as a cathode material for rechargeable Li/Na batteries. However, the poor cycling stability originating from the severe shuttle effect of polyselenides hinders their practical applications. Herein, highly stable Li/Na-Se batteries are developed using ultrathin (≈270 nm, loading of 0.09 mg cm −2 ) cetrimonium bromide (CTAB)/carbon nanotube (CNT)/Ti 3 C 2 T x MXene hybrid modified polypropylene (PP) (CCNT/ MXene/PP) separators. The hybrid separator can immobilize the polyselenides via enhanced Lewis acid-base interactions betwee...

A nitrogen-doped carbon nanotubes/ultrathin MoS2 nanosheet core–shell architecture can chemically immobilize lithium polysulfides and catalyze the conversion of polysulfides.