Multiple negative factors, including the poor electronic conductivity of sulfur, dissolution and shuttling of lithium polysulfides (Li2S n ), and sluggish decomposition of solid Li2S, seriously hinder practical applications of lithium–sulfur (Li–S) batteries. To solve these problems, a general strategy was proposed for enhancing the electrochemical performance of Li–S batteries using surface-functionalized Ti3C2 MXenes. Functionalized Ti3C2T2 (T = N, O, F, S, and Cl) showed metallic conductivity, as bare Ti3C2. Among all Ti3C2T2 investigated, Ti3C2S2, Ti3C2O2, and Ti3C2N2 offered moderate adsorption strength, which effectively suppressed Li2S n dissolution and shuttling. This Ti3C2T2 exhibited effective electrocatalytic ability for Li2S decomposition. The Li2S decomposition barrier was significantly decreased from 3.390 eV to ∼0.4 eV using Ti3C2S2 and Ti3C2O2, with fast Li+ diffusivity. Based on these results, O- and S-terminated Ti3C2 were suggested as promising host materials for S cathodes. In addition, appropriate functional group vacancies could further promote anchoring and catalytic abilities of Ti3C2T2 to boost the electrochemical performance of Li–S batteries. Moreover, the advantages of a Ti3C2T2 host material could be well retained even at high S loading, suggesting the potential of surface-modified MXene for confining sulfur in Li–S battery cathodes.

Sodium (Na) metal batteries have attracted increasing attention and gained rapid development. However, the processing, storing, and application of Na metal anodes are restricted by its inherent stickiness and poor mechanical properties. Herein, an MXene (Ti 3 C 2 T x )-coated carbon cloth (Ti 3 C 2 T x -CC) host is designed and synthesized, which shows a highly metallic conductive and sodiophilic surface. After a thermal infusion treatment, a Na-Ti 3 C 2 T x -CC composite with rigidity and bendability is obtained and employed as a metal anode for Na ion batteries. The Na-Ti 3 C 2 T x -CC electrodes present stable cycling performance and high stripping/plating capacity in both an ether-based (up to 5 mA•h•cm −2 ) and a carbonate-based (up to 8 mA•h•cm −2 ) electrolyte. The fundamental protection mechanism of MXene Ti 3 C 2 T x is investigated. Ti 3 C 2 T x efficiently induces Na's initial nucleation and laterally oriented deposition, which effectively avoids the generation of mossy/dendritic Na. The arrangement of Na atoms deposited on the MXene surface inherits the MXene atomic architecture, leading to a smooth "sheet-like" Na surface. Meanwhile, a flexible Na-based Na-Ti 3 C 2 T x -CC∥Na 3 V 2 (PO 4 ) 3 device is assembled and exhibits capable electrochemical performance. KEYWORDS: sodium metal anode, MXene Ti 3 C 2 T x , induction effect, first-principles calculation, transverse growth

Metallic anodes have high theoretical specific capacities and low electrochemical potentials. However, short-circuit problems caused by dendritic deposition and low Coulombic efficiency limit the cyclic life and safety of metallic anode-based batteries. Herein, dendrite-free and flexible three-dimensional (3D) alkali anodes (Li/Na-Ti 3 C 2 T x -rGO) are constructed by infusing molten lithium (Li) or sodium (Na) metal into 3D porous MXene Ti 3 C 2 T xreduced graphene oxide (Ti 3 C 2 T x -rGO) membranes. Firstprinciples calculations indicate that large fractions of functional groups on the Ti 3 C 2 T x surface lead to the good affinity between the Ti 3 C 2 T x -rGO membrane and molten alkali metal (Li/Na), and the formation of Ti-Li/ Na, O-Li/Na, and F-Li/Na mixed covalent/ionic bonds is extremely critical for uniform electrochemical deposition. Furthermore, the porous structure in Li/Na-Ti 3 C 2 T x -rGO composites results in an effective encapsulation, preventing dendritic growth and exhibiting stable stripping/plating behaviors up to 12 mA•cm −2 and a deeper capacity of 10 mA•h• cm −2 . Stable cycling performances over 300 h (750 cycles) at 5.0 mA•cm −2 for Li-Ti 3 C 2 T x -rGO and 500 h (750 cycles) at 3.0 mA•cm −2 for Na-Ti 3 C 2 T x -GO are achieved. In a full cell with LiFePO 4 cathodes, Li-Ti 3 C 2 T x -rGO electrodes show low polarization and retain 96.6% capacity after 1000 cycles. These findings are based on 2D MXene materials, and the resulting 3D host provides a practical approach for achieving stable and safe alkali metal anodes. KEYWORDS: MXene-Ti 3 C 2 T x , 3D porous film, first-principles calculations, metal anodes (Li/Na), thermal infusion, encapsulation effect

Nb2CF2–VF–Pt is confirmed to be the best bifunctional catalyst toward ORR and OER, with relative low theoretical overpotentials (0.40 V for ORR and 0.37 V for OER).