Two-dimensional transition-metal carbide materials (termed MXene) have attracted huge attention in the field of electrochemical energy storage due to their excellent electrical conductivity, high volumetric capacity, etc. Herein, with inspiration from the interesting structure of pillared interlayered clays, we attempt to fabricate pillared TiC MXene (CTAB-Sn(IV)@TiC) via a facile liquid-phase cetyltrimethylammonium bromide (CTAB) prepillaring and Sn pillaring method. The interlayer spacing of TiC MXene can be controlled according to the size of the intercalated prepillaring agent (cationic surfactant) and can reach 2.708 nm with 177% increase compared with the original spacing of 0.977 nm, which is currently the maximum value according to our knowledge. Because of the pillar effect, the assembled LIC exhibits a superior energy density of 239.50 Wh kg based on the weight of CTAB-Sn(IV)@TiC even under higher power density of 10.8 kW kg. When CTAB-Sn(IV)@TiC anode couples with commercial AC cathode, LIC reveals higher energy density and power density compared with conventional MXene materials.

Two-dimensional transition metal carbide materials called MXenes show potential application for energy storage due to their remarkable electrical conductivity and low Li(+) diffusion barrier. However, the lower capacity of MXene anodes limits their further application in lithium-ion batteries. Herein, with inspiration from the unique metal ion uptake behavior of highly conductive Ti3C2 MXene, we overcome this impediment by fabricating Sn(4+) ion decorated Ti3C2 nanocomposites (PVP-Sn(IV)@Ti3C2) via a facile polyvinylpyrrolidone (PVP)-assisted liquid-phase immersion process. An amorphous Sn(IV) nanocomplex, about 6-7 nm in lateral size, has been homogeneously anchored on the surface of alk-Ti3C2 matrix by ion-exchange and electrostatic interactions. In addition, XRD and TEM results demonstrate the successful insertion of Sn(4+) into the interlamination of an alkalization-intercalated Ti3C2 (alk-Ti3C2) matrix. Due to the possible "pillar effect" of Sn between layers of alk-Ti3C2 and the synergistic effect between the alk-Ti3C2 matrix and Sn, the nanocomposites exhibit a superior reversible volumetric capacity of 1375 mAh cm(-3) (635 mAh g(-1)) at 216.5 mA cm(-3) (100 mA g(-1)), which is significantly higher than that of a graphite electrode (550 mAh cm(-3)), and show excellent cycling stability after 50 cycles. Even at a high current density of 6495 mA cm(-3) (3 A g(-1)), these nanocomposites retain a stable specific capacity of 504.5 mAh cm(-3) (233 mAh g(-1)). These results demonstrate that PVP-Sn(IV)@Ti3C2 nanocomposites offer fascinating potential for high-performance lithium-ion batteries.

Sodium (Na) metal is a promising alternative to lithium metal as an anode material for the next-generation energy storage systems due to its high theoretical capacity, low cost, and natural abundance. However, dendritic/mossy Na growth caused by uncontrollable plating/stripping results in serious safe concerns and rapid electrode degradation. This study presents Sn 2+ pillared Ti 3 C 2 MXene serving as a stable matrix for high-performance dendrite-free Na metal anode. The intercalated Sn 2+ between Ti 3 C 2 layers not only induces Na to nucleate and grow within Ti 3 C 2 interlayers, but also endows the Ti 3 C 2 with larger interlayer space to accommodate the deposited Na by taking advantage of the "pillar effect," contributing to uniform Na deposition. As a result, the pillar-structured MXene-based Na metal electrode could enable high current density (up to 10 mA cm −2 ) along with high areal capacity (up to 5 mAh cm −2 ) over long-term cycling (up to 500 cycles). The full cell using MXene-based Na metal anode exhibits superior electrochemical performance than that using host-less commercial Na. It is believed that the well-controlled MXene-based Na anode not only extends the application scope of MXene, but also provides guidance in designing high-performance Na metal batteries.

No abstract

High ionic conductivity, satisfactory mechanical properties, and wide electrochemical windows are crucial factors for composite electrolytes employed in solid-state lithium-ion batteries (SSLIBs). Based on these considerations, we fabricate MgBO nanowire enabled poly(ethylene oxide) (PEO)-based solid-state electrolytes (SSEs). Notably, these SSEs have enhanced ionic conductivity and a large electrochemical window. The elevated ionic conductivity is attributed to the improved motion of PEO chains and the increased Li migrating pathway on the interface between MgBO and PEO-LiTFSI. Moreover, the interaction between MgBO and -SO- in TFSI anions could also benefit the improvement of conductivity. In addition, the SSEs containing MgBO nanowires exhibit improved the mechanical properties and flame-retardant performance, which are all superior to the pristine PEO-LiTFSI electrolyte. When these multifunctional SSEs are paired with LiFePO cathodes and lithium metal anodes, the SSLIBs show better rate performance and higher cyclic capacity of 150, 106, and 50 mAh g under 0.2 C at 50, 40, and 30 °C. This strategy of employing MgBO nanowires provides the design guidelines of assembling multifunctional SSLIBs with high ionic conductivity, excellent mechanical properties, and flame-retardant performance at the same time.

This review assesses both theoretical and experimental knowledge on sodium nucleation for the first time towards a safe sodium battery.

2D MXenes have been widely applied in the field of electrochemical energy storage owning to their high electrical conductivity and large redox‐active surface area. However, electrodes made from multilayered MXene with small interlayer spacing exhibit sluggish kinetics with low capacity for sodium‐ion storage. Herein, Ti3C2 MXene with expanded and engineered interlayer spacing for excellent storage capability is demonstrated. After cetyltrimethylammonium bromide pretreatment, S atoms are successfully intercalated into the interlayer of Ti3C2 to form a desirable interlayer‐expanded structure via TiS bonding, while pristine Ti3C2 is hardly to be intercalated. When the annealing temperature is 450 °C, the S atoms intercalated Ti3C2 (CT‐S@Ti3C2‐450) electrode delivers the improved Na‐ion capacity of 550 mAh g−1 at 0.1 A g−1 (≈120 mAh g−1 at 15 A g−1, the best MXene‐based Na+‐storage rate performance reported so far), and excellent cycling stability over 5000 cycles at 10 A g−1 by enhanced pseudocapacitance. The enhanced sodium‐ion storage capability has also been verified by theoretical calculations and kinetic analysis. Coupling the CT‐S@Ti3C2‐450 anode with commercial AC cathode, the assembled Na+ capacitor delivers high energy density (263.2 Wh kg−1) under high power density (8240 W kg−1), and outstanding cycling performance.

The use of reticular materials in the electrochemical reduction of carbon dioxide to value-added products has the potential to enable tunable control of the catalytic performance through the modulation of chemical and structural features of framework materials with atomic precision. However, the tunable functional performance of such systems is still largely hampered by their poor electrical conductivities. This work demonstrates the use of four systematic structural analogs of conductive two-dimensional (2D) metal–organic frameworks (MOFs) made of metallophthalocyanine (MPc) ligands linked by Cu nodes with electrical conductivities of 2.73 × 10–3 to 1.04 × 10–1 S cm–1 for the electrochemical reduction of CO2 to CO. The catalytic performance of the MOFs, including the activity and selectivity, is found to be hierarchically governed by two important structural factors: the metal within the MPc (M = Co vs Ni) catalytic subunit and the identity of the heteroatomic cross-linkers between these subunits (X = O vs NH). The activity and selectivity are dominated by the choice of metal within MPcs and are further modulated by the heteroatomic linkages. Among these MOFs, CoPc–Cu–O exhibited the highest selectivity toward CO product (Faradaic efficiency FECO = 85%) with high current densities up to −17.3 mA cm–2 as a composite with carbon black at 1:1 mass ratio) at a low overpotential of −0.63 V. Without using any conductive additives, the use of CoPc–Cu–O directly as an electrode material was able to achieve a current density of −9.5 mA cm–2 with a FECO of 79%. Mechanistic studies by comparison tests with metal-free phthalocyanine MOF analogs supported the dominant catalytic role of the central metal of the phthalocyanine over Cu nodes. Density-functional theory calculations further suggested that, compared with the NiPc-based and NH-linked analogs, CoPc-based and O-linked MOFs have lower activation energies in the formation of carboxyl intermediate, in line with their higher activities and selectivity. The results of this study indicate that the use of 2D MPc-based conductive framework materials holds great promise for achieving efficient CO2 reduction through strategic ligand engineering with multiple levels of tunability.