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HIGHLIGHTS • A novel N-doped strategy of C 2 N 3 − in situ trimerization between the 2D MXene interlayers was first proposed. • The ultra-fast pseudocapacitive behavior of Ti 3 C 2 T x /Na 3 TCM anode was managed and verified. • The as-fabricated sodium-ion capacitor delivers excellent electrochemical performance by anode/cathode mass matching. ABSTRACT 2D MXenes are attractive for energy storage applications because of their high electronic conductivity. However, it is still highly challenging for improving the sluggish sodium (Na)-ion transport kinetics within the MXenes interlayers. Herein, a novel nitrogen-doped Ti 3 C 2 T x MXene was synthesized by introducing the in situ polymeric sodium dicyanamide (Na-dca) to tune the complex terminations and then utilized as intercalation-type pseudocapacitive anode of Na-ion capacitors (NICs). The Na-dca can intercalate into the interlayers of Ti 3 C 2 T x nanosheets and simultaneously form sodium tricyanomelaminate (Na 3 TCM) by the catalyst-free trimerization. The as-prepared Ti 3 C 2 T x /Na 3 TCM exhibits a high N-doping of 5.6 at.% in the form of strong TiN bonding and stabilized triazine ring structure. Consequently, coupling Ti 3 C 2 T x /Na 3 TCM anode with different mass of activated carbon cathodes, the asymmetric MXene//carbon NICs are assembled. It is able to deliver high energy density (97.6 Wh kg −1), high power output (16.5 kW kg −1), and excellent cycling stability (≈ 82.6% capacitance retention after 8000 cycles).

First-principles calculations were performed to investigate the catalytic activity of Ti 2 C MXene material as a potential cathode in Li−O 2 batteries. Stable non-, O-, F-, OHterminated Ti 2 C MXenes are constructed to show the real state of Ti 2 C MXene monolayer in the experiment. Then, the interfacial models of Li x O 2 (x = 4, 2, and 1) molecules adsorbed on Ti 2 C, Ti 2 CO 2 , Ti 2 CF 2 , and Ti 2 C(OH) 2 MXenes were used to simulate the structural evolution during discharging (oxygen reduction reaction, ORR) and charging (oxygen evolution reaction, OER) processes. The catalytic activity was quantitatively assessed by calculating the ORR and OER overpotentials. Among them, Ti 2 CO 2 MXene displays the best catalytic activity with the lowest ORR/OER/TOT overpotential, suggesting that O-terminated Ti 2 C MXene is more favorable for Li−O 2 batteries. This is because the Ti 3d orbital of the Ti 2 CO 2 surface is completely polarized near the Fermi level, showing strong oxidation capability toward O 2 2− . These findings give a theoretical guidance for Ti 2 C MXene used in Li− O 2 batteries and widen the applications of MXene-based materials.

The application prospects of lithium−sulfur (Li−S) batteries are constrained by many challenges, especially the shuttle effect of lithium polysulfides (Li 2 S x ). Recently, microporous covalent organic framework (COF) materials have been used to anchor electrodes in Li−S batteries, because of their preferable characteristics, such as self-design ability, suitable pore size, and various active groups.To identify the ideal anchoring materials that can effectively restrain the shuttle of Li 2 S x species, the anchoring mechanism between COF materials and Li 2 S x species should be investigated in depth. Therefore, we systematically investigated the anchoring mechanism between specific COF nanomaterials (consisting of boron and oxygen atoms and benzene group) and Li 2 S x (x = 1, 2, 4, 6, or 8) species on the surface and inside the pore using density functional theory methods with van der Waals interactions. The detailed analysis of the adsorption energy, difference charge density, charge transfer, and atomic density of states can be used to determine that the COF nanomaterials, with the structure of boroxine connecting to benzene groups and boroxine groups not constructed at the corner of the structure, can effectively anchor the Li 2 S x series. Accordingly, this study provides the theoretical basis for the molecular-scale design of ideal anchoring materials, which can be useful to improve the performance of the Li−S batteries.

A thermocouple based thermal monitoring system, employing a specially designed heat flux sensor to measure temperature and heat flux simultaneously, has been developed for the mould of a round billet caster. Based on the measured data, mould temperature and heat flux distributions are analszed for a wide range of casting conditions. The results show that the heat flux profiles along the mould length and around the mould circumference are frequently variable, especially in the area 70-110 mm below the meniscus where heat flux is much higher and more sensitive to operational parameters such as pouring temperature, casting speed, carbon content and mould powder. The local temperature and heat flux responses are also studied in abnormal heat transfer conditions, i.e. scale. Finally, through analysing the fluctuations of heat flux measurements, an optimised thermocouple layout is proposed. The experimental and analytical results lay the foundation for an intelligent mould with online detection of defects, adjustment of operational parameters, optimisation of the monitoring system and even prediction of abnormal heat transfer.

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The outstanding advantages of lithium−sulfur (Li− S) batteries have made them a potential energy storage device. However, shuttle effect is one of the main problems that restrict the commercial application of Li−S batteries. Apart from it, accelerating the dissociation of Li 2 S to LiS and Li + is also very important to achieve high Coulombic efficiency. Recently, it has been found that metal atoms exhibit strong interactions with lithium polysulfides (Li 2 S x ), which can be used as single-atom catalysts by embedding in the matrix. Covalent organic framework (COF) materials have porous structures and large surface area, which can anchor Li 2 S x by introducing active sites. In this study, we constructed metal phthalocyanine COFs (MPc-COFs, M = Ti, V, Mn, Cu, and Zn) as the cathode, which combined the advantages of metal atoms and COFs. The adsorption and catalysis performance of Li 2 S x species were investigated by MPc-COF cathode using density functional theory. The results show that the strong adsorption capacity of TiPc-COF and VPc-COF and the formation of axial complexes make them unfavorable for Li−S batteries. MnPc-COFs show excellent conductivity and sulfur fixation capacity, which also have a lower energy barrier in the catalytic oxidation of Li 2 S. This work has a guiding significance in the design of catalytic electrodes for Li−S batteries.