Unique layered Ti3C2/Ni–Co–Al layered double hydroxide (LDH) heterostructures alternatively stacked with molecular-level nanosheets are for the first time synthesized by a facile liquid-phase cofeeding and electrostatic attraction heteroassemble strategy between negatively charged Ti3C2 and positively charged Ni–Co–Al-LDH nanosheets. The molecular-level Ti3C2/Ni–Co–Al-LDH heterostructures possessing the merits of both conductive and pseudocapacitive components can show greatly enhanced dynamic behavior in Faradaic reaction, which is significant for obtaining a high power density. Electrons penetrate in Ti3C2 layers, while ions diffuse rapidly along two-dimensional galleries, displaying the shortest diffusion pathway and highest efficiency for charge transfer. The Ti3C2/Ni–Co–Al-LDH heterostructure exhibits a specific capacitance of 748.2 F g–1 at current density of 1 A g–1, showing an enhanced rate capacity. Importantly, a maximum energy density of 45.8 Wh kg–1 is obtained when Ti3C2/Ni–Co–Al-LDH acts as the positive electrode for an all-solid-state flexible asymmetric supercapacitor. The results indicate that molecular-level heterotructure is a promising candidate for future high-energy supercapacitors.

Two types of hard carbon materials were synthesized through direct pyrolysis of commercial polyvinyl chloride (PVC) particles and pyrolysis of PVC nanofibers at 600-800 °C, respectively, where the nanofibers were prepared by an electrospinning PVC precursors method. These as-prepared hard carbon samples were used as anode materials for Na-ion batteries. The hard carbon obtained from PVC nanofibers achieved a high reversible capacity of 271 mAh/g and an initial Coulombic efficiency of 69.9%, which were much superior to the one from commercial PVC, namely, a reversible capacity of 206 mAh/g and an initial Coulombic efficiency of 60.9%. In addition, the hard carbon originated from the PVC nanofibers exhibited good cycling stability and rate performance: the initial discharge capacities were 389, 228, 194, 178, 147 mAh/g at the current density of 12, 24, 60, 120, and 240 mA/g, respectively, retaining 211 mAh/g after 150 cycles. Such excellent cycle performance, high reversible capacity, and good rate capability enabled this hard carbon to be a promising candidate as anode material for Na-ion battery application.

Na 3 V 2Àx Mg x (PO 4 ) 3 /C composites with different Mg 2+ doping contents (x ¼ 0, 0.01, 0.03, 0.05, 0.07 and 0.1) were prepared by a facile sol-gel method. The doping effects on the crystal structure were investigated by XRD, XPS and EXAFS. The results show that low dose doping of Mg 2+ does not alter the structure of the material, and magnesium is successfully substituted for the vanadium site. The Mg doped Na 3 V 2Àx Mg x (PO 4 ) 3 /C composites exhibit significant improvements on the electrochemical performance in terms of the rate capability and cycle performance, especially for the Na 3 V 1.95 Mg 0.05 (PO 4 ) 3 /C. For example, when the current density increased from 1 C to 30 C, the specific capacity only decreased from 112.5 mA h g À1 to 94.2 mA h g À1 showing very good rate capability. Moreover, even cycling at a high rate of 20 C, an excellent capacity retention of 81% is maintained from the initial value of 106.4 mA h g À1 to 86.2 mA h g À1 at the 50th cycle. Enhanced rate capability and cycle performance can be attributed to the optimized particle size, structural stability and enhanced ionic and electronic conductivity induced by Mg doping.batteries for large-scale energy storage system applications is very important. 7 Recently, sodium ion batteries have gained an increasing amount of attention due to their abundant reserves and relatively even geological distribution. 8 Actually, many electrode materials such as Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ), 9 Na[Ni 0.25 Fe 0.5 Mn 0.25 ]O 2 / C, 10 Na[Li 0.05 (Ni 0.25 Fe 0.25 Mn 0.5 ) 0.95 ]O 2 , 11 Na x CoO 2 , 12 Na 2 C 8 H 4 O 4 , 13 Na 3 (VO 1Àx PO 4 ) 2 F 1+2x , 14 Prussian blue analogues, 15,16 phosphorus, 17,18 and TiO 2 19 have been studied as active materials in cathodes and anodes for sodium ion batteries. However, due to the bigger ionic radius of the sodium ion than the lithium ion (1.02Å for Na + vs. 0.76Å for Li + ), its storage performance and cycle performance are much poorer than its lithium counterpart. The larger Na + radius is proved to be a crucial obstacle for Na + diffusion. 20,21 Therefore, improving Na + diffusion is quite important in developing new electrode materials for sodium-ion batteries with good electrochemical performance.NASICON-type Na 3 V 2 (PO 4 ) 3 has recently been investigated as a prospective cathode material for sodium ion batteries. It is worth noting that Na 3 V 2 (PO 4 ) 3 possesses the highly covalent three dimensional framework that generates large interstitial spaces through which sodium ions may diffuse easily. [22][23][24][25][26][27] In addition, the electrochemical response of the Na 3 V 2 (PO 4 ) 3 electrode displays two at plateaus at 3.4 V and 1.6 V vs. Na + /Na; the voltage plateau located at 3.4 V is relatively higher than most other cathode materials for sodium ion batteries in recent reports. 28,29 However, Na 3 V 2 (PO 4 ) 3 also has an inherent deciency. The distorted VO 6 octahedral units in the NASICON † Electronic supplementary information (ESI) available: SEM images of Na 3 V 2Àx Mg x (PO 4 ) ...

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