Rational selection of amorphous or crystalline V2O5 cathode for sodium-ion batteries.

Vanadium oxide (V2O5), as a potential positive electrode for sodium ion batteries (SIBs), has attracted considerable attention from researchers. Herein, amorphous and crystalline V2O5 cathodes on a graphite paper without a binder and conductive additives have been synthesized via facile anodic electrochemical deposition following different heat treatments. Both the amorphous V2O5 (a-V2O5) cathode and crystalline V2O5 (c-V2O5) cathode show good rate cycling performance and long cycling life. After five rate cycles, the reversible capacities of both the cathodes were almost unchanged at different current densities from 40 to 5120 mA g-1. Long cycling tests with 10 000 cycles were carried out and the two cathodes exhibit excellent cycling stability. The c-V2O5 cathode retains a high specific capacity of 54 mA h g-1 after 10 000 cycles at 2560 mA g-1 and can be charged within 80 s. Interestingly, the a-V2O5 cathode possesses higher reversible capacities than the c-V2O5 cathode at low current densities, whereas it is inversed at high current densities. The c-V2O5 cathode shows faster capacity recovery from 5120 to 40 mA g-1 than the a-V2O5 cathode. When discharged at 80 mA g-1 (long discharge time of 140 min) and charged at 640 mA g-1 (short charge time of 17 min), the a-V2O5 cathode shows a higher discharge capacity than its c-V2O5 counterpart. The different electrochemical performance of a-V2O5 and c-V2O5 cathodes during various electrochemical processes can provide a rational selection of amorphous or crystalline V2O5 cathode materials for SIBs in their practical applications to meet the variable requirements.

Ionic liquid electrodeposition of strain-released Germanium nanowires as stable anodes for lithium ion batteries.

With the growing demand for portable and wearable electronic devices, it is imperative to develop high performance Li-ion batteries with long life times. Germanium-based materials have recently demonstrated excellent lithium-ion storage ability and are being considered as the most promising candidates for the anodes of lithium-ion batteries. Nevertheless, the practical implementation of Ge-based materials to Li-ion batteries is greatly hampered by the poor cycling ability that resulted from the huge volume variation during lithiation/delithiation processes. Herein, we develop a simple and efficient method for the preparation of Ge nanowires without catalyst nanoparticles and templates, using ionic liquid electrodeposition with subsequent annealing treatment. The Ge nanowire anode shows improved electrochemical performance compared with the Ge dense film anode. A capacity of ∼1200 mA h g-1 after 200 cycles at 0.1 C is obtained, with an initial coulombic efficiency of 81.3%. In addition, the Ge nanowire anode demonstrates superior rate capability with excellent capacity retention and stability (producing highly stable discharge capacities of about 620 mA h g-1 at 5 C). The improved electrochemical performance is the result of the enhanced electron migration and electron transport paths of the nanowires, and sufficient elasticity to buffer the volume expansion. This approach encompasses a low energy processing method where all the material is electrochemically active and binder free. The improved cycling stability and rate performance characteristics make these anodes highly attractive for the most demanding lithium-ion applications.

Assembly of flexible CoMoO4@NiMoO4·xH2O and Fe2O3 electrodes for solid-state asymmetric supercapacitors.

In this work, CoMoO4@NiMoO4·xH2O core-shell heterostructure electrode is directly grown on carbon fabric (CF) via a feasible hydrothermal procedure with CoMoO4 nanowires (NWs) as the core and NiMoO4 nanosheets (NSs) as the shell. This core-shell heterostructure could provide fast ion and electron transfer, a large number of active sites, and good strain accommodation. As a result, the CoMoO4@NiMoO4·xH2O electrode yields high-capacitance performance with a high specific capacitance of 1582 F g-1, good cycling stability with the capacitance retention of 97.1% after 3000 cycles and good rate capability. The electrode also shows excellent mechanical flexibility. Also, a flexible Fe2O3 nanorods/CF electrode with enhanced electrochemical performance was prepared. A solid-state asymmetric supercapacitor device is successfully fabricated by using flexible CoMoO4@NiMoO4·xH2O as the positive electrode and Fe2O3 as the negative electrode. The asymmetric supercapacitor with a maximum voltage of 1.6 V demonstrates high specific energy (41.8 Wh kg-1 at 700 W kg-1), high power density (12000 W kg-1 at 26.7 Wh kg-1), and excellent cycle ability with the capacitance retention of 89.3% after 5000 cycles (at the current density of 3A g-1).

Preparation of Ge nanotube arrays from an ionic liquid for lithium ion battery anodes with improved cycling stability.

Ge nanotube array anodes are prepared by template-assisted electrodeposition from an ionic liquid to obtain superior cycling performance. They show remarkable cycling ability at 0.2 C, with a very high initial discharge capacity of 1641 mA h g(-1) and a charge capacity of 1260 mA h g(-1). After 250 cycles the capacity retention is 98% relative to that at the 50th cycle.