Wenhao Ren

General synthesis of complex nanotubes by gradient electrospinning and controlled pyrolysis

Nanowires and nanotubes have been the focus of considerable efforts in energy storage and solar energy conversion because of their unique properties. However, owing to the limitations of synthetic methods, most inorganic nanotubes, especially for multi-element oxides and binary-metal oxides, have been rarely fabricated. Here we design a gradient electrospinning and controlled pyrolysis method to synthesize various controllable 1D nanostructures, including mesoporous nanotubes, pea-like nanotubes and continuous nanowires. The key point of this method is the gradient distribution of low-/middle-/high-molecular-weight poly(vinyl alcohol) during the electrospinning process. This simple technique is extended to various inorganic multi-element oxides, binary-metal oxides and single-metal oxides. Among them, Li3V2(PO4)3, Na0.7Fe0.7Mn0.3O2 and Co3O4 mesoporous nanotubes exhibit ultrastable electrochemical performance when used in lithium-ion batteries, sodium-ion batteries and supercapacitors, respectively. We believe that a wide range of new materials available from our composition gradient electrospinning and pyrolysis methodology may lead to further developments in research on 1D systems.

Emerging Prototype Sodium‐Ion Full Cells with Nanostructured Electrode Materials

Due to steadily increasing energy consumption, the demand of renewable energy sources is more urgent than ever. Sodium-ion batteries (SIBs) have emerged as a cost-effective alternative because of the earth abundance of Na resources and their competitive electrochemical behaviors. Before practical application, it is essential to establish a bridge between the sodium half-cell and the commercial battery from a full cell perspective. An overview of the major challenges, most recent advances, and outlooks of non-aqueous and aqueous sodium-ion full cells (SIFCs) is presented. Considering the intimate relationship between SIFCs and electrode materials, including structure, composition and mutual matching principle, both the advance of various prototype SIFCs and the electrochemistry development of nanostructured electrode materials are reviewed. It is noted that a series of SIFCs combined with layered oxides and hard carbon are capable of providing a high specific gravimetric energy above 200 Wh kg-1 , and an NaCrO2 //hard carbon full cell is able to deliver a high rate capability over 100 C. To achieve industrialization of SIBs, more systematic work should focus on electrode construction, component compatibility, and battery technologies.

An electrospun hierarchical LiV3O8 nanowire-in-network for high-rate and long-life lithium batteries

Structural and morphological control of the LiV3O8 material has a significant impact on its electrochemical performance. In order to obtain a favorable structure, a hierarchical LiV3O8 nanowire-in-network is designed and constructed by electrospinning through a polymer crosslinking strategy. The crosslinking effect between poly(vinyl alcohol) (PVA) and poly(ethylene oxide) (PEO) not only benefits electrospinning, but also realizes a mild multi-step degradation process during calcination. Based on temperature-dependent experiments and thermogravimetric (TG) analysis, the function of polymer blends and the formation mechanism of the structure are discussed in detail. As a cathode for lithium batteries, LiV3O8 exhibits a high initial capacity of 320.6 mA h g−1 at 100 mA g−1 and a high-rate capacity of 202.8 mA h g−1 at 2000 mA g−1. This remarkable performance is attributed to its unique structure, which provides a large effective contact area, low charge transfer resistance, and improved structural stability. Our work indicates that the hierarchical LiV3O8 nanowire-in-network material is a promising cathode for use in high-rate and long-life rechargeable lithium batteries.

Activation of Sodium Storage Sites in Prussian Blue Analogues via Surface Etching

Sodium-ion battery technologies are known to suffer from kinetic problems associated with the solid-state diffusion of Na+ in intercalation electrodes, which results in suppressed specific capacity and degraded rate performance. Here, a controllable selective etching approach is developed for the synthesis of Prussian blue analogue (PBA) with enhanced sodium storage activity. On the basis of time-dependent experiments, a defect-induced morphological evolution mechanism from nanocube to nanoflower structure is proposed. Through in situ X-ray diffraction measurement and computational analysis, this unique structure is revealed to provide higher Na+ diffusion dynamics and negligible volume change during the sodiation/desodiation processes. As a sodium ion battery cathode, the PBA exhibits a discharge capacity of 90 mA h g-1, which is in good agreement with the complete low spin FeLS(C) redox reaction. It also demonstrates an outstanding rate capability of 71.0 mA h g-1 at 44.4 C, as well as an unprecedented cycling reversibility over 5000 times.

Alkaline earth metal vanadates as sodium-ion battery anodes

The abundance of sodium resources indicates the potential of sodium-ion batteries as emerging energy storage devices. However, the practical application of sodium-ion batteries is hindered by the limited electrochemical performance of electrode materials, especially at the anode side. Here, we identify alkaline earth metal vanadates as promising anodes for sodium-ion batteries. The prepared calcium vanadate nanowires possess intrinsically high electronic conductivity (> 100 S cm−1), small volume change (< 10%), and a self-preserving effect, which results in a superior cycling and rate performance and an applicable reversible capacity (> 300 mAh g−1), with an average voltage of ∼1.0 V. The specific sodium-storage mechanism, beyond the conventional intercalation or conversion reaction, is demonstrated through in situ and ex situ characterizations and theoretical calculations. This work explores alkaline earth metal vanadates for sodium-ion battery anodes and may open a direction for energy storage.The development of suitable anode materials is essential to advance sodium-ion battery technologies. Here the authors report that alkaline earth metal vanadates are promising candidates due to the favorable electrochemical properties and interesting sodium-storage mechanism.

Hollow spherical LiNi0.5Mn1.5O4 built from polyhedra with high-rate performance via carbon nanotube modification

Lithium nickel manganese oxide spinel (LiNi0.5Mn1.5O4, LNMO) has attracted much attention as the cathode material for rechargeable lithium-ion batteries due to its high energy density and low cost. However, the short cycle life and poor high-rate capability hinder its commercialization. In this study, we synthesized hollow spherical LNMO built from polyhedral particles. The LNMO hollow structure guarantees sufficient contact with electrolyte and rapid diffusion of lithium ions. To enhance the conductivity, we use carbon nanotubes (CNTs) to modify the surface of the cathode. After CNT modification, the LNMO hollow structure manifests outstanding cycling stability and high-rate capability. It delivers a discharge capacity of 127 mA h g−1 at 5 C, maintaining 104 mA h g−1 after 500 cycles. Even at a high rate of 20 C, a capacity of 121 mA h g−1 can be obtained. The excellent electrochemical performance is ascribed to the unique structure and the enhanced conductivity through CNT modification. It is demonstrated that the CNT-modified hollow spherical LNMO is a promising cathode for lithium ion batteries.摘要本文通过调节烧结温度设计构筑了一种纳米多面体颗粒堆积的中空球状LiNi0.5Mn1.5O4材料, 并进一步通过碳纳米管(CNT)的改性来提高材料的循环性能和高倍率性能. 纳米中空结构不仅减少了锂离子的扩散路径, 也保证了电解液和正极材料的充分接触, 三维网状CNT的 改性提高了材料的电子导电率, 从而明显改善了材料的循环和高倍率性能. 最终得到的LNMO-850/CNT材料在5 C的电流密度下初始容量为 127 mA h g−1, 500次循环后容量保持在104 mA h g−1. 而在20 C的高电流密度下容量仍达到121 mA h g−1, 体现了材料优异的循环和高倍率性能.

3.0 V High Energy Density Symmetric Sodium-Ion Battery: Na4V2(PO4)3∥Na3V2(PO4)3

Symmetric sodium-ion batteries (SIBs) are considered as promising candidates for large-scale energy storage owing to the simplified manufacture and wide abundance of sodium resources. However, most symmetric SIBs suffer from suppressed energy density. Here, a superior congeneric Na4V2(PO4)3 anode is synthesized via electrochemical preintercalation, and a high energy density symmetric SIB (Na3V2(PO4)3 as a cathode and Na4V2(PO4)3 as an anode) based on the deepened redox couple of V4+/V2+ is built for the first time. When measured in half cell, both electrodes show stabilized electrochemical performance (over 3000 cycles). The symmetric SIBs exhibit an output voltage of 3.0 V and a cell-level energy density of 138 W h kg-1. Furthermore, the sodium storage mechanism under the expanded measurement range of 0.01-3.9 V is disclosed through an in situ X-ray diffraction technique.

Nonhierarchical Heterostructured Fe2O3/Mn2O3 Porous Hollow Spheres for Enhanced Lithium Storage

High capacity transition-metal oxides play significant roles as battery anodes benefiting from their tunable redox chemistry, low cost, and environmental friendliness. However, the application of these conversion-type electrodes is hampered by inherent large volume variation and poor kinetics. Here, a binary metal oxide prototype, denoted as nonhierarchical heterostructured Fe2 O3 /Mn2 O3 porous hollow spheres, is proposed through a one-pot self-assembly method. Beyond conventional heteromaterial, Fe2 O3 /Mn2 O3 based on the interface of (104)Fe2O3 and (222)Mn2O3 exhibits the nonhierarchical configuration, where nanosized building blocks are integrated into microsized spheres, leading to the enhanced structural stability and boosted reaction kinetics. With this design, the Fe2 O3 /Mn2 O3 anode shows a high reversible capacity of 1075 mA h g-1 at 0.5 A g-1 , an outstanding rate capability of 638 mA h g-1 at 8 A g-1 , and an excellent cyclability with a capacity retention of 89.3% after 600 cycles.