Aqueous MnV2 O6 -Zn Battery with High Operating Voltage and Energy Density.

Aqueous Zn ion batteries (AZIBs), featuring low cost, long-term cycling stability, and superior safety are promising for applications in advanced energy storage devices. However, they still suffer from unsatisfactory energy density and operating voltage, which are closely related to cathode materials used. Herein, the use of monoclinic MnV2 O6 (MVO) is reported, which can be activated for high-capacity Zn ions storage by electrochemically oxidizing part of the Mn2+ to Mn3+ or Mn4+ while the remaining Mn2+ ions act as binders/pillars to hold the layer structure of MVO and maintain its integrity during charging/discharging process. Moreover, after introducing carbon nanotubes (CNT), the MVO:CNT composite not only provides robust 3D Zn-ion diffusion channels but also shows enhanced structural integrity. As a result, a MVO:CNT cathode delivers a high midpoint voltage (1.38 V after 3000 cycles at 2 A g-1 ) and a high energy density of 597.9 W h kg-1 . Moreover, DFT analyses clearly illustrate stepwise Zn ion insertion into the MnV2 O6 lattice, and ex-situ analyses results further verify the highly structural reversibility of the MnV2 O6 cathode upon extended cycling, demonstrating the good potential of MnV2 O6 for the establishment of viable aqueous Zn ion battery systems.

Porous BN Nanofibers Enable Long-Cycling Life Sodium Metal Batteries.

Sodium metal anode, featuring high capacity, low voltage and earth abundance, is desirable for building advanced sodium-metal batteries. However, Na-ion deposition typically leads to morphological instability and notorious chemical reactivity between sodium and common electrolytes still limit its practical application. In this study, a porous BN nanofibers modified sodium metal (BN/Na) electrode is introduced for enhancing Na-ion deposition dynamics and stability. As a result, symmetrical BN/Na cells enable an impressive rate capability and markedly enhanced cycling durability over 600 h at 10 mA cm-2 . Density functional theory simulations demonstrate BN could effectively improve Na-ion adsorption and diffusion kinetics simultaneously. Finite element simulation clearly reveals the intrinsic smoothing effect of BN upon multiple Na-ion plating/stripping cycles. Coupled with a Na3 V2 O2 (PO4 )2 F/Ti3 C2 X cathode, sodium metal full cells offer an ultrastable capacity of 125/63 mA h g-1 (≈420/240 Wh kg-1 ) at 0.05/5 C rate over 500 cycles. These comprehensive analyses demonstrate the feasibility of BN/Na anode for the establishment of high-energy-density sodium-metal full batteries.

Surface-Engineered Black Niobium Oxide@Graphene Nanosheets for High-Performance Sodium-/Potassium-Ion Full Batteries.

Nanoscale surface-engineering plays an important role in improving the performance of battery electrodes. Nb2 O5 is one typical model anode material with promising high-rate lithium storage. However, its modest reaction kinetics and low electrical conductivity obstruct the efficient storage of larger ions of sodium or potassium. In this work, partially surface-amorphized and defect-rich black niobium oxide@graphene (black Nb2 O5- x @rGO) nanosheets are designed to overcome the above Na/K storage problems. The black Nb2 O5- x @rGO nanosheets electrodes deliver a high-rate Na and K storage capacity (123 and 73 mAh g-1 , respectively at 3 A g-1 ) with long-term cycling stability. Besides, both Na-ion and K-ion full batteries based on black Nb2 O5- x @rGO nanosheets anodes and vanadate-based cathodes (Na0.33 V2 O5 and K0.5 V2 O5 for Na-ion and K-ion full batteries, respectively) demonstrate promising rate and cycling performance. Notably, the K-ion full battery delivers higher energy and power densities (172 Wh Kg-1 and 430 W Kg-1 ), comparable to those reported in state-of-the-art K-ion full batteries, accompanying with a capacity retention of ≈81.3% over 270 cycles. This result on Na-/K-ion batteries may pave the way to next-generation post-lithium batteries.

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.

3D Ag@C Cloth for Stable Anode Free Sodium Metal Batteries.

While sodium metal anodes (SMAs) feature many performance advantages in sodium ion batteries (SIBs), severe safety concerns remain for using bulk sodium electrodes. Herein, a 3D Ag@C natrophilic substrate prepared by a facile thermal evaporation deposition method, which can be employed as a much safer "anode-free" SMA, is reported. Initially, there is no bulk sodium on the Ag@C substrate in the assembled SIBs. Upon charging, sodium will be uniformly deposited onto the Ag@C substrate and afterwards functions as a real SMA, thus inheriting the intrinsic merits of SMA and enhancing safety simultaneously. While cycling, the as-synthesized substrate demonstrates superior sodium plating/stripping cycling stability at 1, 2 and 3 mA cm-2 with a capacity of 2 mAh cm-2 . Theoretical simulations reveal that Na ions prefer to bind with Ag and form a Na-Ag network, thus clearly revealing uniform sodium deposition on the Ag@C substrate. More importantly, a full battery based on Ag@C and Prussian white with impressive Coulomb efficiency (CE), high rate capability (from 0.1 C to 5 C) and long-term cycling life is illustrated for the first time, thus making Ag@C feasible for the establishment of "anode-free" SIBs with reduced cost, high gravimetric/volumetric energy density and enhanced safety.

Mechanisms of sodiation in anatase TiO2 in terms of equilibrium thermodynamics and kinetics.

Anatase TiO2 is a promising anode material for sodium-ion batteries (SIBs). However, its sodium storage mechanisms in terms of crystal structure transformation during sodiation/de-sodiation processes are far from clear. Here, by analyzing the redox thermodynamics and kinetics under near-equilibrium states, we observe, for the first time, that upon Na-ion uptake, the anatase TiO2 undergoes a phase transition and then an irreversible crystal structure disintegration. Additionally, unlike previous theoretical studies which investigate only the two end points of the sodiation process (i.e., TiO2 and NaTiO2), we study the progressive crystal structure changes of anatase TiO2 upon step-by-step Na-ion uptake (Na x TiO2, x = 0.0625, 0.125, 0.25, 0.5, 0.75, and 1) for the first time. It is found that the anatase TiO2 goes through a thermodynamically unstable intermediate phase (Na0.25TiO2) before reaching crystalline NaTiO2, confirming the inevitable crystal structure disintegration during sodiation. These combined experimental and theoretical studies provide new insights into the sodium storage mechanisms of TiO2 and are expected to provide useful information for further improving the performance of TiO2-based anodes for SIB applications.

Achieving rapid Li-ion insertion kinetics in TiO2 mesoporous nanotube arrays for bifunctional high-rate energy storage smart windows.

Smart electrochromic windows integrated with electrochemical energy storage capacity are receiving increasing interest for green buildings. However, the fabrication of bifunctional devices that demonstrate high-rate capability with stable and desirable optical modulation still remains a great challenge. Herein, a facile sacrificial template-accelerated hydrolysis approach is presented to prepare a designed lithium-ion insertion-type material layer on a fluorine-doped tin oxide substrate, with TiO2 mesoporous nanotube array (MNTA) film as an example, with rapid Li-ion insertion kinetics and without sacrificing window transparency, to meet requirements. A bifunctional device is assembled to exhibit the optical-electrochemical superiority of MNTA nanostructures. The as-assembled bifunctional smart window exhibits strong electrochromic contrast and high-rate capability in the fast galvanostatic charge/discharge process. For instance, at 1 A g-1, it completes the charge or discharge process within only 232 s and delivers a high, reversible and stable specific capacity of 60 mA h g-1, accompanying obvious transmittance modulation in the visible spectrum, with a typical value of ca. 30.4% at 700 nm, and strong color changes between deep blue and transparency.

Self-supported one-dimensional materials for enhanced electrochromism.

A reversible, persistent electrochromic change in color or optical parameter controlled by a temporarily applied electrical voltage is attractive because of its enormous display and energy-related applications. Due to the electrochemical and structural advantages, electrodes based on self-supported one-dimensional (1D) nanostructured materials have become increasingly important, and their impacts are particularly significant when considering the ease of assembly of electrochromic devices. This review describes recent advances in the development of self-supported 1D nanostructured materials as electrodes for enhanced electrochromism. Current strategies for the design and morphology control of self-supported electrodes fabricated using templates, anodization, vapor deposition, and solution techniques are outlined along with demonstrating the influences of nanostructures and components on the electrochemical redox kinetics and electrochromic performance. The applications of self-supported 1D nanomaterials in the emerging bifunctional devices are further illustrated.

Novel morphology changes from 3D ordered macroporous structure to V2O5 nanofiber grassland and its application in electrochromism.

Because vanadium pentoxide (V2O5) is the only oxide that shows both anodic and cathodic coloration electrochromism, the reversible lithium ion insertion/extraction processes in V2O5 lead to not only reversible optical parameter changes but also multicolor changes for esthetics. Because of the outstanding electrochemical properties of V2O5 nanofibers, they show great potential to enhance V2O5 electrochromism. However, the development and practical application of V2O5 nanofibers are still lacking, because traditional preparation approaches have several drawbacks, such as multiple processing steps, unsatisfactory electrical contact with the substrate, expensive equipment, and rigorous experimental conditions. Herein, we first report a novel and convenient strategy to prepare grass-like nanofiber-stacked V2O5 films by a simple annealing treatment of an amorphous, three-dimensionally ordered macroporous vanadia film. The V2O5 nanofiber grassland exhibits promising transmittance modulation, fast switching responses, and high color contrast because of the outstanding electrochemical properties of V2O5 nanofibers as well as the high Li-ion diffusion coefficients and good electrical contact with the substrate. Moreover, the morphology transformation mechanism is investigated in detail.