The Critical Role of Carbon Nanotubes in Bridging Academic Research to Commercialization of Lithium Batteries.

Rechargeable lithium batteries have been intensively explored due to their potential to deliver a high energy and stable cycling performance. Yet considerable achievements have been reported on battery performance in lab-based research, a broad gap from fundamental research to their industrial application needs to be filled. The significant advances in the field of carbon nanotubes over the past decades make it a promising candidate to bridge such a gap. Nevertheless, a systematic and profound understanding of its roles in Li batteries is lacking. In this review, we discuss the critical role of carbon nanotube in developing several lithium techniques such as Li-ion, Li-sulfur, and Li-air cells. The focus is laid on the elevation of device capacity, energy, and cyclic life to meet the practical demand. We hope this paper, together with other recently-proposed guiding principles, will pave the way for the massive application of carbon nanotube-based lithium batteries.

Structurally Durable Bimetallic Alloy Anodes Enabled by Compositional Gradients.

Metals such as Sb and Bi are important anode materials for sodium-ion batteries because they feature a large capacity and low reaction potential. However, the accumulation of stress and strain upon sodium storage leads to the formation of cracks and fractures, resulting in electrode failure upon extended cycling. In this work, the design and construction of Bix Sb1-x bimetallic alloy films with a compositional gradient to mitigate the intrinsic structural instability is reported. In the gradient film, the top is rich in Sb, contributing to the capacity, while the bottom is rich in Bi, helping to reduce the stress in the interphase between the film and the substrate. Significantly, this gradient film affords a high reversible capacity of ≈500 mAh g-1 and sustains 82% of the initial capacity after 1000 cycles at 2 C, drastically outperforming the solid-solution counterpart and many recently reported alloy anodes. Such a gradient design can open up the possibilities to engineering high-capacity anode materials that are structurally unstable due to the huge volume variation upon energy storage.

Carbon nanotubes for flexible batteries: recent progress and future perspective.

Flexible batteries, which maintain their functions potently under various mechanical deformations, attract increasing interest due to potential applications in emerging portable and wearable electronics. Significant efforts have been devoted to material synthesis and structural designs to realize the mechanical flexibility of various batteries. Carbon nanotubes (CNTs) have a unique one-dimensional (1D) nanostructure and are convenient to further assemble into diverse macroscopic structures, such as 1D fibers, 2D films and 3D sponges/aerogels. Due to their outstanding mechanical and electrical properties, CNTs and CNT-based hybrid materials are superior building blocks for different components in flexible batteries. This review summarizes recent progress on the application of CNTs in developing flexible batteries, from closed-system to open-system batteries, with a focus on different structural designs of CNT-based material systems and their roles in various batteries. We also provide perspectives on the challenges and future research directions for realizing practical applications of CNT-based flexible batteries.

Adsorption and diffusion of magnesium on nitrogen-doped Mo2C monolayer.

The Mg adsorption and diffusion behaviors on nitrogen-doped (N-doped) Mo2C monolayer have been investigated by the first principles based on density functional theory (DFT). To investigate the effect of nitrogen concentration on adsorption energies, Mo2C1-xNx (x=0.0625, 0.125, 0.1875, and 0.25) with four different nitrogen doping concentrations have been considered in the present work. The results show that N-doped Mo2C is benefit for Mg adsorption. In particular, when the doping concentration reaches to 14.29%, the adsorption energies of Mg on Mo2C0.875N0.125 are in the region between -1.639 and -1.517 eV, e.g., the adsorption energies of Mg on TC1 and H2 sites are -1.639 eV and -1.625 eV, which are decreased by 16.49% and 18.43% as compared with the pristine Mo2C. The calculations on diffusion behaviors show that the Mg diffusing between two adjacent favored sites via a high-symmetry site along H3-B-H4 and H1-B-H1 paths possesses the barriers of 0.021 eV and 0.028 eV. Additionally, the partial density of states (PDOS) reveals the interaction between Mg and Mo2C0.875N0.125, and indicates that nitrogen doping causes the PDOS peaks transfer to a lower energy level, which is benefit for the bonding between Mg and Mo2C0.875N0.125. These results suggest that the adsorption and diffusion behaviors of Mg have been enhanced by nitrogen doping.

An Energetic CuS-Cu Battery System Based on CuS Nanosheet Arrays.

The development of low-cost and high-energy aqueous battery technologies is of significance for renewable and stationary energy applications. However, this development has been bottlenecked by poor conductivity, low capacity, and limited cycling stability of existing electrode materials. In this work, we report on an energetic aqueous copper ion system based on CuS nanosheet arrays, taking profit of high conductivity of CuS and efficient charge carrier of copper ions. Electrochemical results reveal a high capacity of 510 mAh g-1, robust rate capability of 497 mAh g-1 at a high rate of 7.5 A g-1, and ultrastable cycling by retaining 91% of the initial capacity over 2500 cycles. The charge-storage mechanism was systematically investigated by ex situ and in situ techniques involving a reversible transition from CuS to Cu7S4 and to Cu2S through the redox of Cu2+/Cu+. Moreover, we demonstrate a hybrid ion battery consisting of CuS positive electrode and Zn negative electrode, which affords an energy and power of 286 Wh kg-1 and 900 W kg-1, respectively, on the basis of both electrodes, exceeding many aqueous battery systems.

Durian-Inspired Design of Bismuth-Antimony Alloy Arrays for Robust Sodium Storage.

Sodium-ion batteries have attracted widespread attention for cost-effective and large-scale electric energy storage. However, their practical deployment has been largely retarded by the lack of choice of efficient anode materials featuring large capacity and electrochemical stability and robustness. Herein, we report a durian-inspired design and template-free fabrication of a robust sodium anode based on triangular pyramid arrays of Bi0.75Sb0.25 alloy electrodeposited on Cu substrates. The Bi0.75Sb0.25 arrays exhibit an appreciable electrochemical robustness for sodium storage, sustaining a reversible capability 335 mAh g-1 at a high rate of 2.5 A g-1 and 87% of the initial capacity over 2000 cycles. We further demonstrate the applicability of the Bi0.75Sb0.25 array anode in sodium full cells by pairing it with a Na3V2(PO4)3/C cathode. This full cell achieves a high specific energy of 203 Wh kg-1 (based on both active electrodes). Such an enhanced performance is attributed to the thorny-durian-like architecture and bimetallic alloy composition. The pyramid tip induces ion enrichment for rapid charge-transfer reaction, while the alloy design reduces the electrode volume swelling for stable Na cycling.

Cathode Architectures for Rechargeable Ion Batteries: Progress and Perspectives.

To satisfy the rising demand for energy, battery electrodes with higher loading, to simultaneously increase areal energy and power, are necessary. Nevertheless, in conventional thin-film electrodes, there is mutual exclusion between energy (capacity) and power. Increasing the thickness of electrodes alone is not feasible since this will lead to reductions in ion-diffusion efficiency, as well as electrode flexibility. To address this difficulty, 3D electrode architectures, especially cathode architectures, are proposed to pave a new path for the design and optimization of battery devices. Recent research suggests that 3D cathode architectures may optimize the configuration and engineering processes of battery technologies. Herein, the state-of-the-art progress of cathode architectures in various rechargeable-ion-battery technologies is summarized. Emphasis is placed on the different architecture strategies, areal loading, and mechanical understanding of 3D electrodes. Upcoming research directions are further outlined for future development in this field.

Rooting binder-free tin nanoarrays into copper substrate via tin-copper alloying for robust energy storage.

The need for high-energy batteries has driven the development of binder-free electrode architectures. However, the weak bonding between the electrode particles and the current collector cannot withstand the severe volume change of active materials upon battery cycling, which largely limit the large-scale application of such electrodes. Using tin nanoarrays electrochemically deposited on copper substrate as an example, here we demonstrate a strategy of strengthening the connection between electrode and current collector by thermally alloying tin and copper at their interface. The locally formed tin-copper alloys are electron-conductive and meanwhile electrochemically inactive, working as an ideal "glue" robustly bridging tin and copper to survive harsh cycling conditions in sodium ion batteries. The working mechanism of the alloy "glue" is further characterized through a combination of electrochemical impedance spectroscopy, atomic structural analysis and in situ X-ray diffraction, presenting itself as a promising strategy for engineering binder-free electrode with endurable performance.

Realizing Stable Artificial Photon Energy Harvesting Based on Perovskite Solar Cells for Diverse Applications.

As the fastest developing photovoltaic device, perovskite solar cells have achieved an extraordinary power conversion efficiency (PCE) of 25.3% under AM 1.5 illumination. However, few studies have been devoted to perovskite solar cells harvesting artificial light, owing to the great challenge in the simultaneous manipulation of bandgap-adjustable perovskite materials, corresponding matched energy band structure of carrier transport materials, and interfacial defects. Herein, through systematic morphology, composition, and energy band engineering, high-quality Cs0.05 MA0.95 PbBrx I3- x perovskite as the light absorber and Nby Ti1- y O2 (Nb:TiO2 ) as the electron transport material with an ideal energy band alignment are obtained simultaneously. The theoretical-limit-approaching record PCEs of 36.3% (average: 34.0 ± 1.2%) under light-emitting diode (LED, warm white) and 33.2% under fluorescent lamp (cold white) are achieved simultaneously, as well as a PCE of 19.5% (average: 18.9 ± 0.3%) under solar illumination. An integrated energy conversion and storage system based on an artificial light response solar cell and sodium-ion battery is established for diverse practical applications, including a portable calculator, quartz clock, and even environmental monitoring equipment. Over a week of stable operation shows its great practical potential and provides a new avenue to promote the commercialization of perovskite photovoltaic devices via integration with ingenious electronic devices.

Ultrastable Sodium Storage in MoO3 Nanotube Arrays Enabled by Surface Phosphorylation.

Molybdenum trioxide (MoO3) has been considered as an appealing choice of anode for sodium-ion batteries because of its high theoretical capacity (1117 mA h g-1). However, the large volume change upon Na+ storage results in poor cycling stability and capacity fade of MoO3. Here, we demonstrate a surface phosphorylation strategy to mitigate the degradation of three-dimensional MoO3 array electrodes. Such a phosphorylation strategy allows MoO3 arrays to sustain a capacity of 265 mA h g-1, or ∼90% of the initial value, at a rate of 2 A g-1 over 1500 cycles, outperforming most reported MoO3 electrodes. Moreover, kinetic analysis unveils a capacitance-dominated Na+ storage feature of MoO3 arrays, owing to the enhanced electron mobility imparted by oxygen vacancies that are simultaneously introduced by phosphorylation. Hence, surface phosphorylation might offer new possibilities to bypass multiple materials challenges facing current sodium electrodes.

Highly Efficient Sodium Storage in Iron Oxide Nanotube Arrays Enabled by Built-In Electric Field.

High-power sodium-ion batteries capable of charging and discharging rapidly and durably are eagerly demanded to replace current lithium-ion batteries. However, poor activity and instable cycling of common sodium anode materials represent a huge barrier for practical deployment. A smart design of ordered nanotube arrays of iron oxide (Fe2 O3 ) is presented as efficient sodium anode, simply enabled by surface sulfurization. The resulted heterostructure of oxide and sulfide spontaneously develops a built-in electric field, which reduces the activation energy and accelerates charge transport significantly. Benefiting from the synergy of ordered architecture and built-in electric field, such arrays exhibit a large reversible capacity, a superior rate capability, and a high retention of 91% up to 200 cycles at a high rate of 5 A g-1 , outperforming most reported iron oxide electrodes. Furthermore, full cells based on the Fe2 O3 array anode and the Na0.67 (Mn0.67 Ni0.23 Mg0.1 )O2 cathode deliver a specific energy of 142 Wh kg-1 at a power density of 330 W kg-1 (based on both active electrodes), demonstrating a great potential in practical application. This material design may open a new door in engineering efficient anode based on earth-abundant materials.

Pencil-Drawing Skin-Mountable Micro-Supercapacitors.

In this study, integrated plaster-like micro-supercapacitors based on medical adhesive tapes are fabricated by a simple pencil drawing process combined with a mild solution deposition of MnO2 . These solid micro-supercapacitors not only exhibit excellent stretchability, flexibility, and biocompatibility, but also possess outstanding electrochemical performances, such as exceptional rate capability and cycling stability. Hence they may act as skin-mountable and thin-film energy storage devices of high efficiency to power miniaturized and wearable electronic devices.

Materials Based on Antimony and Bismuth for Sodium Storage.

Sodium-ion batteries (SIBs) that efficiently store electricity into chemical energy have been extensively pursued because of their great potential for low-cost and large-scale stationary application such as smart grid and renewable energy. Successful deployment of SIBs requires efficient anode materials that could store Na+ ions via a reversible way at reasonable rates. Materials based on antimony and bismuth are capable of storing a high-concentration of Na+ ions via a reversible alloying reaction at suitable redox potentials, and thus have drawn substantial attention. However, these electrode materials are facing significant technical challenges, such as poor conductivity, multiple phase transformation, and severe volume swelling and shrinking, which make efficient materials design a necessity. In this review, we will give a latest overview of research progress in the design and application of electrode materials based on antimony and bismuth, and offer some value insights into their future development in sodium storage.

Effect of distribution, interface property and density of hydrogel-embedded vertically aligned carbon nanotube arrays on the properties of a flexible solid state supercapacitor.

In this paper we fabricate a robust flexible solid-state supercapacitor (FSC) device by embedding a conductive poly(vinyl alcohol) hydrogel into aligned carbon nanotube (CNT) arrays. We carefully investigate the effect of distribution, interface properties and densification of CNTs in the gel matrix on the electrochemical properties of an FSC. The total electrochemical capacitance of the device is measured to be 227 mF cm-3 with a maximum energy density of 0.02 mWh cm-3, which is dramatically enhanced compared with a similar device composed of non-parallel CNTs. Additionally, controllable in situ electrochemical oxidation greatly improved the compatibility between the hydrophobic CNTs and the hydrophilic hydrogel, which decreased the resistance of the device and introduced extra pseudocapacitance. After such oxidation treatment the energy storage ability further doubled to 430 mF cm-3 with a maximum energy density of 0.04 mWh cm-3 . The FSCs based on densified CNT arrays exhibited a much higher volumetric capacitance of 1140 mF cm-3 and a larger energy density of 0.1 mWh cm-3, with a large power density of 14 mW cm-3. All devices show excellent stability of capacitance after at least 10 000 charge-discharge cycles with a loss of less than 2%. These easy-to-assemble hybrid arrays thus potentially provide a new method for manufacturing wearable devices and implantable medical devices.

Boosting Sodium Storage in TiO2 Nanotube Arrays through Surface Phosphorylation.

Sodium-ion batteries (SIBs) offer a promise of a scalable, low-cost, and environmentally benign means of renewable energy storage. However, the low capacity and poor rate capability of anode materials present an unavoidable challenge. In this work, it is demonstrated that surface phosphorylated TiO2 nanotube arrays grown on Ti substrate can be efficient anode materials for SIBs. Fabrication of the phosphorylated nanoarray film is based on the electrochemical anodization of Ti metal in NH4 F solution and subsequent phosphorylation using sodium hypophosphite. The phosphorylated TiO2 nanotube arrays afford a reversible capacity of 334 mA h g-1 at 67 mA g-1 , a superior rate capability of 147 mA h g-1 at 3350 mA g-1 , and a stable cycle performance up to 1000 cycles. In situ X-ray diffraction and transmission electron microscopy reveal the near-zero strain response and robust mechanical behavior of the TiO2 host upon (de)sodiation, suggesting its excellent structural stability in the Na+ storage application.

Highly Reversible and Durable Na Storage in Niobium Pentoxide through Optimizing Structure, Composition, and Nanoarchitecture.

Amorphous, hydrogenated, and self-ordered nanoporous Nb2 O5 films serve as an excellent binder-free electrode for sodium batteries, affording a high and sustainable capacity delivery and robust high-rate capability. This collaborative material engineering of structural order (amorphization), composition (hydrogenation), and architecture (ordered nanopore) opens up new possibilities to develop an energy storage solution that is more accessible, sustainable, and producible.

Hydrogenation Driven Conductive Na2Ti3O7 Nanoarrays as Robust Binder-Free Anodes for Sodium-Ion Batteries.

We present a general and rational approach to fabricate highly accessible and affordable sodium-ion battery anodes by engineering three-dimensional hydrogenated Na2Ti3O7 nanoarrays supported on flexible Ti substrates. The hydrogenated Na2Ti3O7 nanoarrays exhibit desirable properties for sodium storage, such as high surface area, high electrical conductivity, and Na(+) diffusivity. The as-obtained nanoarrays demonstrate remarkably stable and robust Na-storage performance when tested as binder-free anodes for sodium-ion battery. They can afford a high reversible (desodiation) capacity of 227 mAh g(-1) and retain a capacity of 65 mAh g(-1) over 10,000 continuous cycles at a high rate of 35 C. Therefore, through this synergy of array architecture and hydrogenation, it is possible to engineer numerous anodes that can reversibly store Na(+) ions in a fast and stable manner.

Self-Supported Nanotube Arrays of Sulfur-Doped TiO2 Enabling Ultrastable and Robust Sodium Storage.

Self-supported nanotube arrays of sulfur-doped TiO2 on metal substrates are fabricated using electrochemical anodization and subsequent sulfidation. The nanotube arrays can serve as an efficient anode for sodium storage, enabling ultrastable cycling (retaining 91% of the 2nd capacity up to 4400 cycles) and robust rate capability (167 mA h g(-1) at 3350 mA g(-1)), remarkably outperforming any other reported TiO2 -based electrodes.

A Li4Ti5O12/TiO2@CNT Core/Shell Structure for Rechargeable Li Batteries.

Li4Ti5O12 is an important type of anode material for rechargeable Li battery due to its excellent cycling and thermal reliability, but the poor conductivity represents a significant challenge in the scalable application. Here we design a ternary Li4Ti5O12/TiO2@CNT core/shell structure to well mitigate the conductivity issue. The hybrid core/shell structure is fabricated by a facile hydrothermal reaction followed by heat treatment at 600 degrees C. It is comprised of Li4Ti5O12/TiO2 nanocrystals several nanometers in dimension tightly anchored on CNT network. The CNT network provides a fast and robust conductive way for electron transport, while the minor rutile-TiO2 phase improves the kinetics of Li4Ti5O12 toward fast lithium insertion/extraction. The electrochemical results indicate that the core/shell structure displays a high electrochemical activity in terms of reversible capacity and rate capability. The hybrid structure also shows excellent long-term cycling stability when operated at a high rate of 5 C.

Graphene wrapped ordered LiNi0.5Mn1.5O4 nanorods as promising cathode material for lithium-ion batteries.

LiNi0.5Mn1.5O4 nanorods wrapped with graphene nanosheets have been prepared and investigated as high energy and high power cathode material for lithium-ion batteries. The structural characterization by X-ray diffraction, Raman spectroscopy, and Fourier transform infrared spectroscopy indicates the LiNi0.5Mn1.5O4 nanorods prepared from ß-MnO2 nanowires have ordered spinel structure with P4332 space group. The morphological characterization by scanning electron microscopy and transmission electron microscopy reveals that the LiNi0.5Mn1.5O4 nanorods of 100-200 nm in diameter are well dispersed and wrapped in the graphene nanosheets for the composite. Benefiting from the highly conductive matrix provided by graphene nanosheets and one-dimensional nanostructure of the ordered spinel, the composite electrode exhibits superior rate capability and cycling stability. As a result, the LiNi0.5Mn1.5O4-graphene composite electrode delivers reversible capacities of 127.6 and 80.8 mAh g(-1) at 0.1 and 10 C, respectively, and shows 94% capacity retention after 200 cycles at 1 C, greatly outperforming the bare LiNi0.5Mn1.5O4 nanorod cathode. The outstanding performance of the LiNi0.5Mn1.5O4-graphene composite makes it promising as cathode material for developing high energy and high power lithium-ion batteries.