Haoshen Zhou

Large Reversible Li Storage of Graphene Nanosheet Families for Use in Rechargeable Lithium Ion Batteries

The lithium storage properties of graphene nanosheet (GNS) materials as high capacity anode materials for rechargeable lithium secondary batteries (LIB) were investigated. Graphite is a practical anode material used for LIB, because of its capability for reversible lithium ion intercalation in the layered crystals, and the structural similarities of GNS to graphite may provide another type of intercalation anode compound. While the accommodation of lithium in these layered compounds is influenced by the layer spacing between the graphene nanosheets, control of the intergraphene sheet distance through interacting molecules such as carbon nanotubes (CNT) or fullerenes (C60) might be crucial for enhancement of the storage capacity. The specific capacity of GNS was found to be 540 mAh/g, which is much larger than that of graphite, and this was increased up to 730 mAh/g and 784 mAh/g, respectively, by the incorporation of macromolecules of CNT and C60 to the GNS.

The Design of a LiFePO4/Carbon Nanocomposite With a Core–Shell Structure and Its Synthesis by an In Situ Polymerization Restriction Method

Nano-sized electrode materials for lithium-ion batteries have attracted much attention recently because their reduced dimensions enable much higher power. However, the large electrolyte/electrode interface arising from their size leads to more undesired reactions, which result in poor cycling performance. Moreover, some nano-sized cathode materials synthesized by low-temperature methods are poorly crystalline, which also reduces their electrochemical stability. The synthesis of highly crystalline nanomaterials completely coated with conductive carbon (or a carbon shell) would be an effective means of eliminating these problems. Such a synthesis is a significant challenge, however, as the highly crystalline structure and its subsequent coating with conductive carbon have to be achieved at high temperature, where larger crystallite sizes are almost inevitable. Olivine (LiFePO4) is considered to be one of the most promising cathode materials for the next generation of lithium batteries due to its low toxicity, low cost, and high safety. However, its power performance is greatly limited by slow diffusion of lithium ions across the two-phase boundary and/or low conductivity. Many efforts have been made over the past few years to improve the power performance of LiFePO4 by using low-temperature routes to obtain tailored particles or carbon painting to improve the conductivity of the solid phase. However, these previous studies have always focused on the “nano-size” or the “coating with conductive carbon” separately, rather than considering both of them together. Various low-temperature methods (synthesis temperature below 600 8C), such as lowtemperature ceramic routes or hydrothermal syntheses, have been developed to lower the particle size of LiFePO4, although none of them have been able to ensure the conductivity of the carbon coating. Furthermore, some lowtemperature routes are not able to produce the required highly crystalline olivine structure, thus reducing the electrochemical stability of LiFePO4. The high surface area arising from the nano-size of the products also greatly increases the undesirable electrode/electrolyte reactions, which leads to a poor cycling performance From a review of previous studies of nano-sized LiFePO4 (less than 100 nm), we can see that a “perfect” cycle-life (> 200 cycles) at high charge/ discharge depth (90%) is almost unheard of. Approaches based on the thermal decomposition of carbon-containing precursors have also been widely studied for the preparation of carbon-coated LiFePO4 particles. [16–23] However, these methods generally involve a high-temperature treatment, during which an increase in crystallite size is inevitable, to ensure the conductivity of the resulting carbon materials. Accordingly, those approaches based on the thermal decomposition of carbon-containing precursors can only produce LiFePO4 particles with a partial coating of carbon (Figure 1a). As shown in Figure 1a, during the intercalation process, the electrons cannot reach all the positions where Li ion intercalation takes place, thus resulting in polarization of the electrode. In view of the one-dimensional Li ion mobility in the framework, full coating with carbon, which ensures LiFePO4 particles get electrons from all directions, could further alleviate this polarization phenomenon. According to our analysis of previous studies, the ideal structure for high-performance LiFePO4 should contain nano-size particles completely coated with conductive carbon (Figure 1b). It should be noted that many previous studies involving the synthesis of nano-sized LiFePO4 employ Fe 2+ salts as precursors. 11–13,21] However, these salts are much more expensive and unstable than Fe salts, therefore the synthesis of a nano-sized LiFePO4/carbon composite with a core–shell structure from Fe salts would be of great interest. Herein we report an in situ polymerization restriction method for the synthesis of a LiFePO4/carbon composite formed from a highly crystalline LiFePO4 core with a size of about 20–40 nm and a semi-graphitic carbon shell with a thickness of about 1–2 nm. As shown in Figure 1c, our strategy includes one in situ polymerization reaction and two typical restriction processes. The first of these restriction processes involves the addition of Fe ions to a solution containing PO4 3 ions and aniline, where it acts as both a precipitator for PO4 3 and oxidant for aniline. The reaction during this process can be summarized as Equations (1) and (2).

Towards sustainable and versatile energy storage devices: an overview of organic electrode materials

As an alternative to conventional inorganic intercalation electrode materials, organic electrode materials are promising candidates for the next generation of sustainable and versatile energy storage devices. In this paper we provide an overview of organic electrode materials, including their fundamental knowledge, development history and perspective applications. Based on different organics including n-type, p-type and bipolar, we firstly analyzed their working principles, reaction mechanisms, electrochemical performances, advantages and challenges. To understand the development history and trends in organic electrode materials, we elaborate in detail various organics with different structures, including conducting polymers, organodisulfides, thioethers, nitroxyl radical polymers and conjugated carbonyl compounds. The high electrochemical performance, in addition with the unique features of organics such as flexibility, processability and structure diversity, provide them great perspective in various energy storage devices, including rechargeable Li/Na batteries, supercapacitors, thin film batteries, aqueous rechargeable batteries, redox flow batteries and even all-organic batteries. It is expected that organic electrode materials will show their talents in the “post Li-ion battery” era, towards cheap, green, sustainable and versatile energy storage devices.

Synthesis of Single Crystalline Spinel LiMn2O4 Nanowires for a Lithium Ion Battery with High Power Density

How to improve the specific power density of the rechargeable lithium ion battery has recently become one of the most attractive topics of both scientific and industrial interests. The spinel LiMn2O4 is the most promising candidate as a cathode material because of its low cost and nontoxicity compared with commercial LiCoO2. Moreover, nanostructured electrodes have been widely investigated to satisfy such industrial needs. However, the high-temperature sintering process, which is necessary for high-performance cathode materials based on high-quality crystals, leads the large grain size and aggregation of the nanoparticles which gives poor lithium ion battery performance. So there is still a challenge to synthesize a high-quality single-crystal nanostructured electrode. Among all of the nanostructures, a single crystalline nanowire is the most attractive morphology because the nonwoven fabric morphology constructed by the single crystalline nanowire suppresses the aggregation and grain growth at high temperature, and the potential barrier among the nanosize grains can be ignored. However, the reported single crystalline nanowire is almost the metal oxide with an anisotropic crystal structure because the cubic crystal structure such as LiMn2O4 cannot easily grow in the one-dimentional direction. Here we synthesized high-quality single crystalline cubic spinel LiMn2O4 nanowires based on a novel reaction method using Na0.44MnO2 nanowires as a self-template. These single crystalline spinel LiMn2O4 nanowires show high thermal stability because the nanowire structure is maintained after heating to 800 degrees C for 12 h and excellent performance at high rate charge-discharge, such as 20 A/g, with both a relative flat charge-discharge plateau and excellent cycle stability.

Core-Shell-Structured CNT@RuO2Composite as a High-Performance Cathode Catalyst for Rechargeable Li-O2Batteries

A RuO2 shell was uniformly coated on the surface of core CNTs by a simple sol-gel method, and the resulting composite was used as a catalyst in a rechargeable Li-O2 battery. This core-shell structure can effectively prevent direct contact between the CNT and the discharge product Li2 O2 , thus avoiding or reducing the formation of Li2 CO3 , which can induce large polarization and lead to charge failure. The battery showed a high round-trip efficiency (ca. 79 %), with discharge and charge overpotentials of 0.21 and 0.51 V, respectively, at a current of 100 mA gtotal (-1) . The battery also exhibited excellent rate and cycling performance.

Li−Air Rechargeable Battery Based on Metal-free Graphene Nanosheet Catalysts

Metal-free graphene nanosheets (GNSs) were examined for use as air electrodes in a Li-air battery with a hybrid electrolyte. At 0.5 mA cm(-1), the GNSs showed a high discharge voltage that was near that of the 20 wt % Pt/carbon black. This was ascribed to the presence of sp(3) bonding associated with edge and defect sites in GNSs. Moreover, heat-treated GNSs not only provided a similar catalytic activity in reducing oxygen in the air, but also showed a much more-stable cycling performance than GNSs when used in a rechargeable Li-air battery. This improvement resulted from removal of adsorbed functional groups and from crystallization of the GNS surface into a graphitic structure on heat treatment.

Challenges of non-aqueous Li–O2 batteries: electrolytes, catalysts, and anodes

A lot of attention has been paid to Li–O2 batteries in recent years, due to the huge potential specific energy and energy density, and they are extensively studied around the world. Much advance has been achieved, however, the fundamental understanding is still insufficient and challenges remain. Here, we provide a specific perspective on the development of non-aqueous Li–O2 batteries excluding those with aqueous, ionic liquid, hybrid, and solid-state electrolytes, because non-aqueous Li–O2 batteries possess a relatively simple configuration and the research on non-aqueous Li–O2 batteries is the most active of all Li–O2 batteries. The discussion will be focused on non-aqueous electrolytes, cathode catalysts, and anodes, and corresponding perspectives are provided at the end.

Nanomaterials for lithium ion batteries

Nanostructured materials are currently of interest for lithium ion storage devices because of their high surface area, porosity, etc. These characteristics make it possible to introduce new active reactions, decrease the path length for Li ion transport, reduce the specific surface current rate, and improve stability and specific capacity. Moreover, composite nanostructured materials designed to include electronic conductive paths could decrease the inner resistance of lithium ion batteries, leading to higher specific capacities even at high charge/discharge current rates.

Centimeter‐Long V2O5 Nanowires: From Synthesis to Field‐Emission, Electrochemical, Electrical Transport, and Photoconductive Properties

Adv. Mater. 2010, 22, 2547–2552 2010 WILEY-VCH Verlag G One-dimensional nanostructures have attracted considerable attention due to their importance in basic scientific research and potential technologic applications. Among them, vanadium pentoxide (V2O5) nanowires have been extensively studied in recent years because of their prospective applications in chemical sensors, field-emitters, catalysts, lithium-ion batteries, actuators, and electrochromic or other nanodevices. Several different approaches have been explored for the synthesis of V2O5 nanowires, such as thermal evaporation methods, hydrothermal/solvothermal syntheses, sol–gel techniques, and electrodeposition. However, the nanowires synthesized by these methods have typical lengths in the micrometer range (most of them are shorter than 10mm);moreover, if one canmake centimeter-long V2O5 nanowires, which should be much more useful compared to short wires for some specific purposes, such as field-emission (FE), device interconnects, and reinforcing fibers in composites. Herein, we fabricated high-quality single-crystalline centimeter-long V2O5 nanowires ( 80–120 nm in diameter, several centimeters in length; aspect ratio >10–10) using an environmental friendly hydrothermal approach without dangerous reagents, harmful solvents, and surfactants. The FE, electrochemical and electrical transport, and photoconductive properties of the synthesized V2O5 nanowires were then investigated in detail. Our results suggest a high potential of utilizing these novel nanowires in field-emitters, lithium-ion batteries, interconnects, and optoelectronic devices. The representative morphologies of the V2O5 nanowires were investigated by FE scanning electron microscopy (SEM), as shown in Figure 1a. Other SEM images (see the Supporting Information, Fig. S1) also confirm the high-yield fabrication of smooth and straight nanowires of 80–120 nm in diameter. Large portions of the nanowires are usually several millimeters or even up to several centimeters in length (inset of Fig. 1a), resulting in an aspect ratio of 10–10. To the best of our knowledge, this is the first time that such ultra-long V2O5 nanowires have been obtained. An X-ray diffraction (XRD) pattern of the sample is shown in Figure 1b. All the diffraction peaks can be indexed to an orthorhombic V2O5 phase with the lattice parameters of a1⁄4 11.54 A, b1⁄4 3.571 A, and c1⁄4 4.383 A, in good agreement with the literature values (Joint Committee on Powder Diffraction Standards (JCPDS) Card, no. 89-0612). No characteristic peaks of any impurities are detected in this pattern. Figure S2 (Supplementary Information) depicts a room temperature micro-Raman spectrum of the ultralong V2O5 nanowires. The peaks, located at 145, 197, 285, 305, 407, 480, 525, 694, and 990 cm , can be assigned to the Raman signature of V2O5. [18,19] A predominant low-wavelength peak at 145 cm 1 is attributed to the skeleton bent vibration (B3g mode), while the peaks at 197 and 285 cm 1 derive from the bending vibrations of OC V OB bond (Ag and B2g modes). The bending vibration of V OC (Ag mode), the bending vibration of V OB V bond (Ag mode), the stretching vibration of V OB V bond (Ag mode), and the stretching vibration of V OC bond (B2g mode) are regarded at about 305, 407, 525, and 694 cm , respectively. The layered structure of V2O5 is stacked up from distorted trigonal bipyramidal atoms that share edges to form (V2O4)n zigzag double chains along the [001] direction and are cross-linked along the [100] direction through the shared corners. The mode of a skeleton bent, corresponding to the peak at 145 cm , provides an evidence for the layered structure of V2O5. Furthermore, the narrow peak centered at 990 cm , corresponding to the stretching of vanadium atoms connected to oxygen atoms through double bonds (V1⁄4O), is also an additional clue to the layer-type structure of V2O5. [22,23] The detailed microstructures of V2O5 nanowires were further studied by transmission electron microscopy (TEM). Figure 2a shows a TEM image of V2O5 nanowires, which demonstrates that the V2O5 nanowires have uniform diameters throughout their entire lengths. An X-ray energy-dispersive spectrum (EDS) acquired from an individual nanowire exhibits strong V and O peaks. The atomic ratio of V and O is close to the 2:5

A reversible long-life lithium–air battery in ambient air

Electrolyte degradation, Li dendrite formation and parasitic reactions with H₂O and CO₂ are all directly correlated to reversibility and cycleability of Li-air batteries when operated in ambient air. Here we replace easily decomposable liquid electrolytes with a solid Li-ion conductor, which acts as both a catholyte and a Li protector. Meanwhile, the conventional solid air cathodes are replaced with a gel cathode, which contacts directly with the solid catholyte to form a closed and sustainable gel/solid interface. The proposed Li-air cell has sustained repeated cycling in ambient air for 100 cycles (~78 days), with discharge capacity of 2,000 mAh g(-1). The recharging is based largely on the reversible reactions of Li₂CO₃ product, originating from the initial discharge product of Li₂O₂ instead of electrolyte degradation. Our results demonstrate that a reversible long-life Li-air battery is attainable by coordinated approaches towards the focal issues of electrolytes and Li metal.