Eiji Hosono

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).

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.

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.

Pseudocapacitance of MXene nanosheets for high-power sodium-ion hybrid capacitors.

High-power Na-ion batteries have tremendous potential in various large-scale applications. However, conventional charge storage through ion intercalation or double-layer formation cannot satisfy the requirements of such applications owing to the slow kinetics of ion intercalation and the small capacitance of the double layer. The present work demonstrates that the pseudocapacitance of the nanosheet compound MXene Ti2C achieves a higher specific capacity relative to double-layer capacitor electrodes and a higher rate capability relative to ion intercalation electrodes. By utilizing the pseudocapacitance as a negative electrode, the prototype Na-ion full cell consisting of an alluaudite Na2Fe2(SO4)3 positive electrode and an MXene Ti2C negative electrode operates at a relatively high voltage of 2.4 V and delivers 90 and 40 mAh g−1 at 1.0 and 5.0 A g−1 (based on the weight of the negative electrode), respectively, which are not attainable by conventional electrochemical energy storage systems.

Aromatic porous-honeycomb electrodes for a sodium-organic energy storage device

Rechargeable batteries using organic electrodes and sodium as a charge carrier can be high-performance, affordable energy storage devices due to the abundance of both sodium and organic materials. However, only few organic materials have been found to be active in sodium battery systems. Here we report a high-performance sodium-based energy storage device using a bipolar porous organic electrode constituted of aromatic rings in a porous-honeycomb structure. Unlike typical organic electrodes in sodium battery systems, the bipolar porous organic electrode has a high specific power of 10 kW kg(-1), specific energy of 500 Wh kg(-1), and over 7,000 cycle life retaining 80% of its initial capacity in half-cells. The use of bipolar porous organic electrode in a sodium-organic energy storage device would significantly enhance cost-effectiveness, and reduce the dependency on limited natural resources. The present findings suggest that bipolar porous organic electrode provides a new material platform for the development of a rechargeable energy storage technology.

Fast Li-Ion Insertion into Nanosized LiMn2O4 without Domain Boundaries

The effect of crystallite size on Li-ion insertion in electrode materials is of great interest recently because of the need for nanoelectrodes in higher-power Li-ion rechargeable batteries. We present a systematic study of the effect of size on the electrochemical properties of LiMn(2)O(4). Accurate size control of nanocrystalline LiMn(2)O(4), which is realized by a hydrothermal method, significantly alters the phase diagram as well as Li-ion insertion voltage. Nanocrystalline LiMn(2)O(4) with extremely small crystallite size of 15 nm cannot accommodate domain boundaries between Li-rich and Li-poor phases due to interface energy, and therefore lithiation proceeds via solid solution state without domain boundaries, enabling fast Li-ion insertion during the entire discharge process.

Fabrication of morphology and crystal structure controlled nanorod and nanosheet cobalt hydroxide based on the difference of oxygen-solubility between water and methanol, and conversion into Co3O4

Films of brucite-type cobalt hydroxide with nanorod morphology and hydrotalcite-type cobalt hydroxide with nanosheet morphology films were fabricated by heterogeneous nucleation in a chemical bath using water and a mixed solution of water–methanol as solvents, respectively. Since oxygen is around 25 times more soluble in methanol than in water, a methanol solution was used to convert a part of divalent cobalt ions into trivalent cobalt ions through oxidation, due to the amount of dissolved oxygen. The resultant cobalt hydroxides were of the hydrotalcite type, with a sheet-like morphology, and di- and trivalent cobalt ions. On the other hand, brucite-type hydroxides with a rod morphology, constructed using only divalent cobalt ions, were fabricated due to the scarcity of dissolved oxygen in a water-only solvents. Both the brucite and hydrotalcite types of cobalt hydroxide films were transformed into Co3O4 through pyrolysis without nanostructural deformation. The Co3O4 films were porous structures with a large surface area because both rod and sheet were constructed through nanoparticles and nanopores once the self-template was removed.

Synthesis of Triaxial LiFePO4 Nanowire with a VGCF Core Column and a Carbon Shell through the Electrospinning Method

A triaxial LiFePO4 nanowire with a multi wall carbon nanotube (VGCF:Vapor-grown carbon fiber) core column and an outer shell of amorphous carbon was successfully synthesized through the electrospinning method. The carbon nanotube core oriented in the direction of the wire played an important role in the conduction of electrons during the charge-discharge process, whereas the outer amorphous carbon shell suppressed the oxidation of Fe2+. An electrode with uniformly dispersed carbon and active materials was easily fabricated via a single process by heating after the electrospinning method is applied. Mossbauer spectroscopy for the nanowire showed a broadening of the line width, indicating a disordered coordination environment of the Fe ion near the surface. The electrospinning method was proven to be suitable for the fabrication of a triaxial nanostructure.

Facile synthesis of NaV6O15 nanorods and its electrochemical behavior as cathode material in rechargeable lithium batteries

A ternary vanadium bronze compound, NaV6O15 (Na0.33V2O5), constructed by highly ordered nanorod structures, was facilely synthesized via a low temperature hydrothermal route using V2O5, H2O2 and NaCl as the precursors. A reaction mechanism involved in present hydrothermal condition was tentatively proposed. The sample was systemically post-treated at different temperatures and well characterized by various techniques. It was found that the prepared NaV6O15 nanorods had a highly crystallined single phase with a preferred c* orientation growth. When used as the cathode material in rechargeable lithium batteries, the NaV6O15 nanorods exhibited stable lithium-ion insertion/deinsertion reversibility and delivered as high as 328 mAh g−1 lithium cycled at the current density of 0.02 A g−1. In galvanostatic cycling test, a specific discharge capacity of around 300 mAh g−1 could be demonstrated for 70 cycles under 0.05 A g−1 current density. According to its unique crystallographic structure and electrochemical characteristics, it is therefore expected that as-prepared NaV6O15 nanorods may be employed as cathode material in rechargeable lithium, sodium-based batteries.