Laxmikant V. Saraf

Self-assembled TiO2-Graphene Hybrid Nanostructures for Enhanced Li-ion Insertion

We used anionic sulfate surfactants to assist the stabilization of graphene in aqueous solutions and facilitate the self-assembly of in situ grown nanocrystalline TiO2, rutile and anatase, with graphene. These nanostructured TiO2-graphene hybrid materials were used for investigation of Li-ion insertion properties. The hybrid materials showed significantly enhanced Li-ion insertion/extraction in TiO2. The specific capacity was more than doubled at high charge rates, as compared with the pure TiO2 phase. The improved capacity at high charge-discharge rate may be attributed to increased electrode conductivity in the presence of a percolated graphene network embedded into the metal oxide electrodes.

Sodium ion insertion in hollow carbon nanowires for battery applications.

Hollow carbon nanowires (HCNWs) were prepared through pyrolyzation of a hollow polyaniline nanowire precursor. The HCNWs used as anode material for Na-ion batteries deliver a high reversible capacity of 251 mAh g(-1) and 82.2% capacity retention over 400 charge-discharge cycles between 1.2 and 0.01 V (vs Na(+)/Na) at a constant current of 50 mA g(-1) (0.2 C). Excellent cycling stability is also observed at an even higher charge-discharge rate. A high reversible capacity of 149 mAh g(-1) also can be obtained at a current rate of 500 mA g(-1) (2C). The good Na-ion insertion property is attributed to the short diffusion distance in the HCNWs and the large interlayer distance (0.37 nm) between the graphitic sheets, which agrees with the interlayered distance predicted by theoretical calculations to enable Na-ion insertion in carbon materials.

Hierarchically porous graphene as a lithium-air battery electrode.

The lithium-air battery is one of the most promising technologies among various electrochemical energy storage systems. We demonstrate that a novel air electrode consisting of an unusual hierarchical arrangement of functionalized graphene sheets (with no catalyst) delivers an exceptionally high capacity of 15000 mAh/g in lithium-O(2) batteries which is the highest value ever reported in this field. This excellent performance is attributed to the unique bimodal porous structure of the electrode which consists of microporous channels facilitating rapid O(2) diffusion while the highly connected nanoscale pores provide a high density of reactive sites for Li-O(2) reactions. Further, we show that the defects and functional groups on graphene favor the formation of isolated nanosized Li(2)O(2) particles and help prevent air blocking in the air electrode. The hierarchically ordered porous structure in bulk graphene enables its practical applications by promoting accessibility to most graphene sheets in this structure.

A Soft Approach to Encapsulate Sulfur: Polyaniline Nanotubes for Lithium‐Sulfur Batteries with Long Cycle Life

A novel vulcanized polyaniline nanotube/sulfur composite was prepared successfully via an in situ vulcanization process by heating a mixture of polyaniline nanotube and sulfur at 280 °C. The electrode could retain a discharge capacity of 837 mAh g(-1) after 100 cycles at a 0.1 C rate and manifested 76% capacity retention up to 500 cycles at a 1 C rate.

Ternary self-assembly of ordered metal oxide-graphene nanocomposites for electrochemical energy storage.

Surfactant or polymer directed self-assembly has been widely investigated to prepare nanostructured metal oxides, semiconductors, and polymers, but this approach is mostly limited to two-phase materials, organic/inorganic hybrids, and nanoparticle or polymer-based nanocomposites. Self-assembled nanostructures from more complex, multiscale, and multiphase building blocks have been investigated with limited success. Here, we demonstrate a ternary self-assembly approach using graphene as fundamental building blocks to construct ordered metal oxide-graphene nanocomposites. A new class of layered nanocomposites is formed containing stable, ordered alternating layers of nanocrystalline metal oxides with graphene or graphene stacks. Alternatively, the graphene or graphene stacks can be incorporated into liquid-crystal-templated nanoporous structures to form high surface area, conductive networks. The self-assembly method can also be used to fabricate free-standing, flexible metal oxide-graphene nanocomposite films and electrodes. We have investigated the Li-ion insertion properties of the self-assembled electrodes for energy storage and show that the SnO2-graphene nanocomposite films can achieve near theoretical specific energy density without significant charge/discharge degradation.

Reversible Sodium Ion Insertion in Single Crystalline Manganese Oxide Nanowires with Long Cycle Life

Single crystalline Na4Mn9O18 nanowires were synthesized via pyrolysis of polyacrylate salt precursors prepared by in-situ polymerization of the metal salts and acrylate acid, followed by calcinations at an appropriate temperature to achieve good crystalline structure and uniform nanowire morphology with an average diameter of 50 nm. The Na4Mn9O18 nanowires have shown a high, reversible, and near theoretical sodium ion insertion capacity (128 mA h g-1 at 0.1C), excellent long cyclability (77% capacity retention for 1000 cycles at 0.5 C), along with good rate capability. Good capacity and charge-discharge stability are also observed for full cell experiments using a pyrolyzed carbon as the anode, therefore demonstrating the potential of these materials for sodium-ion batteries for large scale energy storage. Furthermore, this research shows that a good crystallinity and small particles are required to enhance the Na-ion diffusion and increase the stability of the electrode materials for long charge-discharge cycles.

Optimization of mesoporous carbon structures for lithium–sulfur battery applications

Mesoporous carbon (MC) with tunable pore sizes (22 nm, 12 nm, 7 nm, and 3 nm) and pore volumes (from 1.3 to 4.8 cm3 g−1) containing sulfur in the pores was studied as a mesoporous carbon–sulfur (MCS) composite electrode for lithium–sulfur (Li–S) batteries. Systematic investigation of these MCS composites reveals that MC with a larger pore volume can hold a higher maximum sulfur loading, but overall the battery performance is very similar for different MCS composites at full sulfur-filling conditions (i.e., the condition at which the sulfur loading approaches the maximum limit set by the pore volume of the individual MC and, therefore, the pores of each MC are fully filled by sulfur). For the same MC, partial sulfur-filling (i.e., the condition at which the sulfur loading is lower than the maximum limit and, therefore, the pores are only partially filled with sulfur) leads to an improved initial discharge capacity and cycle stability, probably because of improved electrical and ionic transport during electrochemical reactions. Based on this understanding, an MCS composite electrode using MC with a large pore volume, partial sulfur filling, and a novel surface modification was designed for Li–S batteries. An initial capacity of ∼1390 mA h g−1 (based on sulfur) and a capacity retention of ∼840 mA h g−1 over 100 cycles at a 0.1 C rate were obtained using MC (22 nm, 4.8 cm3 g−1) with 50 wt% sulfur loading and a commercially available Clevios P (poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate) (PEDT/PSS)) coating.

Manipulating surface reactions in lithium–sulphur batteries using hybrid anode structures

Lithium-sulphur batteries have high theoretical energy density and potentially low cost, but significant challenges such as severe capacity degradation prevent its widespread adoption. Here we report a new design of lithium-sulphur battery using electrically connected graphite and lithium metal as a hybrid anode to control undesirable surface reactions on lithium. Lithiated graphite placed in front of the lithium metal functions as an artificial, self-regulated solid electrolyte interface layer to actively control the electrochemical reactions and minimize the deleterious side reactions, leading to significant performance improvements. Lithium-sulphur cells incorporating this hybrid anodes deliver capacities of >800 mAh g(-1) for 400 cycles at a high rate of 1,737 mA g(-1), with only 11% capacity fade and a Coulombic efficiency >99%. This simple hybrid concept may also provide scientific strategies for protecting metal anodes in other energy-storage devices.

In Situ Transmission Electron Microscopy Observation of Microstructure and Phase Evolution in a SnO2 Nanowire during Lithium Intercalation

Recently we have reported structural transformation features of SnO(2) upon initial charging using a configuration that leads to the sequential lithiation of SnO(2) nanowire from one end to the other (Huang et al. Science2010, 330, 1515). A key question to be addressed is the lithiation behavior of the nanowire when it is fully soaked into the electrolyte (Chiang Science2010, 330, 1485). This Letter documents the structural characteristics of SnO(2) upon initial charging based on a battery assembled with a single nanowire anode, which is fully soaked (immersed) into an ionic liquid based electrolyte using in situ transmission electron microscopy. It has been observed that following the initial charging the nanowire retained a wire shape, although highly distorted. The originally straight wire is characterized by a zigzag structure following the phase transformation, indicating that during the phase transformation of SnO(2) + Li ↔ Li(x)Sn + Li(y)O, the nanowire was subjected to severe deformation, as similarly observed for the case when the SnO(2) was charged sequentially from one end to the other. Transmission electron microscopy imaging revealed that the Li(x)Sn phase possesses a spherical morphology and is embedded into the amorphous Li(y)O matrix, indicating a simultaneous partitioning and coarsening of Li(x)Sn through Sn and Li diffusion in the amorphous matrix accompanied the phase transformation. The presently observed composite configuration gives detailed information on the structural change and how this change takes place on nanometer scale.

Nanoscale Effects on Ion Conductance of Layer-by-Layer Structures of Gadolinia-doped Ceria and Zirconia

Layer-by-layer structures of gadolinia-doped ceria and zirconia have been synthesized on Al2O3(0001) using oxygen plasma-assisted molecular beam epitaxy. Oxygen ion conductivity greatly increased with an increasing number of layers compared to bulk polycrystalline yttria-stabilized zirconia and gadolinia-doped ceria electrolytes. The conductivity enhancement in this layered electrolyte is interesting, yet the exact cause for the enhancement remains unknown. For example, the space charge effects that are responsible for analogous conductivity increases in undoped layered halides are suppressed by the much shorter Debye screening length in layered oxides. Therefore, it appears that a combination of lattice strain and extended defects due to lattice mismatch between the heterogeneous structures may contribute to the enhancement of oxygen ionic conductivity in this layered oxide system.