Taeeun Yim

Spherical Ordered Mesoporous Carbon Nanoparticles with High Porosity for Lithium–Sulfur Batteries

Rechargeable lithium–sulfur (Li–S) batteries are attracting increasing attention due to their high theoretical specific energy density, which is 3 to 5 times higher than that of Li-ion batteries based on intercalation chemistry. Since the electronic conductivity of sulfur is extremely low, conductive carbon materials with high accessible porosity to “wire” and contain the sulfur are an essential component of the positive electrode. During the past decades, attempts have been made to fabricate C/S composites using carbon black, activated carbons (ACs), and carbon nanotubes (CNTs). Although improvements resulted, the cathodes suffered from inhomogeneous contact between the active material and the electronic conductors. A major step forward in fabricating a uniform C/S composite was reported in 2009. Some of us employed CMK-3, an ordered mesoporous carbon (OMC) featuring high specific surface area and large pore volume as a scaffold. As much as 70 wt% sulfur was incorporated into the uniform 3–4 nm mesopores, and the cells exhibited reversible capacities up to 1350 mAhg , albeit at moderate rates. Inspired by this, another OMC, a bulk bimodal mesoporous carbon (BMC-1) was investigated as a Li-S cathode. The favorable pore dimensions and large pore volume greatly improved the rate performance. An electrode with 40 wt% S showed a high initial discharge capacity of 1135 mAhg 1 at a current rate of 1 C (defined as discharge/ charge in one hour). However, similar to other reports, the capacity is sensitive to the sulfur ratio, dropping to 718 mAhg 1 at a sulfur content of 60 wt%. These results suggest that the texture of the mesoporous carbon could be further enhanced. Recently, Archer et al. reported nanoscale hollow porous C/S spheres prepared through vapor infusion. These materials displayed good cyclability and capacity at a C/5 rate, illustrating the advantages of nanosized porous carbon in the sulfur cathodes. Here we report the synthesis of unique nanoscale spherical OMCs with extremely high bimodal porosities. The particles were investigated as a cathode material and sulfur host in Li–S batteries where they showed high initial discharge capacity and good cyclability without sacrificing rate capability. Unlike bulk porous carbons, these carbon– sulfur sphere electrodes did not display significant capacity fading with the increase of sulfur content in the cathodes. We show that the nanoscale morphology of these materials is of key importance for ensuring very efficient use of the sulfur content even at high cycling rates. Morphology control is a central issue in OMC synthesis. There are numerous examples of mesoporous bulk materials obtained either by hard-templating or soft-templating, including thin films, membranes or free fibers. Most syntheses use evaporation-induced self-assembly (EISA) followed by thermal treatment for template-removal and carbonization. It is a challenge to either create solution-based OMC nanoparticle syntheses or to adapt the established EISA methods to nanoparticles. Only few examples of OMC nanoparticles have been reported so far which are mostly unsuitable for applications in Li–S cells due to low pore volume and/or surface area. Approaches include templating with PMMA colloidal crystals or mesoporous silica nanoparticles, aerosol-assisted synthesis, ultrasonic emulsification or hydrothermal synthesis. Ordered arrays of fused mesoporous carbon spheres were reported by Liu et al. using a macroporous silica as template. Recently Lei et al. reported the synthesis of 65 nm mesoporous carbon nanospheres, with both 2.7 nm mesopores and high textural porosity (surface area of 2400 mg ). These showed promising supercapacitor properties. Our spherical OMC nanoparticles of 300 nm in diameter, prepared by a novel method, can be dispersed in water by sonification to form stable colloidal suspensions. The spherical mesoporous carbon nanoparticles were obtained in a twostep casting process. An opal structure of PMMA spheres was cast with a silica precursor solution to form a silica inverse opal. The inverse opal was then used as template for a triconstituent precursor solution containing resol as the carbon precursor, tetraethylorthosilicate (TEOS) as the silica precursor and the block copolymer Pluronic F127 as a structure-directing agent. Carbonization was followed by etching of the silica template and the silica in the carbon/silica nanocomposite, resulting in the formation of OMC with hierarchical porosity. Through the presence of silica in the [*] J. Schuster, B. Mandlmeier, Prof. Dr. T. Bein Department of Chemistry and Center for NanoScience (CeNS), University of Munich (LMU), Butenandtstrasse 5–13 (Gerhard Ertl Building), 81377 Munich (Germany) E-mail: tbein@cup.uni-muenchen.de Homepage: http://bein.cup.uni-muenchen.de G. He, T. Yim, K. T. Lee, Prof. Dr. L. F. Nazar Department of Chemistry, University of Waterloo 200 University Avenue West, Waterloo, Ontario N2L 3G1 (Canada) E-mail: lfnazar@uwaterloo.ca [] These authors contributed equally to this work.

Screening for Superoxide Reactivity in Li-O2 Batteries: Effect on Li2O2/LiOH Crystallization

Unraveling the fundamentals of Li-O(2) battery chemistry is crucial to develop practical cells with energy densities that could approach their high theoretical values. We report here a straightforward chemical approach that probes the outcome of the superoxide O(2)(-), thought to initiate the electrochemical processes in the cell. We show that this serves as a good measure of electrolyte and binder stability. Superoxide readily dehydrofluorinates polyvinylidene to give byproducts that react with catalysts to produce LiOH. The Li(2)O(2) product morphology is a function of these factors and can affect Li-O(2) cell performance. This methodology is widely applicable as a probe of other potential cell components.

A new strategy for integrating abundant oxygen functional groups into carbon felt electrode for vanadium redox flow batteries

The effects of surface treatment combining corona discharge and hydrogen peroxide (H2O2) on the electrochemical performance of carbon felt electrodes for vanadium redox flow batteries (VRFBs) have been thoroughly investigated. A high concentration of oxygen functional groups has been successfully introduced onto the surface of the carbon felt electrodes by a specially designed surface treatment, which is mainly responsible for improving the energy efficiency of VRFBs. In addition, the wettability of the carbon felt electrodes also can be significantly improved. The energy efficiency of the VRFB cell employing the surface modified carbon felt electrodes is improved by 7% at high current density (148 mA cm−2). Such improvement is attributed to the faster charge transfer and better wettability allowed by surface-active oxygen functional groups. Moreover, this method is much more competitive than other surface treatments in terms of processing time, production costs, and electrochemical performance.

Surface Film Formation on LiNi0.5Mn1.5O4 Electrode in an Ionic Liquid Solvent at Elevated Temperature

A comparative study is made on the surface film formation on the high-voltage LiNi 0.5 Mn 1.5 O 4 positive electrode at elevated temperature (55°C) in two different electrolytes; LiPF 6 /organic carbonate and LiTFSI/ionic liquid (propylmethylpyrrolidinium bis(trifluoromethylsulfonyl)imide, PMPyr-TFSI). The surface film derived by a decomposition of the former electrolyte is enriched by inorganic fluorinated species, which becomes thicker with cycling to lead a continued electrode polarization and cell failure. In contrast, organic carbon species are dominant in the film derived from the latter electrolyte. This organic-rich film deposited in the earlier period of cycling seems to effectively passivate the positive electrode presumably due to uniform coverage. As a result, the film does not grow with cycling and electrode polarization is not serious to give a stable cycling behavior.

Linear-Sweep Thermammetry Study on Corrosion Behavior of Al Current Collector in Ionic Liquid Solvent

The corrosion behavior of A1 foil as the current collector for lithium-ion batteries is studied by linear-sweep thermammetry. The onset temperature for Al pitting corrosion depends on Li salt that is dissolved in an ionic liquid solvent; lithium bis(trifluo- romethanesulfonyl)imide (LiTFSI) < lithium bis(perfluoroethanesulfonyl)imide (LiBETI) < LiPF 6 < LiBF 4 , With LiBF 4 , no corrosion current is observed until 1 10°C. X-ray photoelectron spectroscopy study reveals that this Al surface is covered by Al-F compound (presumably AlF 3 ). Due to the formation of a highly passivating AlF 3 layer in this electrolyte, the high voltage LiNi 0.5 Mn 1.5 O 4 positive electrode coated on Al foil can be successfully cycled at 65°C without electrode failure.

Comparative Study on Surface Films from Ionic Liquids Containing Saturated and Unsaturated Substituent for LiCoO2

A comparative study is made on the surface films that are deposited on a LiCoO 2 electrode by oxidative decomposition of room-temperature ionic liquids (RTILs) that have either a saturated (propyl) or an unsaturated (allyl) substituent on the pyrrolidinium cation. The surface film deposited from the former RTIL does not so perfectly cover the LiCoO 2 electrode surface that the film deposition continues with cycling, leading to a gradual increase in the electrode polarization and an eventual capacity fading. From the allyl-containing RTIL, however, a uniformly covered surface film is deposited even after a single charging to give better cyclability to the LiCoO 2 electrode. The latter film contains a larger amount of organic carbon species relative to that found in the former. An enhanced film property is also observed by adding vinylene carbonate that has an unsaturated moiety, ensuring that the unsaturated functional groups are responsible for such favorable film properties.

Why is tris(trimethylsilyl) phosphite effective as an additive for high-voltage lithium-ion batteries?

Tris(trimethylsilyl) phosphite (TMSP) is well known as an effective electrolyte additive that significantly improves the electrochemical performance of high-voltage lithium-ion batteries. This work reveals that TMSP is oxidized more readily than an electrolyte solvent, is more difficult to be reduced, and exhibits high reactivity with HF. The significant role of TMSP in HF removal is demonstrated by full-cell experiments. We suggest a mechanism through which TMSP removes HF molecules from the electrolyte, forms a cathode–electrolyte interphase with outstanding properties on the cathode surface while not undergoing a side reaction at the anode surface. In addition, the fact that TMSP can be oxidized more readily than vinylene carbonate is what results in the synergistic effect observed when the two additives are used together.

Porous carbon spheres as a functional conducting framework for use in lithium–sulfur batteries

Porous carbon spheres with hybrid pore structure have been designed as a promising conducting framework to be used in cathode material for lithium–sulfur batteries. By creating three-dimensionally interconnected micropores and mesopores, sufficient space for sulfur storage, as well as electrolyte pathways, can be secured in the carbon spheres. Sulfur is mainly confined in mesopores with diameters of a few tens of nanometers in the carbon spheres and separated on the mesoscopic domain, which is advantageous for enhancing charge transfer and effectively accommodating volume expansion of sulfur during electrochemical reactions with Li+. The important role of the micropores, with diameters of less than 2 nm, is to extend effective interfacial contact between the sulfur and electrolyte, leading to enhancement of Li+ mobility. The sulfur-porous carbon sphere composite exhibits excellent cyclic performance and rate capability without significant capacity degradation caused by the loss of soluble Li polysulfides or electrical isolation of the active sulfur in the cathode. Importantly, the shape of the porous carbon spheres is advantageous for building robust electrodes with high-energy density. Our observations, based on various structural and electrochemical analyses, will be helpful for understanding and consolidating the fundamental aspects of the electrochemistry of sulfur. Furthermore, our approach is expected to be helpful in designing and tailoring advanced cathode materials with improved performance for lithium–sulfur batteries.

Physically Cross-linked Polymer Binder Induced by Reversible Acid–Base Interaction for High-Performance Silicon Composite Anodes

Silicon is greatly promising for high-capacity anode materials in lithium-ion batteries (LIBs) due to their exceptionally high theoretical capacity. However, it has a big challenge of severe volume changes during charge and discharge, resulting in substantial deterioration of the electrode and restricting its practical application. This conflict requires a novel binder system enabling reliable cyclability to hold silicon particles without severe disintegration of the electrode. Here, a physically cross-linked polymer binder induced by reversible acid-base interaction is reported for high performance silicon-anodes. Chemical cross-linking of polymer binders, mainly based on acidic polymers including poly(acrylic acid) (PAA), have been suggested as effective ways to accommodate the volume expansion of Si-based electrodes. Unlike the common chemical cross-linking, which causes a gradual and nonreversible fracturing of the cross-linked network, a physically cross-linked binder based on PAA-PBI (poly(benzimidazole)) efficiently holds the Si particles even after the large volume changes due to its ability to reversibly reconstruct ionic bonds. The PBI-containing binder, PAA-PBI-2, exhibited large capacity (1376.7 mAh g(-1)), high Coulombic efficiency (99.1%) and excellent cyclability (751.0 mAh g(-1) after 100 cycles). This simple yet efficient method is promising to solve the failures relating with pulverization and isolation from the severe volume changes of the Si electrode, and advance the realization of high-capacity LIBs.

Self-Extinguishing Lithium Ion Batteries Based on Internally Embedded Fire-Extinguishing Microcapsules with Temperature-Responsiveness

User safety is one of the most critical issues for the successful implementation of lithium ion batteries (LIBs) in electric vehicles and their further expansion in large-scale energy storage systems. Herein, we propose a novel approach to realize self-extinguishing capability of LIBs for effective safety improvement by integrating temperature-responsive microcapsules containing a fire-extinguishing agent. The microcapsules are designed to release an extinguisher agent upon increased internal temperature of an LIB, resulting in rapid heat absorption through an in situ endothermic reaction and suppression of further temperature rise and undesirable thermal runaway. In a standard nail penetration test, the temperature rise is reduced by 74% without compromising electrochemical performances. It is anticipated that on the strengths of excellent scalability, simplicity, and cost-effectiveness, this novel strategy can be extensively applied to various high energy-density devices to ensure human safety.