Hyun-Kon Song

Who will drive electric vehicles, olivine or spinel?

Lithium iron phosphate olivine (LFP) and lithium manganese oxide spinel (LMO) are competitive and complementary to each other as cathode materials for lithium ion batteries, especially for use in hybrid electric vehicles and electric vehicles. Interest in these materials, due to their low cost and high safety, has pushed research and development forward and toward high performance in terms of rate capability and capacity retention or cyclability at a high temperature of around 60 °C. From the view point of basic properties, LFP shows a higher gravimetric capacity while LMO has better conductivities, both electrically and ionically. According to our comparison experiments, depending on the material properties and operational potential window, LFP was favored for fast charging while LMO led to better discharge performances. Capacity fading at high temperatures due to metal dissolution was revealed to be the most problematic issue of LFP and LMO-based cells for electric vehicles (EVs), with thicker electrodes, in the case of no additives in the electrolyte and no coating to prevent metal dissolution on cathode materials. Various strategies to enhance the properties of LFP and LMO are ready for the realization of EVs in the near future.

Facile Route to an Efficient NiO Supercapacitor with a Three-Dimensional Nanonetwork Morphology

NiO nanostructures with three distinct morphologies were fabricated by a sol-gel method and their morphology-dependent supercapacitor properties were exploited. The nanoflower- shaped NiO with a distinctive three-dimensional (3D) network and the highest pore volume shows the best supercapacitor properties. The nanopores in flower-shaped nanostructures, offering advantages in contact with and transport of the electrolyte, allow for 3D nanochannels in NiO structure, providing longer electron pathways. The XPS and EIS data of the NiO nanostructure confirm that the flower-shaped NiO, which has the lowest surface area among the three morphologies, was effectively optimized as a superior electrode and yielded the greatest pseudocapacitance. This study indicates that forming a 3D nanonetwork is a straightforward means of improving the electrochemical properties of a supercapacitor.

Ionic liquid modified graphene nanosheets anchoring manganese oxide nanoparticles as efficient electrocatalysts for Zn–air batteries

Ionic liquid (IL) modified reduced graphene oxide (rGO–IL) nanosheets anchoring manganese oxide (Mn3O4) are synthesized via a facile solution-based growth mechanism and applied to a Zn–air battery as an effective electrocatalyst for the oxygen reduction reaction (ORR). In this study, the IL moiety in these composites increases not only the conductivity of the system, but also the electrocatalytic activity compared to pristine rGO, together with the synergic effect of facilitating the ORR with the intrinsic catalytic activity of Mn3O4. Based on the Koutecky–Levich plot, we suggest that the ORR pathway of these composites is tunable with the relative amount of Mn3O4 nanoparticles supported onto the graphene sheets; for example, the ORR mechanism of the system with a lower Mn3O4 (19.2%) nanoparticle content is similar to a Pt/C electrode, i.e., a one-step, quasi-4-electron transfer, unlike that with a higher Mn3O4 (52.5%) content, which undergoes a classical two-step, 2-electron pathway. We also demonstrate the potential of these hybrid rGO–IL/Mn3O4 nanoparticles as efficient catalysts for the ORR in the Zn–air battery with a maximum peak power density of 120 mW cm−2; a higher performance than that from commercial cathode catalysts.

Scalable approach to multi-dimensional bulk Si anodes via metal-assisted chemical etching

Specific design and optimization of the configuration of micro-scale materials can effectively enhance battery performance, including volumetric density. Herein, we employed commercially available low-cost bulk silicon powder to produce multi-dimensional silicon composed of porous nanowires and micro-sized cores, which can be used as anode materials in lithium-ion batteries, by combining a metal deposition and metal-assisted chemical etching process. Nanoporous silicon nanowires of 5–8 μm in length and with a pore size of ∼10 nm are formed in the bulk silicon particle. The silicon electrodes having multi-dimensional structures accommodate large volume changes of silicon during lithium insertion and extraction. These materials show a high reversible charge capacity of ∼2400 mAh g−1 with an initial coulombic efficiency of 91% and stable cycle performance. The synthetic route described herein is simple, low-cost, and mass producible (high yield of 40–50% in tens of gram scale), and thus, provides an effective method for producing high-performance anode materials.

A polymer electrolyte-skinned active material strategy toward high-voltage lithium ion batteries: a polyimide-coated LiNi0.5Mn1.5O4 spinel cathode material case

A facile approach to the surface modification of spinel LiNi0.5Mn1.5O4 (LNMO) cathode active materials for high-voltage lithium ion batteries is demonstrated. This strategy is based on nanoarchitectured polyimide (PI) gel polymer electrolyte (GPE) coating. The PI coating layer successfully wrapped a large area of the LNMO surface via thermal imidization of 4-component (pyromellitic dianhydride/biphenyl dianhydride/phenylenediamine/oxydianiline) polyamic acid. In comparison to conventional metal oxide-based coatings, distinctive features of the unusual PI wrapping layer are the highly continuous surface coverage with nanometre thickness (∼10 nm) and the provision of facile ion transport. The nanostructure-tuned PI wrapping layer served as an ion-conductive protection skin to suppress the undesired interfacial side reactions, effectively preventing the direct exposure of the LNMO surface to liquid electrolyte. As a result, the PI wrapping layer played a crucial role in improving the high-voltage cell performance and alleviating the interfacial exothermic reaction between charged LNMO and liquid electrolyte. Notably, the superior cycle performance (at 55 °C) of the PI-wrapped LNMO (PI-LNMO) was elucidated in great detail by quantitatively analyzing manganese (Mn) dissolution, cell impedance, and chemical composition (specifically, lithium fluoride (LiF)) of byproducts formed on the LNMO surface.

Hybrid multilayer thin film supercapacitor of graphene nanosheets with polyaniline: importance of establishing intimate electronic contact through nanoscale blending

A hybrid electrode consisting of an electric double-layer capacitor of graphene nanosheets and a pseudocapacitor of the conducting polymer polyaniline exhibits a synergistic effect with excellent electrochemical performance for flexible thin film supercapacitors. This hybrid supercapacitor is constructed by a nanoscale blending method of layer-by-layer (LbL) assembly based on the electrostatic interactions between positively charged polyaniline (PANi) and negatively charged graphene oxide (GO) nanosheets. The hybrid electrode provides not only improved electronic conductivity through the intimate contact with the graphene nanosheet, but also enhanced chemical stability during the charge–discharge process. We also investigated the dependence of the electrochemical performance on the various parameters of LbL assembly such as the number of bilayers and the post-thermal and chemical treatments that could affect the degree of reduction of GO and PANi. We found that after thermal treatment, the LbL-assembled thin film of PANi with GO nanosheets exhibited an excellent gravimetric capacitance of 375.2 F g−1 at a discharge current density of 0.5 A g−1 that outperformed many other hybrid supercapacitors reported to date. The hybrid supercapacitor maintained its capacity up to 90.7% over 500 cycles at a high current density of 3.0 A g−1. This study opens up the possibility for the production of diverse graphene-based hybrid nanocomposites that are promising for future flexible supercapacitors.

Electrochemical Regeneration of NADH Enhanced by Platinum Nanoparticles

This work was supported by the Korea Energy Management Corporation (2005-C-CD11-P-04) and the Korea Research Foundation (KRF-2006-331-D00113).

Redox-active charge carriers of conducting polymers as a tuner of conductivity and its potential window

Electric conductivity of conducting polymers has been steadily enhanced towards a level worthy of being called its alias, “synthetic metal”. PEDOT:PSS (poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonate)), as a representative conducting polymer, recently reached around 3,000 S cm−1, the value to open the possibility to replace transparent conductive oxides. The leading strategy to drive the conductivity increase is solvent annealing in which aqueous solution of PEDOT:PSS is treated with an assistant solvent such as DMSO (dimethyl sulfoxide). In addition to the conductivity enhancement, we found that the potential range in which PEDOT:PSS is conductive is tuned wider into a negative potential direction by the DMSO-annealing. Also, the increase in a redox-active fraction of charge carriers is proposed to be responsible for the enhancement of conductivity in the solvent annealing process.

Electronegativity-induced enhancement of thermal stability by succinonitrile as an additive for Li ion batteries

Succinonitrile (SN, CN–[CH2]2–CN) is evaluated as an additive for improving thermal stability in ethylene carbonate (EC)-based electrolytes for lithium ion batteries. Without any sacrifice of performance such as cyclability and capacity, the introduction of SN into an electrolyte with a graphite anode and LixCoO2 cathode leads to (1) reducing the amount of gas emitted at high temperature, (2) increasing the onset temperature of exothermic reactions and (3) decreasing the amount of exothermal heat. The improvement in the thermal stability is considered to be due to strong complex formation between the surface metal atoms of LixCoO2 and nitrile (–CN) groups of SN, from spectroscopic studies based on photoelectrons induced by X-rays and by considering that the exothermic heat and gas evolution are caused by interfacial reactions between the electrolyte and cathode.

High-performance silicon-based multicomponent battery anodes produced via synergistic coupling of multifunctional coating layers

Nanostructured Si-based materials are key building blocks for next-generation energy storage devices. To meet the requirements of practical energy storage devices, Si-based materials should exhibit high-power, low volume change, and high tap density. So far, there have been no reliable materials reported satisfying all of these requirements. Here, we report a novel Si-based multicomponent design, in which the Si core is covered with multifunctional shell layers. The synergistic coupling of Si with the multifunctional shell provides vital clues for satisfying all Si anode requirements for practical batteries. The Si-based multicomponent anode delivers a high capacity of ∼1000 mA h g−1, a highly stable cycling retention (∼65% after 1000 cycles at 1 C), an excellent rate capability (∼800 mA h g−1 at 10 C), and a remarkably suppressed volume expansion (12% after 100 cycles). Our synthetic process is simple, low-cost, and safe, facilitating new methods for developing electrode materials for practical energy storage.