Study on the Nano-structured Materials for Energy Storage System

The Li-oxygen cell is an emerging energy storage system which has a great promise in the near future for its high energy density and use of free oxygen available from the air. This energy system is attractive since it has nine times higher energy density than conventional lithium-ion cells based on LiCoO2 and graphite. However, this deceptively simple system poses many challenges which must be overcome before it could be considered for any practical application. Firstly, the formation of the insulating and insoluble reaction product, lithium peroxide, requires the highly porous, yet electronically well percolated cathode structure for the reasonable performance. Secondly, the decomposition of lithium peroxide usually involves the huge anodic overpotential which requires the development of highly efficient catalyst. Thirdly, a proper electrolyte system should be developed which is resistant to both the superoxide attack during the discharge reaction and the highly oxidizing environment during the charge process where both oxygen radicals and catalysts are present. Lastly, protection of the lithium anode is necessary, otherwise oxygen radicals in the solvent will react with lithium metal spontaneously, which eventually increases the impedance and more importantly depletes lithium metal from the anode. This thesis is primarily focused on the development of highly efficient catalysts for oxygen reduction and oxygen evolution reactions for the rechargeable Li-oxygen battery. To this end, a highly porous and electrically networked cathode film was manufactured by utilizing common plasticizers as pore forming agents and a Li-oxygen testing cells were developed using Swagelok fittings. The Li-oxygen cell test in two different electrolyte systems shows that the reactivity of electrolyte system to superoxide radicals is a key parameter to determine the nature of reaction product. For LiBOB/PC system, both LIBOB and PC are actively decomposed by superoxide radicals to produce lithium oxalate and lithium carbonate as main discharge products. In the case of LiPF6/TEGDME system, both salt and solvent are stable and thus ideal discharge product, lithium peroxide is obtained. Lead ruthenate and bismuth ruthenate with the extended pyrochlore structure show an excellent catalytic activity by increasing discharge capacity and lowering the anodic overpotential considerably during charge process in both electrolyte systems. They reduce the decomposition of electrolyte system and the extent of carbon corrosion, which accounts for more efficient cycling. The excellent catalytic activity of these pyrochlores originates from their intrinsic oxygen vacancies, electronic conductivity and many surface active sites afforded by its morphology. The performance of this catalyst was further increased through gold deposition on the pyrochlore surface, resulting in much increased discharge capacity. The pyrochlore coated carbon was proposed as a type of catalyst for an efficient way to reduce the amount of catalyst and enhance homogenous mixing with other components. The investigation on the lithium peroxide decomposition mechanism shows that carbon corrosion which occurs at around 4.0 V by lithium peroxide makes further decomposition difficult without a catalyst. In the presence of catalyst, almost full decomposition of lithium peroxide occurs with a lowered decomposition potential even though carbon corrosion still occurs. This gives a hint that the generation of a nano-porous structure and the homogenous distribution of catalyst over these pores are very important, as well as use of a highly efficient catalyst for lowering activation overpotentials. In conclusion, although there are still many obstacles present, as listed above, for the commercial application of Li-oxygen cell, these hurdles are surmountable in the near future by intensive research and the results shown in this thesis can be a cornerstone for further research. Supercapacitors based on metal oxide are new energy storage devices for ultrafast charging and discharging with decent energy density over hundreds of thousands of cycles for many commercial electronic devices and power tools. The surface redox nature of these reactions requires the creation of high surface area for better utilization and performance. Hydrous ruthenium dioxide is one of the most attractive supercapacitor materials owing to its high pseudo-capacitance and metallic character, which facilitate fast electron movement. In this thesis, a simple process involving soft liquid crystal templating using cationic surfactant and gentle heat-treatment in the mild temperature was developed to prepare mesoporous ruthenium oxide with a quasi-crystalline wall character, which has a high surface area and controlled water contents in the structure. The electrochemical testing results exhibited a promising performance of high gravimetric capacitance and a good rate capability facilitated by high surface area as well as porous structure.1 yea

Tailoring ion-conducting interphases on magnesium metals for high-efficiency rechargeable magnesium metal batteries

Magnesium (Mg) rechargeable batteries are one of the promising highenergy post-lithium battery chemistries exploiting the multivalent charge carrier. However, the use of magnesium metal has been challenging due to the formation of the ion-blocking passivation layer on magnesium metal in most organic electrolytes. Herein, we propose a new strategy to transform the passivating film into a Mg2+- conductive interphase via simple chemisorption of sulfur dioxide molecules on magnesium metal. The facile chemical tuning converts the magnesium oxide-based passivation layer into a magnesium sulfate-like phase, which greatly enhances the chargetransfer capability of multivalent Mg2+ ions. The reduced surface resistance of the magnesium metal results in efficient magnesium stripping/deposition reactions even under conventional ether-based electrolytes. Theoretical calculations support that the facile ionic conduction is attributed to the relatively low Mg2+ dissociation and migration energies in the tailored interphases. Furthermore, we elucidate the degradation mechanism of magnesium electrodes by combining various experimental analyses with computational calculations.11Nsciescopu

Tungsten Carbide as a Highly Efficient Catalyst for Polysulfide Fragmentations in Li-S Batteries

The sluggish disproportionation of short-chain lithium polysulfides, Li2Sx is known to be one of the major causes to limit the rate capability of lithium sulfur batteries. Herein, we report that tungsten carbide not only affords strong sulfiphilic surface moieties but also provides an efficient catalysis to enhance the polysulfide fragmentation, leading to a drastic improvement in the electrode kinetics. We show that tungsten carbide acts as a superb anchoring material for the long-chain polysulfide and also promotes the dissociation of short-chain polysulfide during the electroreduction process. This leads to a high-rate performance of the composite cathode loaded with tungsten carbide, delivering a markedly enhanced discharge capacity of 780 mA h g(-1) at a high current rate of 5 C, when it is applied with a combination of a carbon-coated separator for the polysulfide confinement. Hence, this work presents a new strategic approach for a high-power lithium-sulfur battery.11Nsciescopu

Simultaneous etching and transfer-Free multilayer graphene sheets derived from C60 thin films

Despite the advantage of chemical vapor deposition (CVD) for realization of large area epitaxial growth of graphene on transition metal catalysts, both etching and transfer process of CVD-grown graphene sheets still remain a big challenge. Here we demonstrate the formation of multilayer graphene (MLG) sheets tailored from C60 thin films on the top of Si/Ni substrate without etching and transfer steps based on Ni films. This self-assembled process separates the MLG sheets from the conductive Ni catalyst, embarking a possibility for direct characterizations of MLG sheets. The fine-tuned C60 films (30 nm) are transformed into approximately 17 MLG sheets, thus making it large-area MLG sheets for a variety of direct applications. © 2018 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.11sciekc

Direct, Soft Chemical Route to Mesoporous Metallic Lead Ruthenium Pyrochlore and Investigation of its Electrochemical Properties

Mesoporous, nanocrystalline metallic lead ruthenium oxide (a pyrochlore) was synthesized through the formation of a mesostructured cohydroxide network via liquid crystal templating, and subsequent “soft” chemical oxidation that crystallizes the oxide at low temperature. The stable S⁺I^– interaction chemistry responsible for the templating methodology is elucidated. The formation of a disordered mesoporous structure with a pore volume of 0.18 cm³/g and walls comprised of nanocrystallites was confirmed by X-ray diffraction and conductivity, N₂ isotherm measurements, and TEM observations. The resistivity of mesoporous oxide at room temperature was 0.046 Ω·cm, only 2 orders of magnitude less than the single crystal value, and one of the only two porous metallic oxides that are known to date. The electrocatalytic properties of this material for oxygen reduction and evolution in aqueous and nonaqueous media were evaluated by cyclic voltammetry, chronoamperometry, and linear sweep voltammetry. These techniques show that the synthesized pyrochlore lowers the overall oxidation voltage by 0.7 V relative to carbon in nonaqueous, Li⁺-containing electrolyte. This is the result of its ability to both completely oxidize Li₂O₂ (at a relatively low potential) <i>and</i> electrocatalytically oxidize all known side-products formed from electrolyte decomposition in the Li–O₂ battery. This further helps to explain the nature of “electrocatalysis” in this system

Operando Visualization of Morphological Evolution in Mg Metal Anode: Insight into Dendrite Suppression for Stable Mg Metal Batteries

© Rechargeable Mg-metal batteries (RMBs) are considered promising alternatives to conventional Li-ion batteries owing to their high volumetric capacity and low cost. In addition, Mg anodes for RMBs do not suffer from metal dendritic growth or internal short circuit. However, the notion that Mg anodes are indeed dendrite-free has recently been under debate, and further clarification is crucial for advancing practical RMBs. In this work, we closely investigated Mg dendrite behaviors under various electrochemical test conditions using operando observation techniques. The critical current density inducing fatal Mg dendritic growth was defined by directly monitoring the dendritic growth process leading to a short circuit. We further propose a new strategy to regulate the dendrite growth by introducing magnesiophilic sites of Au nanoseeds on a substrate. We not only elucidated the effect of the applied current density and capacity utilization on the Mg growth behaviors but also demonstrated the effect of magnesiophilic seeds in suppressing Mg dendrite growth.11Nsciescopu

Heterogeneous Catalysis for Lithium-Sulfur Batteries: Enhanced Rate Performance by Promoting Polysulfide Fragmentations

A spatial confiment of polysulfides using the metal compound additives having polar surfaces has been considered to be a promising approach to address the insufficient rate capability and cyclability of lithium-sulfur batteries. Herein, we report a more effective approach outperforming this conventional one: a heterogeneous catalysis to promote polysulfide fragmentations. It was revealed using combined computational and experimental approaches that an ultrastrong adsorption of elemental sulfur on TiN surfaces resulted in a spontaenous fragmentation into shorter chains of polysulfides. This heterogeneous catalysis reaction improved the sluggish kinetics of polysulfide reduction because of the chemical disproportionation at the second plateau. A markedly enhanced rate capability was finally obtained, exhibiting a discharge capacity of 700 mAh g(-1) at a scan rate of 5C.11Nsciescopu

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

Unraveling the fundamentals of Li-O₂ 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 O₂^–, 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 Li₂O₂ product morphology is a function of these factors and can affect Li-O₂ cell performance. This methodology is widely applicable as a probe of other potential cell components

Critical role of elemental copper for enhancing conversion kinetics of sulphur cathodes in rechargeable magnesium batteries

Despite recent remarkable progress associated with the electrolyte, understanding of the reaction mechanism of magnesium-sulphur batteries is not yet mature. In particular, the lethargic redox reactions involved in the electrochemical conversion of sulphur and MgS in the cathode need to be overcome. Here, we unveil the reaction mechanism involving copper (Cu) metal, a common current collector for electrodes in rechargeable batteries. Specifically, Cu can undergo chemical reactions with polysulphides produced from the reaction of sulphur or MgS with Mg2+. Throughout the conversion reaction, these Cu-polysulphide reactions play a critical role to improve reaction kinetics markedly. The present investigation opens new avenues to the emerging Mg-S battery technology, that is, the incorporation of various metals that can speed up the conversion reaction between sulphur and Mg.

Nanoscale Zirconium-Abundant Surface Layers on Lithium- and Manganese-Rich Layered Oxides for High-Rate Lithium-Ion Batteries

Battery performance, such as the rate capability and cycle stability of lithium transition metal oxides, is strongly correlated with the surface properties of active particles. For lithium-rich layered oxides, transition metal segregation in the initial state and migration upon cycling leads to a significant structural rearrangement, which eventually degrades the electrode performance. Here, we show that a fine-tuning of surface chemistry on the particular crystal facet can facilitate ionic diffusion and thus improve the rate capability dramatically, delivering a specific capacity of ∼110 mAh g^–1 at 30C. This high rate performance is realized by creating a nanoscale zirconium-abundant rock-salt-like surface phase epitaxially grown on the layered bulk. This surface layer is spontaneously formed on the Li⁺-diffusive crystallographic facets during the synthesis and is also durable upon electrochemical cycling. As a result, Li-ions can move rapidly through this nanoscale surface layer over hundreds of cycles. This study provides a promising new strategy for designing and preparing a high-performance lithium-rich layered oxide cathode material