Electrochemical Performance and Failure Mechanism of Phosphorus/Graphite Composite as an Anode for Na-ion Batteries

Department of Energy Engineering (Battery Science and Technology)These days, there are large applications of lithium ion batteries (LIBs) as energy conversion and storage devices, from portable devices (e.g. mobile devices) to large scale energy storage devices (e.g. power devices – electrical vehicles (EVs), military parts, and stationary electrical energy storage systems – energy storage system (ESS)). As expanding applications of the LIBs, the amount of demanding the LIBs is growing fast. But lithium reserves are not so much in Earth’s crust and distributed in limited locations. Therefore, in the not too distant future, lithium prices will be raised inevitably. At this point in time, Na-ion batteries (NIBs) are re-spotlighted and nominated for one of the post LIBs. The NIBs have typical two advantages of cost reduction. One is that sodium resources are abundant materials in Earth’s crust. The other is that using Aluminum current collector is available on the anode for NIBs. However, at the present time, there are limitations for practical using NIBs. Many materials for NIBs have not been developed up to now. On the cathode materials, comparing with cathode materials for LIB, the cathode materials for NIBs have lower operating voltage and lower specific capacity. And on the cathode side, it has limitations for the intercalation chemistry. Hence, the development of the anode materials for NIBs should be conducted. Among them, the anode materials for NIBs should have lower operating voltage and higher specific capacity to increase energy density for NIBs. In this situation, an amorphous red phosphorus/carbon composite was introduced as a promising anode material for NIBs. Because the red phosphorus has poor electronic conductivity, the carbon acts as a conductive material. It showed the highest reversible capacity and suitable operating voltage for NIBs. But the composite has poor cycle-ability in terms of long cycling. Because alloying compounds (in this case, phosphorus) have severe volume expansion and shrinkage that cause contact loss, and then capacity fading is happened. The carbon couldn’t sufficiently act as a buffer matrix in this case. Therefore, to improve cycle-ability for phosphorus composite materials, a new composite structure is adopted and an electrolyte additive is used in this research. Firstly, a phosphorus/graphite composite is used as an anode material for NIBs. The composite is obtained via a facile and simple mechanochemical ball milling process. The composites are tested by two kinds of different weight ratio. The two composites show that high reversible capacities and good cycle-ability comparing with previously reported the amorphous red phosphorus/carbon composite. And by having different graphite contents, the two composites have different structures which affect the reversible capacity, the cycle-ability, low changing ratio of electrode thickness, surface area, and pore volume. Hence, optimizing weight ratio of the phosphorus/graphite composite, higher reversible capacity and good cycle-ability can be achieved simultaneously. Secondly, fluoroehtylene carbonate (FEC) is used as an electrolyte additive, surfaces of electrodes and Na metals are analyzed. The FEC used case shows better electrochemical performances than the FEC unused case. To define different points between the two cases, EIS and XPS analysis are carried out on the electrodes and the Na metals. In EIS result, charge transfer resistances have different behaviors between the electrodes and the Na metals. To investigate these phenomena, XPS analysis is conducted, and the XPS result shows that compositions of solid electrolyte interphase (SEI) layers and surface are different between the cases. The FEC additive makes stable SEI layers and minimizes oxidation of phosphorus on the electrode. Through the results in this research, the phosphorus/graphite composite can fulfill increasing electronic conductivity and buffer matrix structure. Phosphorus is coated by graphite, exposure of phosphorus is minimized. And the composite can be one of the promising anode materials for NIBs by having high reversible capacity and low operating voltage. Additionally, using the FEC additive can further enhance the electrochemical performances by making stable surface for the composite.ope

Microstructure and adhesion characteristics of a silver nanopaste screen-printed on Si substrate

The microstructural evolution and the adhesion of an Ag nanopaste screen-printed on a silicon substrate were investigated as a function of sintering temperature. Through the two thermal analysis methods, such as differential scanning calorimeter and thermo-gravimetric analysis, the sintering conditions were defined where the temperature was raised from 150°C to 300°C, all with a fixed sintering time of 30 min. The microstructure and the volume of the printed Ag nanopaste were observed using a field emission scanning electron microscope and a 3-D surface profiler, respectively. The apparent density of the printed Ag nanopaste was calculated depending on the sintering conditions, and the adhesion was evaluated by a scratch test. As the sintering temperature increased from 150°C to 300°C, the apparent density and the adhesion increased by 22.7% and 43%, respectively. It is confirmed that the printed Ag nanopaste sintered at higher temperatures showed higher apparent density in the microstructural evolution and void aggregation, resulting in the lower electrical resistivity and various scratched fractures

The Value of Waiting: Foreign Direct Investment with Uncertainty and Imperfect Local Knowledge

This paper examines the role of uncertainty and imperfect local knowledge in foreign direct investment. The main idea comes from the literature on investment under uncertainty, such as Pindyck (1991) and Dixit and Pindyck (1994). We empirically test .the value of waiting. with a dataset on foreign direct investment (FDI). Many factors (e.g., political and economic regulations) as well as uncertainty and the risks due to imperfect local knowledge, determine the attractiveness of FDI. The uncertainty and irreversibility of FDI links the time interval between permission and actual execution of such FDI with explanatory variables, including information on foreign (home) countries and domestic industries. Common factors, such as regulatory change and external shocks, may affect the uncertainty when foreign investors make irreversible FDI decisions. We derive testable hypotheses from models of investment under uncertainty to determine those possible factors that induce delays in FDI, using Korean data over 1962 to 2001

Sodium Biphenyl as Anolyte for Sodium-Seawater Batteries

Sodium-based battery systems have recently attracted increasing research interest due to the abundant resources employed. Among various material candidates for the negative electrode, sodium metal provides the highest capacity of theoretically 1165 mAh g(-1) and a very low redox potential of -2.71 versus the standard hydrogen electrode. However, the high reactivity of sodium metal toward the commonly used electrolytes results in severe side reactions, including the evolution of gaseous decomposition products, and, in addition, the risk of dendritic sodium growth, potentially causing a disastrous short circuit of the cell. Herein, the use of sodium biphenyl (Na-BP) as anolyte for the Na-seawater batteries (Na-SWB) is investigated. The catholyte for the open-structured positive electrode is natural seawater with sodium cations dissolved therein. Remarkably, the significant electronic and ionic conductivities of the Na-BP anolyte enable a low overpotential for the sodium deposition upon charge, allowing for high capacity and excellent capacity retention for 80 cycles in full Na-SWB. Additionally, the Na-BP anolyte suppresses gas evolution and dendrite growth by forming a homogeneous surface layer on the metallic negative electrode

Effects Of Patent Pools On Innovation Investment Ex Ante Perspectives

Recently Patent Pooling has a fast growing interest as a good alternative means to decrease transaction costs between IPRs owners and promote technology commercialization and diffusion. In this paper we attempt to shed light on the effects of patent pooling on the ex-ante innovation investment or incentive using the game theoretical economic model. We generalize the model by including many vertical integrated firms, research laboratories, and specialized manufacturing firms. Main results of this paper are: 1) Patent Pools can affect on the innovation incentives of vertically integrated firms(I-firms) and of research laboratories(R-firms) differently, and the effect depends on the number of I-firms owning essential  patents and the number of specially manufacturing firms(M-firms). But in the presence of many I-firms owning essential patents, the instruction of patent pooling increases  I-firms’ ex-ante innovation incentive or investments with independence of M-firms. 2) There is strategic complementary relationship between innovation investments of I-firms and those of R-firms, so I-firms’ increased ex-ante innovation investments make R-firms’ ex-ante innovation investments increasing. 3) In the case of R-firms maximizing private profit, the best aspect is to license independently their patent technology when I-firms make up patent pools. But this aspect is not desirable for I-firms because I-firms’ gross profit is smaller than that of I-firms which license their patent technologies independently. However, we show that in the cases of many I-firms owning essential patent technologies, patent pools including only I-firms(IP) or all upstream firms(CP) can affect asymmetrically on the I-firms’ or R-firms’ innovation investments. Nonetheless, any types of patent pools make the innovation investments of I-firms and R-firms higher than those of all firms which license independently. In summary, nowadays under general aspects that production of final goods requires many complex technologies and that many I-firms and R-firms attend R&D for essential technologies, competition authorities’ deregulation for patent pooling or government policy supporting the patent pooling can promote upstream firms’ innovation incentives or investments and compulsory licensing about R-firms is not necessary for enhancing upstream firms’ innovation investments or incentives.

Solvent‐free Ternary Polymer Electrolytes with High Ionic Conductivity for Stable Sodium‐based Batteries at Room Temperature

Transitioning to solid-state batteries using polymer electrolytes results in inherently safer devices and can facilitate the use of sodium metal anodes enabling higher energy densities. In this work, solvent-free ternary polymer electrolytes based on cross-linked polyethylene oxide (PEO), sodium bis(fluorosulfonyl) imide (NaFSI) or sodium bis(trifluoromethanesulfonyl) imide (NaTFSI) and N-butyl-N-methyl-pyrrolidinium-based ionic liquids (ILs, Pyr$_{14}$FSI or Pyr$_{14}$TFSI) are developed. Synthesized polymer membranes are thoroughly characterized, verifying their good thermal and electrochemical stability, as well as a low glass transition and crystallinity, thus high segmental mobility of the polymer matrix. The latter results in good ionic conductivities around 1×10$^{−3}$ S cm$^{−1}$ at 20 °C. The polymer electrolytes are successfully employed in sodium-metal battery (SMB) cells operating at room temperature (RT) and using P2-Na$_{2/3}$Ni$_{1/3}$Mn$_{2/3}$O$_2$ layered oxide as cathode. The electrochemical performance strongly depends on the choice of anion in the conducting sodium salt and plasticizing IL. Furthermore, this solid-state SMB approach mitigates capacity fading drivers for the P2-Na$_{2/3}$Ni$_{1/3}$Mn$_{2/3}$O$_2$, resulting in high Coulombic efficiency (99.91 %) and high capacity retention (99 % after 100 cycles) with good specific capacity (140 mAh g$^{−1}$)

Anode-less seawater batteries with a Na-ion conducting solid-polymer electrolyte for power to metal and metal to power energy storage

Seawater batteries (SWBs) have been mostly researched for large scale energy storage and (sub-)marine applications. In a SWB, the aqueous catholyte (seawater) and a non-aqueous anolyte (aprotic solvent solution) are physically separated by a NASICON solid electrolyte membrane. Given the practically unlimited Na+ ion supply from seawater, the energy storage is only limited by the amount of Na stored in the negative electrode. Therefore, the highest volumetric and gravimetric energy densities can be achieved by storing Na metal without the need for a host material. To achieve safe realization of such a cell, a compact, metal-less anode design is herein demonstrated for the first time. The anode compartment integrates the NASICON solid electrolyte, a Na-ion conductive solid-state polymer electrolyte (Na-SPE), having a high ionic conductivity (over 1 mS cm−1 at moderate room temperature), and a negative electrode current collector. The reactive Na metal is not employed in the cell construction, but it is harvested from seawater upon charge (power to metal) and reconverted into energy upon the discharge process (metal to power). The overall round-trip energy efficiency (RTE) of the devices is over 85% at room temperature

Characteristics of Nanophase WC and WC-3 wt% (Ni, Co, and Fe) Alloys Using a Rapid Sintering Process for the Application of Friction Stir Processing Tools

Microstructures and mechanical characteristics of tungsten carbide- (WC-) based alloys, that is, WC, WC-3 wt% Ni, WC-3 wt% Co, and WC-3 wt% Fe, fabricated using a spark plasma sintering (SPS) method for the application of friction stir processing tools were evaluated. The sintered bodies with a diameter of 66 mm showed relative densities of up to 99% with an average particle size of 0.26~0.41 μm under a pressure condition of 60 MPa with an electric current for 35 min without noticeable grain growth during sintering. Even though no phase changes were observed after the ball milling process the phases of W2C and WC1-x appeared in all sintered samples after sintering. The Vickers hardness and fracture toughness of the WC, WC-3 wt% Ni, WC-3 wt% Co, and WC-3 wt% Fe samples ranged from 2,240 kg mm2 to 2,730 kg mm2 and from 6.3 MPa·m1/2 to 9.1 MPa·m1/2, respectively

Dialogizer: Context-aware Conversational-QA Dataset Generation from Textual Sources

To address the data scarcity issue in Conversational question answering (ConvQA), a dialog inpainting method, which utilizes documents to generate ConvQA datasets, has been proposed. However, the original dialog inpainting model is trained solely on the dialog reconstruction task, resulting in the generation of questions with low contextual relevance due to insufficient learning of question-answer alignment. To overcome this limitation, we propose a novel framework called Dialogizer, which has the capability to automatically generate ConvQA datasets with high contextual relevance from textual sources. The framework incorporates two training tasks: question-answer matching (QAM) and topic-aware dialog generation (TDG). Moreover, re-ranking is conducted during the inference phase based on the contextual relevance of the generated questions. Using our framework, we produce four ConvQA datasets by utilizing documents from multiple domains as the primary source. Through automatic evaluation using diverse metrics, as well as human evaluation, we validate that our proposed framework exhibits the ability to generate datasets of higher quality compared to the baseline dialog inpainting model.Comment: Accepted to EMNLP 2023 main conferenc