Yongseon Kim

Lithium Nickel Cobalt Manganese Oxide Synthesized Using Alkali Chloride Flux: Morphology and Performance As a Cathode Material for Lithium Ion Batteries

Li(Ni(0.8)Co(0.1)Mn(0.1))O(2) (NCM811) was synthesized using alkali chlorides as a flux and the performance as a cathode material for lithium ion batteries was examined. Primary particles of the powder were segregated and grown separately in the presence of liquid state fluxes, which induced each particle to be composed of one primary particle with well-developed facet planes, not the shape of agglomerates as appears with commercial NCMs. The new NCM showed far less gas emission during high temperature storage at charged states, and higher volumetric capacity thanks to its high bulk density. The material is expected to provide optimal performances for pouch type lithium ion batteries, which require high volumetric capacity and are vulnerable to deformation caused by gas generation from the electrode materials.

Synthesis of High-Density Nickel Cobalt Aluminum Hydroxide by Continuous Coprecipitation Method

Spherical nickel cobalt aluminum hydroxide (Ni(0.80)Co(0.15)Al(0.05)-hydroxide, NCA) was prepared by a continuous coprecipitation method. A new design of the Al solution and the feeding method was applied, which enabled to prevent rapid precipitation of Al(OH)(3) and to obtain spherical NCA with large enough particle size and high density. The active material (LiNi(0.80)Co(0.15)Al(0.05)O(2) or LNCA) prepared from it showed higher tap-density than that made from NCA prepared by general processes, and homogeneity of Al-distribution was also improved. It is expected that the electrode density of lithium ion batteries adopting LNCA could be improved with the new process proposed in this study.

First-principles thermodynamic calculations and experimental investigation of Sr–Si–N–O system—synthesis of Sr2Si5N8:Eu phosphor

Thermodynamic stabilities of the phases of Sr–Si–N–O system were evaluated by simulating phase diagrams at various conditions based on first-principles density functional theory calculations. Synthesis conditions and stability of the compounds belonging to the system, in which oxidation and nitridation reactions are involved complicatedly, could be interpreted through this first systematic investigation on the two-gas system. Practical synthetic methods of nitrides, such as hydrogen-reduction and nitridation or carbothermal reduction and nitridation reactions, were studied with special attention. This study enabled us to calculate proper conditions for synthesis of the Sr2Si5N8 phase, which is drawing attention as a new phosphor material for light emitting diodes. The types of impurities appearing with deviation from the proper synthetic conditions were also investigated, which may provide information about optimizing synthesis conditions. Synthesis of Sr2Si5N8:Eu phosphor using SiO2, instead of conventionally used Si3N4, was predicted by first-principles calculations, and we succeeded in synthesizing Sr2Si5N8:Eu phosphor for the first time using all oxide raw materials under normal pressure on this basis. The results of this study are expected to provide useful guidelines for synthesis of nitrides and the established simulation method may effectively be applied to other multi-gas systems.

Calculation of Formation Energy of Oxygen Vacancy in ZnO Based on Photoluminescence Measurements

The formation energy of an oxygen vacancy in ZnO was calculated. The photoluminescence intensity of green emission was used as a measure of vacancy concentration, and its variation as a function of reduction temperature was monitored. This enabled a thermodynamic approach based on experimental data, which is in contrast with most previous studies, which focused on a theoretical treatment based on first principles methods. Two reduction conditions were used: hydrogen gas flow and a CO/CO(2) atmosphere generated using activated carbon. The two cases were compared, and mechanisms for the formation of oxygen vacancies during thermal treatment were investigated. The similar results obtained for the two cases indicate that the proposed models of V(O) formation are reasonable.

Investigation of the gas evolution in lithium ion batteries: effect of free lithium compounds in cathode materials

The evolution of gas in lithium ion batteries (LIBs) was investigated. The large amount of gas emission related to a charged cathode has been a critical issue because it causes deformation and performance degradation of LIBs. This study examined the effect of free lithium compounds such as Li2CO3 or LiOH on gas generation, which revealed several different features comparing with gas generation related to the cathode active materials themselves: CO2 was the main gas generated, chain-structured carbonate solvents such as dimethyl carbonate or ethyl methyl carbonate generated more gas than cyclic-structured ethylene carbonate, and the gas generation did not occur without LiPF6 in the electrolyte solution. These were found to be the main reason for the different gas-generating behaviors between LiCoO2 (LCO) and LiNi0.85Co0.12Al0.03O2 (NCA) cathodes. For LCO, which has a very small amount of free lithium compounds on the surface, the gas was generated mainly by a reaction between delithiated LCO itself and the electrolyte solution, whereas a considerable amount of gas was generated by surface free lithium for NCA. Therefore, the removal of free lithium compounds is essential, particularly for NCA, to prevent the swelling of LIBs.

Mechanism of gas evolution from the cathode of lithium-ion batteries at the initial stage of high-temperature storage

The evolution of gas in lithium-ion batteries (LIBs) at a charged state is one of the main problems in the industry because it causes significant distortion or swelling of the batteries. The mechanism of the gas-generating reaction related to the cathode at a charged state of LIBs was investigated. A side reaction between the electrolyte solution and free lithium compounds, such as Li2CO3 or LiOH in the cathode, is considered as the main cause of gas evolution at early stages of the storage test. Both Li2CO3 and LiOH generated CO2 mainly by a HF-mediated reaction, but the evolution of CO2 could be triggered by addition of H2O without a fluorine source for LiOH. Ni-based cathode materials generated more gas than conventional LiCoO2 at the initial stage because they contain more free lithium compounds, but the rate of gas evolution slowed down with time, suggesting that Ni-based active materials might be more appropriate for reducing long-term gas evolution in LIBs if the free lithium compounds could be removed effectively from the surface.

Thermodynamic investigation of Ti doping in MgAl2O4 based on the first-principles method

Ti doping of MgAl2O4 crystals is investigated using a theoretical thermodynamic approach. A number of types of Ti-doped MgAl2O4 are produced by combining possible Ti oxidation states (Ti2+, Ti3+, and Ti4+), doping sites (Al and Mg sites), and additional point defects (antisites and vacancies). Crystal models containing each doping type are prepared, and by treating them as independent phases, phase diagrams of the Mg–Al–Ti–O system are simulated based on first-principles calculations. This enables the study of stable doping types as a function of synthesis conditions and chemical compositions. Under air or H2 conditions, only the type is stable in the phase diagram, whereas and appear to be stable in a strongly reducing CO/CO2 atmosphere at high temperatures. A quantitative analysis model is established for calculating the fractions of certain doping types based on considerations of formation energy and mixing entropy, and the proportions of doping types are calculated. Changes in the Ti3+ : Ti4+ ratio are simulated, and the effects of temperature, oxygen partial pressure, Mg/Al ratio, and surface are examined. The results obtained show good agreement with XPS results and photoluminescence measurements. This study provides guidelines to the design and property tuning of Ti-doped MgAl2O4 phosphors for LED applications.

First-principles investigation of the gas evolution from the cathodes of lithium-ion batteries during the storage test

Gas evolution related to the positive electrode of charged lithium-ion batteries during the storage test was investigated using a first-principle method. The distribution of lithium during the delithiation process was simulated based on the density functional theory calculations of the energy required to remove the lithium from the surface or bulk crystal of lithium nickel cobalt manganese oxide (NCM) and lithium cobalt oxide (LCO). Lithium coverage of the surface was smaller for LCO than NCM at a highly charged state. The energy required to form an oxygen vacancy in NCM and LCO crystal was also calculated. The results showed that LCO was more apt to emit oxygen than NCM as the delithiation percentage was increased. The results suggest that the gas-generating side reactions related to the emission of oxygen would be more significant for LCO with high voltage charging. Experimental result showed a considerable portion of the gas was generated at the initial stages of storage for NCM, whereas LCO showed slow but steady gas evolution with increasing storage time. A large amount of Li2CO3 or LiOH on the surface of NCM appears to cause an immediate gas-generating side reaction, whereas LCO produces slow side reaction related to the emission of oxygen from the LCO material itself.

Eu2+-Activated Phase-Pure Oxonitridosilicate Phosphor in a Ba–Si–O–N System via Facile Silicate-Assisted Routes Designed by First-Principles Thermodynamic Simulation

Eu(2+)-activated single phase Ba(2+)-oxonitridosilicate phosphors were prepared under a mild synthetic condition via silicate precursors, and their luminescent properties were investigated. Both the preferred oxonitridosilicate formation as for the available host compounds and thermodynamic stability within the Ba-Si-O-N system were elucidated in detail by the theoretical simulation based on the first-principles density functional theory. Those results can visualize the optimum synthetic conditions for Eu(2+)-activated highly luminescent Ba(2+)-oxonitridosilicates, especially Ba3Si6O12N2, as promising conversion phosphors for white LEDs, including Ba3Si6O9N4 and BaSi2O2N2 phases. To prove the simulated design rule, we synthesized the Ba3Si6O12N2:Eu(2+) phosphor using various silicate precursors, Ba2Si4O10, Ba2Si3O8, and BaSiO3, in a carbothermal reduction ambient and finally succeeded in obtaining a phase of pure highly luminescent oxonitridosilicate phosphor without using any solid-state nitride addition and/or high pressure synthetic procedures. Our study provides useful guidelines for robust synthetic procedures for developing thermally stable rare-earth-ion activated oxonitridosilicate phosphors and an established simulation method that can be effectively applied to other multigas systems.