Dong-Hee Yeon

Ab initio study of doping effects on LiMnO2 and Li2MnO3 cathode materials for Li-ion batteries

For the over-lithiated-oxides (OLOs), a composite of layered Li2MnO3 and LiMO2 (M = Mn, Co, Ni), the Li2MnO3 part is not stable after the 1st charge–discharge cycle and partly transforms into layered LiMnO2, which in practice indicates that the phase used is actually a mixture of both Li2MnO3 and LiMnO2. In the present work, the influence of 10 cationic (Mg, Ti, V, Nb, Fe, Ru, Co, Ni, Cu, and Al) and 2 anionic (N and F) dopants on the phase stability, redox potential, ionic and electronic conductivity of both Li2MnO3 and LiMnO2 is investigated in detail using density functional theory. The calculations show that all the cationic dopants and F can be thermodynamically stable in the layered structures. The redox potential of both oxides is quite sensitive to some of the dopants, like V, Nb, and Ru, due to the appearance of gap states introduced by those dopants. The Jahn–Teller effect has a strong influence on the Li vacancy diffusion behavior in both LiMnO2 and its doped phases. Li vacancy diffusion behavior in Li2MnO3, including both interlayer and intralayer pathways, is relatively more complex and some dopants like Mg, Ti, Nb, and Ru can decrease the barriers of the diffusion paths. The calculations also show the evidence of hole polaron formation in LiMnO2 and electron polaron formation in Li2MnO3 which should be the reason why these phases have low electronic conductivities. Based on these findings, possible ways to improve the electronic conductivity through the doping process are discussed.

Highly conductive polymer-decorated Cu electrode films printed on glass substrates with novel precursor-based inks and pastes

Novel inks and pastes based on a copper(II) formate tetrahydrate precursor were formulated with controllable viscosities in the range 5000–10 000 cP for use in printed electrodes. In particular, the addition of ethyl cellulose increased the adhesion of the printed paste films to the glass substrates. For the facile fabrication of electrodes with improved performance, the formulated pastes were printed on glass substrates under ambient conditions by a doctor-blade method. The printed films were thermally sintered at 170–250 °C in air and subsequently reduced under a formic acid atmosphere. The phase and microstructural evolution of the electrode films were systematically investigated by X-ray diffraction (XRD) and cross-sectional, focused ion beam scanning electron microscopy (FIB-SEM) in each processing step. Highly adhesive, polycrystalline Cu electrode films decorated by ethyl cellulose with a vermicular microstructure and large interconnected pore channels were well formed on the glass substrates. The electrode films sintered for 1 min in air and then reduced for 5 min under formic acid atmosphere at 250 °C showed the lowest electrical resistivity of ∼8 μΩ cm (electrical conductivity of ∼125 000 Ω cm−1, equivalent to ∼22% of bulk Cu), despite their maximum porosity of 27.31%.

Enhancement of Seebeck Coefficient in Bi0.5Sb1.5Te3 with High-Density Tellurium Nanoinclusions

Bi0.5Sb1.5Te3 films with homogeneously dispersed ~15 nm Te nanoparticles were prepared by the alternate deposition of Bi0.5Sb1.5Te3 layers and Te nanoparticles. As the amount of Te nanoinclusions increased to 15 vol %, the Seebeck coefficient increased from 169 to 248 µV/K. The authors concluded that the high-density Te nanoinclusions result in a carrier energy filtering effect in Bi0.5Sb1.5Te3. Consequently, the thermoelectric power factor was enhanced by 30% despite a reduction in electrical conductivity. The improvement of the power factor implies the enhancement of the thermoelectric figure of merit ZT, providing the possibility of further ZT improvement by embedding Te nanoinclusions in Bi0.5Sb1.5Te3 bulk materials.

Stabilization of high-cobalt-content perovskites for use as cathodes in solid oxide fuel cells

B-site modification of the high-cobalt-content perovskite Ba0.5Sr0.5Co0.8Fe0.2O3−δ reduces its thermal expansion coefficient and stabilizes its structure, allowing its long-term operation without degradation of the area-specific resistance, and thus, maintaining the high electrode performance.