Electrochemical Properties and Crystal Structure of Li+ / H+ Cation-exchanged LiNiO2

LiNiO2 has high energy density but easily reacts with moisture in the atmosphere and deteriorates. We performed qualitative and quantitative evaluations of the degraded phase of LiNiO2 and the influence of the structural change on the electrochemical properties of the phase. Li1-xHxNiO2 phase with cation exchange between Li+ and H+ was confirmed by thermogravimetric analysis and Karl Fischer titration measurement. As the H concentration in LiNiO2 increased, the rate capability deteriorated, especially in the low-temperature range and under low state of charge. Experimental and density functional theory (DFT) calculation results suggested that this outcome was due to increased activation energy of Li+ diffusion owing to cation exchange. Rietveld analysis of X-ray diffraction and DFT calculation confirmed that the c lattice parameter and Li-O layer reduced because of the Li+/H+ cation exchange. These results indicate that LiNiO2 modified in the atmosphere has a narrowed Li-O layer, which is the Li diffusion path, and the rate characteristics are degraded.Comment: 8 pages, 11 figure

Quantum Monte Carlo Study of Molecular Crystals

Open House, ISM in Tachikawa, 2011.7.14統計数理研究所オープンハウス（立川）、H23.7.14ポスター発

First-Principles-Based Insight into Electrochemical Reactivity in a Cobalt-Carbonate-Hydrate Pseudocapacitor

Cobalt carbonate hydroxide (CCH) is a pseudocapacitive material with remarkably high capacitance and cycle stability. Previously, it was reported that CCH pseudocapacitive materials are orthorhombic in nature. Recent structural characterization has revealed that they are hexagonal in nature; however, their H positions still remain unclear. In this work, we carried out first-principles simulations to identify the H positions. Through the simulations, we could consider various fundamental deprotonation reactions inside the crystal and computationally evaluate the electromotive forces (EMF) of the deprotonation ($V_\mathrm{dp}$). Compared with the experimental potential window of the reaction ($< 0.6$ V (vs. saturated calomel electrode (SCE))), the computed $V_\mathrm{dp}$ (vs. SCE) value ($3.05$ V) was beyond the potential window, indicating that deprotonation never occurred inside the crystal. This may be attributed to the strongly formed hydrogen-bonds (H-bonds) in the crystal, thereby leading to the structural stabilization. We further investigated the crystal anisotropy in an actual capacitive material by considering the growth mechanism of the CCH crystal. By associating our X-ray diffraction (XRD) peak simulations with experimental structural analysis, we found that the H-bonds formed between CCH $\{(\bar{1}\bar{1}\bar{1}), (2\bar{1}\bar{1}), (2\bar{1}1)\}$ planes (approximately parallel to $ab$-plane) can result in 1-D growth (stacked along with $c$-axis)