Minoru Otani

We present an efficient implementation of an effective screening medium method combined with a reference interaction site model (ESM-RISM). The ESM-RISM theory is a straightforward extension of the three-dimensional RISM (3D-RISM) theory, which solves the Laue-represented RISM equation to describe atomistic-scale phenomena at the electrified electrode/electrolyte interface. However, the original ESM-RISM approach employed an approximate solution susceptibility, which leads to deviations of the ESM-RISM results from the 3D-RISM level and requires higher computational costs than 3D-RISM. In this study, we analytically addressed these limitations and applied the present implementation to a water molecule in bulk solution and a Pt(111)/HCl aqueous solution interface. The results for a hydrated water molecule show that the current implementation cures the different approximation levels between ESM-RISM and 3D-RISM. In addition, the analytical reformulation enables us to reduce the computational complexity of evaluating the Laue-represented RISM equation from O(N^4/3) to O(NlogN). Subsequently, we re-examined the computational flow of ESM-RISM and found that the present scheme reasonably reduces the wall time required to reach convergence in the overall ESM-RISM calculation. The results of the application to the Pt(111)/HCl aqueous solution interface demonstrated that the current implementation enabled us to capture the properties of the electrified interface.

High-voltage Li-ion batteries have been extensively studied to increase the energy density of batteries. To achieve this goal, 5 V class cathode materials have been developed, but the long-term stable operation has not been achieved. Recently, we reported that the poor cycle stability comes not only from the oxidative degradation of the electrolyte and/or cathode active material, but also from side reactions caused by the insertion of anions into the interlayer of a cathode conductive carbon [1]. However, the detailed mechanism of anion insertion into the conductive carbon is still not well understood. In this study, we theoretically investigated the anion insertion process at the interface between a graphite and EC-based electrolyte solution (1 M LiPF₆ / EC). We adopted a density functional theory (DFT) in combination with the effective screening medium and reference interaction site model (ESM-RISM) method to analyze the insertion process of a PF₆ ^- anion into the graphite [2]. The simulation cells are composed of graphite electrode (C₂₅₂(PF₆)₉H₃₆) and the 1 M LiPF₆ / EC electrolyte, where the entire graphite electrode are treated quantum mechanically, whereas the electrolyte components are treated classically by the RISM method. The activation energy was calculated by computing the grand potential (Ω) at each point while moving a PF₆ ^- anion from its initial position on the electrolyte side (r = 20 Å) to the electrode interior (r = 0 Å). These calculations were performed at two different electrode potentials, the potential of zero charge (PZC) and the equilibrium potential of the anion insertion reaction, respectively. All the DFT and ESM-RISM calculations were conducted using the Quantum Espresso v.6.1 code. The figure shows the profiles of Ω along the insertion process of the PF₆ ^– anion into the graphite layer. The activation energies were estimated to be 0.49 eV (Ω_PZC, black line) and 0.37 eV (Ω_eq, red line) at the PZC and the equilibrium potential, respectively, considerably smaller than that of Li⁺ (0.6 eV) [3]. Importantly, the coordination number of EC molecules around PF₆ ^– (N _EC, blue line) decreases from about 10 Å and Ω becomes correspondingly unstable, indicating that the desolvation of PF₆ ^– contributes significantly to the activation barrier. Furthermore, systematic calculations of the desolvation energies for different solvent species show that the calculated values tend to increase as the dipole moment of the solvent molecule decreases. Therefore, a use of solvents with smaller dipole moments would increase the desolvation energy, thus suppressing the anion insertion. 1) S. Ko et al., Joule, 5, 998 (2021). 2) S. Nishihara et al., Phys. Rev. B, 96, 115429 (2017). 3) J. Haruyama et al., J. Phys. Chem. C, 122, 9804 (2018). Figure 1

Understanding the properties of electrochemical interfaces leads to improving the efficiency and durability of energy and structural materials such as secondary batteries, fuel cells, catalysts, and corrosion. First-principles calculations based on the density functional theory (DFT) are useful simulation technique to those materials and are widely used to understand physicochemical phenomena such as charge and electron transfer and reaction kinetics. However, the simulation costs for the full DFT calculation are quite large. Thus, we need to reduce it to obtain as many results as possible within the limited calculation resources. Combining the DFT and the classical liquid theory is one of a possible way to reduce the costs. We have been developed a series of simulation techniques to hybridize the DFT and the reference interaction site model (RISM) [1-4]. In the presentation, after providing a brief introduction of our simulation technique, we will show the simulation results for electrolyte decompositions and battery degradation [5], a confined effect of water molecule in the MXene electrode [6], and a water dissociation reaction on Cu surface using an automated reaction path search method [7,8]. References: Otani and O. Sugino, Phys. Rev. B, 73, 115407 (2006). Bonnet, T. Morishita, O. Sugino, and M. Otani, Phys. Rev. Lett., 109, 266101, (2012). Nishihara and M. Otani, Phys. Rev. B, 96, 115429 (2017). Haruyama, T. Ikeshoji, and M. Otani, Phys. Rev. Mater., 2, 095801 (2018). Hagiwara, J. Haruyama, M. Otani, Y. Umemura, T. Takeuchi and H. Sakaebe, Electrochemistry, 90, 107002 (2022). Sugahara, Y. Ando, S. Kajiyama, K. Yazawa, K. Gotoh, M. Otani, M. Okubo, A. Yamada, Nat. Commun., 10 850, (2019). S. Maeda, K. Ohio, and K. Morokuma, Phys. Chem. Chem. Phys. 15, 3683 (2013) Hasegawa, S. Hagiwara, M. Otani, and S. Maeda, submitted to J. Phys. Chem. Lett.

As part of the concerted effort for development of energy storage technologies, dual‐ion batteries (DIBs) or dual‐carbon batteries (DCBs) are attracting interest, owing primarily to their eco‐friendly active materials. The use of carbon as the active materials of DCBs brings about several challenges involving capacity and stability. This contribution aims to provide an in‐depth understanding of the structural and electronic properties of Ge‐doped graphene (Germagraphene) as a novel cathode material for DCBs. Density functional theory (DFT) calculations are combined with the effective screening medium (ESM) method for analyzing the electronic and band structure of PF₆⁻anion‐adsorbed Germagraphene under a potential bias. These theoretical investigations indicate that the use of Ge as a dopant for graphene has a positive impact on the adsorption of the anion on the cathode under both neutral and electrically biased conditions.

The lithium/vanadium tetra sulfide (Li|VS4) battery can be considered a promising next-generation battery because of its high theoretical energy density, and it is expected to overcome the problems inherent in Li-S batteries. However, the charge/discharge cycle degradation of the Li|VS4 battery strongly depends on the choice of the organic solvent. We investigated the equilibrium potentials of the decomposition reactions involving the VS4 electrode and organic solvent molecules using the density functional and classical solution theories. We first modelled the decomposition reactions between VS4 and different organic solvent molecules, such as ethylene, dimethyl, and propylene carbonates (EC, DMC, and PC). Next, we calculated the change in the Gibbs free energy of the decomposition reaction by assuming a thermodynamic cycle and estimated the equilibrium potential vs. Li/Li+. From the equilibrium potential, the overpotential of the DMC against the potential plateau of the Li|VS4 battery is negative and shows the lowest value in the considered solvents. This result suggests that the battery cycle with DMC deteriorates more quickly than that with EC and PC. This suggestion explains the experimental tendency of battery cycle degradation and will be a useful guide for improving the electrochemical performance of Li|VS4 batteries.

The dielectrically consistent reference interaction site model (DRISM) is one of the methods used to solve the well-known drawback of the reference interaction site model (RISM) theory: That it underestimates the dielectric constant of the solutions. Recently, Nishihara and Otani developed the first-principles effective screening medium (ESM) method combined with RISM theory, called ESM-RISM, to simulate physical properties at the electrode/electrolyte interface [ Phys. Rev. B 96, 115429 (2017)]. In this study, we combined DRISM with the ESM-RISM framework to increase the accuracy of electrochemical interface simulations, which was applied to the Pt(111)/aqueous water interface as a benchmark calculation. The electrochemical properties at the interface, such as the potential of zero charge, standard hydrogen electrode potential, and double-layer capacitance, all reasonably agree with previous experiments. The thickness of the contact layer was found to correlate well with double-layer capacitance. We believe that the present method is a useful tool to better model the physical properties at electrochemical interfaces.