Atomistic structures and transport phenomena at interfaces in lithium battery materials

Lithium-ion batteries (LIBs) are widely used in mobile devices. Increasingly they are also being scaled up for use as energy-storage devices in hybrid vehicles and fully electric ones. All-solid-state LIBs are expected to be one of the candidates as next generation LIBs because the LIBs can realize higher energy density and safety without liquid electrolytes. In these devices, the solid-solid interfaces, specifically those between electrodes and an electrolyte and grain boundaries within each component, are thought to strongly affect the battery performance. Despite their clear scientific and technical importance, however, so far, few studies have focused on the internal interfaces in battery materials. In this report, we present investigation combining scanning transmission electron microscope (STEM) observations with atomic resolution and theoretical calculations based on density functional theory (DFT) regarding interfaces in materials of LIBs Lithium-ion batteries (LIBs) are widely used in mobile devices. Increasingly they are also being scaled up for use as energy-storage devices in hybrid vehicles and fully electric ones. All-solid-state LIBs are expected to be one of the candidates as next generation LIBs because the LIBs can realize higher energy density and safety without liquid electrolytes. In these devices, the solid-solid interfaces, specifically those between electrodes and an electrolyte and grain boundaries within each component, are thought to strongly affect the battery performance. Despite their clear scientific and technical importance, however, so far, few studies have focused on the internal interfaces in battery materials. In this report, we present investigation combining scanning transmission electron microscope (STEM) observations with atomic resolution and theoretical calculations based on density functional theory (DFT) regarding interfaces in materials of LIBs. Figure 1 shows a HAADF-STEM image of a twin boundary in LiCoO2, a typical cathode of LIBs[1]. Bright spots correspond to columns of Co atoms. Figure 2 is a HAADF-STEM image of the edge-on atomic structures of a 90° domain boundary along the [100]p direction in La0.62Li0.16TiO3 samples which is a solid electrolyte of LIBs[2]. Bright spots correspond to columns of La atoms. Both interfaces seem to have smooth contact. However, our DFT calculations reveal that Li ion migration are strongly affected even by these coherent interfaces

Empirical interatomic potentials for ZrO2 and YSZ polymorphs: Application to a tetragonal ZrO2 grain boundary

Yttria stabilized zirconia (YSZ) ceramics have been used for various engineering applications including structural ceramics, biomedical materials, and thermal barrier coatings. The versatile and excellent properties of YSZ stem from its unique microstructure consisting of monoclinic, tetragonal, and cubic phases, whose stability depends on yttria concentration and temperature. However, there are no empirical interatomic potentials (EIPs) that can reproduce the structures and energies of ZrO2 and YSZ polymorphs, limiting the atomic-scale investigation of lattice defect structures and their interactions that affect the YSZ microstructure and properties. Here, using a genetic algorithm and ab initio training datasets, we have optimized EIPs to sufficiently reproduce the structures and stability of ZrO2 and YSZ polymorphs, as well as the properties of the tetragonal and cubic phases at finite temperature. The potentials have also been applied to the search for a tetragonal grain boundary structure, showing that the obtained grain boundary structure is consistent with that obtained by ab initio calculations. The developed EIPs will aid in revealing the microstructure-property relationships in YSZ by performing large-scale and systematic calculations, which are practically difficult to perform with ab initio and machine-learning-potential calculations.Fujii S., Shimazaki K., Kuwabara A.. Empirical interatomic potentials for ZrO2 and YSZ polymorphs: Application to a tetragonal ZrO2 grain boundary. Acta Materialia 262, 119460 (2024); https://doi.org/10.1016/j.actamat.2023.119460

First principles calculations of defect clustering in acceptor-doped BaZrO3

Acceptor-doped BaZrO3 shows high proton conductivity under wet atmosphere conditions and is a promising material used for a proton conductive electrolyte. Similar to other kinds of ionic conductors, however, carrier trapping by dopant occurs and suppresses conductivity of the acceptor-doped BaZrO3 [1]. The carrier trapping is an unavoidable phenomenon for ionic conductors because formation of charge carriers for ionic conduction is attributed to dopants with opposite charge states to the carriers. We have to understand and to control the carrier trapping behavior to optimize properties of ionic conductors. Please click Additional Files below to see the full abstract

On the weights of end-pairs in n-end catenoids of genus zero

First principles calculations are carried out to analyze adsorption of CO and H2 molecules on a Pt (111) surface and the effect of surface strain on the adsorption energy. A CO molecule is more adsorptive on the Pt (111) surface than a H2 molecule under an ordinary condition. Surface expansion enhances CO poisoning on a Pt (111) surface. On the contrary, a compressive strain reduces adsorptive strength of a CO molecule. Similar tendency is also found in adsorption of a H2 molecule on the bridge, fcc-hollow, and hcp-hollow sites. However, H2 adsorption on the top site is less affected by the strain. As a consequence, the difference of adsorption energies between CO and H2 molecules becomes smaller when compressive strain is introduced into the Pt (111) surface. Based on thermodynamics, surface coverage ratio is quantitatively evaluated with taking into account the effect of surface strain and partial pressure of gas phase. It is revealed that compressive strain improves probability of H2 adsorption on Pt surface

First-principles calculations of lattice dynamics in CdTiO3 and CaTiO3: Phase stability and ferroelectricity

First-principles calculations of various phases of CdTiO3 carried out with the aim of obtaining insights into the mechanism of the ferroelectric phase transition and the structure of the low-temperature ferroelectric phase are reported. The results indicate that the preferred symmetry of the low-temperature phase is Pna2_1, rather than P21ma, corresponding to a small relative shift of the Ti and O ions in the paraelectric Pnma phase with the polarization axis parallel to the long axis. Calculated phonon dispersion curves show a distinct soft mode at the Γ point of the Pnma phase, which vanishes in the Pna2_1 phase, confirming that the transition to the ferroelectric phase is of the soft-mode displacive type. Calculations of perovskite CaTiO3, which also has an orthorhombic Pnma structure at room temperature but, unlike CdTiO3, does not exhibit a ferroelectric phase transition down to 4.2 K, were also carried out to help characterize the factors controlling ferroelectric phase transitions in perovskite titanates

Oxygen affinity: the missing link enabling prediction of proton conductivities in doped barium zirconates

Proton-conducting oxides, specifically doped barium zirconates, have garnered much attention as electrolytes for solid-state electrochemical devices operable at intermediate temperatures (400–600 °C). In chemical terms, hydration energy, E_(hyd), and proton–dopant association energy, E_(as), are two key parameters that determine whether an oxide exhibits fast proton conduction, but to date ab initio studies have for the most part studied each parameter separately, with no clear correlation with proton conductivity identified in either case. Here, we demonstrate that the oxygen affinity, E_(O.dopant), defined as the energy released when an oxide ion enters an oxygen vacancy close to a dopant atom, is the missing link between these two parameters and correlates well with experimental proton conductivities in doped barium zirconates. Ab initio calculations of point defects and their complexes in Sc-, In-, Lu-, Er-, Y-, Gd-, and Eu-doped barium zirconates are used to determine E_(hyd), E_(as), E_(O.dopant), and the hydrogen affinity, EH.host, of each system. These four energy terms are related by E_(hyd) = E_(O.dopant) + 2E_(H.host) + 2E_(as). Complementary impedance spectroscopy measurements reveal that the stronger the calculated oxygen affinity of a system, the higher the proton conductivity at 350 °C. Although the proton trapping site is also an important factor, the results show that oxygen affinity is an excellent predictor of proton conductivity in these materials