An important issue about developing all solid-state Li-ion batteries is to lower the high ionic interfacial resistance between a cathode and an electrolyte. An origin of the interfacial resistance is hypothesized due to a Li-depleted layer at the interface. Our computation has shown that the Li-depleted layer was the result of redox reaction at the interface in the charging process. In this subsequent theoretical study, we validate this redox reaction between the FePO4 phase and the Li3PS4 phase from the viewpoint of their band alignment through the density functional theory with the hybrid functional (HSE06). In addition, we demonstrate that the Li-depleted layer grows up to a defective layer at a Li3PS4/FePO4 interface by exothermic radical polymerization of PS4 anions in the oxidized Li3PS4 phase with the volume reduction. This decrease in Li-ion sites due to the PS4 polymerization makes the Li-depleted region long-lived and has the potential as an origin of the resistance against the Li-ion diffusion near the interface.

An important issue about developing all solid-state Li-ion batteries is to lower the high ionic interfacial resistance between a cathode and an electrolyte. An origin of the interfacial resistance is hypothesized due to a Li-depleted layer at the interface. Our computation has shown that the Li-depleted layer was the result of redox reaction at the interface in the charging process. In this subsequent theoretical study, we validate this redox reaction between the FePO4 phase and the Li3PS4 phase from the viewpoint of their band alignment through the density functional theory with the hybrid functional (HSE06). In addition, we demonstrate that the Li-depleted layer grows up to a defective layer at a Li3PS4/FePO4 interface by exothermic radical polymerization of PS4 anions in the oxidized Li3PS4 phase with the volume reduction. This decrease in Li-ion sites due to the PS4 polymerization makes the Li-depleted region long-lived and has the potential as an origin of the resistance against the Li-ion diffusion near the interface.

We have found the disproportion between the intermediate spin (IS) and low spin (LS) configurations of Co atoms at a Li3PO4/LiCoO2 (104) interface through density functional molecular dynamics (DF-MD). The manifold of the spin state at the interface, however, does not affect the band alignment between the Li3PO4 and LiCoO2 regions.

•A hole appears in the LiFePO4 phase without depending on a Li vacancy position.•Li atoms near the interface are extracted in the initial stage of charging.•The space charge layer may be negligible in this system.We have investigated the introduction of Li vacancy (VLi× in the Kröger–Vink notation) into a Li3PO4 (100)/LiFePO4 (010) coherent interface with the density functional theory in order to gain an insight into the initial stage of the charging process. The VLi× introduction results in the formation of a Li ion vacancy (VLi′) and hole (h). When one VLi× is introduced in each bulk LiFePO4 and Li3PO4 materials (both VLi′ and h are donated to the position in the proximity of the initial VLi× position), the formation energies of the Li vacancy (EV) are quite different between them. On the other hand, when introduced into the Li3PO4/LiFePO4 interface system, the values of EV in the LiFePO4 and Li3PO4 bulk regions of the interface become almost equal. This is because irrespective of the introduced VLi× position, the associated h is always donated to the LiFePO4 region while the Li3PO4 region keeps its insulating properties. Although the Li atoms near the interface have smaller EV than in the bulk regions, it is suggested that only a fraction of them may be extracted at the initial stage of charging which is not enough to lead to the Li depletion at the interface and as a result the effect of the space charge layer may be negligible in the Li3PO4/LiFePO4 interface.

Interfaces between cathodes and sulfide electrolytes exhibit high resistance in all-solid-state lithium ion batteries. In this paper, to elucidate the origin of the high interface resistance we have theoretically investigated the properties of the cathode interfaces with the sulfide electrolyte and oxide electrolyte for comparison. From the density functional molecular dynamics simulations of the LiFePO4/Li3PS4 interface in both discharged and charged states, we have demonstrated the instability of the sulfide interface in the charged state, that is, the lithium depletion and oxidation on the sulfide side near the interface, in contrast to the oxide interfaces. The obtained results imply the formation of a Li-depleted layer around the sulfide interfaces during charging and support the validity of the insertion of oxide buffer layers at the interface to reduce the interface resistance.

The crystal and electronic structures, electrochemical properties and diffusion mechanism of NASICON-type Na3V2(PO4)3 have been investigated based on the hybrid density functional Heyd–Scuseria–Ernzerhof (HSE06). A polaron–Na vacancy complex model for revealing the diffusion mechanism is proposed for the first time in the field of Na-ion batteries. The bound polaron is found to favorably form at the first nearest V site to the Na vacancy. Consequently, the movement of the Na vacancy will be accompanied by the polaron. Three preferable diffusion pathways are revealed; these are two intra-layer diffusion pathways and one inter-layer pathway. The activation barriers for the intra-layer and inter-layer pathways are 353 meV and 513 meV, respectively. For further comparison, the generalized gradient approximation with an onsite Coulomb Hubbard U (GGA+U) is also employed.

We report a first-principles study on the initial deposition of Li2O2 on three rutile oxide surfaces, RuO2(110)-(1×1)-O, TiO2(110), and SnO2(110). The intermediate discharge product in a Li–air battery, LiO2, is found to be less stable on all rutile surfaces and will be further reduced to Li2O2 through disproportionation reaction. For the first and second layers of deposited Li2O2, the adsorption energy is comparable to the cohesive energy of bulk Li2O2, suggesting Li2O2 will likely wet the oxide surfaces and grow into thin films rather than particles. Electronic structure analyses of interfaces demonstrate Li2O2/TiO2(110) is metallic and Li2O2/SnO2(110) is semiconducting with a bandgap of 0.2 eV, substantially smaller than in bulk Li2O2. The large lattice mismatch at these interfaces could create amorphousness of Li2O2 and grain boundaries might form abundantly thereafter, both of which can provide charge and ion transport channels needed for oxygen reduction and evolution reactions in Li–air batteries. Therefore, coating nanostructured carbon cathode with thin films of TiO2 or employing mesoporous TiO2 nanostructures as cathode could possibly lead to the formation of low-resistance Li2O2 thin films and thereby enhance the rate capacity of Li–air batteries.

The dominant discharge product in Li–air batteries, lithium peroxide (Li2O2), is intrinsically a wide band gap insulator as a perfect crystal. Recent density functional theory studies have suggested both vacancy- and polaron-mediated electron transportation mechanisms. We here show computational evidence from both semilocal and hybrid density functional calculations that the Σ3(11̅00)[112̅0] tilt grain boundaries (GBs) in Li2O2 can produce spin-polarized gap states. For each type of Σ3 GBs, GB1, GB2, and GB2* which has different atomic layer as the mirror plane, we have examined stoichiometric and a number of O-rich chemistry and find that stable geometry can take both forms. We find stoichiometric GBs disturb negligibly the electronic structure of Li2O2, yet the O-rich GBs produce spin-polarized gap states in a similar manner to free surface cases. Lithium deficiency leads to compression of interfacial O–O bonds, enlarges the πp–πp* split, and pushes up the antibonding πp* to (GB2) or beyond (GB2*) the Fermi energy. As a result, GB2 becomes half-metallic and GB2* becomes semiconducting with a small band gap of 1.0 eV. In both cases, spin polarization of O ions help to stabilize the GB by leaving the up spin of its gap states shifted down below the Fermi level and the down spin states open. Since Li2O2 is always polycrystalline as a discharge product, the presence of GBs may enhance conductivity.

Carbon coating substantially promotes electron transport through LiFePO4 nanoparticles at the positive electrode in Li-ion batteries and approaches to forming thin, tightly attached, and uniform carbon layers are extremely desirable. On the basis of our recent first principles computational discovery that the graphite crystallites are more likely standing on the LiFePO4 (010) surface via C–O bonding rather than lying on van der Waals forces, we have in this work searched promising co-coating substances which can reinforce the perpendicular orientation of graphite crystallites by strengthening the C/LiFePO4 binding. Among the three supervalent elements we have studied, Sc (3+), Ti (4+), and V (5+), Sc and Ti are found to show good performance. The binding energy of perpendicular graphite on the LiFePO4 (010) surface increases noticeably from 0.52 to 0.58 (Sc) and 0.61 (Ti) eV/Å, whereas the binding of a parallel graphene sheet with the same surface is nearly unchanged. A strengthened perpendicular graphite orientation is very welcome for leaving diffusion channels of Li unblocked.

We present a theoretical study on electron and hole trap states in the bulk and (001) surface of anatase titanium dioxide using screened hybrid density functional calculations. In both the bulk and surface, calculations suggest that the neutral and ionized oxygen vacancies are possible electron traps. The doubly ionized oxygen vacancy is the most stable in the bulk, and is a candidate for a shallow donor in colorless anatase crystals. The hole trap states are localized at oxygen anions in both the bulk and surface. The self-trapped electron centered at a titanium cation cannot be produced in the bulk, but can be formed at the surface. The electron trap level at the surface oxygen vacancy is consistent with observations by photoelectron spectroscopy. The optical absorptions and luminescence in UV-irradiated anatase nanoparticles are found to come from the surface self-trapped hole and the surface oxygen vacancy.

We have theoretically studied the adsorption of a thiophenethiolate (C4H3S–S) molecule on the Au(1 1 1) surface by first-principles calculations. It is found that the bridge site is the most stable adsorption site with the adsorption energy of 1.02 eV. In the optimized adsorption geometry, the bond between the head S atom and the connected C atom in the tail thiophene molecule is tilted by 57.2° from the surface normal. In addition, the adsorption of thiophenethiolate induces large relaxations of the surface Au atoms around it. Furthermore, weak interactions between the S atom in the tail thiophene ring and the Au atoms also contribute to the adsorption on the Au surface.

We present a new coupling method for hybrid simulations in which the system is partitioned into covalently linked quantum mechanical (QM) and molecular mechanical (MM) regions. Our method, called the “link molecule method (LMM),” is substantially different from the link atom methods in that LMM is free from the delicate issue of how to remove the additional degrees of freedom with respect to the position of the virtual atoms linking the QM and the MM regions. The force acting on the atom at the regional boundary is obtained in a simple form based on the total energy conservation. The accuracy of LMM is demonstrated in detail using a system of silicon partitioned into the QM and the MM region at the (1 0 0) boundary plane. This condition has been difficult to simulate by conventional methods employing the link atoms because of the strong repulsion between the nearby link atoms.

We present a theoretical study on electron and hole trap states in the bulk and (001) surface of anatase titanium dioxide using screened hybrid density functional calculations. In both the bulk and surface, calculations suggest that the neutral and ionized oxygen vacancies are possible electron traps. The doubly ionized oxygen vacancy is the most stable in the bulk, and is a candidate for a shallow donor in colorless anatase crystals. The hole trap states are localized at oxygen anions in both the bulk and surface. The self-trapped electron centered at a titanium cation cannot be produced in the bulk, but can be formed at the surface. The electron trap level at the surface oxygen vacancy is consistent with observations by photoelectron spectroscopy. The optical absorptions and luminescence in UV-irradiated anatase nanoparticles are found to come from the surface self-trapped hole and the surface oxygen vacancy.

The crystal and electronic structures, electrochemical properties and diffusion mechanism of NASICON-type Na3V2(PO4)3 have been investigated based on the hybrid density functional Heyd–Scuseria–Ernzerhof (HSE06). A polaron–Na vacancy complex model for revealing the diffusion mechanism is proposed for the first time in the field of Na-ion batteries. The bound polaron is found to favorably form at the first nearest V site to the Na vacancy. Consequently, the movement of the Na vacancy will be accompanied by the polaron. Three preferable diffusion pathways are revealed; these are two intra-layer diffusion pathways and one inter-layer pathway. The activation barriers for the intra-layer and inter-layer pathways are 353 meV and 513 meV, respectively. For further comparison, the generalized gradient approximation with an onsite Coulomb Hubbard U (GGA+U) is also employed.

Based on density functional theory, we have systematically studied the crystal and electronic structures, and the diffusion mechanism of the NASICON-type solid electrolyte Na3Zr2Si2PO12. Four possible elementary processes are addressed: three inner-chain and one inter-chain processes. In inner-chain processes, Na tends to move inside the Na diffusion chain, while Na moves across the Na diffusion chain in the inter-chain process. The activation energies for the inner-chain and inter-chain processes are 230 meV and 260 meV, respectively. By combining possible elementary processes, three preferable pathways along a, b, and c directions are found.

We have found the disproportion between the intermediate spin (IS) and low spin (LS) configurations of Co atoms at a Li3PO4/LiCoO2 (104) interface through density functional molecular dynamics (DF-MD). The manifold of the spin state at the interface, however, does not affect the band alignment between the Li3PO4 and LiCoO2 regions.