First-principles calculations were performed to investigate the electrochemical stability of lithium solid electrolyte materials in all-solid-state Li-ion batteries. The common solid electrolytes were found to have a limited electrochemical window. Our results suggest that the outstanding stability of the solid electrolyte materials is not thermodynamically intrinsic but is originated from kinetic stabilizations. The sluggish kinetics of the decomposition reactions cause a high overpotential leading to a nominally wide electrochemical window observed in many experiments. The decomposition products, similar to the solid-electrolyte-interphases, mitigate the extreme chemical potential from the electrodes and protect the solid electrolyte from further decompositions. With the aid of the first-principles calculations, we revealed the passivation mechanism of these decomposition interphases and quantified the extensions of the electrochemical window from the interphases. We also found that the artificial coating layers applied at the solid electrolyte and electrode interfaces have a similar effect of passivating the solid electrolyte. Our newly gained understanding provided general principles for developing solid electrolyte materials with enhanced stability and for engineering interfaces in all-solid-state Li-ion batteries.

Garnet-type solid-state electrolytes have attracted extensive attention due to their high ionic conductivity, approaching 1 mS cm, excellent environmental stability, and wide electrochemical stability window, from lithium metal to ∼6 V. However, to date, there has been little success in the development of high-performance solid-state batteries using these exceptional materials, the major challenge being the high solid-solid interfacial impedance between the garnet electrolyte and electrode materials. In this work, we effectively address the large interfacial impedance between a lithium metal anode and the garnet electrolyte using ultrathin aluminium oxide (AlO) by atomic layer deposition. LiLaCaZrNbO (LLCZN) is the garnet composition of choice in this work due to its reduced sintering temperature and increased lithium ion conductivity. A significant decrease of interfacial impedance, from 1,710 Ω cm to 1 Ω cm, was observed at room temperature, effectively negating the lithium metal/garnet interfacial impedance. Experimental and computational results reveal that the oxide coating enables wetting of metallic lithium in contact with the garnet electrolyte surface and the lithiated-alumina interface allows effective lithium ion transport between the lithium metal anode and garnet electrolyte. We also demonstrate a working cell with a lithium metal anode, garnet electrolyte and a high-voltage cathode by applying the newly developed interface chemistry.

Lithium solid electrolytes can potentially address two key limitations of the organic electrolytes used in today's lithium-ion batteries, namely, their flammability and limited electrochemical stability. However, achieving a Li(+) conductivity in the solid state comparable to existing liquid electrolytes (>1 mS cm(-1)) is particularly challenging. In this work, we reveal a fundamental relationship between anion packing and ionic transport in fast Li-conducting materials and expose the desirable structural attributes of good Li-ion conductors. We find that an underlying body-centred cubic-like anion framework, which allows direct Li hops between adjacent tetrahedral sites, is most desirable for achieving high ionic conductivity, and that indeed this anion arrangement is present in several known fast Li-conducting materials and other fast ion conductors. These findings provide important insight towards the understanding of ionic transport in Li-ion conductors and serve as design principles for future discovery and design of improved electrolytes for Li-ion batteries.

This study provides the understanding and design strategy of solid electrolyte–electrode interfaces to improve electrochemical performance of all-solid-state Li-ion batteries.

The continued drive for high performance lithium batteries has imposed stricter requirements on the electrolyte materials.1, 2 Solid electrolytes comprising lithium super ionic conductor materials exhibit good safety and stability, and are promising to replace current organic liquid electrolytes. One major limitation in the application of Li-ion conductors is that their typical conductivity is less than 10 -4 S/cm at room temperature. Recently, Kamaya et al. reported a new Li super ionic conductor Li 10 GeP 2 S 12 (LGPS), which has the highest conductivity ever achieved among solid lithium electrolytes of 12 mS/cm at room temperature (comparable conductivity with liquid electrolytes), and outstanding electrochemical performance in Li batteries. The high conductivity in LGPS is attributed to the fast diffusion of Li + in its crystal structural framework, which consists of (Ge 0.5 P 0.5 )S 4 tetrahedra, PS 4 tetrahedra, LiS 6 octahedra, and LiS 4 tetrahedra. Kamaya et al. proposed that diffusion in LGPS occurs along one-dimension (1D) with diffusion pathways along the c axis.3 The authors also proposed that Li atoms in LiS 4 tetrahedra enable fast diffusion along the c direction, while Li atoms in LiS 6 octahedra are not active for diffusion. This hypothetical diffusion mechanism in LGPS has been inferred from the large anisotropic thermal factors and the Li disorder in the 1D channels, but has not been directly proven. Understanding this LGPS material is important to improve its performance, and may provide insight into designing new Li super ionic conductor materials.Using first principles modeling, we investigated the diffusivity, stability, and electrochemical window of LGPS. We provide a hypothesis for the observed wide electrochemical window of LGPS. We also identified the diffusion pathways and calculated the corresponding activation energies and diffusion coefficient.All calculations in this study were performed using the Vienna Ab initio Simulation Package (VASP) 4 within the projector augmented-wave approach.5 Unless otherwise noted, all calculations were performed using the Perdew-BurkeErnzerhof generalized-gradient approximation (GGA) to density functional theory (DFT). 6 We assessed the phase stability of LGPS by constructing the quaternary Li-Ge-P-S phase diagram using all known Li-Ge-P-S compounds in the Inorganic Crystal Structure Database 7 , all Li x P y S z compounds compiled by Holzwarth et al., 10 and the calculated ground state of LGPS. As the refined structure has partial occupancies, we ordered the arrangement of Li, Ge and P atoms in LGPS using an electrostatic energy criterion.8 Of the ten orderings with the lowest electrostatic energy, the structure with the lowest calculated DFT energy was selected as the representative ground state. The calculation input parameters are based on those used in the Materials Project 9 to leverage on the large set of computed data available in that database.Our calculated phase diagram predicts LGPS to be thermodynamically unstable at 0K with respect to ...

Super-ionic conductor materials have great potential to enable novel technologies in energy storage and conversion. However, it is not yet understood why only a few materials can deliver exceptionally higher ionic conductivity than typical solids or how one can design fast ion conductors following simple principles. Using ab initio modelling, here we show that fast diffusion in super-ionic conductors does not occur through isolated ion hopping as is typical in solids, but instead proceeds through concerted migrations of multiple ions with low energy barriers. Furthermore, we elucidate that the low energy barriers of the concerted ionic diffusion are a result of unique mobile ion configurations and strong mobile ion interactions in super-ionic conductors. Our results provide a general framework and universal strategy to design solid materials with fast ionic diffusion.

Macroscopic laws of friction do not generally apply to nanoscale contacts. Although continuum mechanics models have been predicted to break down at the nanoscale, they continue to be applied for lack of a better theory. An understanding of how friction force depends on applied load and contact area at these scales is essential for the design of miniaturized devices with optimal mechanical performance. Here we use large-scale molecular dynamics simulations with realistic force fields to establish friction laws in dry nanoscale contacts. We show that friction force depends linearly on the number of atoms that chemically interact across the contact. By defining the contact area as being proportional to this number of interacting atoms, we show that the macroscopically observed linear relationship between friction force and contact area can be extended to the nanoscale. Our model predicts that as the adhesion between the contacting surfaces is reduced, a transition takes place from nonlinear to linear dependence of friction force on load. This transition is consistent with the results of several nanoscale friction experiments. We demonstrate that the breakdown of continuum mechanics can be understood as a result of the rough (multi-asperity) nature of the contact, and show that roughness theories of friction can be applied at the nanoscale.

We present an investigation of the phase stability, electrochemical stability and Li + conductivity in the Li 10±1 MP 2 X 12 (M = Ge, Si, Sn, Al or P, and X = O, S or Se) family of but at the expense of reduced electrochemical stability. We also studied the effect of lattice parameter changes on the Li + conductivity and found the same asymmetry in behavior between increases and decreases in the lattice parameters, i.e., decreases in the lattice parameters lower the Li + conductivity significantly, while increases in the lattice parameters increase the Li + conductivity only marginally. Based on these results, we conclude that the size of the S 2− is near optimal for Li + conduction in this structural framework.