Design of a trigonal halide superionic conductor by regulating cation order-disorder.

Lithium-metal-halides have emerged as a class of solid electrolytes that can deliver superionic conductivity comparable to that of state-of-the-art sulfide electrolytes, as well as electrochemical stability that is suitable for high-voltage (>4 volt) operations. We show that the superionic conduction in a trigonal halide, such as Li3MCl6 [where metal (M) is Y or Er], is governed by the in-plane lithium percolation paths and stacking interlayer distance. These two factors are inversely correlated with each other by the partial occupancy of M, serving as both a diffusion inhibitor and pillar for maintaining interlayer distance. These findings suggest that a critical range or ordering of M exists in trigonal halides, and we showcase the achievement of high ionic conductivity by adjusting the simple M ratio (per Cl or Li). We provide general design criteria for superionic trigonal halide electrolytes.

Coupling structural evolution and oxygen-redox electrochemistry in layered transition metal oxides.

Lattice oxygen redox offers an unexplored way to access superior electrochemical properties of transition metal oxides (TMOs) for rechargeable batteries. However, the reaction is often accompanied by unfavourable structural transformations and persistent electrochemical degradation, thereby precluding the practical application of this strategy. Here we explore the close interplay between the local structural change and oxygen electrochemistry during short- and long-term battery operation for layered TMOs. The substantially distinct evolution of the oxygen-redox activity and reversibility are demonstrated to stem from the different cation-migration mechanisms during the dynamic de/intercalation process. We show that the π stabilization on the oxygen oxidation initially aids in the reversibility of the oxygen redox and is predominant in the absence of cation migrations; however, the π-interacting oxygen is gradually replaced by σ-interacting oxygen that triggers the formation of O-O dimers and structural destabilization as cycling progresses. More importantly, it is revealed that the distinct cation-migration paths available in the layered TMOs govern the conversion kinetics from π to σ interactions. These findings constitute a step forward in unravelling the correlation between the local structural evolution and the reversibility of oxygen electrochemistry and provide guidance for further development of oxygen-redox layered electrode materials.

Challenges and Strategies towards Practically Feasible Solid-State Lithium Metal Batteries.

Remarkable improvement of the ionic conductivity of inorganic solid electrolytes (SEs) exceeding 10 mS cm-1 at room temperature has opened up the opportunities to realize the commercialization of solid-state batteries (SSBs). The transition to the intrinsically inflammable SEs also promises that SSBs would successfully utilize lithium metal anode thus achieving the high-energy-density lithium metal batteries without the risk of a safety hazard. However, the practical operation of solid-state lithium metal batteries (SSLMBs) still faces the challenges of the poor cycle stability and the low energy efficiency, which are coupled with the interface stability and even with the dendrite growth of lithium metal. This article overviews current understandings regarding the underlying origins of the issues in employing the lithium metal anode in SSLMBs from the five main standpoints: i) the chemical/electrochemical interfacial stability, ii) the microscopic evolution of interfacial morphology, iii) the intrinsic diffusivity of lithium atom/vacancy at the interface, iv) imperfections (defect/pores), and v) non-negligible electronic conductivity of SEs. The discussions are followed on the state-of-the-art efforts and strategies to overcome these respective challenges. Finally, the authors provide their perspectives for the future research directions toward achieving the commercial level of high-energy SSLMBs.

Aqueous ionic effect on electrochemical breakdown of Si-dielectric-electrolyte interface.

The breakdown of thin dielectric films (SiO2, Si3N4, HfO2) immersed in aqueous electrolyte was investigated. The current and the kinetics of dielectric breakdown caused by large cathodic electric field applied across the dielectric layer reveal the electrochemical nature of dielectric materials. Electrolytes play a huge role in the established dielectric-electrolyte interface with respect to the overall electrical behavior of the system. Although aqueous cations are considered as spectator ions in most electrochemical systems, in dielectric interfaces the current-potential characteristics depend on the type of cation. Computer simulation based on density functional theory and molecular dynamics showed cations affect the dielectric strength. The responses of various dielectric films to solution components provide invaluable information for dielectric-incorporated electrochemical systems.

Planting Repulsion Centers for Faster Ionic Diffusion in Superionic Conductors.

The successful launch of solid-state batteries relies on the discovery of solid electrolytes with remarkably high ionic conductivity. Extensive efforts have identified several important superionic conductors (SICs) and broadened our understanding of their superionic conductivity. Herein, we propose a new design strategy to facilitate ionic conduction in SICs by planting immobile repulsion centers. Our ab initio molecular dynamics simulations on the model system Na11 Sn2 PS12 demonstrate that the sodium ionic conductivity can be increased by approximately one order of magnitude by simply doping large Cs ions as repulsion centers in the characteristic vacant site of Na11 Sn2 PS12 . Planting immobile repulsion centers locally induces the formation of high-energy sites, leading to a fast track for ionic conduction owing to the unique interactions among mobile ions in SICs. Seemingly non-intuitive approaches tailor the ionic diffusion by exploiting these immobile repulsion centers.

Synergistic multi-doping effects on the Li7La3Zr2O12 solid electrolyte for fast lithium ion conduction.

Here, we investigate the doping effects on the lithium ion transport behavior in garnet Li7La3Zr2O12 (LLZO) from the combined experimental and theoretical approach. The concentration of Li ion vacancy generated by the inclusion of aliovalent dopants such as Al(3+) plays a key role in stabilizing the cubic LLZO. However, it is found that the site preference of Al in 24d position hinders the three dimensionally connected Li ion movement when heavily doped according to the structural refinement and the DFT calculations. In this report, we demonstrate that the multi-doping using additional Ta dopants into the Al-doped LLZO shifts the most energetically favorable sites of Al in the crystal structure from 24d to 96 h Li site, thereby providing more open space for Li ion transport. As a result of these synergistic effects, the multi-doped LLZO shows about three times higher ionic conductivity of 6.14 × 10(-4) S cm(-1) than that of the singly-doped LLZO with a much less efforts in stabilizing cubic phases in the synthetic condition.