Capacity degradation by phase changes and oxygen evolution has been the largest obstacle for the ultimate commercialization of high‐capacity LiNiO2‐based cathode materials. The ultimate thermodynamic and kinetic reasons of these limitations are not yet systematically studied, and the fundamental mechanisms are still poorly understood. In this work, both phenomena are studied by density functional theory simulations and validation experiments. It is found that during delithiation of LiNiO2, decreased oxygen reduction induces a strong thermodynamic driving force for oxygen evolution in bulk. However, oxygen evolution is kinetically prohibited in the bulk phase due to a large oxygen migration kinetic barrier (2.4 eV). In contrast, surface regions provide a larger space for oxygen migration leading to facile oxygen evolution. These theoretical results are validated by experimental studies, and the kinetic stability of bulk LiNiO2 is clearly confirmed. Based on these findings, a rational design strategy for protective surface coating is proposed.

Two-dimensional transition metal dichalcogenides (TMDs) are promising low-dimensional materials which can produce diverse electronic properties and band alignment in van der Waals heterostructures. Systematic density functional theory (DFT) calculations are performed for 24 different TMD monolayers and their bilayer heterostacks. DFT calculations show that monolayer TMDs can behave as semiconducting, metallic or semimetallic depending on their structures; we also calculated the band alignment of the TMDs to predict their alignment in van der Waals heterostacks. We have applied the charge equilibration model (CEM) to obtain a quantitative formula predicting the highest occupied state of any type of bilayer TMD heterostacks (552 pairs for 24 TMDs). The CEM predicted values agree quite well with the selected DFT simulation results. The quantitative prediction of the band alignment in the TMD heterostructures can provide an insightful guidance to the development of TMD-based devices.

Metal-insulator transitions in low-dimensional materials under ambient conditions are rare and worth pursuing due to their intriguing physics and rich device applications. Monolayer MoTe2 and WTe2 are distinguished from other TMDs by the existence of an exceptional semimetallic distorted octahedral structure (T') with a quite small energy difference from the semiconducting H phase. In the process of transition metal alloying, an equal stability point of the H and the T' phase is observed in the formation energy diagram of monolayer WxMo1-xTe2. This thermodynamically driven phase transition enables a controlled synthesis of the desired phase (H or T') of monolayer WxMo1-xTe2 using a growth method such as chemical vapor deposition (CVD) and molecular beam epitaxy (MBE). Furthermore, charge mediation, as a more feasible method, is found to make the T' phase more stable than the H phase and induce a phase transition from the H phase (semiconducting) to the T' phase (semimetallic) in monolayer WxMo1-xTe2 alloy. This suggests that a dynamic metal-insulator phase transition can be induced, which can be exploited for rich phase transition applications in two-dimensional nanoelectronics.

Despite these advantages, the large-scale applications of Li-S batteries are hindered by some major challenges originating from the multielectron and multiphase S electrochemistry. [3] The electronic/ionic insulating nature of bulk S and Li sulfides (Li 2 S 2 /Li 2 S) results in sluggish reaction kinetics and low utilization of the active material. [4] Moreover, dissolved Li polysulfide intermediates (Li 2 S n , n = 4-8) undergo reduction at the Li anode and diffuse back to the cathode. This induces the detrimental "shuttle effect" that results in low Coulombic efficiency, continuous electrode degradation, and rapid capacity decay. [5] To address the aforementioned issues, significant efforts have been devoted to the development of various conducting/ polar host materials that can encapsulate sulfur. [6] However, these "inside" modification strategies for the cathode cannot prevent pronounced diffusion of Li 2 S n through the separator to the Li anode over time. This is attributed to the inevitable Li 2 S n dissolution and escape from the S carriers. It is necessary to develop an effective cathode "outside" design strategy to block the shuttle pathway. [7] Extensive research has been conducted, based on this concept, on the modification of separator with various derived carbonbased materials. The objective of the research is to localize the soluble Li 2 S n on the cathode side and enable the reutilization of the trapped active S. [8] 2D porous carbon nanosheets exhibit a high surface area and close-packing laminar structure. Critical drawbacks, including sluggish redox kinetics and undesirable shuttling of polysulfides (Li2 S n , n = 4-8), seriously deteriorate the electrochemical performance of high-energy-density lithium-sulfur (Li-S) batteries. Herein, these challenges are addressed by constructing an integrated catalyst with dual active sites, where single-atom (SA)-Fe and polar Fe 2 N are co-embedded in nitrogen-doped graphene (SA-Fe/Fe 2 N@NG). The SA-Fe, with plane-symmetric Fe-4N coordination, and Fe 2 N, with triangular pyramidal Fe-3N coordination, in this well-designed configuration exhibit synergistic adsorption of polysulfides and catalytic selectivity for Li 2 S n lithiation and Li 2 S delithiation, respectively. These characteristics endow the SA-Fe/Fe 2 N@NG-modified separator with an optimal polysulfides confinement-catalysis ability, thus accelerating the bidirectional liquid-solid conversion (Li 2 S n ↔Li 2 S) and suppressing the shuttle effect. Consequently, a Li-S battery based on the SA-Fe/Fe 2 N@NG separator achieves a high capacity retention of 84.1% over 500 cycles at 1 C (pure S cathode, S content: 70 wt%) and a high areal capacity of 5.02 mAh cm −2 at 0.1 C (SA-Fe/Fe 2 N@NG-supported S cathode, S loading = 5 mg cm −2 ). It is expected that the outcomes of the present study will facilitate the design of high-efficiency catalysts for long-lasting Li-S batteries.

The limited grain size (<200 nm) for transition metal dichalcogenides (TMDs) grown by molecular beam epitaxy (MBE) reported in the literature thus far is unsuitable for high-performance device applications. In this work, the fundamental nucleation and growth behavior of WSe 2 is investigated through a detailed experimental design combined with on-lattice, diffusion-based first principles kinetic modeling to enable large area TMD growth. A three-stage adsorption-diffusion-attachment mechanism is identified and the adatom stage is revealed to play a significant role in the nucleation behavior. To limit the nucleation density and promote 2D layered growth, it is necessary to have a low metal flux in conjunction with an elevated substrate temperature. At the same time, providing a Serich environment further limits the formation of W-rich nuclei which suppresses vertical growth and promotes 2D growth. The fundamental understanding gained through this investigation has enabled an increase of over one order of magnitude in grain size for WSe 2 thus far, and provides valuable insight into improving the growth of other TMD compounds by MBE and other growth techniques such as chemical vapor deposition (CVD).