Jun Liu

Measuring and controlling the density of states (DOS) and defect states of 2D van der Waals materials is essential for understanding their unique physical properties and for advancing their future practical applications. However, this typically requires experiments performed at cryogenic temperatures and/or in ultra‐high vacuum conditions, severely constraining efforts to monitor the electronic structure evolution of these materials under useful device operating conditions. Here, a new electrochemical quantum capacitance spectroscopy (EQCS) technique for detecting the absolute energies of defect states and band edges in an ambient environment is developed. Its applicability is demonstrated using a variety of 2D material systems, with the ability to easily extend to many more. The highest energy resolution achieved at room temperature, 116 meV, approaches the theoretical limit of 91 meV (3.5 k _B T ). The in situ EQCS platform can monitor and manipulate the DOS in real‐time, enabling controlled enhancement of electrochemical reactions. Notably, new in‐gap states resulting from the formation of sulfur vacancies can activate the relatively inert basal plane of monolayer molybdenum disulfide and increase the catalytic activity for hydrogen generation by nearly threefold. The EQCS platform provides a powerful new method for probing and manipulating the intrinsic DOS and defect states of 2D materials in ambient environments.

Polymer additives [like polyethylene oxide (PEO)] are widely used for smooth electrode deposition in aqueous zinc and many other battery systems. However, the precise mechanism by which they regulate morphology and suppress dendrite formation remains unclear. In this study, the knowledge gap is addressed by using in situ electrochemical atomic force microscopy to directly observe the interfacial evolution during Zn electrodeposition and polymer adsorption on Cu substrates in the presence of varying concentrations of ZnSO ₄ and PEO. Contrary to previous literature assumptions, which emphasize the binding to the growing Zn crystal surfaces or Zn ²⁺ ions, the results demonstrate that PEO smooths Zn films by promoting nucleation of (002)‐oriented Zn platelets through interactions with the Cu substrate. Density functional theory simulations support this finding by showing that PEO adsorption on Cu modifies the interfacial energy of Zn/Cu/electrolyte interfaces, favoring the stabilization of Zn (002) on the Cu substrate, as well as confines Zn electrodeposition to a narrow near‐surface region. These findings elucidate a novel design principle for electrode smoothing, emphasizing the importance of substrate selection paired with polymer additives that exhibit an attractive interaction with the substrate but minimal interaction with growing crystals, offering a mechanistic perspective for improved battery performance.

New electrolytes are critical for high‐energy lithium (Li)–sulfur (S) batteries (LSBs) to ensure their stability against Li metal anode and polysulfides (PSs) shuttling which hinder the large‐scale application of LSBs. In this study, the design principle of moderately solvating electrolytes (MSEs) for LSBs is demonstrated by using a multiple‐solvent system comprising of a highly solvating solvent, a weakly solvating solvent, and a non‐solvating solvent to create a well‐balanced electrolyte system. This resulting electrolyte significantly improves the cycle life of LSBs, achieving 300 cycles, which is twice as long as that of similar cells with the conventional electrolyte and it also ensures stable calendar life for at least seven months. The optimal MSE forms robust passivation layers enhancing the structural integrity of both S and Li metal electrodes after cycling. These virtues effectively hinder parasitic side reactions and self‐discharge behavior of LSBs. This electrolyte design principle is versatile and can be applied to other battery chemistries, providing a potential path toward the development of a more efficient and stable battery system. By addressing key challenges such as the instability of electrodes and shuttling of polysulfides, this electrolyte approach offers promising solutions for advancing LSB technology.

The performance of all-solid-state lithium batteries (ASSLBs) is significantly impacted by lithium interfacial instability, which originates from the dynamic chemical, morphological, and mechanical changes during deep Li plating and stripping. In this study, we introduce a facile approach to generate a conductive and regenerative solid interface, enhancing both the Li interfacial stability and overall cell performance. The regenerative interface is primarily composed of nanosized lithium iodide (nano-LiI), which originates in situ from the adopted solid-state electrolyte (SSE). During cell operation, the nano-LiI interfacial layer can reversibly diffuse back and forth in synchronization with Li plating and stripping. The interface and dynamic process improve the adhesion and Li⁺ transport between the Li anode and SSE, facilitating uniform Li plating and stripping. As a result, the metallic Li anode operates stably for over 1000 h at high current densities and even under elevated temperatures. By using metallic Li as the anode directly, we demonstrate stable cycling of all-solid-state Li-sulfur batteries for over 250 cycles at an areal capacity of >2 mA h cm^-2 and room temperature. This study offers insights into the design of regenerative and Li⁺-conductive interfaces to tackle solid interfacial challenges for high-performance ASSLBs.

Measuring and controlling the density of states (DOS) and defect states of two-dimensional van der Waals materials is of profound importance for understanding their unique physical properties, and for advancing their future practical applications. However, probing their defect states typically requires experiments performed at cryogenic temperatures and/or in ultra-high vacuum conditions, severely constraining efforts to monitor the electronic structure evolution of these materials under useful device operating conditions. Here, we develop a new electrochemical quantum capacitance spectroscopy (EQCS) technique for detecting the absolute energies of defect states and band edges in an ambient environment. We demonstrate the viability of this method with a variety of two-dimensional material systems, with the ability to easily extend to many more. The highest energy resolution achieved at room temperature, 116 meV, approaches the theoretical limit of 91 meV (3.5k_BT). The in-situ EQCS platform can be further used to monitor and manipulate the DOS in real-time, enabling a controlled enhancement of electrochemical reactions. Notably, band shifts driven by as little as ≈ 1% mechanical strain can increase of the catalytic activity for hydrogen generation by half an order of magnitude. The EQCS platform provides a powerful new method for probing and manipulating the intrinsic DOS and defect states of 2D materials in ambient environments.

Lithium metal batteries (LMB) have high energy densities and are crucial for clean energy solutions. The characterization of the lithium metal interphase is fundamentally and practically important but technically challenging. Taking advantage of synchrotron X-ray, which has the unique capability of analyzing crystalline/amorphous phases quantitatively with statistical significance, we study the composition and dynamics of the LMB interphase for a newly developed important LMB electrolyte that is based on fluorinated ether. Pair distribution function analysis revealed the sequential roles of the anion and solvent in interphase formation during cycling. The relative ratio between Li₂O and LiF first increases and then decreases during cycling, suggesting suppressed Li₂O formation in both initial and long extended cycles. Theoretical studies revealed that in initial cycles, this is due to the energy barriers in many-electron transfer. In long extended cycles, the anion decomposition product Li₂O encourages solvent decomposition by facilitating solvent adsorption on Li₂O which is followed by concurrent depletion of both. This work highlights the important role of Li₂O in transitioning from an anion-derived interphase to a solvent-derived one.