Houlong L. Zhuang

Prediction and characterization of MXene nanosheet anodes for non-lithium-ion batteries.

Rechargeable non-lithium-ion (Na(+), K(+), Mg(2+), Ca(2+), and Al(3+)) batteries have attracted great attention as emerging low-cost and high energy-density technologies for large-scale renewable energy storage applications. However, the development of these batteries is hindered by the limited choice of high-performance electrode materials. In this work, MXene nanosheets, a class of two-dimensional transition-metal carbides, are predicted to serve as high-performing anodes for non-lithium-ion batteries by combined first-principles simulations and experimental measurements. Both O-terminated and bare MXenes are shown to be promising anode materials with high capacities and good rate capabilities, while bare MXenes show better performance. Our experiments clearly demonstrate the feasibility of Na- and K-ion intercalation into terminated MXenes. Moreover, stable multilayer adsorption is predicted for Mg and Al, which significantly increases their theoretical capacities. We also show that O-terminated MXenes can decompose into bare MXenes and metal oxides when in contact with Mg, Ca, or Al. Our results provide insight into metal ion storage mechanisms on two-dimensional materials and suggest a route to preparing bare MXene nanosheets.

Softened elastic response and unzipping in chemical vapor deposition graphene membranes

We use atomic force microscopy to image grain boundaries and ripples in graphene membranes obtained by chemical vapor deposition. Nanoindentation measurements reveal that out-of-plane ripples effectively soften graphenes in-plane stiffness. Furthermore, grain boundaries significantly decrease the breaking strength of these membranes. Molecular dynamics simulations reveal that grain boundaries are especially weakening when subnanometer voids are present in the lattice. Finally, we demonstrate that two graphene membranes brought together form membranes with higher resistance to breaking.

Angle-Resolved Raman Imaging of Interlayer Rotations and Interactions in Twisted Bilayer Graphene

Few-layer graphene is a prototypical layered material, whose properties are determined by the relative orientations and interactions between layers. Exciting electrical and optical phenomena have been observed for the special case of Bernal-stacked few-layer graphene, but structure-property correlations in graphene which deviates from this structure are not well understood. Here, we combine two direct imaging techniques, dark-field transmission electron microscopy (DF-TEM) and widefield Raman imaging, to establish a robust, one-to-one correlation between twist angle and Raman intensity in twisted bilayer graphene (tBLG). The Raman G band intensity is strongly enhanced due to a previously unreported singularity in the joint density of states of tBLG, whose energy is exclusively a function of twist angle and whose optical transition strength is governed by interlayer interactions, enabling direct optical imaging of these parameters. Furthermore, our findings suggest future potential for novel optical and optoelectronic tBLG devices with angle-dependent, tunable characteristics.

Enhanced Li–S Batteries Using Amine-Functionalized Carbon Nanotubes in the Cathode

The rechargeable lithium-sulfur (Li-S) battery is an attractive platform for high-energy, low-cost electrochemical energy storage. Practical Li-S cells are limited by several fundamental issues, including the low conductivity of sulfur and its reduction compounds with Li and the dissolution of long-chain lithium polysulfides (LiPS) into the electrolyte. We report on an approach that allows high-performance sulfur-carbon cathodes to be designed based on tethering polyethylenimine (PEI) polymers bearing large numbers of amine groups in every molecular unit to hydroxyl- and carboxyl-functionalized multiwall carbon nanotubes. Significantly, for the first time we show by means of direct dissolution kinetics measurements that the incorporation of CNT-PEI hybrids in a sulfur cathode stabilizes the cathode by both kinetic and thermodynamic processes. Composite sulfur cathodes based the CNT-PEI hybrids display high capacity at both low and high current rates, with capacity retention rates exceeding 90%. The attractive electrochemical performance of the materials is shown by means of DFT calculations and physical analysis to originate from three principal sources: (i) specific and strong interaction between sulfur species and amine groups in PEI; (ii) an interconnected conductive CNT substrate; and (iii) the combination of physical and thermal sequestration of LiPS provided by the CNT=PEI composite.

Computational Screening of 2D Materials for Photocatalysis.

Two-dimensional (2D) materials exhibit a range of extraordinary electronic, optical, and mechanical properties different from their bulk counterparts with potential applications for 2D materials emerging in energy storage and conversion technologies. In this Perspective, we summarize the recent developments in the field of solar water splitting using 2D materials and review a computational screening approach to rapidly and efficiently discover more 2D materials that possess properties suitable for solar water splitting. Computational tools based on density-functional theory can predict the intrinsic properties of potential photocatalyst such as their electronic properties, optical absorbance, and solubility in aqueous solutions. Computational tools enable the exploration of possible routes to enhance the photocatalytic activity of 2D materials by use of mechanical strain, bias potential, doping, and pH. We discuss future research directions and needed method developments for the computational design and optimization of 2D materials for photocatalysis.

Ab Initio Prediction of Piezoelectricity in Two-Dimensional Materials

Two-dimensional (2D) materials present many unique materials concepts, including material properties that sometimes differ dramatically from those of their bulk counterparts. One of these properties, piezoelectricity, is important for micro- and nanoelectromechanical systems applications. Using symmetry analysis, we determine the independent piezoelectric coefficients for four groups of predicted and synthesized 2D materials. We calculate with density-functional perturbation theory the stiffness and piezoelectric tensors of these materials. We determine the in-plane piezoelectric coefficient d11 for 37 materials within the families of 2D metal dichalcogenides, metal oxides, and III-V semiconductor materials. A majority of the structures, including CrSe2, CrTe2, CaO, CdO, ZnO, and InN, have d11 coefficients greater than 5 pm/V, a typical value for bulk piezoelectric materials. Our symmetry analysis shows that buckled 2D materials exhibit an out-of-plane coefficient d31. We find that d31 for 8 III-V semiconductors ranges from 0.02 to 0.6 pm/V. From statistical analysis, we identify correlations between the piezoelectric coefficients and the electronic and structural properties of the 2D materials that elucidate the origin of the piezoelectricity. Among the 37 2D materials, CdO, ZnO, and CrTe2 stand out for their combination of large piezoelectric coefficient and low formation energy and are recommended for experimental exploration.

Hybrid cathode architectures for lithium batteries based on TiS2 and sulfur

Great efforts have been devoted towards development of rechargeable lithium–sulfur (Li–S) battery designs that offer extended cycling. The poor electronic and ionic conductivity of sulfur and its reduction compounds with lithium, solubility of some sulfides in the most commonly used Li–S electrolyte solvents, and the volume changes that accompany lithiation and delithiation processes in a sulfur cathode, pose significant challenges that have so far impeded commercialization of rechargeable Li–S batteries. We demonstrate that TiS2, which uses an intercalation chemistry and is known for its high rate capability and stable performance as a lithium battery cathode, and sulfur, which has very high capacity (1675 mA h g−1) when coupled in a conversion-based electrochemical reaction with lithium metal, can be combined to produce hybrid cathodes in which the two materials function synergistically in the same electrolyte and within the same working voltage. We further show that co-generation of interconnected S8/TiS2 hybrid foams through thermal reaction of Ti precursor foams and S8 yields 3D hybrid cathode structures in which sulfur is infused into porous TiS2 foams. In galvanostatic electrochemical cycling studies the hybrid cathodes demonstrate high areal specific capacity (9 mA h cm−2) and high retention ratios, even at a relatively large areal mass loading of ∼40 mg sulfur per cm2 and high current density (10 mA cm−2). We attribute the improved performance of the materials to the synergistic effect in which TiS2 not only improves the conductivity and rate capability of the cathode, but exerts a strong affinity for soluble lithium polysulfides, which limit their loss to the electrolyte.

Electronic structures of single-layer boron pnictides

Single-layer materials such as graphene and boron nitride promise alternative routes to electronic devices. Hybrid density functional calculations for single-layer boron pnictides boron nitride (BN), boron phosphide (BP), boron arsenide (BAs), and boron antimonide (BSb) show that these materials exhibit a direct bandgap of 6.1, 1.4, 1.2, and 0.6 eV, respectively, that originates from the energy difference of the pz orbitals of the species and is tunable by strain. The bandgap linearly decreases with strain for BN, while it increases non-linearly for BP, BAs, and BSb. The calculated natural band offsets between the various boron pnictides are all of type I.

Ultrathin nanosheets of CrSiTe3: a semiconducting two-dimensional ferromagnetic material

Finite range ferromagnetism and antiferromagnetism in two-dimensional (2D) systems within an isotropic Heisenberg model at non-zero temperature were originally proposed to be impossible. However, recent theoretical studies using an Ising model have shown that 2D magnetic crystals can exhibit magnetism. Experimental verification of existing 2D magnetic crystals in this system has remained exploratory. In this work we exfoliated CrSiTe3, a bulk ferromagnetic semiconductor, to mono- and few-layer 2D crystals onto a Si/SiO2 substrate. Raman spectra indicate good stability and high quality of the exfoliated flakes, consistent with the computed phonon spectra of 2D CrSiTe3, giving strong evidence for the existence of 2D CrSiTe3 crystals. When the thickness of the CrSiTe3 crystals is reduced to a few layers, we observed a clear change in resistivity at 80–120 K, consistent with theoretical calculations of the Curie temperature (Tc) of ∼80 K for the magnetic ordering of 2D CrSiTe3 crystals. The ferromagnetic mono- and few-layer 2D CrSiTe3 indicated here should enable numerous applications in nano-spintronics.

Regulating Li deposition at artificial solid electrolyte interphases

Developing high-energy lithium-metal batteries (LMBs) as a high priority for next-generation electrochemical energy storage. The unstable solid electrolyte interphase (SEI) formed on the anode and the subsequent nucleation of uneven lithium electrodeposits are now known to be the key limitations to progress in the field. These challenges are shared with multiple other metal anodes, implying that fundamentally-based solutions are broadly interesting. Here, we report a novel strategy of promoting uniform lithium electrodeposition by establishing an artificial SEI layer on the lithium anode via magnetron sputtering deposition. We show by means of density functional theory (DFT) simulations that an artificial SEI enriched in lithium fluoride (LiF) salt provides a low energy barrier (Eb = 0.19 eV in vacuum) for the diffusion of lithium ions, in comparison to SEI components formed spontaneously at the interface by Li reaction with electrolyte components. We also compute the electron localization function (ELF) for Li adsorbates on LiF and show for the first time that degeneracy in the adsorption energy is responsible for the low in-plane diffusion barriers experienced by Li ad atoms on LiF. On this basis we create artificial SEIs based on LiF on Li electrodes and find that the electrodes exhibit exceptional ability to promote smooth Li deposition during charging at a high current density of 1 mA cm−2. Through post-mortem SEM analysis of patterned lithium electrodes, we find visual support for the effectiveness of the artificial SEI. By means of impedance and XPS analyses, we further show that the artificial SEI retains its integrity upon repeated cycles.