Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Fast parallel algorithms for short-range molecular dynamics

We developed two-step solution-phase reactions to form hybrid materials of Mn3O4 nanoparticles on reduced graphene oxide (RGO) sheets for lithium ion battery applications. Mn3O4 nanoparticles grown selectively on RGO sheets over free particle growth in solution allowed for the electrically insulating Mn3O4 nanoparticles wired up to a current collector through the underlying conducting graphene network. The Mn3O4 nanoparticles formed on RGO show a high specific capacity up to ~900mAh/g near its theoretical capacity with good rate capability and cycling stability, owing to the intimate interactions between the graphene substrates and the Mn3O4 nanoparticles grown atop. The Mn3O4/RGO hybrid could be a promising candidate material for high-capacity, low-cost, and environmentally friendly anode for lithium ion batteries. Our growth-on-graphene approach should offer a new technique for design and synthesis of battery electrodes based on highly insulating materials.

Mn3O4-Graphene Hybrid as a High Capacity Anode Material for Lithium Ion Batteries

Covalent organic frameworks (COFs) are a class of crystalline porous organic polymers with permanent porosity and highly ordered structures. Unlike other polymers, a significant feature of COFs is that they are structurally predesignable, synthetically controllable, and functionally manageable. In principle, the topological design diagram offers geometric guidance for the structural tiling of extended porous polygons, and the polycondensation reactions provide synthetic ways to construct the predesigned primary and high-order structures. Progress over the past decade in the chemistry of these two aspects undoubtedly established the base of the COF field. By virtue of the availability of organic units and the diversity of topologies and linkages, COFs have emerged as a new field of organic materials that offer a powerful molecular platform for complex structural design and tailor-made functional development. Here we target a comprehensive review of the COF field, provide a historic overview of the chemistry of the COF field, survey the advances in the topology design and synthetic reactions, illustrate the structural features and diversities, scrutinize the development and potential of various functions through elucidating structure-function correlations based on interactions with photons, electrons, holes, spins, ions, and molecules, discuss the key fundamental and challenging issues that need to be addressed, and predict the future directions from chemistry, physics, and materials perspectives.

Covalent Organic Frameworks: Design, Synthesis, and Functions.

Notably, many significant breakthroughs for a new generation of supercapacitors have been reported in recent years, related to theoretical understanding, material synthesis and device designs. Herein, we summarize the state-of-the-art progress toward mechanisms, new materials, and novel device designs for supercapacitors. Firstly, fundamental understanding of the mechanism is mainly focused on the relationship between the structural properties of electrode materials and their electrochemical performances based on some in situ characterization techniques and simulations. Secondly, some emerging electrode materials are discussed, including metal–organic frameworks (MOFs), covalent organic frameworks (COFs), MXenes, metal nitrides, black phosphorus, LaMnO3, and RbAg4I5/graphite. Thirdly, the device innovations for the next generation of supercapacitors are provided successively, mainly emphasizing flow supercapacitors, alternating current (AC) line-filtering supercapacitors, redox electrolyte enhanced supercapacitors, metal ion hybrid supercapacitors, micro-supercapacitors (fiber, plane and three-dimensional) and multifunctional supercapacitors including electrochromic supercapacitors, self-healing supercapacitors, piezoelectric supercapacitors, shape-memory supercapacitors, thermal self-protective supercapacitors, thermal self-charging supercapacitors, and photo self-charging supercapacitors. Finally, the future developments and key technical challenges are highlighted regarding further research in this thriving field.

Latest advances in supercapacitors: from new electrode materials to novel device designs.

Lithium-sulfur (Li-S) batteries have attracted tremendous interest because of their high theoretical energy density and cost effectiveness. The target of Li-S battery research is to produce batteries with a high useful energy density that at least outperforms state-of-the-art lithium-ion batteries. However, due to an intrinsic gap between fundamental research and practical applications, the outstanding electrochemical results obtained in most Li-S battery studies indeed correspond to low useful energy densities and are not really suitable for practical requirements. The Li-S battery is a complex device and its useful energy density is determined by a number of design parameters, most of which are often ignored, leading to the failure to meet commercial requirements. The purpose of this review is to discuss how to pave the way for reliable Li-S batteries. First, the current research status of Li-S batteries is briefly reviewed based on statistical information obtained from literature. This includes an analysis of how the various parameters influence the useful energy density and a summary of existing problems in the current Li-S battery research. Possible solutions and some concerns regarding the construction of reliable Li-S batteries are comprehensively discussed. Finally, insights are offered on the future directions and prospects in Li-S battery field.

More Reliable Lithium-Sulfur Batteries: Status, Solutions and Prospects.

A high lithium conductive MoS2 /Celgard composite separator is reported as efficient polysulfides barrier in Li-S batteries. Significantly, thanks to the high density of lithium ions on MoS2 surface, this composite separator shows high lithium conductivity, fast lithium diffusion, and facile lithium transference. When used in Li-S batteries, the separator is proven to be highly efficient for depressing polysulfides shuttle, leading to high and long cycle stability. With 65% of sulfur loading, the device with MoS2 /Celgard separator delivers an initial capacity of 808 mAh g-1 and a substantial capacity of 401 mAh g-1 after 600 cycles, corresponding to only 0.083% of capacity decay per cycle that is comparable to the best reported result so far. In addition, the Coulombic efficiency remains more than 99.5% during all 600 cycles, disclosing an efficient ionic sieve preventing polysulfides migration to the anode while having negligible influence on Li+ ions transfer across the separator. The strategy demonstrated in this work will open the door toward developing efficient separators with flexible 2D materials beyond graphene for energy-storage devices.

MoS2 /Celgard Separator as Efficient Polysulfide Barrier for Long-Life Lithium-Sulfur Batteries.

Conventional technology for the purification of organic solvents requires massive energy consumption, and to reduce such expending calls for efficient filtration membranes capable of high retention of large molecular solutes and high permeance for solvents. Herein, we report a surface-initiated polymerization strategy through C–C coupling reactions for preparing conjugated microporous polymer (CMP) membranes. The backbone of the membranes consists of all-rigid conjugated systems and shows high resistance to organic solvents. We show that 42-nm-thick CMP membranes supported on polyacrylonitrile substrates provide excellent retention of solutes and broad-spectrum nanofiltration in both non-polar hexane and polar methanol, the permeance for which reaches 32 and 22 l m−2 h−1 bar−1, respectively. Both experiments and simulations suggest that the performance of CMP membranes originates from substantially open and interconnected voids formed in the highly rigid networks. Membranes with high selectivity and high permeance that allows rapid passage of solvent molecules are desirable for efficient separation processes. Microporous conjugated-polymer membranes have now been fabricated through surface-initiated polymerization. These membranes are capable of ultrafast organic-solvent nanofiltration because of the high porosity and pore interconnectivity originating from the rigid skeleton.

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Microporous membranes comprising conjugated polymers with rigid backbones enable ultrafast organic-solvent nanofiltration

Efficient Polysulfide Chemisorption in Covalent Organic Frameworks for High-Performance Lithium-Sulfur Batteries

Covalent organic frameworks (COFs) are a novel class of porous crystalline organic materials assembled from molecular building blocks. Due to their high surface area and ability to precisely control electro-active groups in their pores, COFs have emerged as an ideal candidate for energy storage devices. Here we present a two dimensional (2D) COF with redox active pyridine units that exhibit reversible electrochemical processes. When used as an electrode in supercapacitors, this redox COF shows faradaic behavior and excellent chemical stability even after 6000 charge–discharge cycles. This strategy may shed light on designing new redox active COF based faradaic supercapacitors and other electrochemical devices.

A redox-active 2D covalent organic framework with pyridine moieties capable of faradaic energy storage

Membrane technology is one of the most promising technologies for separation and purification that is routinely and commercially employed in aqueous solutions. In comparison, its applications in organic solvents are severely underdeveloped mainly due to the poor stability of traditional polymer membranes in organic solvents. The emerging materials such as crosslinked polymers, covalent organic frameworks, metal-organic frameworks, conjugated microporous polymers, carbon molecular sieves, and graphene provide the solutions to address this problem. The membranes constructed with these novel materials show outstanding separation performance in regard to both high selectivity and solvent permeability, greatly pushing forward utilization of membrane technology in organic media. Here, an overview of the most important organic mixtures that need to be separated, the major separation processes adopted nowadays in organic solvents, and the recent progress in new developed membranes is provided.

Bin Liang

Papers

MoS2 /Celgard Separator as Efficient Polysulfide Barrier for Long-Life Lithium-Sulfur Batteries.

Microporous membranes comprising conjugated polymers with rigid backbones enable ultrafast organic-solvent nanofiltration

Efficient Polysulfide Chemisorption in Covalent Organic Frameworks for High-Performance Lithium-Sulfur Batteries

A redox-active 2D covalent organic framework with pyridine moieties capable of faradaic energy storage

Membrane Separation in Organic Liquid: Technologies, Achievements, and Opportunities.