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

Each cell of a battery stores electrical energy as chemical energy in two electrodes, a reductant (anode) and an oxidant (cathode), separated by an electrolyte that transfers the ionic component of the chemical reaction inside the cell and forces the electronic component outside the battery. The output on discharge is an external electronic current I at a voltage V for a time Δt. The chemical reaction of a rechargeable battery must be reversible on the application of a charging I and V. Critical parameters of a rechargeable battery are safety, density of energy that can be stored at a specific power input and retrieved at a specific power output, cycle and shelf life, storage efficiency, and cost of fabrication. Conventional ambient-temperature rechargeable batteries have solid electrodes and a liquid electrolyte. The positive electrode (cathode) consists of a host framework into which the mobile (working) cation is inserted reversibly over a finite solid–solution range. The solid–solution range, which is...

The Li-ion rechargeable battery: a perspective.

Electrochemical energy storage technology is based on devices capable of exhibiting high energy density (batteries) or high power density (electrochemical capacitors). There is a growing need, for current and near-future applications, where both high energy and high power densities are required in the same material. Pseudocapacitance, a faradaic process involving surface or near surface redox reactions, offers a means of achieving high energy density at high charge–discharge rates. Here, we focus on the pseudocapacitive properties of transition metal oxides. First, we introduce pseudocapacitance and describe its electrochemical features. Then, we review the most relevant pseudocapacitive materials in aqueous and non-aqueous electrolytes. The major challenges for pseudocapacitive materials along with a future outlook are detailed at the end.

https://hal.archives-ouvertes.fr/hal-01171774/document

Pseudocapacitive oxide materials for high-rate electrochemical energy storage

The lithium metal battery is strongly considered to be one of the most promising candidates for high-energy-density energy storage devices in our modern and technology-based society. However, uncontrollable lithium dendrite growth induces poor cycling efficiency and severe safety concerns, dragging lithium metal batteries out of practical applications. This review presents a comprehensive overview of the lithium metal anode and its dendritic lithium growth. First, the working principles and technical challenges of a lithium metal anode are underscored. Specific attention is paid to the mechanistic understandings and quantitative models for solid electrolyte interphase (SEI) formation, lithium dendrite nucleation, and growth. On the basis of previous theoretical understanding and analysis, recently proposed strategies to suppress dendrite growth of lithium metal anode and some other metal anodes are reviewed. A section dedicated to the potential of full-cell lithium metal batteries for practical applicatio...

Toward Safe Lithium Metal Anode in Rechargeable Batteries: A Review.

Owing to high specific energy, low cost, and environmental friendliness, lithium–sulfur (Li–S) batteries hold great promise to meet the increasing demand for advanced energy storage beyond portable electronics, and to mitigate environmental problems. However, the application of Li–S batteries is challenged by several obstacles, including their short life and low sulfur utilization, which become more serious when sulfur loading is increased to the practically accepted level above 3–5 mg cm−2. More and more efforts have been made recently to overcome the barriers toward commercially viable Li–S batteries with a high sulfur loading. This review highlights the recent progress in high-sulfur-loading Li–S batteries enabled by hierarchical design principles at multiscale. Particularly, basic insights into the interfacial reactions, strategies for mesoscale assembly, unique architectures, and configurational innovation in the cathode, anode, and separator are under specific concerns. Hierarchy in the multiscale design is proposed to guide the future development of high-sulfur-loading Li–S batteries.

Review on High-Loading and High-Energy Lithium–Sulfur Batteries

Li-S batteries have attracted enormous interest due to their potentially high energy density, non-toxicity and the low cost of sulfur. The main challenges of sulfur cathodes are the short cycling life and limited power density caused by the low conductivity of sulfur and dissolution of Li polysulfides. Here we design a new double-hierarchical sulfur host to address these problems. Hierarchical carbon spheres (HCSs), constructed from building blocks of hollow carbon nanobubbles for loading sulfur, are sealed within a thin, polar MoS2 coating composed of ultrathin nanosheets. Experimental and theoretical studies show a strong absorption of the MoS2 nanoshell to polysulfides, and the excellent stability of the MoS2 nanosheets after the adsorption of polysulfides. Moreover, MoS2 promotes the electrochemical redox kinetics in Li-S batteries. Benefiting from the unique hierarchical, hollow and compositional characteristics of the host, the S/MoS2@HCS composite electrode shows a high capacity of 1048 mA h g-1 at 0.2C, good rate capacity and long cycling life with a slow capacity decay of 0.06% per cycle.

/pdf/a-highly-efficient-double-hierarchical-sulfur-host-for-4escxp07ha.pdf

A highly efficient double-hierarchical sulfur host for advanced lithium–sulfur batteries

The lithium insertion behavior of nanoparticle (3-D) and nanosheet (2-D) architectures of TiO2(B) is quite different, as observed by differential capacity plots derived from galvanostatic charging/discharge experiments. DFT+U calculations show unique lithiation mechanisms for the different nanoarchitectures. For TiO2(B) nanoparticles, A2 sites near equatorial TiO6 octahedra are filled first, followed by A1 sites near axial TiO6 octahedra. No open-channel C site filling is observed in the voltage range studied. Conversely, TiO2(B) nanosheets incrementally fill C sites, followed by A2 and A1. DFT+U calculations suggest that the different lithiation mechanisms are related to the elongated geometry of the nanosheet along the a-axis that reduces Li+–Li+ interactions between C and A2 sites. The calculated lithiation potentials and degree of filling agree qualitatively with the experimentally observed differential capacity plots.

/pdf/morphological-dependence-of-lithium-insertion-in-2sbhn5ej9b.pdf

Morphological Dependence of Lithium Insertion in Nanocrystalline TiO2(B) Nanoparticles and Nanosheets

A single-phase crystalline Na3V2O2(PO4)2F material has been prepared by the solvothermal method. Partial ion exchange between Na and Li was then used to form Na3–xLixV2O2(PO4)2F. The two materials were studied as positive cathodes by physical characterization, electrochemical measurements, and simulation. With density functional theory calculations, four stable phases of NaxV2O2(PO4)2F were identified at the Na concentrations of x = 0, 1, 2, 3. The transitions between these phases give rise to three values of the Na chemical potential and three voltage plateaus for Na intercalation. The lower two voltages, corresponding to removal of the first two Na per formula unit, agree well with the corresponding experimental electrochemical measurements. Removal of the third Na, however, is not observed experimentally, because it is outside of the (4.8 V) stability window of the electrolyte. This observation is consistent with our calculations that show that the last Na will only be removed at 5.3 V, owing to the st...

/pdf/theoretical-and-experimental-study-of-vanadium-based-3s2d4t32rb.pdf

Theoretical and Experimental Study of Vanadium-Based Fluorophosphate Cathodes for Rechargeable Batteries

The Prussian Blue analog, NaxFeMn(CN)6, is a potential new cathode material for Na-ion batteries. During Na intercalation, the dehydrated material exhibits a monoclinic to rhombohedral phase transition, while the hydrated material remains in the monoclinic phase. With density functional theory calculations, the phase transition is explained in terms of a competition between Coulomb attraction, Pauli repulsion, and d–π covalent bonding. The interstitial Na cations have a strong Coulomb attraction to the N anions in the host material, which tend to bend the Mn–N bonds and reduce the volume of the structure. The presence of lattice H2O enhances the Pauli repulsion so that the volume reduction is suppressed. The calculated volume change, as it depends upon the presence of lattice H2O, is consistent with experimental measurements. Additionally, a new LiFeMn(CN)6 phase is predicted where MnN6 octahedra decompose into LiN4 and MnN4 edge-sharing tetrahedra.

http://pubs.acs.org/doi/pdf/10.1021/acs.chemmater.5b01132

Theoretical Study of the Structural Evolution of a Na2FeMn(CN)6 Cathode upon Na Intercalation

DOI: 10.1002/aenm.201501933 one of the most promising lithium-based batteries, the Li-S batteries are appealing as both the sulfur cathode and the lithiummetal anode offer an order of magnitude higher charge-storage capacity compared to the currently used insertion-compound electrodes. [ 16,17 ] In addition, sulfur is abundant and environmentally benign while lithium metal offers a desirable low negative electrochemical potential. Unfortunately, the lithium-metal anode suffers from nonuniform metal redeposition and unstable surface chemistry in organic electrolytes, which lead to a continuous breakdown and reformation of the solid electrolyte interphase (SEI) layer during cycling. [ 18 ] To stabilize the lithium-metal anode, various electrolyte systems that target the reduction of the amount of free solvent molecules causing unwanted side reactions have been proposed. [ 19–21 ] Stable high-rate performance with lithium-metal anode has been reported by employing electrolytes with high lithium-salt concentrations, but the practical application of these electrolyte systems in Li-S batteries has not been verifi ed. [ 22 ] Alternatively, the concept of artifi cial SEI layers has been proposed, e.g., isolation of the lithium anode by hollow carbon nanosphere or lithiated graphite fi lms. [ 23,24 ] Albeit increased cycling effi ciency, the stability of the artifi cial SEIs in Li-S cells and their large-scale production remain a challenge for industrial applications. [ 25 ]

Penghao Xiao

Papers

A highly efficient double-hierarchical sulfur host for advanced lithium–sulfur batteries

Morphological Dependence of Lithium Insertion in Nanocrystalline TiO2(B) Nanoparticles and Nanosheets

Theoretical and Experimental Study of Vanadium-Based Fluorophosphate Cathodes for Rechargeable Batteries

Theoretical Study of the Structural Evolution of a Na2FeMn(CN)6 Cathode upon Na Intercalation

Breaking Down the Crystallinity: The Path for Advanced Lithium Batteries